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Journal Cover Journal of Geophysical Research : Space Physics
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   ISSN (Online) 2169-9402
   Published by AGU Homepage  [17 journals]
  • Investigating the Development of Localized Neutral Density Increases
           During the 24 August 2005 Geomagnetic Storm
    • Authors: Ildiko Horvath; Brian C. Lovell
      Abstract: We investigate localized thermospheric neutral density increases (or density spikes) developed during the 24 August 2005 geomagnetic storm and utilize a multi-instrument database for monitoring the underlying thermospheric, ionospheric, magnetospheric, and solar wind conditions. We study five scenarios occurring under eastward/westward BY dominations and demonstrating the development of density spikes during various events. These scenarios show (1) a northern density spike associated with enhanced antisunward polar flows driven by dayside merging along old-open field lines during the initial phase, (2) some southern density spikes associated with enhanced sunward auroral flows caused by auroral brightening events during the storm initial phase and main phase, (3) two pairs of southern dayside-nightside density spike configuration associated with enhanced antisunward polar flows caused by dayside merging and nightside magnetotail reconnections along old-open field lines during the underlying substorm activity, and (4) a series of enhanced polar sunward flows developed in the polar cap driven by different processes during the recovery phase. We provide a detailed analysis of these events including the specification of underlying field aligned current systems, flow channel types, and auroral forms. Observational results demonstrate the various density spikes and their associated auroral forms and/or flow channels and thus provide a better understanding of their development during this intense geomagnetic storm.
      PubDate: 2017-11-24T21:56:34.091408-05:
      DOI: 10.1002/2017JA024362
       
  • On the Origin of Ionospheric Hiss: A Conjugate Observation
    • Authors: Zeren Zhima; Lunjin Chen, Ying Xiong, Jinbin Cao, Huishan Fu
      Abstract: We present a conjugate observation on whistler mode electromagnetic hiss from the low Earth orbit satellite Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) and the high-altitude elliptical orbit spacecraft Time History of Events and Macroscale Interactions during Substorms (THEMIS). The conjugate observation was performed at 14:51:10 to 15:12:00 UT on 15 June 2010, when DEMETER was flying across the L shell region from ~1.39 to 2.80 at an altitude of ~660 km; meanwhile, THEMIS probes were passing through the L shell region from ~1.64 to 1.91 at altitudes from ~1.6 to 2.0 RE. The conjugated observations demonstrate similar time-frequency structures between the ionospheric hiss (~350 to 800 Hz) captured by DEMETER and the plasmaspheric hiss (~350 to 900 Hz) recorded by THEMIS probes, including similar peak frequencies (~500 to 600 Hz), similar lower cutoff frequencies (~350 to 400 Hz), and upper cutoff frequencies (~730 to 800 Hz). The wave vector analyses show that the ionospheric hiss propagates obliquely downward to the Earth and slightly equatorward with right-handed polarization, suggesting that its source comes from higher altitudes. Ray tracing simulations with the constraint of observations verify that the connection between ionospheric and plasmaspheric hiss is physically possible through wave propagation. This study provides direct observational evidence to support the mechanism that high-altitude plasmaspheric hiss is responsible for the generation of low-altitude ionospheric hiss.
      PubDate: 2017-11-24T21:55:51.981193-05:
      DOI: 10.1002/2017JA024803
       
  • Seasonal Changes in Hydrogen Escape From Mars Through Analysis of HST
           Observations of the Martian Exosphere Near Perihelion
    • Authors: D. Bhattacharyya; J. T. Clarke, J. Y. Chaufray, M. Mayyasi, J. L. Bertaux, M. S. Chaffin, N. M. Schneider, G. L. Villanueva
      Abstract: Hubble Space Telescope (HST) observations of the Martian hydrogen exosphere in Lyman α are presented in this paper for a period when Mars passed perihelion and southern summer solstice in its orbit. The peak intensity in the exospheric Lyman α brightness was recorded after Mars went past its perihelion, slightly after southern summer solstice. The increase in brightness as Mars approached perihelion was found to not be symmetric around the peak, making it impossible to fit the H escape flux trend with a single sinusoidal curve with the peak at perihelion. While the short-term (~30 Earth days) changes were not directly correlated with changes in the solar Lyman α flux, the long-term (~10 Earth years) trend in the data does show some correlation with solar activity. This suggests that the short-term changes brought about in the exosphere could be due to intrinsic changes occurring within the lower atmosphere. For example, thermospheric heating by dust can alter the cold-trapping mechanism for water vapor resulting in it being present in large quantities at higher altitudes (60–80 km), possibly enhancing the escape flux of H. Therefore, it is important to understand the drivers of atmospheric dynamics in the Martian atmosphere, which produce the yearly enhanced seasonal changes observed at Mars around periapsis and southern summer solstice in order to accurately determine the total amount of water lost over its history.
      PubDate: 2017-11-22T23:05:45.076826-05:
      DOI: 10.1002/2017JA024572
       
  • An Artificial Neural Network-Based Ionospheric Model to Predict NmF2 and
           hmF2 Using Long-Term Data Set of FORMOSAT-3/COSMIC Radio Occultation
           Observations: Preliminary Results
    • Authors: V. Sai Gowtam; S. Tulasi Ram
      Abstract: Artificial Neural Networks (ANNs) are known to be capable of solving linear as well as highly nonlinear problems. Using the long-term and high-quality data set of Formosa Satellite-3/Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC, in short F3/C) from 2006 to 2015, an ANN-based two-dimensional (2-D) Ionospheric Model (ANNIM) is developed to predict the ionospheric peak parameters, such as NmF2 and hmF2. In this pilot study, the ANNIM results are compared with the original F3/C data, GRACE (Gravity Recovery and Climate Experiment) observations as well as International Reference Ionosphere (IRI)-2016 model to assess the learning efficiency of the neural networks used in the model. The ANNIM could well predict the NmF2 (hmF2) values with RMS errors of 1.87 × 105 el/cm3 (27.9 km) with respect to actual F3/C; and 2.98 × 105 el/cm3 (40.18 km) with respect to independent GRACE data. Further, the ANNIM predictions found to be as good as IRI-2016 model with a slightly smaller RMS error when compared to independent GRACE data. The ANNIM has successfully reproduced the local time, latitude, longitude, and seasonal variations with errors ranging ~15–25% for NmF2 and 10–15% for hmF2 compared to actual F3/C data, except the postsunset enhancement in hmF2. Further, the ANNIM has also captured the global-scale ionospheric phenomena such as ionospheric annual anomaly, Weddell Sea Anomaly, and the midlatitude summer nighttime anomaly. Compared to IRI-2016 model, the ANNIM is found to have better represented the fine longitudinal structures and the midlatitude summer nighttime enhancements in both the hemispheres.
      PubDate: 2017-11-22T22:55:54.693168-05:
      DOI: 10.1002/2017JA024795
       
  • Plasma sheet pressure variations in the near Earth magnetotail during
           substorm growth phase: THEMIS observations
    • Authors: W. J. Sun; S. Y. Fu, Y. Wei, Z. H. Yao, Z. J. Rong, X. Z. Zhou, J. A. Slavin, W. X. Wan, Q. G. Zong, Z. Y. Pu, Q. Q. Shi, X. C. Shen
      Abstract: We investigate the plasma sheet pressure variations in the near Earth magnetotail (Radius distance, R, from 7.5 RE to 12 RE and Magnetic Local Time, MLT, from 18:00 to 06:00) during substorm growth phase with Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations. It is found that, during the substorm growth phase, about 39.4% (76/193) of the selected events display a phenomenon of equatorial plasma pressure (Peq) decrease. The occurrence rates of Peq decrease cases are higher in the dawn (04:00 to 06:00) and dusk (18:00 to 20:00) flanks (> 50%) than in the midnight region (20:00 to 04:00, < 40%). The mean values of the maximum percentages of Peq decrease during the substorm growth phases are larger in the dawn and dusk flanks (~ - 20%) than in the midnight region (~> - 16%). The mean value of Peq increase percentages at the end of substorm growth phase is the highest (~ 40%) in the pre-midnight MLT bin (22:00 to 00:00) and is almost unchanged in the dawn and dusk flanks. Further investigations show that 13.0% of the events have more than 10% of Peq decrease at the end of substorm growth phase comparing to the value before the growth phase, and ~ 28.0% of the events have small changes (< 10%), and ~ 59.0% events have a 10% increase. This study also reveals the importance of electron pressure (Pe) in the variation of Peq in the substorm growth phase. The Pe variations often account for more than 50% of the Peq changes, and the ratios of Pe to ion pressure often display large variations (~ 50%). Among the investigated events, during the growth phase, an enhanced equatorial plasma convection flow is observed, which diverges in the midnight tail region and propagates azimuthally towards the dayside magnetosphere with velocity of ~ 20 km/s. It is proposed that the Peq decreases in the near Earth plasma sheet during the substorm growth phase may be due to the transport of closed magnetic flux towards the dayside magnetosphere driven by dayside magnetopause reconnection. Both solar wind and ionospheric conductivity effects may influence the distributions of occurrence rates for Peq decrease events and the Peq increase percentages in the investigated region.
      PubDate: 2017-11-22T09:20:35.580939-05:
      DOI: 10.1002/2017JA024603
       
  • Long-term variability of Jupiter's magnetodisk and implications for the
           aurora
    • Authors: Marissa F. Vogt; Emma J. Bunce, Jonathan D. Nichols, John T. Clarke, William S. Kurth
      Abstract: Observations of Jupiter's UV auroral emissions collected over several years show that the ionospheric positions of the main emission and the Ganymede footprint can vary by as much as 3 degrees in latitude. One explanation for this shift is a change of Jupiter's current sheet current density, which would alter the amount of field line stretching and displace the ionospheric mapping of field lines from a given radial distance in the magnetosphere. In this study we measure the long-term variability of Jupiter's magnetodisk using Galileo magnetometer data collected from 1996 to 2003. Using the Connerney et al. [1981] current sheet model, we calculate the current sheet density parameter that gives the best fit to the data from each orbit and find that the current density parameter varies by about 15 percent of its average value during the Galileo era. We investigate possible relationships between the observed current sheet variability and quantities such as Io's plasma torus production rate inferred from volcanic activity and external solar wind conditions extrapolated from data at 1 AU, but find only a weak correlation. Finally, we trace Khurana [1997] model field lines to show that the observed changes in Jupiter's current sheet are sufficient to shift the ionospheric footprint of Ganymede and main auroral emission by a few degrees of latitude, consistent with the magnitude of auroral variability observed by HST. However, we find that the measured auroral shifts in HST images are not consistent with concurrent changes in the current density parameter measured by Galileo.
      PubDate: 2017-11-22T09:15:39.660371-05:
      DOI: 10.1002/2017JA024066
       
  • Influence of auroral streamers on rapid evolution of ionospheric SAPS
           flows
    • Authors: Bea Gallardo-Lacourt; Y. Nishimura, L. R. Lyons, E. V. Mishin, J. M. Ruohoniemi, E. F. Donovan, V. Angelopoulos, N. Nishitani
      Abstract: Subauroral Polarization Streams (SAPS) often show large, rapid enhancements above their slowly-varying component. We present simultaneous observations from ground-based all-sky imagers (ASI) and flows from the SuperDARN radars to investigate the relationship between auroral phenomena and flow enhancement. We first identified auroral streamers approaching the equatorward boundary of the auroral oval to examine how often the subauroral flow increased. We also performed the reverse query starting with subauroral flow enhancements and then evaluating the auroral conditions. In the forward study, 98% of the streamers approaching the equatorward boundary were associated with SAPS flow enhancements reaching ~700 m/s and typically 100's m/s above background speeds. The reverse study reveals that flow enhancements associated with streamers (60%) and enhanced larger-scale convection (37%) contribute to SAPS flow enhancements. The strong correlation of auroral streamers with rapid evolution (~minutes) of SAPS flows suggests that transient fast earthward plasma sheet flows can often lead to westward SAPS flow enhancements in the subauroral region, and that such enhancements are far more common than only during substorms because of the much more frequent occurrences of streamers under various geomagnetic conditions. We also found a strong correlation between flow duration and streamer duration, and a weak correlation between SAPS flow velocity and streamer intensity. This result suggests that intense flow bursts in the plasma sheet (which correlate with intense streamers) are associated with intense SAPS ionospheric flows perhaps by enhancing the ring current pressure and localized pressure gradients when they are able to penetrate close enough to Earth.
      PubDate: 2017-11-21T08:25:21.847195-05:
      DOI: 10.1002/2017JA024198
       
  • How much flux does a flux transfer event transfer'
    • Authors: R. C. Fear; L. Trenchi, J. C. Coxon, S. E. Milan
      Abstract: Flux transfer events are bursts of reconnection at the dayside magnetopause, which give rise to characteristic signatures observed by a range of magnetospheric/ionospheric instrumentation. One outstanding problem is that there is a fundamental mismatch between space-based and ionospheric estimates of the flux that is opened by each flux transfer event âĂŞ- in other words, their overall significance in the Dungey cycle. Spacecraft-based estimates of the flux content of individual FTEs correspond to each event transferring flux equivalent to approximately 1% of the open flux in the magnetosphere, whereas studies based on global-scale radar and auroral observations suggest this figure could be of the order of 10%. In the former case, flux transfer events would be a minor detail in the Dungey cycle, but in the latter they could be its main driver. We present observations of two conjunctions between flux transfer events observed by the Cluster spacecraft, and pulsed ionospheric flows observed by the SuperDARN network. In both cases, a similar number of FTE signatures were observed by Cluster and one of the SuperDARN radars, but the conjunctions differ in the azimuthal separation of the spacecraft and ionospheric observations (i.e. the distance of the spacecraft from the cusp throat). We argue that the reason for the existing mismatch in flux estimates is due to implicit assumptions made about FTE structure, which tacitly ignore the majority of flux opened in mechanisms based on longer reconnection lines. If the effects of such mechanisms are considered, a much better match is found.
      PubDate: 2017-11-20T09:40:44.110514-05:
      DOI: 10.1002/2017JA024730
       
  • The relationship of high-latitude thermospheric wind with ionospheric
           horizontal current, as observed by CHAMP satellite
    • Authors: Tao Huang; Hermann Lühr, Hui Wang, Chao Xiong
      Abstract: The relationship between high-latitude ionospheric currents (Hall current and field-aligned current) and thermospheric wind is investigated. The 2D patterns of horizontal wind and equivalent current in the Northern Hemisphere derived from the CHAMP satellite are considered for the first time simultaneously. The equivalent currents show strong dependences on both interplanetary magnetic field (IMF) By and Bz components. However, IMF By orientation is more important in controlling the wind velocity patterns. The duskside wind vortex as well as the anti-sunward wind in the morning polar cap are more evident for positive By. To better understand their spatial relation in different sectors, a systematic superposed epoch analysis is applied. Our results show that in the dusk sector the vectors of the zonal wind and equivalent current are anti-correlated. And both of them form a vortical flow pattern for different activity levels. The currents and zonal wind are intensified with the increase of merging electric field. However, on the dawnside, where the relation is less clear, anti-sunward zonal winds dominate. Plasma drift seems to play a less important role for the wind than neutral forces in this sector. In the noon sector the best anti-correlation between equivalent current and wind is observed for a positive IMF By component and it is less obvious for negative By. A clear seasonal effect with current intensities increasing from winter to summer is observed in the noon sector. Different from the currents, the zonal wind intensity shows little dependence on seasons. Our results indicate that the plasma drift and the neutral forces are of comparable influence on the zonal wind at CHAMP altitude in the noon sector.
      PubDate: 2017-11-20T09:40:30.404687-05:
      DOI: 10.1002/2017JA024614
       
  • Ion friction and quantification of the geomagnetic influence on gravity
           wave propagation and dissipation in the thermosphere-ionosphere
    • Authors: Alexander S. Medvedev; Erdal Yiğit, Paul Hartogh
      Abstract: Motions of neutrals and ions in the thermosphere-ionosphere (TI) do not, generally, coincide due to the presence of the geomagnetic field and associated electromagnetic forces affecting plasma. Collisions of ions with gravity wave (GW)-induced motions of neutrals impose damping on the latter. We derive a practical formula for the vertical damping rate of GW harmonics that accounts for the geometry of the geomagnetic field and the direction of GW propagation. The formula can be used in parameterizations of GW effects developed for general circulation models extending from the lower atmosphere into the mesosphere and thermosphere. Vertical damping of GW harmonics by ion-neutral interactions in the TI depends on the geometry of the geomagnetic field, but not the strength of the latter. The ion damping of harmonics propagating in the meridional direction (in the geomagnetic coordinates) maximizes over the poles and reduces to zero over the equator. Waves propagating in the zonal direction are uniformly affected by ions at all latitudes. Accounting for the anisotropy produces changes in the GW drag in the F-region of more than 100 m s−1 d−1, cooling/heating rates of more than 15 K d−1, and in GW temperature variance of disturbances by more than 5 K.
      PubDate: 2017-11-20T09:40:20.704832-05:
      DOI: 10.1002/2017JA024785
       
  • Examining the magnetic signal due to gravity and plasma pressure gradient
           current with the TIE-GCM
    • Authors: A. Maute; A. D. Richmond
      Abstract: Accurate magnetic field measurements at ground and Low-Earth Orbit (LEO) are crucial to describe Earth's magnetic field. One of the challenges with processing LEO magnetic field measurements to study Earthâ's magnetic field is that the satellite flies in regions of highly varying ionospheric currents which needs to be characterized accurately. The present study focuses on ionospheric current systems due to gravity and plasma pressure gradient forcing, and aims to provide guidance on the estimation of their magnetic effect at LEO altitudes with the help of numerical modeling. We assess the diamagnetic approximation which estimates the magnetic signal of the plasma pressure gradient current. The simulations indicate that the diamagnetic effect should not be removed from LEO magnetic observations without considering the gravity current effect, as this will lead to an error larger than the magnetic signal of these currents. We introduce and evaluate a method to capture the magnetic effect of the gravity driven current. The diamagnetic and gravity current approximations ignore the magnetic effect from currents set up by the induced electric field. The combined gravity and plasma pressure gradient magnetic effect tends to cancel above the F-region peak, however between approximately 300 km and the peak it exhibits a significant height and latitudinal variation with magnitudes up to 8nT. During solar minimum the combined magnetic signal is less than 1nT above 300 km. In addition to the solar cycle dependence, the magnetic signal strength varies with longitude (approximately by 50%) and season (up to 80%) at solar maximum.
      PubDate: 2017-11-20T09:30:31.003944-05:
      DOI: 10.1002/2017JA024841
       
  • Density structures, dynamics, and seasonal and solar cycle modulations of
           Saturn's inner plasma disk
    • Authors: M. K. G. Holmberg; O. Shebanits, J.-E. Wahlund, M. W. Morooka, E. Vigren, N. André, P. Garnier, A. M. Persoon, V. Génot, L. K. Gilbert
      Abstract: We present statistical results from the Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe (LP) recorded during the time interval from orbit 3 (2005-02-01) to 237 (2016-06-29). A new and improved data analysis method to obtain ion density from the Cassini LP measurements is used to study the asymmetries and modulations found in the inner plasma disk of Saturn, between 2.5 and 12 Saturn radii (1 RS = 60,268 km). The structure of Saturn's plasma disk is mapped and the plasma density peak, nmax, is shown to be located at ∼4.6 RS and not at the main neutral source region at 3.95 RS. The shift in the location of nmax is due to that the hot electron impact ionization rate peaks at ∼4.6 RS.Cassini RPWS plasma disk measurements show a solar cycle modulation. However, estimates of the change in ion density due to varying EUV flux is not large enough to describe the detected dependency, which implies that an additional mechanism, still unknown, is also affecting the plasma density in the studied region. We also present a dayside/nightside ion densities asymmetry, with nightside densities up to a factor of 2 larger than on the dayside. The largest density difference is found in the radial region 4 to 5 RS. The dynamic variation in ion density increases towards Saturn, indicating an internal origin of the large density variability in the plasma disk, rather than being caused by an external source origin in the outer magnetosphere.
      PubDate: 2017-11-20T09:30:21.596123-05:
      DOI: 10.1002/2017JA024311
       
  • Modeling the proton radiation belt with Van Allen Probes REPT data
    • Authors: R. S. Selesnick; D. N. Baker, S. G. Kanekal, V. C. Hoxie, X. Li
      Abstract: An empirical model of the proton radiation belt is constructed from data taken during 2013–2017 by the Relativistic Electron-Proton Telescopes on the Van Allen Probes satellites. The model intensity is a function of time, kinetic energy in the range 18–600 MeV, equatorial pitch angle, and L-shell of proton guiding centers. Data are selected, on the basis of energy deposits in each of the 9 silicon detectors, to reduce background caused by hard proton energy spectra at low L. Instrument response functions are computed by Monte Carlo integration, using simulated proton paths through a simplified structural model, to account for energy loss in shielding material for protons outside the nominal field-of-view. Overlap of energy channels, their wide angular response, and changing satellite orientation require the model dependencies on all three independent variables be determined simultaneously. This is done by least-squares minimization with a customized steepest-descent algorithm. Model uncertainty accounts for statistical data error and systematic error in the simulated instrument response. A proton energy spectrum is also computed from data taken during the Jan 8, 2014 solar event, to illustrate methods for the simpler case of an isotropic and homogeneous model distribution. Radiation belt and solar proton results are compared to intensities computed with a simplified, on-axis response that can provide a good approximation under limited circumstances.
      PubDate: 2017-11-14T09:25:27.136694-05:
      DOI: 10.1002/2017JA024661
       
  • Ionospheric electron densities at Mars: Comparison of Mars Express
           ionospheric sounding and MAVEN local measurements
    • Authors: F. Němec; D. D. Morgan, C. M. Fowler, A. J. Kopf, L. Andersson, D. A. Gurnett, D. J. Andrews, V. Truhlík
      Abstract: We present the first direct comparison of Martian ionospheric electron densities measured by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) topside radar sounder on board the Mars Express spacecraft and by the Langmuir Probe and Waves (LPW) instrument on board the Mars Atmosphere and Volatile Evolution Mission (MAVEN) spacecraft. As low electron densities are not measured by MARSIS due to the low power radiated at low sounding frequencies, MARSIS electron density profiles between the local electron density and the first data point from the ionospheric sounding (≈104 cm−3) rely on an empirical electron density profile shape. We use the LPW electron density measurements to improve this empirical description, and thereby the MARSIS derived electron density profiles. We further analyze four coincident events, where the two instruments were measuring within a five degree solar zenith angle (SZA) interval within one hour. The differences between the electron densities measured by the MARSIS and LPW instruments are found to be within a factor of two in 90% of measurements. Taking into account the measurement precision and different locations and times of the measurements, these differences are within the estimated uncertainties.
      PubDate: 2017-11-13T09:15:22.175899-05:
      DOI: 10.1002/2017JA024629
       
  • The Contribution of Compressional Magnetic Pumping to the Energization of
           the Earth's Outer Electron Radiation Belt during High-Speed-Stream-Driven
           Storms
    • Authors: Joseph E. Borovsky; Richard B. Horne, Nigel P. Meredith
      Abstract: Compressional magnetic pumping is an interaction between cyclic magnetic compressions and pitch-angle scattering with the scattering acting as a catalyst to allow the cyclic compressions to energize particles. Compressional magnetic pumping of the outer electron radiation belt at geosynchronous orbit in the dayside magnetosphere is analyzed by means of computer simulations, wherein solar-wind compressions of the dayside magnetosphere energize electrons with electron pitch-angle scattering by chorus waves and by EMIC. The magnetic pumping is found to produce a weak bulk heating of the electron radiation belt, and it also produces an energetic tail on the electron energy distribution. The amount of energization depends on the robustness of the solar-wind compressions and on the amplitude of the chorus and/or EMIC waves. Chorus-catalyzed pumping is better at energizing medium-energy (50 - 200 keV) electrons than it is at energizing higher energy electrons; at high energies (500 keV - 2 MeV) EMIC-catalyzed pumping is a stronger energizer. The magnetic-pumping simulation results are compared with energy-diffusion calculations for chorus waves in the dayside magnetosphere; in general compressional magnetic pumping is found to be weaker at accelerating electrons than is chorus-driven energy diffusion. In circumstances when solar-wind compressions are robust and when EMIC waves are present in the dayside magnetosphere without the presence of chorus, EMIC-catalyzed magnetic pumping could be the dominant energization mechanism in the dayside magnetosphere, but at such times loss-cone losses will be strong.
      PubDate: 2017-11-13T09:06:06.40635-05:0
      DOI: 10.1002/2017JA024607
       
  • CMEs' speed, travel time and temperature: A Thermodynamic approach
    • Authors: Héctor J. Durand-Manterola; Alberto Flandes, Ana Leonor Rivera, Alejandro Lara, Tatiana Niembro
      Abstract: Due to their important role in Space weather, Coronal Mass Ejections or CMEs have been thoroughly studied in order to forecast their speed and transit time from the Sun to the Earth. We present a Thermodynamic analytical model that describes the dynamics of CMEs. The thermodynamic approach has some advantages with respect to the hydrodynamic approach. First, it deals with the energy involved, which is a scalar quantity. Second, one may calculate the work done by the different forces separately and sum all contributions to determine the changes in speed, which simplifies the problem and allows us to obtain fully rigorous results.Our model considers the drag force, which dominates the dynamics of CMEs and the solar gravitational force, which has a much smaller effect, but it is, still, relevant enough to be considered.We derive an explicit analytical expression for the speed of CMEs in terms of its most relevant parameters and obtain an analytical expression for the CME temperature. The model is tested with a CME observed at three different heliocentric distances with three different spacecraft (SOHO, ACE an Ulysses); also, with a set of 11 CMEs observed with the SOHO, Wind and ACE spacecraft and, finally, with two events observed with the STEREO spacecraft. In all cases, we have a consistent agreement between the theoretical and the observed speeds and transit times. Additionally, for the set of 11 events, we estimate their temperatures at their departure position from their temperatures measured near the orbit of the Earth.
      PubDate: 2017-11-13T08:50:29.099208-05:
      DOI: 10.1002/2017JA024369
       
  • Nightside Pi2 wave properties during an extended period with stable
           plasmapause location and variable geomagnetic activity
    • Authors: M. D. Hartinger; S. Zou, K. Takahashi, X. Shi, R. Redmon, J. Goldstein, W. Kurth, J. W. Bonnell
      Abstract: The frequencies and amplitudes of inner magnetosphere Pi2 waves are affected by the radial plasma density profile. Variable geomagnetic activity and external driving conditions can affect both wave properties and density profiles simultaneously. When interpreting observations, this can lead to ambiguity about whether changing wave properties are due to changing external conditions, density profiles, or a combination of factors. We present a case study using multi-point ground-based and in situ measurements to examine Pi2 wave properties during a period of variable geomagnetic activity. Multiple satellite passes demonstrate the density profile and plasmapause location is stable for at least two hours over a wide range of MLT. This stability allows us to examine how factors besides the radial density profile affect Pi2 wave properties. We find evidence for Pi2 waves with a broadband frequency spectrum as well as a discrete frequency plasmaspheric virtual resonance (PVR) that is observed at low, mid, and high-latitudes and both inside and outside the plasmapause. The PVR is excited in repeated bursts before, during and after (1) the development of a substorm, (2) several auroral intensifications, (3) the development of Sub-Auroral Polarization Stream (SAPS) flows/electric fields/conductivities, and (4) variable Interplanetary Magnetic Field (IMF) conditions. Through all these changes the PVR frequency remains remarkably stable (8.2 +/− 0.53 mHz, based on low latitude ground magnetometer observations), suggesting these variations have little effect on the frequency. This is consistent with PVR model predictions for a stationary plasmapause.
      PubDate: 2017-11-13T08:45:22.528431-05:
      DOI: 10.1002/2017JA024708
       
  • Electron Drift Resonance in the MHD-coupled Comprehensive Inner
           Magnetosphere Ionosphere model
    • Authors: C. M. Komar; A. Glocer, M. D. Hartinger, K. R. Murphy, M.-C. Fok, S.-B. Kang
      Abstract: Relativistic electrons in the outer radiation belt are highly dynamic and respond to interplanetary solar wind structures interacting with the Earth's magnetic field. A known mechanism dictating electron dynamics is the drift-resonant interaction with ultra-low frequency (ULF) waves. The present work simulates the ring current and radiation belt electron populations in the bounce-averaged, kinetic Comprehensive Inner Magnetosphere-Ionosphere (CIMI) model coupled with the Block Adaptive Tree Solar Wind Roe-type Upwind Scheme (BATS-R-US) global magnetospheric magnetohydrodynamic (MHD) code using an idealized ULF wave solar wind density driver. ULF waves generated with 10 minute periods (at 1.67 mHz frequencies) in the MHD model are characterized and the corresponding energization of electrons and radial transport of electron phase space density is presented. The drift-resonant electron energy is determined in the simulation and is consistent with the electron resonance conditions in dipolar magnetic fields. The present results will be an important component of understanding inner magnetospheric dynamics and how these inner magnetospheric populations interact with ULF waves resulting from interplanetary solar wind structures.
      PubDate: 2017-11-10T18:06:59.907346-05:
      DOI: 10.1002/2017JA024163
       
  • Longitudinal variation of the lunar tide in the equatorial electrojet
    • Authors: Yosuke Yamazaki; Claudia Stolle, Jürgen Matzka, Tarique A. Siddiqui, Hermann Lühr, Patrick Alken
      Abstract: The atmospheric lunar tide is one known source of ionospheric variability. The subject received renewed attention as recent studies found a link between stratospheric sudden warmings and amplified lunar tidal perturbations in the equatorial ionosphere. There is increasing evidence from ground observations that the lunar tidal influence on the ionosphere depends on longitude. We use magnetic field measurements from the CHAMP satellite during July 2000–September 2010 and from the two Swarm satellites during November 2013–February 2017 to determine, for the first time, the complete seasonal-longitudinal climatology of the semidiurnal lunar tidal variation in the equatorial electrojet intensity. Significant longitudinal variability is found in the amplitude of the lunar tidal variation, while the longitudinal variability in the phase is small. The amplitude peaks in the Peruvian sector (∼285∘E) during the Northern-Hemisphere winter and equinoxes, and in the Brazilian sector (∼325∘E) during the Northern-Hemisphere summer. There are also local amplitude maxima at ∼55∘E and ∼120∘E. The longitudinal variation is partly due to the modulation of ionospheric conductivities by the inhomogeneous geomagnetic field. Another possible cause of the longitudinal variability is neutral wind forcing by nonmigrating lunar tides. A tidal spectrum analysis of the semidiurnal lunar tidal variation in the equatorial electrojet reveals the dominance of the westward-propagating mode with zonal wavenumber 2 (SW2), with secondary contributions by westward-propagating modes with zonal wavenumber 3 (SW3) and 4 (SW4). Eastward-propagating waves are largely absent from the tidal spectrum. Further study will be required for the relative importance of ionopsheric conductivities and nonmigrating lunar tides.
      PubDate: 2017-11-10T18:03:57.180714-05:
      DOI: 10.1002/2017JA024601
       
  • Sunward strahl: A method to unambiguously determine open solar flux from
           in situ spacecraft measurements using suprathermal electron data
    • Authors: M. J. Owens; M. Lockwood, P. Riley, J. Linker
      Abstract: A fraction of the magnetic flux which threads the photosphere reaches sufficient coronal altitude to be dragged out by the solar wind and form the heliospheric magnetic field (HMF). Directly measuring this “open solar flux” (OSF) component, however, is difficult. While OSF can be extrapolated from photospheric magnetic field measurements, the most direct method is from in-situ spacecraft measurements of the HMF. The difficultly is unambiguously distinguishing between HMF which connects directly back to the Sun (the OSF) and that which is locally distorted by waves, turbulence and near-Sun reconnection. Suitable temporal filtering of the data can remove such “noise”, but the level of filtering cannot be known a priori and varies with solar cycle, solar wind types, etc. Here, we use the suprathermal electron beam, or “strahl”, to distinguish between different HMF topologies. As strahl moves antisunward on global scales, times when strahl is observed to be moving sunward indicate that the HMF is locally inverted. By subtracting the inverted HMF we compute the OSF without need for arbitrary filtering of the data. We find that the OSF obtained in this manner is slightly larger than the proposed “kinematic correction” based on observed solar wind velocity structure, though in general agreement. Our new OSF estimate agrees with methods based wholly on HMF data, if the data are first used to compute approximately 1-day averages during solar minimum and approximately 3-day averages during solar maximum, stressing the point that the filter method is unreliable because the required characteristics vary.
      PubDate: 2017-11-09T09:06:34.031058-05:
      DOI: 10.1002/2017JA024631
       
  • Properties of the equatorial magnetotail flanks ∼50 − 200RE
           downtail.
    • Authors: A. V. Artemyev; V. Angelopoulos, A. Runov, C.-P. Wang, L. M. Zelenyi
      Abstract: In space, thin boundaries separating plasmas with different properties serve as a free energy source for various plasma instabilities and determine the global dynamics of large-scale systems. In planetary magnetopauses and shock waves, classical examples of such boundaries, the magnetic field makes a significant contribution to the pressure balance and plasma dynamics. The configuration and properties of such boundaries have been well investigated and modeled. However, much less is known about boundaries that form between demagnetized plasmas where the magnetic field is not important for pressure balance. The most accessible example of such a plasma boundary is the equatorial boundary layer of the Earth's distant magnetotail. Rather limited measurements since its first encounter in the late 1970's by the International Sun-Earth Explorer-3 spacecraft revealed the basic properties of this boundary, but its statistical properties and structure have not been studied to date. In this study, we use Geotail and Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) missions to investigate the equatorial boundary layer from lunar orbit (∼55 Earth radii, RE, downtail) to as far downtail as ∼200RE. Although the magnetic field has almost no effect on the structure of the boundary layer, the layer separates well the hot, rarefied plasma sheet from dense cold magnetosheath plasmas. We suggest that the most important role in plasma separation is played by polarization electric fields, which modify the efficiency of magnetosheath ion penetration into the plasma sheet. We also show that the total energies (bulk flow plus thermal) of plasma sheet ions and magnetosheath ions are very similar, i.e., magnetosheath ion thermalization (e.g., via ion scattering by magnetic field fluctuations) is sufficient to produce hot plasma sheet ions without any additional acceleration.
      PubDate: 2017-11-09T09:05:30.106044-05:
      DOI: 10.1002/2017JA024723
       
  • How to define the mean square amplitude of solar wind fluctuations with
           respect to the local mean magnetic field
    • Authors: John J. Podesta
      Abstract: Over the last decade it has become popular to analyze turbulent solar wind fluctuations with respect to a coordinate system aligned with the local mean magnetic field. This useful analysis technique has provided new information and new insights about the nature of solar wind fluctuations and provided some support for phenomenological theories of MHD turbulence based on the ideas of Goldreich and Sridhar. At the same time it has drawn criticism suggesting that the use of a scale dependent local mean field is somehow inconsistent or irreconcilable with traditional analysis techniques based on second order structure functions and power spectra which, for stationary time series, are defined with respect to the constant (scale independent) ensemble average magnetic field. Here it is shown that for fluctuations with power-law spectra, such as those observed in solar wind turbulence, it is possible to define the local mean magnetic field in a special way such that the total mean square amplitude (trace amplitude) of turbulent fluctuations is approximately the same, scale by scale, as that obtained using traditional second order structure functions or power spectra. This fact should dispel criticism concerning the physical validity or practical usefulness of the local mean magnetic field in these applications.
      PubDate: 2017-11-09T09:05:27.104755-05:
      DOI: 10.1002/2017JA023864
       
  • Solar illumination control of the polar wind
    • Authors: L. Maes; R. Maggiolo, J. D Keyser, M. André, A. Eriksson, S. Haaland, K. Li, S. Poedts
      Abstract: Polar wind outflow is an important process through which the ionosphere supplies plasma to the magnetosphere. The main source of energy driving the polar wind is solar illumination of the ionosphere. As a result, many studies have found a relation between polar wind flux densities and solar EUV intensity, but less is known about their relation to the solar zenith angle at the ionospheric origin, certainly at higher altitudes. The low energy of the outflowing particles and spacecraft charging means it is very difficult to measure the polar wind at high altitudes. We take advantage of an alternative method that allows estimations of the polar wind flux densities far in the lobes. We analyze measurements made by the Cluster spacecraft at altitudes from 4 up to 20 RE. We observe a strong dependence on the solar zenith angle in the ion flux density and see that both the ion velocity and density exhibit a solar zenith angle dependence as well. We also find a seasonal variation of the flux density.
      PubDate: 2017-11-09T09:05:25.326726-05:
      DOI: 10.1002/2017JA024615
       
  • Ring/Shell Ion Distributions at Geosynchronous Orbit
    • Authors: M. F. Thomsen; M. H. Denton, S. P. Gary, Kaijun Liu, Kyungguk Min
      Abstract: One year's worth of plasma observations from geosynchronous orbit is examined for ion distributions that may simultaneously be subject to the ion Bernstein (IB) instability (generating fast magnetosonic waves) and the Alfvén cyclotron (AC) instability (generating electromagnetic ion cyclotron waves). Confirming past analyses, distributions with robust ∂fp(v⊥)/∂v⊥>0 near v =0, which we denote as “ring/shell” distributions, are commonly found primarily on the dayside of the magnetosphere. A new approach to high-fidelity representation of the observed ring/shell distribution functions in a form readily suited to both analytical moments calculation and linear dispersion analysis is presented, which allows statistical analysis of the ring/shell properties. The ring/shell temperature anisotropy is found to have a clear upper limit that depends on the parallel beta of the ring/shell (® r) in a manner that is diagnostic of the operation of the AC instability. This upper limit is only reached in the post-noon events, which are primarily produced by the energy- and pitch-angle dependent magnetic drifts of substorm-injected ions. Further, it is primarily the leading edge of such injections, where the distribution is strongly ring-like, that the AC instability appears to be operating. By contrast, the ratio of the ring energy to the Alfvén energy remains well within the range 0.25-4.0 suitable for IB instability throughout essentially all of the events, except those that occur in denser cold plasma of the outer plasmasphere.
      PubDate: 2017-11-08T17:55:34.750672-05:
      DOI: 10.1002/2017JA024612
       
  • Electron cooling and isotropization during magnetotail current sheet
           thinning: Implications for parallel electric fields
    • Authors: San Lu; A. V. Artemyev, V. Angelopoulos
      Abstract: Magnetotail current sheet thinning is a distinctive feature of substorm growth phase, during which magnetic energy is stored in the magnetospheric lobes. Investigation of charged particle dynamics in such thinning current sheets is believed to be important for understanding the substorm energy storage and the current sheet destabilization responsible for substorm expansion phase onset. We use THEMIS B and C observations in 2008 and 2009 at ~18 − 25 RE to show that during magnetotail current sheet thinning, the electron temperature decreases (cooling), and the parallel temperature decreases faster than the perpendicular temperature, leading to a decrease of the initially strong electron temperature anisotropy (isotropization). This isotropization cannot be explained by pure adiabatic cooling or by pitch-angle scattering. We use test particle simulations to explore the mechanism responsible for the cooling and isotropization. We find that during the thinning, a fast decrease of a parallel electric field (directed toward the Earth) can speed up the electron parallel cooling, causing it to exceed the rate of perpendicular cooling, and thus lead to isotropization, consistent with observation. If the parallel electric field is too small or does not change fast enough, the electron parallel cooling is slower than the perpendicular cooling, so the parallel electron anisotropy grows, contrary to observation. The same isotropization can also be accomplished by an increasing parallel electric field directed toward the equatorial plane. Our study reveals the existence of a large-scale parallel electric field which plays an important role in magnetotail particle dynamics during the current sheet thinning process.
      PubDate: 2017-11-08T08:47:24.922618-05:
      DOI: 10.1002/2017JA024712
       
  • Flows, fields, and forces in the Mars-solar wind interaction
    • Authors: J. S. Halekas; D. A. Brain, J. G. Luhmann, G. A. DiBraccio, S. Ruhunusiri, Y. Harada, C. M. Fowler, D. L. Mitchell, J. E. P. Connerney, J. R. Espley, C. Mazelle, B. M. Jakosky
      Abstract: We utilize suprathermal ion and magnetic field measurements from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, organized by the upstream magnetic field, to investigate the morphology and variability of flows, fields, and forces in the Mars-solar wind interaction. We employ a combination of case studies and statistical investigations to characterize the interaction in both quasi-parallel and quasi-perpendicular regions and under high and low solar wind Mach number conditions. For the first time, we include a detailed investigation of suprathermal ion temperature and anisotropy. We find that the observed magnetic fields and suprathermal ion moments in the magnetosheath, bow shock, and upstream regions have observable asymmetries controlled by the interplanetary magnetic field, with particularly large asymmetries found in the ion parallel temperature and anisotropy. The greatest temperature anisotropies occur in quasi-perpendicular regions of the magnetosheath and under low Mach number conditions. These results have implications for the growth and evolution of wave-particle instabilities and their role in energy transport and dissipation. We utilize the measured parameters to estimate the average ion pressure gradient, J × B, and v × B macroscopic force terms. The pressure gradient force maintains nearly cylindrical symmetry, while the J × B force has larger asymmetries and varies in magnitude in comparison to the pressure gradient force. The v × B force felt by newly produced planetary ions exceeds the other forces in magnitude in the magnetosheath and upstream regions for all solar wind conditions.
      PubDate: 2017-11-08T08:40:45.487475-05:
      DOI: 10.1002/2017JA024772
       
  • Automated Identification and Shape Analysis of Chorus Elements in the Van
           Allen Radiation Belts
    • Authors: Ananya Sen Gupta; Craig Kletzing, Robin Howk, William Kurth, Morgan Matheny
      Abstract: An important goal of the Van Allen Probes mission is to understand wave-particle interaction by chorus emissions in terrestrial Van Allen radiation belts. To test models, statistical characterization of chorus properties, such as amplitude variation and sweep rates, is an important scientific goal. The EMFISIS instrumentation suite provides measurements of wave electric and magnetic fields as well as DC magnetic fields for the Van Allen Probes mission. However, manual inspection across terrabytes of EMFISIS data is not feasible and as such, introduces human confirmation bias. We present signal processing techniques for automated identification, shape analysis and sweep rate characterization of high-amplitude whistler-mode chorus elements in the Van Allen radiation belts. Specifically, we develop signal processing techniques based on the Radon transform that disambiguate chorus elements with a dominant sweep rate against hiss-like chorus. We present representative results validating our techniques and also provide statistical characterization of detected chorus elements across a case study of a 6 second epoch.
      PubDate: 2017-11-07T19:11:14.081349-05:
      DOI: 10.1002/2017JA023949
       
  • A comprehensive analysis of multi-scale field aligned currents:
           Characteristics, controlling parameters, and relationships
    • Authors: Ryan M. McGranaghan; Anthony J. Mannucci, Colin Forsyth
      Abstract: We explore the characteristics, controlling parameters, and relationships of multi-scale field aligned currents (FACs) using a rigorous, comprehensive, and cross-platform analysis. Our unique approach combines FAC data from the Swarm satellites and the Advanced Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) to create a database of small-scale (∼10-150 km, 250 km) FACs. We examine these data for the repeatable behavior of FACs across scales (i.e., the characteristics), the dependence on the interplanetary magnetic field (IMF) orientation, and the degree to which each scale ‘departs’ from nominal large-scale specification. We retrieve new information by utilizing magnetic latitude and local time dependence, correlation analyses, and quantification of the departure of smaller from larger scales. We find that: 1) FACs characteristics and dependence on controlling parameters do not map between scales in a straight forward manner; 2) relationships between FAC scales exhibit local time dependence; and 3) the dayside high-latitude region is characterized by remarkably distinct FAC behavior when analyzed at different scales, and the locations of distinction correspond to ‘anomalous’ ionosphere-thermosphere (IT) behavior. Comparing with nominal large-scale FACs, we find that differences are characterized by a horseshoe shape, maximizing across dayside local times, and that difference magnitudes increase when smaller scale observed FACs are considered. We suggest that both new physics and increased resolution of models are required to address the multi-scale complexities. We include a summary table of our findings to provide a quick reference for differences between multi-scale FACs.
      PubDate: 2017-11-07T19:10:40.398802-05:
      DOI: 10.1002/2017JA024742
       
  • IMF control of Alfvénic energy transport and deposition at high
           latitudes
    • Authors: Spencer M. Hatch; James LaBelle, William Lotko, Christopher C. Chaston, Binzheng Zhang
      Abstract: We investigate the influence of the interplanetary magnetic field (IMF) clock angle ϕIMF on high-latitude inertial Alfvén wave (IAW) activity in the magnetosphere-ionosphere transition region using FAST satellite observations. We find evidence that negative IMF Bz coincides with nightside IAW power generation and enhanced rates of IAW-associated electron energy deposition, while positive IMF Bz coincides with enhanced dayside wave and electron energy deposition. Large (≳5 nT) negative IMF By coincides with enhanced postnoon IAW power, while large positive IMF By coincides with enhanced but relatively weaker prenoon IAW power. For each ϕIMF orientation we compare IAW Poynting flux and IAW-associated electron energy flux distributions with previously published distributions of Alfvénic Poynting flux over ∼2–22 mHz, as well as corresponding wave–driven electron energy deposition derived from Lyon-Fedder-Mobarry (LFM) global MHD simulations. We also compare IAW Poynting flux distributions with distributions of broad and diffuse electron number flux, categorized using an adaptation of the Newell et al. [2009] precipitation scheme for FAST. Under negative IMF Bz in the vicinity of the cusp (9.5–14.5 MLT), regions of intense dayside IAW power correspond to enhanced diffuse electron number flux but relatively weaker broadband electron precipitation. Differences between cusp-region IAW activity and broadband precipitation illustrate the need for additional information, such as fields or pitch-angle measurements, to identify the physical mechanisms associated with electron precipitation in the vicinity of the cusp.
      PubDate: 2017-11-07T09:25:44.709118-05:
      DOI: 10.1002/2017JA024175
       
  • Van Allen Probes measurements of energetic particle deep penetration into
           the low L region (L
    • Authors: H. Zhao; D. N. Baker, S. Califf, X. Li, A. N. Jaynes, T. Leonard, S. G. Kanekal, J. B. Blake, J. F. Fennell, S. G. Claudepierre, D. L. Turner, G. D. Reeves, H. E. Spence
      Abstract: Using measurements from the Van Allen Probes, a penetration event of 10s – 100s of keV electrons and 10s of keV protons into the low L-shells (L
      PubDate: 2017-11-06T14:40:24.960403-05:
      DOI: 10.1002/2017JA024558
       
  • Dependence of electromagnetic ion cyclotron wave occurrence on north-south
           orientation of interplanetary magnetic field: THEMIS observations
    • Authors: Jong-Sun Park; Jih-Hong Shue, Khan-Hyuk Kim
      Abstract: We investigate the L-MLT (i.e., L-shall versus magnetic local time) distributions of electromagnetic ion cyclotron (EMIC) waves, using the Time History of Events and Macroscale Interactions during Substorms (THEMIS) data from 2009 to 2014, under prolonged (more than four hours) intervals of northward and southward interplanetary magnetic fields (IMFs). H-band EMIC waves show high occurrence in the dawn-to-postnoon sector (0400-1600 MLT) at L> ~7 for northward IMFs. On the other hand, for southward IMFs H-band EMIC waves occur in the morning-to-afternoon sector (0700-1500 MLT), and no strong wave activity is found in the dawn sector (0400-0700 MLT). H-band EMIC waves are frequently observed in the region where the ambient temperature anisotropy for the energetic ions (~1-25 keV) is high. He-band EMIC waves show high occurrence in the noon-to-dusk sector (1100-2000 MLT) for both IMF BZ conditions. In the dusk sector, however, the high occurrence region is shifted into the inner L-shell for southward IMFs (L < 8) in contrast to that observed for northward IMFs (L < 10). He-band EMIC waves are shown to be generated in the region where the total plasma density is high (especially near the plasmapause). The occurrence rates of EMIC waves are lower overall for southward IMFs than for northward IMFs for both H-band and He-band waves. We suggest that an increase of heavy ion (i.e., He+ and O+) concentration is likely associated with low occurrence rates in both frequency bands for southward IMFs.
      PubDate: 2017-11-06T14:40:22.12747-05:0
      DOI: 10.1002/2017JA024507
       
  • An empirical modification of the force field approach to describe the
           modulation of galactic cosmic rays close to Earth in a broad range of
           rigidities
    • Authors: J. Gieseler; B. Heber, K. Herbst
      Abstract: On their way through the heliosphere, Galactic Cosmic Rays (GCRs) are modulated by various effects before they can be detected at Earth. This process can be described by the Parker equation, which calculates the phase space distribution of GCRs depending on the main modulation processes: convection, drifts, diffusion and adiabatic energy changes. A first order approximation of this equation is the force field approach, reducing it to a one-parameter dependency, the solar modulation potential ϕ. Utilizing this approach, it is possible to reconstruct ϕ from ground based and spacecraft measurements. However, it has been shown previously that ϕ depends not only on the Local Interstellar Spectrum (LIS) but also on the energy range of interest. We have investigated this energy dependence further, using published proton intensity spectra obtained by PAMELA as well as heavier nuclei measurements from IMP-8 and ACE/CRIS. Our results show severe limitations at lower energies including a strong dependence on the solar magnetic epoch. Based on these findings, we will outline a new tool to describe GCR proton spectra in the energy range from a few hundred MeV to tens of GeV over the last solar cycles. In order to show the importance of our modification, we calculate the global production rates of the cosmogenic radionuclide 10Be which is a proxy for the solar activity ranging back thousands of years.
      PubDate: 2017-11-06T14:30:40.579865-05:
      DOI: 10.1002/2017JA024763
       
  • The plasma sheet as natural symmetry plane for dipolarization fronts in
           the Earth's magnetotail
    • Authors: D. Frühauff; K.-H. Glassmeier
      Abstract: In this work, observations of multi-spacecraft mission THEMIS are used for statistical investigation of dipolarization fronts in the near-Earth plasma sheet of the magnetotail. Using very stringent criteria 460 events are detected in almost 10 years of mission data. Minimum variance analysis is used to determine the normal directions of the phase fronts, providing evidence for the existence of a natural symmetry of these phenomena, given by the neutral sheet of the magnetotail. This finding enables the definition of a local coordinate system based on the Tsyganenko model, reflecting the intrinsic orientation of the neutral sheet and, therefore, the dipolarization fronts. In this way, the comparision of events with very different background conditions is improved. Through this study, the statistical results of Liu et al. [2013a] are both confirmed and extended.In a case study, the knowledge of this plane of symmetry helps to explain the concave curvature of dipolarization fronts in the XZ-plane through phase propagation speeds of magnetoacoustic waves. A second case study is presented to determine the central current system of a passing dipolarization front through a constellation of three spacecraft. With this information, a statistical analysis of spacecraft observations above and below the neutral sheet is used to provide further evidence for the neutral sheet as the symmetry plane and the central current system. Furthermore, it is shown that the signatures of dipolarization fronts are under certain conditions closely related to that of flux ropes, indicating a possible relationship between these two transient phenomena.
      PubDate: 2017-11-06T14:30:28.370496-05:
      DOI: 10.1002/2017JA024682
       
  • Electron flux dropouts at L ∼ 4.2 from Global Positioning System
           satellites: Occurrences, magnitudes, and main driving factors
    • Authors: R. J. Boynton; D. Mourenas, M. A. Balikhin
      Abstract: Dropouts in electron fluxes at L ∼ 4.2 were investigated for a broad range of energies from 120 keV to 10 MeV, using 16 years of electron flux data from Combined X-ray Dosimeter (CXD) onboard Global Positioning System (GPS) satellites. Dropouts were defined as flux decreases by at least a factor 4 in 12 hours, or 24 hours during which a decrease by at least a factor of 1.5 must occur during each 12-hours time bin. Such fast and strong dropouts were automatically identified from the GPS electron flux data and statistics of dropout magnitudes and occurrences were compiled as a function of electron energy. Moreover, the Error Reduction Ratio (ERR) analysis was employed to search for nonlinear relationships between electron flux dropouts and various solar wind and geomagnetic activity indices, in order to identify potential external causes of dropouts. At L ∼ 4.2, the main driving factor for the more numerous and stronger 1-10 MeV electron dropouts turns out to be the southward Interplanetary Magnetic Field Bs, suggesting an important effect from precipitation loss due to combined EMIC and whistler-mode waves in a significant fraction of these events, supplementing magnetopause shadowing and outward radial diffusion which are also effective at lower energies.
      PubDate: 2017-11-06T14:20:21.849238-05:
      DOI: 10.1002/2017JA024523
       
  • The effect of a guide field on local energy conversion during asymmetric
           magnetic reconnection: Particle-in-cell simulations
    • Authors: P. A. Cassak; K. J. Genestreti, J. L. Burch, T.-D. Phan, M. A. Shay, M. Swisdak, J. F. Drake, L. Price, S. Eriksson, R. E. Ergun, B. J. Anderson, V. G. Merkin, C. M. Komar
      Abstract: We use theory and simulations to study how the out-of-plane (guide) magnetic field strength modifies the location where the energy conversion rate between the electric field and the plasma is appreciable during asymmetric magnetic reconnection, motivated by observations (Genestreti et al. , this issue). For weak guide fields, energy conversion is maximum on the magnetospheric side of the X-line, midway between the X-line and electron stagnation point. As the guide field increases, the electron stagnation point gets closer to the X-line, and energy conversion occurs closer to the electron stagnation point. We motivate one possible non-rigorous approach to extend the theory of the stagnation point location to include a guide field. The predictions are compared to two-dimensional particle-in-cell (PIC) simulations with vastly different guide fields. The simulations have upstream parameters corresponding to three events observed with MMS. The predictions agree reasonably well with the simulation results, capturing trends with the guide field. The theory correctly predicts that the X-line and stagnation points approach each other as the guide field increases. The results are compared to MMS observations, Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) observations of each event, and a global resistive magnetohydrodynamics simulation of the 2015 Oct 16 event. The PIC simulation results agree well with the global observations and simulation, but differ in the strong electric fields and energy conversion rates found in MMS observations. The observational, theoretical, and numerical results suggest that the strong electric fields observed by MMS do not represent a steady global reconnection rate.
      PubDate: 2017-10-31T18:26:24.953552-05:
      DOI: 10.1002/2017JA024555
       
  • Oxygen ions O+ energized by kinetic Alfvén eigenmode during
           dipolarizations of intense substorms
    • Authors: Suping Duan; Lei Dai, Chi Wang, Zhaohai He, Chunlin Cai, Y. C. Zhang, I. Dandouras, H. Reme, M. André, Y. V. Khotyaintsev
      Abstract: Singly charged oxygen ions, O+, energized by kinetic Alfvén wave eigenmode (KAWE) in the plasma sheet boundary layer during dipolarizations of two intense substorms,10:07 UT on 31 August 2004 and 18:24 UT on 14 September 2004, are investigated by Cluster spacecraft in the magnetotail. It is found that after the beginning of the expansion phase of substorms, O+ ions are clearly energized in the direction perpendicular to the magnetic field with energy larger than 1 keV in the near-Earth plasma sheet (NEPS) during magnetic dipolarizations. The pitch angle distribution of these energetic O+ ions is significantly different from that of O+ ions with energy less than 1 keV before substorm onset which is in the quasi-parallel direction along the magnetic field. The KAWE with the large perpendicular unipolar electric field, Ez ~ -20 mV/m, significantly accelerate O+ ions in the direction perpendicular to the background magnetic field. We present good evidences that O+ ions origin from the ionosphere along the magnetic field line in the northward lobe can be accelerated in the perpendicular direction during substorm dipolarizations. The change of the move direction of O+ ions is useful for O+ transferring from the lobe into the central plasma sheet in the magnetotail. Thus KAWE can play an important role in O+ ions transfer process from the lobe into the plasma sheet during intense substorms.
      PubDate: 2017-10-31T18:25:58.651711-05:
      DOI: 10.1002/2017JA024418
       
  • The effect of solar wind variations on the escape of oxygen ions from Mars
           through different channels. MAVEN observations
    • Authors: E. Dubinin; M. Fraenz, M. Pätzold, J. McFadden, J. S. Halekas, G. A. DiBraccio, J. E. P. Connerney, F. Eparvier, D. Brain, B. M. Jakosky, O. Vaisberg, L. Zelenyi
      Abstract: We present multi-instrument observations of the effects of solar wind on ion escape fluxes on Mars based on the MAVEN data from November 1, 2014 to May 15 2016. Losses of oxygen ions through different channels (plasma sheet, magnetic lobes, boundary layer, ion plume) as a function of the solar wind and the interplanetary magnetic field variations were studied. We have utilized the modified MSE-coordinate system for separation of the different escape routes. Fluxes of the low-energy (≤30 eV) and high-energy (≥30 eV) ions reveal different trends with changes in the solar wind dynamic pressure, the solar wind flux and the motional electric field. Major oxygen fluxes occur through the tail of the induced magnetosphere. The solar wind motional electric field produces an asymmetry in the ion fluxes and leads to different relations between ion fluxes supplying the tail from the different hemispheres and the solar wind dynamic pressure (or flux) and the motional electric field. The main driver for escape of the high-energy oxygen ions is the solar wind flux (or dynamic pressure). On the other hand, the low-energy ion component shows the opposite trend: ion flux decreases with increasing solar wind flux. As a result, the averaged total oxygen ion fluxes reveal a low variability with the solar wind strength. The large standard deviations from the averages values of the escape fluxes indicate the existence of mechanisms which can enhance or suppress the efficiency of the ion escape. It is shown that the Martian magnetosphere possesses the properties of a combined magnetosphere (Dubinin et al., 1980) which contains different classes of field lines. The existence of the closed magnetic field lines in the near-Mars tail might be responsible for suppression of the ion escape fluxes.
      PubDate: 2017-10-30T14:01:22.025263-05:
      DOI: 10.1002/2017JA024741
       
  • Structure and Dissipation Characteristics of an Electron Diffusion Region
           Observed by MMS during a Rapid, Normal-incidence Magnetopause Crossing
    • Authors: R. B. Torbert; J. L. Burch, M. R. Argall, L. Alm, C. J. Farrugia, T. G. Forbes, B. L. Giles, A. Rager, J. Dorelli, R. J. Strangeway, R. E. Ergun, F. D. Wilder, N. Ahmadi, P.-A. Lindqvist, Y. Khotyaintsev
      Abstract: On October 22, 2016, the Magnetospheric Multiscale (MMS) spacecraft encountered the electron diffusion region (EDR) when the magnetosheath field was southward, and there were signatures of fast reconnection, including flow jets, Hall fields, and large power dissipation. One rapid, normal-incidence crossing, during which the EDR structure was almost stationary in the boundary frame, provided an opportunity to observe the spatial structure for the zero guide-field case of magnetic reconnection. The reconnection electric field was determined unambiguously to be 2-3 mV/m. There were clear signals of fluctuating parallel electric fields, up to 6 mV/m on the magnetosphere side of the diffusion region, associated with a Hall-like parallel current feature on the electron scale. The width of the main EDR structure was determined to be ~2km (1.8 de). Although the MMS spacecraft were in their closest tetrahedral separation of ~8km, the divergences and curls for these thin current structures could therefore not be computed in the usual manner. A method is developed to determine these quantities on a much smaller scale and applied to compute the normal component of terms in the generalized Ohm's law for the positions of each individual spacecraft (not a barocentric average). Although the gradient pressure term has a qualitative dependence that follows the observed variation of E+VexB, the quantitative magnitude of these terms differs by more than a factor of 2, which is shown to be greater than the respective errors. Thus, future research is required to find the manner in which Ohm's law is balanced.
      PubDate: 2017-10-30T14:00:46.441373-05:
      DOI: 10.1002/2017JA024579
       
  • Particle Precipitation Effects on Convection and the Magnetic Reconnection
           Rate in Earth's Magnetosphere
    • Authors: Joseph B. Jensen; Joachim Raeder, Kristofor Maynard, W. Douglas Cramer
      Abstract: We investigate the connection between particle precipitation in the ionosphere to both the subsolar distance of the magnetopause and the reconnection rate using the OpenGGCM-CTIM-RCM model. We simulated two events, a calm period on May 4, 2005 and a storm period on March 17, 2013. We find that scaling the precipitation energy flux by several orders of magnitude, conductivities in the auroral oval are influenced which, in turn, influence the cross polar cap potentials. With the change in conductance, magnetospheric convection is enhanced or reduced, and the location of the subsolar distance of the magnetopause can change by up to one RE. The investigation of the reconnection rate for the varying precipitation simulations using the Hesse-Forbes-Bern method shows that particle precipitation affects the magnetic reconnection rate in these two events. The most notable differences, up to 40%, occur on short time scales, i.e., hours. A relation for longer time scales (tens of hours) between precipitation and reconnection for these two events is more difficult to ascertain. Differences in CPCP and R can be explained by viscous interactions and polar cap saturation. We find that when precipitation was decreased, resulting in low polar conductance, viscous interactions are strong and CPCP is higher than R. For high precipitation, high conductance cases the polar cap is in the saturation regime and CPCP is lower than R. We find hemispheric asymmetries in the cross polar cap potential and in the calculated reconnection rate derived from the northern and southern hemispheres.
      PubDate: 2017-10-30T14:00:39.914952-05:
      DOI: 10.1002/2017JA024030
       
  • Distribution of field-aligned electron events in the high-altitude polar
           region: Cluster observations
    • Authors: Jiankui Shi; Ziying Zhang, Klaus Torkar, Zhengwei Cheng, Andrew Farzakeley, Malcolm Dunlop, Chris Carr
      Abstract: Field Aligned Electrons (FAEs) are important for the energy transport in the solar wind-magnetosphere-ionosphere coupling. However, the distribution of FAEs and the concerning physical mechanism in different altitudes of the polar region are still unclear. In this paper, data from the Cluster spacecraft were used to study the characteristics of FAEs in high altitude polar region. We selected FAE events with a flux higher than 3×108(cm2﹒s)-1 for our analysis. Their distribution was double peaked around the auroral oval. The main peak occurred around the cusp region (MLT 0700-1500) which leaned to the dawnside. The other peak appeared in the evening sector with MLT 2100-2300 just before midnight. The durations of the FAE events covered a wide range from 4 to 475 seconds, with most of the FAE events lasting less than 40 seconds. The possible physical mechanisms are discussed, namely that the downward FAEs may consist of decelerated solar wind and reflected up-flowing ionospheric electrons in the potential drops, whereas the upward ones may be mirrored solar wind electrons and accelerated ionospheric up-flowing electrons.
      PubDate: 2017-10-30T14:00:36.646818-05:
      DOI: 10.1002/2017JA024360
       
  • Relativistic electron increase during chorus wave activities on the 6–8
           March 2016 geomagnetic storm
    • Authors: H. Matsui; R. B. Torbert, H. E. Spence, M. R. Argall, L. Alm, C. J. Farrugia, W. S. Kurth, D. N. Baker, J. B. Blake, H. O. Funsten, G. D. Reeves, R. E. Ergun, Yu. V. Khotyaintsev, P.-A. Lindqvist
      Abstract: There was a geomagnetic storm on 6–8 March 2016, in which Van Allen Probes A and B separated by ∼2.5 h measured increase of relativistic electrons with energies ∼ several hundred keV to 1 MeV. Simultaneously, chorus waves were measured by both Van Allen Probes and Magnetospheric Multiscale (MMS) mission. Some of the chorus elements were rising-tones, possibly due to nonlinear effects. These measurements are compared with a nonlinear theory of chorus waves incorporating the inhomogeneity ratio and the field equation. From this theory, a chorus wave profile in time and one-dimensional space is simulated. Test particle calculations are then performed in order to examine the energization rate of electrons. Some electrons are accelerated, although more electrons are decelerated. The measured time scale of the electron increase is inferred to be consistent with this nonlinear theory.
      PubDate: 2017-10-30T14:00:33.941639-05:
      DOI: 10.1002/2017JA024540
       
  • MMS observation of magnetic reconnection in the turbulent magnetosheath
    • Authors: Z. Vörös; E. Yordanova, A. Varsani, K. J. Genestreti, Yu V. Khotyaintsev, W. Li, D. B. Graham, C. Norgren, R. Nakamura, Y. Narita, F. Plaschke, W. Magnes, W. Baumjohann, D. Fischer, A. Vaivads, E. Eriksson, P.-A. Lindqvist, G. Marklund, R. E. Ergun, M. Leitner, M. P. Leubner, R. J. Strangeway, O. Le Contel, C. Pollock, B. J. Giles, R. B. Torbert, J. L. Burch, L. A. Avanov, J. C. Dorelli, D. J. Gershman, W. R. Paterson, B. Lavraud, Y. Saito
      Abstract: In this paper we use the full armament of the MMS (Magnetospheric Multiscale) spacecraft to study magnetic reconnection in the turbulent magnetosheath downstream of a quasi-parallel bow shock. Contrarily to the magnetopause and magnetotail cases, only a few observations of reconnection in the magnetosheath have been reported. The case study in this paper presents, for the first time, both fluid-scale and kinetic-scale signatures of an ongoing reconnection in the turbulent magnetosheath. The spacecraft are crossing the reconnection inflow and outflow regions and the ion diffusion region (IDR). Inside the reconnection outflows D-shape ion distributions are observed. Inside the IDR mixing of ion populations, crescent-like velocity distributions and ion accelerations are observed. One of the spacecraft skims the outer region of the electron diffusion region (EDR), where parallel electric fields, energy dissipation/conversion, electron pressure tensor agyrotropy, electron temperature anisotropy and electron accelerations are observed. Some of the difficulties of the observations of magnetic reconnection in turbulent plasma are also outlined.
      PubDate: 2017-10-30T14:00:28.37034-05:0
      DOI: 10.1002/2017JA024535
       
  • The storm time evolution of the ionospheric disturbance plasma drifts
    • Authors: Ruilong Zhang; Libo Liu, Huijun Le, Yiding Chen, Jiawei Kuai
      Abstract: In this paper, we use the C/NOFS and ROCSAT-1 satellites observations to analyze the storm time evolution of the disturbance plasma drifts in a 24-hour local time scale during three magnetic storms driven by long-lasting southward IMF Bz. The disturbance plasma drifts during the three storms present some common features in the periods dominated by the disturbance dynamo. The newly formed disturbance plasma drifts are upward and westward at night, and downward and eastward during daytime. Further, the disturbance plasma drifts are gradually evolved to present significant local time shifts. The westward disturbance plasma drifts gradually migrate from nightside to dayside. Meanwhile, the dayside downward disturbance plasma drifts become enhanced and shift to later local time. The local time shifts in disturbance plasma drifts are suggested to be mainly attributed to the evolution of the disturbance winds. The strong disturbance winds arisen around midnight can constantly corotate to later local time. At dayside the westward and equatorward disturbance winds can drive the F region dynamo to produce the poleward and westward polarization electric fields (or the westward and downward disturbance drifts). The present results indicate that the disturbance winds corotated to later local time can affect the local time features of the disturbance dynamo electric field.
      PubDate: 2017-10-30T14:00:24.184275-05:
      DOI: 10.1002/2017JA024637
       
  • Model-based assessments of magnetic reconnection and Kelvin-Helmholtz
           instability at Jupiter's magnetopause
    • Authors: A. Masters
      Abstract: The interaction between the solar wind and Jupiter's magnetic field confines the planetary field to the largest magnetosphere in the Solar System. However, the full picture of when and where key processes operate at the magnetopause boundary of the system remains unclear. This is essential for testing understanding with observations, and for determining the relative importance of different drivers of Jovian magnetospheric dynamics. Here we present a global analytical model of Jovian magnetopause conditions under steady state, which forms the basis of boundary process assessments. Sites of magnetic reconnection at Jupiter's magnetopause are expected to be in regions of sufficiently high magnetic shear across the boundary, controlled by the orientation of the Interplanetary Magnetic Field (IMF). Reconnection rates are also most sensitive to changes in the highly variable IMF, followed by changes in the solar wind plasma mass density. The largest plasma flow shear across the boundary is in the equatorial dawn region, producing a region that is typically unstable to growth of the Kelvin-Helmholtz (K-H) instability. Compared to magnetopause reconnection site locations, this K-H-unstable region at dawn is less sensitive to changing conditions. Motion of K-H boundary perturbations typically includes dawn-to-dusk motion across the subsolar region. Model-predicted reconnection voltages are typically hundreds of kV, but rely on steady solar wind conditions on a timescale that is longer than typical at Jupiter's orbit. How the reconnection voltage compares to the voltage applied due to the “viscous-like” interaction involving K-H instability remains unclear.
      PubDate: 2017-10-30T14:00:20.827233-05:
      DOI: 10.1002/2017JA024736
       
  • The Radiation Belt Electron Scattering by Magnetosonic Wave: Dependence on
           Key Parameters
    • Authors: Mingda Lei; Lun Xie, Jinxing Li, Zuyin Pu, Suiyan Fu, Binbin Ni, Man Hua, Lunjin Chen, Wen Li
      Abstract: Magnetosonic (MS) waves have been found capable of creating radiation belt electron butterfly distributions in the inner magnetosphere. To investigate the physical nature of the interactions between radiation belt electrons and MS waves, and to explore a preferential condition for MS waves to scatter electrons efficiently, we performed a comprehensive parametric study of MS wave-electron interactions using test particle simulations (TPS). The diffusion coefficients simulated by varying the MS wave frequency show that the scattering effect of MS waves is frequency-insensitive at low harmonics (f < 20fcp), which has great implications on modeling the electron scattering caused by MS waves with harmonic structures. The electron scattering caused by MS waves is very sensitive to wave normal angles, and MS waves with off-90° wave normal angles scatter electrons more efficiently. By simulating the diffusion coefficients and the electron phase space density (PSD) evolution at different L-shells under different plasma environment circumstances, we find that MS waves can readily produce electron butterfly distributions in the inner part of the plasmasphere where the ratio of electron plasma-to-gyro frequency (fpe/fce) is large, while they may essentially form a two-peak distribution outside the plasmapause and in the inner radiation belt where fpe/fce is small.
      PubDate: 2017-10-27T13:05:26.394669-05:
      DOI: 10.1002/2016JA023801
       
  • An investigation of the ionospheric disturbances due to the 2014 sudden
           stratospheric warming events over Brazilian sector
    • Authors: R. Jesus; I. S. Batista, O. F. Jonah, A. J. Abreu, P. R. Fagundes, K. Venkatesh, C. M. Denardini
      Abstract: The present study investigates the ionospheric F region response in the Brazilian sector due to sudden stratospheric warming (SSW) events of 2014. The data used for this work are obtained from GPS receivers and magnetometer measurements during DOY 01 to 120, 2014 at different stations in the equatorial and low latitude regions in the Brazilian sector. In addition, the data obtained from C/NOFS satellites during DOY 01 to 75 of 2014 are used. The main novelty of this research is that, during the 2014 SSW events, day time Vertical Total Electron Content (VTEC) show a strong increase of the order of about 23% and 11% over the equatorial and low latitude regions respectively. We also observed that the night time VTEC (SSW days) is increased by 8% and 33% over equatorial and low latitude regions respectively. The magnetometer measurements show a strong counter-electrojet (CEJ) during the SSW days. The results show an amplification of the 0.5 day and ~2-16 day periods in the VTEC and EEJ during the SSWs. The occurrences of ionospheric irregularities during the SSW events are around 84% and 53 % in the equatorial and low latitude regions respectively, which is less frequent when compared with those during the pre-SSW periods.
      PubDate: 2017-10-27T13:05:22.283547-05:
      DOI: 10.1002/2017JA024560
       
  • Global Ionospheric and thermospheric effects of the June 2015 geomagnetic
           disturbances: multi-instrumental observations and modeling
    • Authors: E. Astafyeva; I. Zakharenkova, J. D. Huba, E. Doornbos, J. IJssel
      Abstract: By using data from multiple instruments, we investigate ionospheric/thermospheric behavior during the period from 21 to 23 June 2015, when three interplanetary shocks (IS) of different intensities arrived at Earth. The first IS was registered at 16:45UT on 21 June and caused ~50nT increase in the SYM-H index. The second IS arrived at 5:45UT on 22 June and induced an enhancement of the auroral/substorm activity that led to rapid increase of thermospheric neutral mass density and ionospheric vertical total electron content at high-latitudes. Several hours later, topside electron content and electron density increased at low-latitudes on the night side. The third and much larger IS arrived at 18:30UT on 22 June and initiated a major geomagnetic storm that lasted for many hours. The storm provoked significant effects in the thermosphere and ionosphere on both day and night-sides. In the thermosphere, the dayside neutral mass density exceeded the quiet-time levels by 300-500%, with stronger effects in the summer hemisphere. In the ionosphere, both positive and negative storm effects were observed on both day and night sides. We compared the ionospheric observations with simulations by the coupled SAMI3/RCM model. We find rather good agreement between the data and the model for the first phase of the storm, when the prompt penetration electric field (PPEF) was the principal driver. At the end of the storm main phase, when the ionospheric effects were, most likely, driven by a combination of PPEF and thermospheric winds, the modeling results agree less with the observations.
      PubDate: 2017-10-26T17:47:24.93269-05:0
      DOI: 10.1002/2017JA024174
       
  • A Single Deformed Bow Shock for Titan-Saturn System
    • Authors: N. Omidi; A. H. Sulaiman, W. Kurth, H. Madanian, T. Cravens, N. Sergis, M. K. Dougherty, N. J. T. Edberg
      Abstract: ssDuring periods of high solar wind pressure, Saturn's bow shock is pushed inside Titan's orbit exposing the moon and its ionosphere to the solar wind. The Cassini spacecraft's T96 encounter with Titan occurred during such a period and showed evidence for shocks associated with Saturn and Titan. It also revealed the presence of two foreshocks: one prior to the closest approach (foreshock-1) and one after (foreshock-2). Using electromagnetic hybrid (kinetic ions, fluid electrons) simulations and Cassini observations we show that the origin of foreshock-1 is tied to the formation of a single deformed bow shock for the Titan-Saturn system. We also report the observations of a structure in foreshock-1 with properties consistent with those of spontaneous hot flow anomalies (SHFAs) formed in the simulations and previously observed at Earth, Venus and Mars. The results of hybrid simulations also show the generation of oblique fast magnetosonic waves upstream of the outbound Titan bow shock in agreement with the observations of large amplitude magnetosonic pulsations in foreshock-2. We also discuss the implications of a single deformed bow shock for new particle acceleration mechanisms and also Saturn's magnetopause and magnetosphere.
      PubDate: 2017-10-24T16:35:24.09384-05:0
      DOI: 10.1002/2017JA024672
       
  • Empirical modeling of the plasmasphere dynamics using neural networks
    • Authors: Irina S. Zhelavskaya; Yuri Y. Shprits, Maria Spasojević
      Abstract: We propose a new empirical model for reconstructing the global dynamics of the cold plasma density distribution based only on solar wind data and geomagnetic indices. Utilizing the density database obtained using the NURD (Neural-network-based Upper hybrid Resonance Determination) algorithm for the period of October 1, 2012 - July 1, 2016, in conjunction with solar wind data and geomagnetic indices, we develop a neural network model that is capable of globally reconstructing the dynamics of the cold plasma density distribution for 2≤L≤6 and all local times. We validate and test the model by measuring its performance on independent datasets withheld from the training set and by comparing the model predicted global evolution with global images of He+ distribution in the Earth's plasmasphere from the IMAGE Extreme UltraViolet (EUV) instrument. We identify the parameters that best quantify the plasmasphere dynamics by training and comparing multiple neural networks with different combinations of input parameters (geomagnetic indices, solar wind data, and different durations of their time history). The optimal model is based on the 96-hour time history of Kp, AE, SYM-H, and F10.7 indices. The model successfully reproduces erosion of the plasmasphere on the night side and plume formation and evolution. We demonstrate results of both local and global plasma density reconstruction. This study illustrates how global dynamics can be reconstructed from local in-situ observations by using machine learning techniques.
      PubDate: 2017-10-24T12:50:45.555912-05:
      DOI: 10.1002/2017JA024406
       
  • Ground-based optical measurements of quiet-time thermospheric wind and
           temperature: atmospheric scattering corrections
    • Authors: Brian J. Harding; Jianqi Qin, Jonathan J. Makela
      Abstract: Ground-based measurements of thermospheric wind and temperature are known to be affected by tropospheric scattering during geomagnetically active times, when horizontal airglow gradients are large. In this work, we present an analysis of the effects during quiet times, when horizontal airglow gradients can be assumed to be negligible, and we derive corrections to be applied to historical and future data sets. These corrections are easy to apply, depending only upon the optical thickness of the atmosphere and the measured wind, not the viewing direction or temperature. If these corrections are not applied, all winds estimated from ground-based observatories are underestimated by about 10% (depending on the optical thickness), and temperatures are overestimated by a couple K. We present observational evidence of the effect of atmospheric scattering on temperature measurements using five years of data from the North American Thermosphere Ionosphere Observing Network (NATION). We find that the temperature measured to the east and west are statistically larger than the temperatures measured to the zenith. This is consistent with our analysis of the effect of atmospheric scattering, though the difference in the measurements is slightly larger than the theoretical prediction.
      PubDate: 2017-10-23T20:16:21.983742-05:
      DOI: 10.1002/2017JA024705
       
  • The MMS dayside magnetic reconnection locations during Phase 1 and their
           relation to the predictions of the Maximum Magnetic Shear model
    • Authors: K. J. Trattner; J. L. Burch, R. Ergun, S. Eriksson, S. A. Fuselier, B. L. Giles, R. G. Gomez, E. W. Grimes, W. S. Lewis, B. Mauk, S. M. Petrinec, C. T. Russell, R. J. Strangeway, L. Trenchi, F. D. Wilder
      Abstract: Several studies have validated the accuracy of the Maximum Magnetic Shear model to predict the location of the reconnection site at the dayside magnetopause. These studies found agreement between model and observations for 74% to 88% of events examined. It should be noted that, of the anomalous events that failed the prediction of the model, 72% shared a very specific parameter range. These events occurred around equinox for an IMF clock angle of about 240°. This study investigates if this remarkable grouping of events is also present in data from the recently launched MMS mission. The MMS magnetopause encounter data base from the first dayside phase of the mission includes about 4500 full and partial magnetopause crossings and FTEs. We use the known reconnection line signature of switching accelerated ion beams in the magnetopause boundary layer to identify encounters with the reconnection region and identify 302 events during phase 1a when the spacecraft are at reconnection sites. These confirmed reconnection locations are compared with the predicted location from the Maximum Magnetic Shear model and revealed an 80% agreement. The study also revealed the existence of anomalous cases as mentioned in an earlier study. The anomalies are concentrated for times around the equinoxes together with IMF clock angles around 140° and 240°. Another group of anomalies for the same clock angle ranges was found during December events.
      PubDate: 2017-10-23T20:16:19.169328-05:
      DOI: 10.1002/2017JA024488
       
  • A Statistical Study of the Spatial Extent of Relativistic Electron
           Precipitation with Polar Orbiting Environmental Satellites.
    • Authors: Sapna Shekhar; Robyn Millan, David Smith
      Abstract: Relativistic Electron Precipitation (REP) in the atmosphere can contribute significantly to electron loss from the outer radiation belts. In order to estimate the contribution to this loss, it is important to estimate the spatial extent of the precipitation region. We observed REP with the zenith pointing (0o) Medium Energy Proton Electron Detector (MEPED) on board Polar Orbiting Environmental Satellites (POES), for 15 years (2000-2014) and used both single and multi satellite measurements to estimate an average extent of the region of precipitation in L shell and Magnetic Local Time (MLT). In the duration of 15 years (2000-2014), 31035 REP events were found in this study. Events were found to split into two classes; one class of events coincided with proton precipitation in the P1 channel (30-80 keV), were located in the dusk and early morning sector, and were more localized in L shell (dL
      PubDate: 2017-10-23T20:16:15.847138-05:
      DOI: 10.1002/2017JA024716
       
  • The effect of a guide field on local energy conversion during asymmetric
           magnetic reconnection: MMS observations
    • Authors: K. J. Genestreti; J. L. Burch, P. A. Cassak, R. B. Torbert, R. E. Ergun, A. Varsani, T. D. Phan, B. L. Giles, C. T. Russell, S. Wang, M. Akhavan-Tafti, R. C. Allen
      Abstract: We compare case studies of Magnetospheric Multiscale (MMS)-observed magnetopause electron diffusion regions (EDRs) to determine how the rate of work done by the electric field, J·(E+ve×B)≡J·E′ varies with shear angle. We analyze MMS-observed EDR event with a guide field approximately the same size as the magnetosheath reconnecting field, which occurred on 8 December 2015. We find that J·E′ was largest and positive near the magnetic field reversal point, though patchy lower-amplitude J·E′ also occurred on the magnetosphere-side EDR near the electron-crescent point. The current associated with the large J·E′ near the X-point was carried by electrons with a velocity distribution function (VDF) resembling the magnetosheath inflow, shifted in the −v∥ direction. At the magnetosphere-side EDR, the current was carried by electrons with a crescent-like VDF. We compare this 8 December event to 10 other EDRs with different guide field strengths. The dual-region J·E′ was observed in three other moderate-shear EDR events, whereas three high-shear events had a strong positive J·E′ near the electron-crescent point and one low-shear event had a strong positive J·E′ only near the BL=0 point. The dual-region J·E′>0 was seen for one of three “intermediate"-shear EDRs with guide fields of ∼0.2–0.3. We propose a physical relationship between the shear angle and mode of energy conversion where (a) a guide field provides an efficient mechanism for carrying a current at the field reversal point (streaming) and (b) a guide field may limit the formation of crescent eVDFs, limiting the current carried near the stagnation point.
      PubDate: 2017-10-23T20:16:09.043211-05:
      DOI: 10.1002/2017JA024247
       
  • Lower ionosphere sensitivity to solar X-ray flares over a complete solar
           cycle evaluated from VLF signal measurements
    • Authors: Edith L. Macotela; Jean-Pierre Raulin, Jyrki Manninen, Emília Correia, Tauno Turunen, Antonio Magalhães
      Abstract: The daytime lower ionosphere behaves as a solar X-ray flare detector, which can be monitored using Very Low Frequency (VLF) radio waves that propagate inside the Earth-ionosphere waveguide. In this paper, we infer the lower ionosphere sensitivity variation over a complete solar cycle by using the minimum X-ray fluence (FXmin) necessary to produce a disturbance of the quiescent ionospheric conductivity. FXmin is the photon energy flux integrated over the time interval from the start of a solar X-ray flare up to the beginning of the ionospheric disturbance recorded as amplitude deviation of the VLF signal. FXmin is computed for ionospheric disturbances that occurred in the time interval December-January from 2007 to 2016 (Solar Cycle 24). The computation of FXmin uses the X-ray flux in the wavelength band below 0.2 nm and the amplitude of VLF signals transmitted from France (HWU), Turkey (TBB) and USA (NAA); which were recorded in Brazil, Finland and Peru. The main result of this study is that the long-term variation of FXmin is correlated with the level of solar activity, having FXmin values in the range (1 − 12) × 10−7J/m2. Our result suggests that FXmin is anti-correlated with the lower ionosphere sensitivity, confirming that the long-term variation of the ionospheric sensitivity is anti-correlated with the level of solar activity. This result is important to identify the minimum X-ray fluence that an external source of ionization must overcome in order to produce a measurable ionospheric disturbance during daytime.
      PubDate: 2017-10-23T20:15:58.191087-05:
      DOI: 10.1002/2017JA024493
       
  • Mass and energy transfer across the Earth's magnetopause caused by
           vortex-induced reconnection
    • Authors: T. K. M. Nakamura; S. Eriksson, H. Hasegawa, S. Zenitani, W. Y. Li, K. J. Genestreti, R. Nakamura, W. Daughton
      Abstract: When the interplanetary magnetic field (IMF) is strongly northward, a boundary layer that contains a considerable amount of plasma of magnetosheath origin is often observed along and earthward of the low-latitude magnetopause. Such a pre-existing boundary layer, with a higher density than observed in the adjacent magnetosphere, reduces the local Alfvén speed and allows the Kelvin-Helmholtz instability (KHI) to grow more strongly. We employ a three-dimensional fully kinetic simulation to model an event observed by the Magnetospheric Multiscale (MMS) mission in which the spacecraft detected substantial KH waves between a pre-existing boundary layer and the magnetosheath during strong northward IMF. Initial results of this simulation [Nakamura et al., 2017] have successfully demonstrated ion-scale signatures of magnetic reconnection induced by the non-linearly developed KH vortex, which are quantitatively consistent with MMS observations. We further quantify the simulated mass and energy transfer processes driven by this vortex-induced reconnection (VIR) and show that during this particular MMS event (i) mass enters a new mixing layer formed by the VIR more efficiently from the pre-existing boundary layer side than from the magnetosheath side, (ii) mixed plasmas within the new mixing layer convect tailward along the magnetopause at more than half the magnetosheath flow speed, and (iii) energy dissipation in localized VIR dissipation regions results in a strong parallel electron heating within the mixing layer. The quantitative agreements between the simulation and MMS observations allow new predictions that elucidate how the mass and energy transfer processes occur near the magnetopause during strong northward IMF.
      PubDate: 2017-10-23T10:35:25.74671-05:0
      DOI: 10.1002/2017JA024346
       
  • Redistribution of H atoms in the upper atmosphere during geomagnetic
           storms
    • Authors: Jianqi Qin; Lara Waldrop, Jonathan J. Makela
      Abstract: Geocoronal H emission data acquired by NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics mission are analyzed to quantify the H density distribution over the entire magnetosphere-ionosphere-thermosphere region in order to investigate the response of the atmospheric system as a whole to geomagnetic storms. It is shown that at low and middle latitudes the H density averaged over storm times in the thermosphere-exosphere transition region decreases by ∼30%, while the H density at exospheric altitudes above ∼1–2 RE increases by up to ∼40% relative to quiet times. We postulate that enhanced ion-neutral charge exchange in the topside ionosphere and inner plasmasphere is the primary driver of the observed H redistribution. Specifically, charge exchange reactions between H atoms and ionospheric/plasmaspheric O+ lead to direct H loss, while those between thermal H and H+ yield kinetically energized H atoms which populate gravitationally bound satellite orbits. The resulting H density enhancements in the outer exosphere would enhance the charge exchange rates in the ring current and the associated energetic neutral atom production. Regardless of the underlying mechanisms, H redistribution should be considered as an important process in the study of storm time atmospheric evolution, and the resultant changes in the geocoronal H emissions potentially could be used to monitor geomagnetic storms.
      PubDate: 2017-10-23T03:16:10.230469-05:
      DOI: 10.1002/2017JA024489
       
  • Eigenmode analysis of compressional poloidal modes in a self-consistent
           magnetic field
    • Authors: Zhiyang Xia; Lunjin Chen, Liheng Zheng, Anthony A. Chan
      Abstract: In this study, we simulate a self-consistent magnetic field that satisfies force balance with a model ring current that is radially localized, axisymmetric, and has anisotropic plasma pressure. We find that the magnetic field dip forms near the high plasma pressure region with plasma β>∼ 0.6, and the formed magnetic dip becomes deeper for larger plasma β and also slightly deeper for larger anisotropy. We perform linear analysis on a ppol of self-consistent equilibria for second harmonic compressional poloidal modes of sufficiently high azimuthal wave number. We investigate the effect of anisotropic pressure on the eigenfrequency of the poloidal modes and the characteristics of the compressional magnetic field component. We find that the eigenfrequency is reduced at the outer edge of the thermal pressure peak and increased at the inner edge. The compressional magnetic field component occurs primarily within 10° of the equator on both the inner and outer edges, with stronger compressional magnetic field component on the outer edge. Larger β and smaller anisotropy can increase the change of eigenfrequency and the strength of the compressional magnetic field component. The critical condition on plasma β and pressure anisotropy of an Alfvén ballooning instability is also identified.
      PubDate: 2017-10-23T03:15:45.661946-05:
      DOI: 10.1002/2017JA024376
       
  • A Three-Dimensional Model of Pluto's Interaction With the Solar Wind
           During the New Horizons Encounter
    • Authors: Moritz Feyerabend; Lucas Liuzzo, Sven Simon, Uwe Motschmann
      Abstract: We apply a hybrid (kinetic ions and fluid electrons) simulation model to study Pluto's plasma environment during the New Horizons encounter on 14 July 2015. We show that Pluto's plasma interaction is dominated by significant north-south asymmetries, driven by large pickup ion gyroradii on the order of 200 Pluto radii. The transition region from the ambient solar wind to the population of plutogenic ions (called the “Plutopause”) also shows considerable asymmetries that cannot be explained by a fluid picture. Since the New Horizons spacecraft does not carry a magnetometer, we use our model to estimate the strength and direction of the interplanetary magnetic field (IMF) at the time of the flyby by comparing output from the hybrid simulation to the plasma signatures observed during the New Horizons encounter. We find that an IMF strength of at least 0.24 nT is required to generate the observed plasma signatures. An IMF orientation either parallel or antiparallel to Pluto's orbital motion is able to explain the observed plasma densities and velocities along the New Horizons trajectory. Our simulations are able to quantitatively reproduce all key features of the plasma observations, specifically the gradual slowing of the solar wind, as well as the location and thickness of the Plutopause and bow shock.
      PubDate: 2017-10-21T02:01:03.777194-05:
      DOI: 10.1002/2017JA024456
       
  • Kinetics of sub-ion scale magnetic holes in the near-Earth plasma sheet
    • Authors: X.-J. Zhang; A. Artemyev, V. Angelopoulos, R. B. Horne
      Abstract: In collisionless space plasmas, the energy cascade from larger to smaller scales requires effective interactions between ions and electrons. These interactions are organized by sub-ion scale plasma structures in which strong electric fields connect demagnetized ions to magnetized electrons. We consider one such structure, magnetic holes, observed by THEMIS spacecraft in the dipolarized hot plasma sheet. Magnetic holes are localized depressions of the magnetic field with strong currents at their boundaries. Taking advantage of slow plasma convection (∼10–20 km/s), we reconstruct the electron velocity distribution within magnetic holes and demonstrate that the current at their boundaries is predominantly carried by magnetized thermal electrons. The motion of these electrons is the combination of diamagnetic drift and E × B drift in a Hall electric field. Magnetic holes can effectively modulate the intensity of electron cyclotron harmonic waves, and thus the spatial distribution of thermal electron precipitation. They may also contain field-aligned currents with magnitudes of ∼5 nA/m2 (1 order of magnitude smaller than the cross-field current density). Therefore, sub-ion scale magnetic holes can be important for ionosphere-magnetosphere coupling.
      PubDate: 2017-10-21T01:55:40.91901-05:0
      DOI: 10.1002/2017JA024197
       
  • Turbulence in Three-Dimensional Simulations of Magnetopause Reconnection
    • Authors: L. Price; M. Swisdak, J. F. Drake, J. L. Burch, P. A. Cassak, R. E. Ergun
      Abstract: We present detailed analysis of the turbulence observed in three-dimensional particle-in-cell simulations of magnetic reconnection at the magnetopause. The parameters are representative of an electron diffusion region encounter of the Magnetospheric Multiscale (MMS) mission. The turbulence is found to develop around both the magnetic x-line and separatrices, is electromagnetic in nature, is characterized by a wavevector k given by kρe∼(meTe/miTi)0.25 with ρe the electron Larmor radius, and appears to have the ion pressure gradient as its source of free energy. Taken together, these results suggest the instability is a variant of the lower-hybrid drift instability. The turbulence produces electric field fluctuations in the out-of-plane direction (the direction of the reconnection electric field) with an amplitude of around ±10 mV/m, which is much greater than the reconnection electric field of around 0.1 mV/m. Such large values of the out-of-plane electric field have been identified in the MMS data. The turbulence in the simulations controls the scale lengths of the density profile and current layers in asymmetric reconnection, driving them closer to ρeρi than the ρe or de scalings seen in 2D reconnection simulations, and produces significant anomalous resistivity and viscosity in the electron diffusion region.
      PubDate: 2017-10-19T16:35:39.741715-05:
      DOI: 10.1002/2017JA024227
       
  • Voyager 1/UVS Lyman α measurements at the distant heliosphere
           (90-130 AU): unknown source of additional emission
    • Authors: O. A. Katushkina; E. Quémerais, V. V. Izmodenov, R. Lallement, B. R. Sandel
      Abstract: In this work, we present for the first time the Lyman α intensities measured by Voyager 1/UVS in 2003-2014 (at 90-130 AU from the Sun). During this period Voyager 1 measured the Lyman α emission in the outer heliosphere at an almost fixed direction close to the upwind (that is towards the interstellar flow). The data show an unexpected behavior in 2003-2009: the ratio of observed intensity to the solar Lyman α flux is almost constant. Numerical modelling of these data is performed in the frame of a state-of-art self-consistent kinetic-MHD model of the heliospheric interface (Izmodenov & Alexashov, 2015). The model results, for various interstellar parameters, predict a monotonic decrease of intensity not seen in the data. We propose two possible scenarios that explain the data qualitatively. The first is the formation of a dense layer of hydrogen atoms near the heliopause. Such a layer would provide an additional backscattered Doppler shifted Lyman α emission, which is not absorbed inside the heliosphere and may be observed by Voyager. About 35 R of intensity from the layer is needed. The second scenario is an external non-heliospheric Lyman α component, which could be galactic or extragalactic. Our parametric study shows that ∼25 R of additional emission leads to a good qualitative agreement between the Voyager 1 data and the model results.
      PubDate: 2017-10-19T16:35:36.678156-05:
      DOI: 10.1002/2017JA024205
       
  • On the effect of geomagnetic storms on relativistic electrons in the outer
           radiation belt: Van Allen Probes observations
    • Authors: Pablo. S. Moya; Víctor A. Pinto, David G. Sibeck, Shrikanth G. Kanekal, Daniel N. Baker
      Abstract: Using Van Allen Probes ECT-REPT observations we performed a statistical study on the effect of geomagnetic storms on relativistic electrons fluxes in the outer radiation belt for 78 storms between September 2012 and June 2016. We found that the probability of enhancement, depletion and no change in flux values depends strongly on L and energy. Enhancement events are more common for ∼ 2 MeV electrons at L ∼ 5, and the number of enhancement events decreases with increasing energy at any given L shell. However, considering the percentage of occurrence of each kind of event, enhancements are more probable at higher energies, and the probability of enhancement tends to increases with increasing L shell. Depletion are more probable for 4-5 MeV electrons at the heart of the outer radiation belt, and no change events are more frequent at L < 3.5 for E∼ 3 MeV particles. Moreover, for L> 4.5 the probability of enhancement, depletion or no-change response presents little variation for all energies. Because these probabilities remain relatively constant as a function of radial distance in the outer radiation belt, measurements obtained at Geosynchronous orbit may be used as a proxy to monitor E≥1.8 MeV electrons in the outer belt.
      PubDate: 2017-10-19T16:35:34.865505-05:
      DOI: 10.1002/2017JA024735
       
  • Whistler wave propagation through the ionosphere of Venus
    • Authors: F. J. Pérez-Invernón; N. G. Lehtinen, F. J. Gordillo-Vázquez, A. Luque
      Abstract: We investigate the attenuation of whistler waves generated by hypotetical venusian lightning occurring at the altitude of the cloud layer under different ionospheric conditions. We use the Stanford Full Wave Method (FWM) for stratified media of ('Lehtinen2008/JGR) to model wave propagation through the ionosphere of Venus. This method calculates the electromagnetic field created by an arbitrary source in a plane-stratified medium (i.e. uniform in the horizontal direction). We see that the existence of holes in electronic densities and the magnetic field configuration caused by solar wind play an important role in the propagation of electromagnetic waves through the venusian ionosphere.
      PubDate: 2017-10-19T16:35:32.498766-05:
      DOI: 10.1002/2017JA024504
       
  • Magnetosheath High-Speed Jets: Internal Structure and Interaction With
           Ambient Plasma
    • Authors: F. Plaschke; T. Karlsson, H. Hietala, M. Archer, Z. Vörös, R. Nakamura, W. Magnes, W. Baumjohann, R. B. Torbert, C. T. Russell, B. L. Giles
      Abstract: For the first time, we have studied the rich internal structure of a magnetosheath high-speed jet. Measurements by the Magnetospheric Multiscale (MMS) spacecraft reveal large-amplitude density, temperature, and magnetic field variations inside the jet. The propagation velocity and normal direction of planar magnetic field structures (i.e., current sheets and waves) are investigated via four-spacecraft timing. We find structures to mainly convect with the jet plasma. There are indications of the presence of a tangential discontinuity. At other times, there are small cross-structure flows. Where this is the case, current sheets and waves overtake the plasma in the jet's core region; ahead and behind that core region, along the jet's path, current sheets are overtaken by the plasma; that is, they move in opposite direction to the jet in the plasma rest frame. Jet structures are found to be mainly thermal and magnetic pressure balance structures, notwithstanding that the dynamic pressure dominates by far. Although the jet is supermagnetosonic in the Earth's frame of reference, it is submagnetosonic with respect to the plasma ahead. Consequently, we find no fast shock. Instead, we find some evidence for (a series of) jets pushing ambient plasma out of their way, thereby stirring the magnetosheath and causing anomalous sunward flows in the subsolar magnetosheath. Furthermore, we find that jets modify the magnetic field in the magnetosheath, aligning it with their propagation direction.
      PubDate: 2017-10-19T11:21:27.428247-05:
      DOI: 10.1002/2017JA024471
       
  • Simultaneous Remote Observations of Intense Reconnection Effects by DMSP
           and MMS Spacecraft during a Storm-time Substorm
    • Authors: A. Varsani; R. Nakamura, V. A. Sergeev, W. Baumjohann, C. J. Owen, A. A. Petrukovich, Z. Yao, T. K. M. Nakamura, M. V. Kubyshkina, T. Sotireli, J. L. Burch, K. J. Genestreti, Z. Vörös, M. Andriopoulou, D. J. Gershman, L. A. Avanov, W. Magnes, C. T. Russell, F. Plaschke, Y. V. Khotyaintsev, B. L. Giles, V. N. Coffey, J. Dorelli, R. J. Strangeway, R. B. Torbert, P.-A. Lindqvist, R. Ergun
      Abstract: During a magnetic storm on 23 June 2015, several very intense substorms took place, with signatures observed by multiple spacecraft including DMSP and MMS. At the time of interest, DMSP F18 crossed inbound through a poleward-expanding auroral bulge boundary at 23.5h MLT, while MMS was located duskward of 22h MLT during an inward crossing of the expanding plasma sheet boundary. The two spacecraft observed a consistent set of signatures as they simultaneously crossed the reconnection separatrix layer during this very intense reconnection event. These include: 1) energy dispersion of the energetic ions and electrons travelling Earthwards, accompanied with high electron energies in the vicinity of the separatrix; 2) energy dispersion of polar rain electrons, with a high-energy cutoff; and 3) intense inward convection of the magnetic field lines at the MMS location. The high temporal resolution measurements by MMS provide unprecedented observations of the outermost electron boundary layer. We discuss the relevance of the energy dispersion of the electrons, and their pitch angle distribution, to the spatial and temporal evolution of the boundary layer. The results indicate that the underlying magnetotail magnetic reconnection process was an intrinsically impulsive and the active X-line was located relatively close to the Earth, approximately at 16-18 RE.
      PubDate: 2017-10-18T19:10:30.233288-05:
      DOI: 10.1002/2017JA024547
       
  • Lower-hybrid drift waves and electromagnetic electron space-phase holes
           associated with dipolarization fronts and field-aligned currents observed
           by the Magnetospheric Multiscale mission during a substorm
    • Authors: O. Le Contel; R. Nakamura, H. Breuillard, M. R. Argall, D. B. Graham, D. Fischer, A. Retinò, M. Berthomier, R. Pottelette, L. Mirioni, T. Chust, F. D. Wilder, D. J. Gershman, A. Varsani, P.-A. Lindqvist, Yu. V. Khotyaintsev, C. Norgren, R. E. Ergun, K. A. Goodrich, J. L. Burch, R. B. Torbert, J. Needell, M. Chutter, D. Rau, I. Dors, C. T. Russell, W. Magnes, R. J. Strangeway, K. R. Bromund, H. Y. Wei, F. Plaschke, B. J. Anderson, G. Le, T. E. Moore, B. L. Giles, W. R. Paterson, C. J. Pollock, J. C. Dorelli, L. A. Avanov, Y. Saito, B. Lavraud, S. A. Fuselier, B. H. Mauk, I. J. Cohen, D. L. Turner, J. F. Fennell, T. Leonard, A. N. Jaynes
      Abstract: We analyse two ion scale dipolarization fronts associated with field-aligned currents detected by the Magnetospheric Multiscale mission during a large substorm on August 10, 2016. The first event corresponds to a fast dawnward flow with an anti-parallel current and could be generated by the wake of a previous fast earthward flow. It is associated with intense lower-hybrid drift waves detected at the front and propagating dawnward with a perpendicular phase speed close to the electric drift and the ion thermal velocity. The second event corresponds to a flow reversal: from southwward/dawnward to northward/duskward associated with a parallel current consistent with a brief expansion of the plasma sheet before the front crossing, and with a smaller lower-hybrid drift wave activity. Electromagnetic electron phase-space holes are detected near these low-frequency drift waves during both events. The drift waves could accelerate electrons parallel to the magnetic field and produce the parallel electron drift needed to generate the electron holes. Yet, we cannot rule out the possibility that the drift waves are produced by the anti-parallel current associated with the fast flows, leaving the source for the electron holes unexplained.
      PubDate: 2017-10-16T16:30:53.105102-05:
      DOI: 10.1002/2017JA024550
       
  • Classification of Solar Wind with Machine Learning
    • Authors: Enrico Camporeale; Algo Carè, Joseph E. Borovsky
      Abstract: We present a four-category classification algorithm for the solar wind, based on Gaussian Process. The four categories are the ones previously adopted in Xu and Borovsky [2015]: ejecta, coronal hole origin plasma, streamer belt origin plasma, and sector reversal origin plasma. The algorithm is trained and tested on a labeled portion of the OMNI dataset. It uses seven inputs: the solar wind speed Vsw, the temperature standard deviation σT, the sunspot number R, the f10.7 index, the Alfven speed vA, the proton specific entropy Sp and the proton temperature Tp compared to a velocity-dependent expected temperature. The output of the Gaussian Process classifier is a four element vector containing the probabilities that an event (one reading from the hourly-averaged OMNI database) belongs to each category. The probabilistic nature of the prediction allows for a more informative and flexible interpretation of the results, for instance being able to classify events as 'undecided'. The new method has a median accuracy larger than 90% for all categories, even using a small set of data for training. The Receiver Operating Characteristic curve and the reliability diagram also demonstrate the excellent quality of this new method. Finally, we use the algorithm to classify a large portion of the OMNI dataset, and we present for the first time transition probabilities between different solar wind categories. Such probabilities represent the 'climatological' statistics that determine the solar wind baseline.
      PubDate: 2017-10-16T16:30:50.075918-05:
      DOI: 10.1002/2017JA024383
       
  • An empirical model of Titan's magnetic environment during the Cassini era:
           evidence for seasonal variability
    • Authors: Slawa Kabanovic; Sven Simon, Fritz M. Neubauer, Zachary Meeks
      Abstract: Based on the magnetic field data collected during the Cassini era, we construct an empirical model of the ambient magnetospheric field conditions along the orbit of Saturn's largest moon Titan. Observations from Cassini's close Titan flybys as well as 191 non-targeted crossings of Titan's orbit are taken into account. For each of these events we apply the classification technique of Simon et al. [2010a] to categorize the ambient magnetospheric field as current sheet, lobe-like, magnetosheath, or an admixture of these regimes. Independent of Saturnian season, Titan's magnetic environment around noon Saturn local time is dominated by the perturbed fields of Saturn's broad magnetodisk current sheet. Only observations from the nightside magnetosphere reveal a slow, but steady change of the background field from southern lobe-type to northern lobe-type on a time scale of several years. This behavior is consistent with a continuous change in the curvature of the bowl-shaped magnetodisk current sheet over the course of the Saturnian year. We determine the occurrence rate of each magnetic environment category along Titan's orbit as a function of Saturnian season and local time.
      PubDate: 2017-10-16T16:30:46.114232-05:
      DOI: 10.1002/2017JA024402
       
  • On the Dawn-Dusk Asymmetry of the Kelvin-Helmholtz Instability Between
           2007-2013
    • Authors: Z. W. Henry; K. Nykyri, T. W. Moore, A. P. Dimmock, X. Ma
      Abstract: Using data from Time History of Events and Macroscale Interactions during Substorms (THEMIS), a statistical study was performed to determine whether a dawn-dusk asymmetry exists in the occurrence rates of the Kelvin-Helmholtz (KH) instability during Parker-Spiral (PS) and Ortho-Parker-Spiral (OPS) orientations of the interplanetary magnetic field (IMF). It is determined from the data that there is a strong preference toward the dawn side during PS orientation, and although a preference to the dusk side during OPS is suggested, this requires further study for an unambiguous confirmation. The uncertainty in the OPS result is due to a low number of events which satisfied our selection criteria. Because IMF is statistically in PS orientation, the KHI preference for the dawn-flank during this orientation may help explain the origin of the plasma sheet asymmetry of cold-component ions because it has been shown that KHI can drive kinetic-scale wave activity capable of ion heating.
      PubDate: 2017-10-16T16:30:44.081471-05:
      DOI: 10.1002/2017JA024548
       
  • The Variability of Atmospheric Deuterium Brightness at Mars: Evidence for
           Seasonal Dependence
    • Authors: Majd Mayyasi; John Clarke, Dolon Bhattacharyya, Justin Deighan, Sonal Jain, Michael Chaffin, Edward Thiemann, Nick Schneider, Bruce Jakosky
      Abstract: The enhanced ratio of deuterium to hydrogen on Mars has been widely interpreted as indicating the loss of a large column of water into space, and the hydrogen content of the upper atmosphere is now known to be highly variable. The variation in the properties of both deuterium and hydrogen in the upper atmosphere of Mars is indicative of the dynamical processes that produce these species and propagate them to altitudes where they can escape the planet. Understanding the seasonal variability of D is key to understanding the variability of the escape rate of water from Mars. Data from a 15-month observing campaign, made by the MAVEN Imaging Ultraviolet Spectrograph high-resolution echelle channel are used to determine the brightness of deuterium as observed at the limb of Mars. The D emission is highly variable, with a peak in brightness just after southern summer solstice. The trends of D brightness are examined against extrinsic as well as intrinsic sources. It is found that the fluctuations in deuterium brightness in the upper atmosphere of Mars (up to 400 km), corrected for periodic solar variations, vary on timescales that are similar to those of water vapor fluctuations lower in the atmosphere (20-80 km). The observed variability in deuterium may be attributed to seasonal factors such as regional dust storm activity and subsequent circulation lower in the atmosphere.
      PubDate: 2017-10-16T16:30:42.132565-05:
      DOI: 10.1002/2017JA024666
       
  • Ionospheric response to 22-23 June 2015 storm as investigated using ground
           based ionosondes and GPS receivers over India
    • Authors: Ram Singh; S. Sripathi
      Abstract: In this paper, we present response of equatorial and low latitude ionosphere to 22/23 June 2015 geomagnetic storm using chain of ground based ionosondes located at Tirunelveli (8.73°N,77.70°E; geom: 0.32°N), Hyderabad (17.36°N, 78.47°E; geom 8.76°N), and Allahabad (25.45°N, 81.85°E; geom 16.5°N) along with chain of GPS receivers. Uniqueness of this storm is that in contrast to the equatorial plasma bubbles that were detected in the European sector, we see suppression of plasma bubbles in the Indian sector. The observations suggest that westward penetration during local midnight caused abrupt decrease of virtual height (h’F (km)) to ~200 km and suppressed plasma bubbles due to undershielding. Later, the layer increased to 500 km simultaneously due to overshielding effect. On 23rd, we observed negative storm in the northern hemisphere while positive storm in the southern hemisphere. In addition, absence of equatorial Es layers at Tirunelveli and presence of F3 layer at Tirunelveli/Hyderabad seem to be associated with EEJ/CEJ variations. However, on 24th, we observed strong negative storm effects at Allahabad/Hyderabad, while positive storm effect at Tirunelveli. Simultaneous enhancement of h’F (km) at all three ionosonde stations at 20:30 UT on 23rd during recovery phase suggest eastward DD Electric field that caused pre-sunrise spread F at Hyderabad/Allahabad but void of spread F at Tirunelveli suggesting its mid-latitude origin. Periodogram analysis of foF2 and h’F (km) in the present analysis suggest the presence of shorter periods (~2 hours) associated with PP Electric Fields while larger periods (>2 hours) associated with DD Electric field/winds.
      PubDate: 2017-10-16T16:30:40.006387-05:
      DOI: 10.1002/2017JA024460
       
  • Is the Dst Index Sufficient to Define All Geospace Storms'
    • Authors: Joseph E. Borovsky; Yuri Shprits
      Abstract: The purpose of this commentary is (1) to raise awareness about some shortcomings of the use of the Dst index to identify storms, to gauge storm intensity, and to represent stormtime space-weather phenomena and (2) to initiate discussions about different types of storms and about improved identifiers for different types of storms.
      PubDate: 2017-10-16T16:30:37.187623-05:
      DOI: 10.1002/2017JA024679
       
  • Spacecraft and instrument photoelectrons measured by the Dual Electron
           Spectrometers on MMS
    • Authors: Daniel J. Gershman; Levon A. Avanov, Scott A. Boardsen, John C. Dorelli, Ulrik Gliese, Alexander C. Barrie, Conrad Schiff, William R. Paterson, Roy B. Torbert, Barbara L. Giles, Craig J. Pollock
      Abstract: Secondary electrons are continuously generated via photoemission from sunlit spacecraft and instrument surfaces. These particles can subsequently contaminate low energy channels of electron sensors. Spacecraft photoelectrons are measured at energies below that of a positive spacecraft potential and can be removed at the expense of energy resolution. However, fluxes of photoelectrons generated inside electron instruments are independent of spacecraft potential and must be fully characterized in order to correct electron data. Here we present observations of spacecraft and instrument photoelectron populations measured with the Dual Electron Spectrometers (DES) on NASA's Magnetospheric Multiscale (MMS) mission. We leverage observations from Earth's nightside plasma sheet taken during MMS commissioning and develop an empirical model of instrument photoelectrons. This model is used with DES velocity distribution functions to correct plasma moments, and has been made publicly available on the MMS science data center for use by the scientific community.
      PubDate: 2017-10-16T02:30:39.067298-05:
      DOI: 10.1002/2017JA024518
       
  • Simultaneous FPI and TMA measurements of the lower-thermospheric wind in
           the vicinity of the poleward-expanding aurora after substorm onset
    • Authors: Shin-ichiro Oyama; Ken Kubota, Takatoshi Morinaga, Takuo T. Tsuda, Junichi Kurihara, Miguel F. Larsen, Masayuki Yamamoto, Lei Cai
      Abstract: Lower-thermospheric wind fluctuations in the vicinity of an auroral arc immediately before and after a substorm onset was examined by analyzing data from a ground-based green-line Fabry-Perot interferometer (FPI; optical wavelength of 557.7 nm) at Tromsø, Norway, and in-situ measurements from a trimethyl aluminum (TMA) trail released from a sounding rocket launched during the Dynamics and Energetics of the Lower Thermosphere in Aurora 2 (DELTA-2) campaign on 26 January 2009. Soon after the rocket launch but before disappearance of the TMA trail, a substorm onset occurred. The DELTA-2 TMA experiment appears to be the first case in which the substorm onset occurred during the TMA wind measurement. It is known that energy dissipation induced by the ionospheric closure current is compacted at the poleward side of the discrete arc in the ionospheric morning cell. Both FPI and TMA measurements were made at the poleward side, but the FPI measured winds nearer to the poleward edge of the arc than the TMA by 110-130 km. The FPI winds at distance of 53-74 km relative to the arc edge showed clear fluctuations immediately after the substorm onset, but there was no obvious similar fluctuation in the TMA-measured winds. The difference in the response at the two locations suggests that energy dissipation sufficient to be detected as the FPI/TMA wind perturbations was confined to the area from the poleward edge of the arc to a relative distance shorter than 163-203 km but longer than 53-74 km in this event.
      PubDate: 2017-10-13T13:35:29.021665-05:
      DOI: 10.1002/2017JA024613
       
  • Examining coherency scales, substructure, and propagation of whistler-mode
           chorus elements with Magnetospheric Multiscale (MMS)
    • Authors: D. L. Turner; J. H. Lee, S. G. Claudepierre, J. F. Fennell, J. B. Blake, A. N. Jaynes, T. Leonard, F. D. Wilder, R. E. Ergun, D. N. Baker, I. J. Cohen, B. H. Mauk, R. J. Strangeway, D. P. Hartley, C. A. Kletzing, H. Breuillard, O. Le Contel, Yu V. Khotyaintsev, R. B. Torbert, R. C. Allen, J. L. Burch, O. Santolik
      Abstract: Whistler-mode chorus waves are a naturally occurring electromagnetic emission observed in Earth's magnetosphere. Here, for the first time, data from NASA's Magnetospheric Multiscale (MMS) mission were used to analyze chorus waves in detail, including the calculation of chorus wave normal vectors, k. A case study was examined from a period of substorm activity around the time of a conjunction between the MMS constellation and NASA's Van Allen Probes mission on 07 April 2016. Chorus wave activity was simultaneously observed by all six spacecraft over a broad range of L-shells (5.5 < L < 8.5), magnetic local time (06:00 < MLT < 09:00), and magnetic latitude (-32° < MLat < -15°), implying a large chorus active region. Eight chorus elements and their substructure were analyzed in detail with MMS. These chorus elements were all lower band and rising tone emissions, right-handed and nearly circularly polarized, and propagating away from the magnetic equator when they were observed at MMS (MLat ~ -31°). Most of the elements had “hook” like signatures on their wave power spectra, characterized by enhanced wave power at flat or falling frequency following the peak, and all the elements exhibited complex and well organized substructure observed consistently at all four MMS spacecraft at separations up to 70 km (60 km perpendicular and 38 km parallel to the background magnetic field). The waveforms in field-aligned coordinates also demonstrated that these waves were all phase coherent allowing for the direct calculation of k. Error estimates on calculated k revealed that the plane wave approximation was valid for six of the eight elements and most of the subelements. The wave normal vectors were within 20-30° from the direction anti-parallel to the background field for all elements and changed from subelement to subelement through at least two of the eight elements. The azimuthal angle of k in the perpendicular plane was oriented earthward and was oblique to that of the Poynting vector, which has implications for the validity of cold plasma theory.
      PubDate: 2017-10-13T13:35:26.742971-05:
      DOI: 10.1002/2017JA024474
       
  • Substorm overshielding electric field at low latitude on the nightside as
           observed by the HF Doppler sounder and magnetometers
    • Authors: K. K. Hashimoto; T. Kikuchi, I. Tomizawa, T. Nagatsuma
      Abstract: The convection electric field increases during the growth phase of substorms, driving the DP2 ionospheric currents at high-to-equatorial latitudes, intensifying the eastward equatorial electrojet (EEJ) on the dayside. During the expansion phase, the electric field is often reversed, i.e., overshielding occurs at subauroral-to-equatorial latitudes where the EEJ turns to the westward counterelectrojet (CEJ). In this paper, we show that the HF Doppler sounders detected the eastward overshielding electric field at low latitudes on the nightside simultaneously with the CEJ on the dayside. We also show that the overshielding often occurs during the substorm recovery due to the convection reduction, resulting in a two-step form in both the dayside CEJ and nightside electric field. The opposite direction of the electric field on the day- and night-sides is consistent with the dusk-to-dawn potential electric field associated with the Region-2 field-aligned currents intensified by the substorm. The overshielding electric field was found to drive an eastward electrojet with appreciable magnitude in the nighttime equatorial ionosphere, which in turn causes an equatorial enhancement of the midnight positive bay.
      PubDate: 2017-10-13T13:35:22.6959-05:00
      DOI: 10.1002/2017JA024329
       
  • Lower Band Cascade of Whistler Waves Excited by Anisotropic Hot Electrons:
           One-dimensional PIC Simulations
    • Authors: Huayue Chen; Xinliang Gao, Quanming Lu, Yangguang Ke, Shui Wang
      Abstract: Based on THEMIS waveform data, Gao et al. [2016a] have reported two special multiband chorus events, where upper-band waves are located at harmonics of lower-band waves. And they proposed a new generation mechanism to explain this multiband chorus wave, named as lower band cascade. With a 1-D PIC simulation model, we have investigated the lower band cascade of whistler waves excited by anisotropic hot electrons. During each simulation, lower-band whistler-mode waves are firstly excited by the anisotropy of hot electrons. Later, upper-band harmonic waves are generated through the nonlinear coupling between the electromagnetic and electrostatic components of lower-band waves, which supports the scenario of lower band cascade. Moreover, the peak wave number (or frequency) of lower-band waves will continuously drift to smaller values due to the decline of the anisotropy of hot electrons. While, the peak wave number of upper-band harmonic waves will be kept nearly unchanged, but their amplitude continues to decrease after their saturation. We further find the magnetic amplitude of upper-band harmonic waves tends to increase with the increase of the wave normal angle of lower-band waves or the anisotropy of hot electrons. Besides, the amplitude ratio between upper-band and lower-band waves is positively correlated with the wave normal angle of lower-band waves, but is anti-correlated with the anisotropy of hot electrons. Our study has provided a more comprehensive understanding of the lower band cascade of whistler waves.
      PubDate: 2017-10-13T13:35:19.898228-05:
      DOI: 10.1002/2017JA024513
       
  • Variabilities of low-latitude migrating and non-migrating tides in GPS-TEC
           and TIMED-SABER temperature during the sudden stratospheric warming event
           of 2013
    • Authors: S. Sridharan
      Abstract: The Global Positioning System (GPS) deduced total electron content (TEC) data at 15°N (geomagnetic), which is the crest region of equatorial ionization anomaly, are used to study tidal variabilities during the 2013 sudden stratospheric warming (SSW) event. The results from space-time spectral analysis reveal that the amplitudes of migrating diurnal (DW1) and semi-diurnal (SW2) tides are larger than those of non-migrating tides. After the SSW onset, the amplitudes of DW1, SW2, SW1, and DS0 increase. Moreover, they show 16-day variations similar to the periodicity of the high-latitude stratospheric planetary wave (PW) suggesting that the non-migrating tides (SW1 and DS0) are possibly generated due to non-linear interaction of migrating tides with PW. Similar spectral analysis on temperature at 10°N obtained from the Sounding of Atmosphere by Broadband Emission Radiometry (SABER) shows that the SW2 enhances at stratospheric heights and the SW2 is more dominant at 80-90 km, but its amplitude decreases around 100 km. The amplitudes of non-migrating tides become comparable to those of SW2 around 100 km and their contribution become increasingly important at higher heights. This suggests that the non-linear interaction between migrating tides and PW occur at low-latitude upper mesospheric heights, as SW2 exhibits 16-day periodicity in SABER temperature at 100 km as observed in TEC. Besides, it is observed that the eastward propagating tides are less dominant than westward propagating tides in both TEC and SABER temperature.
      PubDate: 2017-10-12T18:10:28.406896-05:
      DOI: 10.1002/2017JA024283
       
  • Characteristics of seasonal variation and solar activity dependence of the
           geomagnetic solar quiet daily variation
    • Authors: Atsuki Shinbori; Yukinobu Koyama, Masahito Nosé, Tomoaki Hori, Yuichi Otsuka
      Abstract: Characteristics of seasonal variation and solar activity dependence of the X- and Y-components of the geomagnetic solar quiet (Sq) daily variation at Memambetsu in mid-latitudes and Guam near the equator have been investigated using long-term geomagnetic field data with 1-h time resolution from 1957 to 2016. The monthly mean Sq variation in the X and Y components (Sq-X and Sq-Y) shows a clear seasonal variation and solar activity dependence. The amplitude of seasonal variation increases significantly during high solar activities, and is proportional to the solar F10.7 index. The pattern of the seasonal variation is quite different between Sq-X and Sq-Y. The result of the correlation analysis between the solar F10.7 index and Sq-X and Sq-Y shows almost the linear relationship, but the slope of the linear fitted line varies as function of local time and month. This implies that the sensitivity of Sq-X and Sq-Y to the solar activity is different for different local times and seasons. The pattern of the local time and seasonal variations of Sq-Y at Guam shows good agreement with that of a magnetic field produced by inter-hemispheric field-aligned currents (FACs), which flow from the summer to winter hemispheres in the dawn and dusk sectors and from the winter to summer hemispheres in the pre-noon to afternoon sectors. The direction of the inter-hemispheric FAC in the dusk sector is opposite to the concept of Fukushima's model.
      PubDate: 2017-10-12T18:10:24.977937-05:
      DOI: 10.1002/2017JA024342
       
  • Asymmetries in the magnetosheath field draping on Venus' night side
    • Authors: M. Delva; M. Volwerk, R. Jarvinen, C. Bertucci
      Abstract: Draping features of the interplanetary magnetic field around non-magnetic bodies, especially Venus, have been studied in detail in numerical simulations and also from observations. Existing analytical and numerical work for non-perpendicular IMF and solar wind velocity direction show a kink in the draped fieldlines in the near magnetosheath on the quasi-parallel side of the bow shock. Here, long-term magnetic field data from the Venus Express mission (2006-2014) are analyzed in the near night-side region of the magnetosheath, searching for differences in the draping pattern between the quasi-parallel- and quasi-perpendicular side of the shock. From these MAG data, the kink in the fieldlines occurring only on the quasi-parallel side is clearly identified from the change of sign in the field component parallel to the solar wind velocity. Furthermore, an asymmetry in the deflection of the out-of-plane field component due to the slipping of the fieldlines over the planetary obstacle is also found, which confirms predictions from numeral studies and from earlier work.
      PubDate: 2017-10-10T18:40:24.157847-05:
      DOI: 10.1002/2017JA024604
       
  • Application of Manley-Rowe relation in analyzing nonlinear interactions
           between planetary waves and the solar semidiurnal tide during 2009 Sudden
           Stratospheric Warming event
    • Authors: Maosheng He; Jorge Luis Chau, Gunter Stober, Chris M. Hall, Masaki Tsutsumi, Peter Hoffmann
      Abstract: Upper mesospheric winds observed by the Svalbard specular meteor radar (16.01°E,78.16°N) are analyzed to study the tidal variabilities during the 2009 SSW. We report a textbook case of nonlinear interactions between planetary waves (PWs) and the SW2 tide (SWm denotes semidiurnal westward-propagating tidal mode with zonal wavenumber m). The Lomb-Scargle algorithm, bispectrum, wavelet spectra, and Manley-Rowe relations are combined to explore the frequency match, phase coherence, energy budget, and wavenumber relations among the interacting waves and their temporal evolution. Our results suggest that (1) 5-, 10-, 16-day PW normal modes interact with SW2 generating significant sidebands (S2Ss) at frequencies lower and higher than SW2, known as SW1 and SW3 enhancements, respectively, (2) SW2 is the main energy supplier for both SW1 and SW3, hence shrinks in the interactions, (3) whereas the PWs export relatively negligible energy to SW3 but accept energy from SW2 in generating SW1, therefore the PWs is not subject to the interactions but controlled by external dynamics, which might in turn act as a key in switching on/off the SW1 and SW3 interactions independently, (4) the SW1 enhancement could be explained as a byproduct of the planetary wave amplification by stimulated tidal decay (PASTIDE), (5) PASTIDE contributes energy to the secondary PW in the late SSW stage reported in previous studies, and (6) one SW1 component associated with the 16-day PW is very close to the semidiurnal lunar mode in frequency, which might contaminate the estimation of the lunar tidal amplification in previous studies.
      PubDate: 2017-10-09T19:40:51.763233-05:
      DOI: 10.1002/2017JA024630
       
  • Cold ion outflow modulated by the solar wind energy input and tilt of the
           geomagnetic dipole
    • Authors: Kun Li; Y. Wei, M. André, A. Eriksson, S. Haaland, E. A. Kronberg, H. Nilsson, L. Maes, Z. J. Rong, W. X. Wan
      Abstract: The solar wind energy input into the Earth's magnetosphere-ionosphere system drives ionospheric outflow, which plays an important role in both the magnetospheric dynamics and evolution of the atmosphere. However, little is known about the cold ion outflow with energies lower than a few tens of eV, as the direct measurement of cold ions is difficult because a spacecraft gains a positive electric charge due to the photoemission effect, which prevents cold ions from reaching the onboard detectors. A recent breakthrough in the measurement technique using Cluster spacecraft revealed that cold ions dominate the ion population in the magnetosphere. This new technique yields a comprehensive data set containing measurements of the velocities and densities of cold ions for the years 2001-2010. In this paper, this data set is used to analyze the cold ion outflow from the ionosphere. We found that about 0.1% of the solar wind energy input is transformed to the kinetic energy of cold ion outflow at the topside ionosphere. We also found that the geomagnetic dipole tilt can significantly affect the density of cold ion outflow, modulating the outflow rate of cold ion kinetic energy. These results give us clues to study the evolution of ionospheric outflow with changing global magnetic field and solar wind condition in the history.
      PubDate: 2017-10-09T19:40:49.245379-05:
      DOI: 10.1002/2017JA024642
       
  • Revisiting Ionosphere-Thermosphere Responses to Solar Wind Driving in
           Superstorms of November 2003 and 2004
    • Authors: O. P. Verkhoglyadova; A. Komjathy, A. J. Mannucci, M. Mlynczak, L. Hunt, L. J. Paxton
      Abstract: We revisit three complex superstorms of 19-20 November 2003, 7-8 November 2004 and 9-11 November 2004 to analyze ionosphere-thermosphere (IT) effects driven by different solar wind structures associated with complex Interplanetary Coronal Mass Ejections (ICMEs) and their upstream sheaths. The efficiency of the solar wind-magnetosphere connection throughout the storms is estimated by coupling functions. The daytime IT responses to the complex driving are characterized by combining and collocating (where possible) measurements of several physical parameters (total electron content or TEC, thermospheric infrared nitric oxide emission and composition ratio) from multiple satellite platforms and ground-based measurements. A variety of metrics are utilized to examine global IT phenomena at ~1 hour time scales. The role of direct driving of IT dynamics by solar wind structures and the role of IT pre-conditioning in these storms, which feature complex unusual TEC responses, are examined and contrasted. Furthermore, IT responses to ICME magnetic clouds and upstream sheaths are separately characterized. We identify IT feedback effects that can be important for long-lasting strong storms. The role of the IMF By component on ionospheric convection may not be well captured by existing coupling functions. Mechanisms of thermospheric overdamping and consequential ionospheric feedback need to be further studied.
      PubDate: 2017-10-09T19:40:46.869771-05:
      DOI: 10.1002/2017JA024542
       
  • Impact of oscillating IMF Bz during 17 March 2013 storm on the
           distribution of plasma over Indian low and mid latitude ionospheric
           regions
    • Authors: P. R. Shreedevi; R. K. Choudhary
      Abstract: We show evidence of a positive ionospheric storm occurring simultaneously at the equatorial, low and mid-latitude ionospheric regions in the Indian sector in response to an intense geomagnetic storm on 17 March 2013. The storm had its onset time coinciding with the local noon. An on-site Digisonde at Trivandrum (dip-equator) recorded a sharp decrease in the height of F-region peak in the afternoon, which is a signature of westward electric field associated with a CEJ. Coupled with it was a simultaneous increase in TEC in the entire Indian region. The magnitude of increase in TEC decreased slowly northward of the dip-equator and had almost no change near the anomaly crest. Further northward of the anomaly crest, the TEC started increasing again and at Shimla, a mid-latitude station, it had a value close to 2 times its monthly mean. We surmise that the westward electric field resulting from the CEJ pushed the F-layer at the dip-equator down to the altitude regions where recombination and diffusion played minimal roles. No loss of plasma due to diffusion while photo-production of ions was still taking place, led to an enhancement in the electron density near the equatorial / low-latitude region. The Joule heating of the thermosphere, on the other hand, gave rise to the TADs which pushed the plasma up in altitude at the equatorial anomaly region. It supplemented the loss of plasma at the anomaly crest region resulting in no change in the TEC and a marked increase in the TEC in the mid-latitude ionosphere.
      PubDate: 2017-10-09T19:40:42.04705-05:0
      DOI: 10.1002/2017JA023980
       
  • SC-associated electric field variations in the magnetosphere and
           ionospheric convective flows
    • Authors: S.-I. Kim; K.-H. Kim, H.-J. Kwon, H. Jin, E. Lee, G. Jee, N. Nishitani, T. Hori, M. Lester, J. R. Wygant
      Abstract: We examine magnetic and electric field perturbations associated with a sudden commencement (SC), caused by an interplanetary (IP) shock passing over the Earth's magnetosphere on 16 February 2013. The SC was identified in the magnetic and electric field data measured at THEMIS-E (THE-E: MLT = 12.4, L = 6.3), Van Allen Probe-A (VAP-A: MLT = 3.2, L = 5.1), and Van Allen Probe-B (VAP-B: MLT = 0.2. L= 4.9) in the magnetosphere. During the SC interval, THE-E observed a dawnward-then-duskward electric (E) field perturbation around noon, while VAP-B observed a duskward E-field perturbation around midnight. VAP-A observed a dawnward-then-duskward E-field perturbation in the postmidnight sector, but the duration and magnitude of the dawnward E-perturbation are much shorter and weaker than that at THE-E. That is, the E-field signature changes with local time during the SC interval. The SuperDARN radar data indicate that the ionospheric plasma motions during the SC are mainly due to the E-field variations observed in space. This indicates that the SC-associated E-field in space plays a significant role in determining the dynamic variations of the ionospheric convection flow. By comparing previous SC MHD simulations and our observations, we suggest that the E-field variations observed at the spacecraft are produced by magnetospheric convection flows due to deformation of the magnetosphere as the IP shock sweeps the magnetopause.
      PubDate: 2017-10-09T19:40:38.686833-05:
      DOI: 10.1002/2017JA024611
       
  • Coseismic Travelling Ionospheric Disturbances (CTIDs) during the Mw 7.8
           Gorkha, Nepal Earthquake on 25 April 2015 from ground and space borne
           observations
    • Authors: S. Tulasi Ram; P. S. Sunil, M. Ravi Kumar, S. -Y. Su, L. C. Tsai, C. H. Liu
      Abstract: Coseismic travelling ionospheric disturbances (CTIDs) and their propagation characteristics during Mw 7.8 Gorkha earthquake in Nepal on 25 April 2015 have been investigated using a suite of ground based GPS receivers and broad band seismometers along with the space borne Radio Occultation observations over the Indian subcontinent region. Depletion in vertical total electron content (VTEC), a so called ionospheric hole, is observed near the epicenter ~9 – 11 minutes after the onset of earthquake. A positive pulse preceding the depletion, similar to N-shaped perturbation, propagating with an apparent velocity of ~2.4 km/s is observed on the South. Further, the CTIDs in the southward direction are found to split in to fast (~2.4 – 1.7 km/s) and slow (~680 – 520 m/s) propagating modes at epicentral distances greater than ~800 km. However, the velocities of fast mode CTIDs are significantly smaller than the surface Rayleigh wave velocity (~3.7 km/s), indicating that they are not the true imprint of Rayleigh wave, instead, can probably be attributed to the superimposed wave front formed by the mixture of acoustic waves excited by main shock and propagating Rayleigh wave. The southward CTIDs are found to propagate at F2-region altitudes of ~300 – 440 km captured by COSMIC Radio Occultation observations. The CTIDs with periods of ~4 – 6 minutes are observed in all directions with significantly larger amplitudes and faster propagation velocities in South and East directions. The observed azimuthal asymmetry in the amplitudes and velocities of CTIDs are discussed in terms of the alignment with geomagnetic field and nature of surface crustal deformation during the earthquake.
      PubDate: 2017-10-09T19:40:33.434015-05:
      DOI: 10.1002/2017JA023860
       
  • Data Assimilation of Ground-based GPS and Radio Occultation Total Electron
           Content for Global Ionospheric Specification
    • Authors: C. Y. Lin; T. Matsuo, J. Y. Liu, C. H. Lin, J. D. Huba, H. F. Tsai, C. Y. Chen
      Abstract: This study presents an approach based on the Gauss-Markov Kalman filter to assimilate the total electron content (TEC) observed from ground-based GPS receivers and space-based radio occultation instrumentations (such as FORMOSAT-3/COSMIC (F3/C) and FORMOSAT-7/COSMIC-2 (F7/C2)) into the International Reference Ionosphere. Observing System Simulation Experiments (OSSEs) show that the data assimilation procedure consisting of the forecast and the measurement update steps can better improve the accuracy of the data assimilation analysis than the assimilation procedure using the measurement update alone. Compared with F3/C, the denser F7/C2 occultation observations can improve the analysis accuracy significantly as suggested by OSSEs. The real data assimilation results are further validated with Global Ionosphere Maps, the global ground-based GPS measurements, and the ionospheric F2-peak height and electron density sounded by ionosondes. Both the OSSEs and validation results confirm that a number of improvements to the data assimilation procedure presented in this paper can indeed be used to reconstruct the three-dimensional ionospheric electron density adequately.
      PubDate: 2017-10-09T19:40:30.12018-05:0
      DOI: 10.1002/2017JA024185
       
  • MMS observations and hybrid simulations of surface ripples at a marginally
           quasi-parallel shock
    • Authors: Imogen Gingell; Steven J. Schwartz, David Burgess, Andreas Johlander, Christopher T. Russell, James L. Burch, Robert E. Ergun, Stephen Fuselier, Daniel J. Gershman, Barbara L. Giles, Katherine A. Goodrich, Yuri V. Khotyaintsev, Benoit Lavraud, Per-Arne Lindqvist, Robert J. Strangeway, Karlheinz Trattner, Roy B. Torbert, Hanying Wei, Frederick Wilder
      Abstract: Simulations and observations of collisionless shocks have shown that deviations of the nominal local shock normal orientation, i.e. surface waves or ripples, are expected to propagate in the ramp and overshoot of quasi-perpendicular shocks. Here, we identify signatures of a surface ripple propagating during a crossing of Earth's marginally quasi-parallel (θBn∼45∘) or quasi-parallel bow shock shock on 2015-11-27 06:01:44 UTC by the Magnetspheric Multiscale (MMS) mission, and determine the ripple's properties using multi-spacecraft methods. Using two-dimensional hybrid simulations, we confirm that surface ripples are a feature of marginally quasi-parallel and quasi-parallel shocks under the observed solar wind conditions. In addition, since these marginally quasi-parallel and quasi-parallel shocks are expected to undergo a cyclic reformation of the shock front, we discuss the impact of multiple sources of non-stationarity on shock structure. Importantly, ripples are shown to be transient phenomena, developing faster than an ion gyroperiod and only during the period of the reformation cycle when a newly developed shock ramp is unaffected by turbulence in the foot. We conclude that the change in properties of the ripple observed by MMS is consistent with the reformation of the shock front over a timescale of an ion gyro-period.
      PubDate: 2017-10-06T17:40:49.10092-05:0
      DOI: 10.1002/2017JA024538
       
  • Thermal conductivity of the multicomponent neutral atmosphere
    • Authors: A. V. Pavlov
      Abstract: Approximate expressions for the thermal conductivity coefficient of the multicomponent neutral atmosphere consisting of N2, O2, O, He, and H are analyzed and evaluated for the atmospheric conditions by comparing them with that given by the rigorous hydrodynamic theory. The new approximations of the thermal conductivity coefficients of simple gases N2, O2, O, He, and H are derived and used. It is proved that the modified Mason and Saxena approximation of the atmospheric thermal conductivity coefficient is more accurate in reproducing the atmospheric values of the rigorous hydrodynamic thermal conductivity coefficient in comparison with those that are generally accepted in atmospheric studies. This approximation of the thermal conductivity coefficient is recommended to use in calculations of the neutral temperature of the atmosphere.
      PubDate: 2017-10-06T17:40:25.686866-05:
      DOI: 10.1002/2017JA024397
       
  • Systematic evaluation of low frequency hiss and energetic electron
           injections
    • Authors: Run Shi; Wen Li, Qianli Ma, G. D. Reeves, C. A. Kletzing, W. S. Kurth, G. B. Hospodarsky, H. E. Spence, J. B. Blake, J. F. Fennell, S. G. Claudepierre
      Abstract: The excitation of low frequency (LF) plasmaspheric hiss, over the frequency range from 20 Hz to 100 Hz, is systematically investigated by comparing the hiss wave properties with electron injections at energies from tens of keV to several hundred keV. Both particle and wave data from the Van Allen Probes during the period from September 2012 to June 2016 are used in the present study. Our results demonstrate that the intensity of LF hiss has a clear day-night asymmetry, and increases with increasing geomagnetic activity, similar to the behavior of normal hiss (~100 Hz to several kHz). The occurrence rate of LF hiss in association with electron injections is up to 80% in the outer plasmasphere (L> 4) on the dayside, and the strong correlation extends to lower L shells for more active times. In contrast, at lower L shells (L < 3.5), LF hiss is seldom associated with electron injections. The LF hiss with Poynting flux directed away from the equator is dominant at higher magnetic latitudes and higher L shells, suggesting a local amplification of LF hiss in the outer plasmasphere. The averaged electron fluxes are larger at higher L shells where significant LF hiss wave events are observed. Our study suggests the importance of electron injections and their drift trajectories towards the dayside plasmasphere in locally amplifying the LF hiss waves detected by the Van Allen Probes.
      PubDate: 2017-10-05T13:40:31.502205-05:
      DOI: 10.1002/2017JA024571
       
  • Using the Coronal Evolution to Successfully Forward Model CMEs' In Situ
           Magnetic Profiles
    • Authors: C. Kay; N. Gopalswamy
      Abstract: Predicting the effects of a coronal mass ejection (CME) impact requires knowing if impact will occur, which part of the CME impacts, and its magnetic properties. We explore the relation between CME deflections and rotations, which change the position and orientation of a CME, and the resulting magnetic profiles at 1 AU. For 45 STEREO-era, Earth-impacting CMEs, we determine the solar source of each CME, reconstruct its coronal position and orientation, and perform a ForeCAT [Kay et al., 2015a] simulation of the coronal deflection and rotation. From the reconstructed and modeled CME deflections and rotations we determine the solar cycle variation and correlations with CME properties. We assume no evolution between the outer corona and 1 AU and use the ForeCAT results to drive the FIDO in situ magnetic field model [Kay et al., 2017a], allowing for comparisons with ACE and Wind observations. We do not attempt to reproduce the arrival time. On average FIDO reproduces the in situ magnetic field for each vector component with an error equivalent to 35% of the average total magnetic field strength when the total modeled magnetic field is scaled to match the average observed value. Random walk best fits distinguish between ForeCAT's ability to determine FIDO's input parameters and the limitations of the simple flux rope model. These best fits reduce the average error to 30%. The FIDO results are sensitive to changes of order a degree in the CME latitude, longitude, and tilt, suggesting that accurate space weather predictions require accurate measurements of a CME's position and orientation.
      PubDate: 2017-10-05T13:40:26.526437-05:
      DOI: 10.1002/2017JA024541
       
  • Low Altitude Emission of Energetic Neutral Atoms: Multiple Interactions
           and Energy Loss
    • Authors: K. LLera; J. Goldstein, D. J. McComas, P. W. Valek
      Abstract: Low Altitude Emissions (LAEs) are the energetic neutral atom (ENA) signature of ring current ions precipitating along the magnetic field to an altitude of 200-800 km. This altitude region is considered to be “optically thick” because ring current ions undergo multiple charge changing interactions (MCCIs) with Earth's dense oxygen exosphere. While each interaction involves an energy loss of ~36 eV, no prior study has determined the accumulated energy lost by 1-100 keV H+ emerging as LAEs. We have developed a 2-D model with a geomagnetic dipole that captures the net effects in energy loss and pitch angle evolution as a result of MCCIs without the computational requirements of a full Monte-Carlo simulation. Dependent on the amount of latitudinal migration, the energy loss is greater than 20% for ions below 60 keV for equatorward moving particles (30 keV for poleward). Since the ENA travels ballistically across a geomagnetic dipole, upon reionization, ion velocity along the local field increases (anti-parallel in the northern hemisphere). Redirecting the particle upward through MCCIs is most effective during poleward ENA motion. The net effect is to redirect precipitating ions (below 2,500 km) to eventually emerge from the optically thick region either as an ion or ENA. Precipitation is a joint ion-neutral process, affecting both the energy and pitch angle distribution through the transverse motion of ENA segments in a converging field. For particles that enter the MCCI regime, the energy loss and evolution of the pitch angle distribution must be considered within a realistic magnetic field.
      PubDate: 2017-10-04T17:06:48.829401-05:
      DOI: 10.1002/2017JA024016
       
  • Interpreting Observations of Large-Scale Traveling Ionospheric
           Disturbances by Ionospheric Sounders
    • Authors: L. H. Pederick; M. A. Cervera, T. J. Harris
      Abstract: From July to October 2015, the Australian Defence Science and Technology Group (DST Group) conducted an experiment during which a vertical incidence sounder (VIS) was set up at Alice Springs Airport. During September 2015 this VIS observed the passage of many large-scale traveling ionospheric disturbances (TIDs). By plotting the measured virtual heights across multiple frequencies as a function of time, the passage of the TID can be clearly displayed. Using this plotting method we show that all the TIDs observed during the campaign by the VIS at Alice Springs show an apparent downward phase progression of the crests and troughs. The passage of the TID can be more clearly interpreted by plotting the true height of iso-ionic contours across multiple plasma frequencies; the true heights can be obtained by inverting each ionogram to obtain an electron density profile. These plots can be used to measure the vertical phase speed of a TID, and also reveal a time lag between events seen in true height compared to virtual height. To the best of our knowledge this style of analysis has not previously been applied to other swept-frequency sounder observations. We develop a simple model to investigate the effect of the passage of a large-scale TID on a VIS. The model confirms that for a TID with a downward vertical phase progression, the crests and troughs will appear earlier in virtual height than in true height, and will have a smaller apparent speed in true height than in virtual height.
      PubDate: 2017-10-04T17:06:44.866185-05:
      DOI: 10.1002/2017JA024337
       
  • Shock-induced disappearance and subsequent recovery of plasmaspheric hiss:
           Coordinated observations of RBSP, THEMIS and POES satellites
    • Authors: Nigang Liu; Zhenpeng Su, Zhonglei Gao, G. D. Reeves, Huinan Zheng, Yuming Wang, Shui Wang
      Abstract: Plasmaspheric hiss is an extremely low frequency whistler-mode emission contributing significantly to the loss of radiation belt electrons. There are two main competing mechanisms for the generation of plasmaspheric hiss: excitation by local instability in the outer plasmasphere and origination from chorus outside the plasmasphere. Here, on the basis of the analysis of an event of shock-induced disappearance and subsequent recovery of plasmaspheric hiss observed by RBSP, THEMIS and POES missions, we attempt to identify its dominant generation mechanism. In the pre-shock plasmasphere, the local electron instability was relatively weak and the hiss waves with bidirectional Poynting fluxes mainly originated from the dayside chorus waves. On arrival of the shock, the removal of pre-existing dayside chorus and the insignificant variation of low-frequency wave instability caused the prompt disappearance of hiss waves. In the next few hours, the local instability in the plasmasphere was greatly enhanced due to the substorm injection of hot electrons. The enhancement of local instability likely played a dominant role in the temporary recovery of hiss with unidirectional Poynting fluxes. These temporarily recovered hiss waves were generated near the equator and then propagated toward higher latitudes. In contrast, both the enhancement of local instability and the recurrence of pre-noon chorus contributed to the substantial recovery of hiss with bidirectional Poynting fluxes.
      PubDate: 2017-10-04T17:06:35.82178-05:0
      DOI: 10.1002/2017JA024470
       
  • Quiet and Disturbed Time Characteristics of Blanketing Es (Esb) during
           Solar Cycle 23
    • Authors: V. Yadav; B. Kakad, A. Bhattacharyya, T. K. Pant
      Abstract: A sporadic E layer with dense ionization blocks the upper ionospheric layers in ionosonde observations and it is called, blanketing sporadic E (Esb). Although earlier studies have demonstrated that Esb occurrence is dependent on solar activity, seasons, local time and equatorial electrojet/counter electrojet (EEJ/CEEJ) strength, the physics behind this dependence is not well established particularly at the dip equatorial stations. Moreover, fewer comprehensive studies are available on Esb. Present work is a detailed statistical study of Esb occurrence and its characteristics during solar cycle 23 (1996-2006) at Indian dip equatorial station Trivandrum (dip latitude 0.5° N). In present study, solar flux dependence of Esb occurrence is clearly evident not only for magnetically quiet, but also for the disturbed periods with maximum during low solar activity. Known seasonal peak Esb occurrence during summer (May-Aug) is found to be dominated by larger percentage of total blanketing events (fbEs ≥ 8 MHz) on magnetically disturbed days as compared to quiet days. We noticed prevalent Esb occurrence in the post-midnight (00-06 IST) periods during low solar activity. Besides Esb characteristics, the present study aims to investigate effect of solar flux on association of Esb and CEEJ. Coexistence of Esb and CEEJ is emphasized in earlier observational studies. However, any dependence of their association on solar flux is not yet examined. We find that their association is weaker during low solar activity as compared to high solar activity in summer, indicating that Esb occurrence is highly likely on CEEJ days during high solar activity periods.
      PubDate: 2017-10-03T17:16:18.7822-05:00
      DOI: 10.1002/2017JA023911
       
  • Quantifying the Precipitation Loss of Radiation Belt Electrons during a
           Rapid Dropout Event
    • Authors: K. H. Pham; W. Tu, Z. Xiang
      Abstract: Relativistic electron flux in the radiation belt can drop by orders of magnitude within the timespan of hours. In this study, we used the drift-diffusion model that includes azimuthal drift and pitch angle diffusion of electrons to simulate low-altitude electron distribution observed by POES/MetOp satellites for rapid radiation belt electron dropout event occurring on May 1, 2013. The event shows fast dropout of MeV energy electrons at L>4 over a few hours, observed by the Van Allen Probes mission. By simulating the electron distributions observed by multiple POES satellites, we resolve the precipitation loss with both high spatial and temporal resolution and a range of energies. We estimate the pitch angle diffusion coefficients as a function of energy, pitch angle, and L-shell, and calculate corresponding electron lifetimes during the event. The simulation results show fast electron precipitation loss at L>4 during the electron dropout, with estimated electron lifetimes on the order of half an hour for MeV energies. The electron loss rate show strong energy dependence with faster loss at higher energies, which suggest that this dropout event is dominated by quick and localized scattering process that prefers higher energy electrons. The improved temporal and spatial resolution of electron precipitation rates provided by multiple low-altitude observations can resolve fast-varying electron loss during rapid electron dropouts (over a few hours), which occur too fast for a single low-altitude satellite. The capability of estimating the fast-varying electron lifetimes during rapid dropout events is an important step in improving radiation belt model accuracy.
      PubDate: 2017-10-03T17:16:12.571081-05:
      DOI: 10.1002/2017JA024519
       
  • Empirical Model of Precipitating Ion Oval
    • Authors: J. Goldstein
      Abstract: In this brief technical report published maps of ion integral flux are used to constrain an empirical model of the precipitating ion oval. The ion oval is modeled as a Gaussian function of ionospheric latitude that depends on local time and the Kp geomagnetic index. The three parameters defining this function are the centroid latitude, width, and amplitude. The local time dependences of these three parameters are approximated by Fourier series expansions whose coefficients are constrained by the published ion maps. The Kp dependence of each coefficient is modeled by a linear fit. Optimization of the number of terms in the expansion is achieved via minimization of the global standard deviation between the model and the published ion map at each Kp. The empirical model is valid near the peak flux of the auroral oval; inside its centroid region the model reproduces the published ion maps with standard deviations of less than 5% of the peak integral flux. On the subglobal scale, average local errors (measured as a fraction of the point-to-point integral flux) are below 30% in the centroid region. Outside its centroid region the model deviates significantly from the H89 integral flux maps. The model's performance is assessed by comparing it with both local and global data from a 17 April 2002 substorm event. The model can reproduce important features of the macroscale auroral region, but none of its subglobal structure, and not immediately following a substorm.
      PubDate: 2017-10-03T17:16:08.976858-05:
      DOI: 10.1002/2017JA024622
       
  • Structure and Properties of the Foreshock at Venus
    • Authors: N. Omidi; G. Collinson, D. Sibeck
      Abstract: The interaction of the solar wind with Venus is dominated by the planet's ionosphere which acts as an obstacle to the flow resulting in an induced magnetosphere and bow shock much smaller than their terrestrial counterparts. This study presents a 3-D electromagnetic hybrid (kinetic ions, fluid electrons) simulation of the solar wind interaction with an unmagnetized obstacle to examine the structure and properties of the Cytherean foreshock during periods of near radial IMF, i.e. when it lies upstream of the ionosphere. The interaction between the backstreaming ions and the solar wind results in the generation of two classes of ULF waves: (1) parallel propagating sinusoidal waves with periods ~20-30 seconds and (2) highly oblique fast magnetosonic waves. The joint nonlinear evolution of these waves result in the formation of structures called foreshock cavitons with dimensions comparable to the size of the planet. Foreshock cavitons are also present in the terrestrial foreshock. The excavation of plasma and magnetic field from their cores leads to lower average densities and magnetic field strengths in the foreshock. As in the case of Earth, this excavation results in the formation of a fast magnetosonic pulse/shock at the edge of the foreshock named the foreshock compressional boundary. Also similar to Earth, is the formation of spontaneous hot flow anomalies (SHFAs) as foreshock cavitons approach the bow shock. The size and properties of SHFAs at Venus are comparable to those at Earth and their existence has recently been established at Mars and Venus in a companion paper.
      PubDate: 2017-10-03T17:15:53.818966-05:
      DOI: 10.1002/2017JA024180
       
  • Magnetospheric ion evolution across the low-latitude boundary layer
           separatrix
    • Authors: S. K. Vines; S. A. Fuselier, K. J. Trattner, J. L. Burch, R. C. Allen, S. M. Petrinec, B. J. Anderson, J. M. Webster, R. E. Ergun, B. L. Giles, P.-A. Lindqvist, C. T. Russell
      Abstract: On 20 September 2015, the Magnetospheric Multiscale (MMS) spacecraft crossed the dusk magnetopause after a compression of the magnetosphere. Enhanced densities and fluxes of both colder (≤10 eV) and hotter (>1 keV) magnetospheric and magnetosheath heavy ion species were observed reaching the magnetopause. The evolution of the velocity distributions for H+, He+, and O+ measured by the Hot Plasma Composition Analyzer (HPCA) on MMS during this magnetopause crossing is presented. In particular, this study focuses on the changes in the species’ distribution functions as MMS passes from the magnetosphere through the electron edge of the low-latitude boundary layer (LLBL) separatrix and then into the LLBL. Two types of processes are suggested to play a role in the heating of colder magnetospheric ions across the LLBL separatrix in the region between the separatrix and the electron and ion edges of the LLBL. One mechanism leads to the formation and enhancement of ring distributions in this layer of the LLBL as the magnetospheric ions propagate across the separatrix. A second mechanism leading first to perpendicular heating and then to parallel heating of colder protons may arise from a possible two-stream instability as the magnetospheric ions first encounter the warmer magnetosheath electrons in the electron layer and then the warmer magnetosheath ions between the electron and ion edges of the LLBL separatrix. Perpendicular heating of He+ and O+ is seen more so in the main reconnection exhaust, due to non-adiabatic behavior of these ions as they are accelerated up to the bulk flow speed.
      PubDate: 2017-10-03T17:15:50.251083-05:
      DOI: 10.1002/2017JA024061
       
  • The Evolution of the Plasma Sheet Ion Composition: Storms and Recoveries
    • Authors: M. H. Denton; M. F. Thomsen, G. D. Reeves, B. A. Larsen, M. G. Henderson, V. K. Jordanova, P. A. Fernandes, R. H. W. Friedel, R. M. Skoug, H. O. Funsten, E. A. MacDonald, H. A. Spence
      Abstract: The ion plasma sheet (~few hundred eV to ~few 10s keV) is usually dominated by H+ ions. Here, changes in ion composition within the plasma sheet are explored both during individual events, and statistically during 54 calm-to-storm events and during 21 active-to-calm events. Ion composition data from the HOPE (Helium, Oxygen, Proton, Electron) instruments onboard Van Allen Probes satellites provide exceptional spatial and temporal resolution of the H+, O+, and He+ ion fluxes in the plasma sheet. H+ shown to be the dominant ion in the plasma sheet in the calm-to-storm transition. However, the energy-flux of each ion changes in a quasi-linear manner during extended calm intervals. Heavy ions (O+ and He+) become increasingly important during such periods as charge-exchange reactions result in faster loss for H+ than for O+ or He+. Results confirm previous investigations showing that the ion composition of the plasma sheet can be largely understood (and predicted) during calm intervals from knowledge of: (a) the composition of previously injected plasma at the onset of calm conditions, and (b) use of simple drift-physics models combined with calculations of charge-exchange losses.
      PubDate: 2017-10-03T17:15:45.259241-05:
      DOI: 10.1002/2017JA024475
       
  • Diffusive transport of several hundred keV electrons in the Earth's slot
           region
    • Authors: Q. Ma; W. Li, R. M. Thorne, J. Bortnik, G. D. Reeves, H. E. Spence, D. L. Turner, J. B. Blake, J. F. Fennell, S. G. Claudepierre, C. A. Kletzing, W. S. Kurth, G. B. Hospodarsky, D. N. Baker
      Abstract: We investigate the gradual diffusion of energetic electrons from the inner edge of the outer radiation belt into the slot region. The Van Allen Probes observed slow inward diffusion and decay of ~200-600 keV electrons following the intense geomagnetic storm that occurred on 17 March 2013. During the 10-day non-disturbed period following the storm, the peak of electron fluxes gradually moved from L~2.7 to L~2.4, and the flux levels decreased by a factor of ~2-4 depending on the electron energy. We simulated the radial intrusion and decay of electrons using a 3-dimentional diffusion code, which reproduced the energy-dependent transport of electrons from ~100 keV to 1 MeV in the slot region. At energies of 100-200 keV, the electrons experience fast transport across the slot region due to the dominance of radial diffusion; at energies of 200-600 keV, the electrons gradually diffuse and decay in the slot region due to the comparable rate of radial diffusion and pitch angle scattering by plasmaspheric hiss; at energies of E> 700 keV, the electrons stopped diffusing near the inner edge of outer radiation belt due to the dominant pitch angle scattering loss. In addition to plasmaspheric hiss, magnetosonic waves and VLF transmitters can cause the loss of high pitch angle electrons, relaxing the sharp ‘top-hat' shaped pitch angle distributions created by plasmaspheric hiss. Our simulation indicates the importance of balance between radial diffusion and loss through pitch angle scattering in forming the diffusive intrusion of energetic electrons across the slot region.
      PubDate: 2017-09-29T18:55:55.297351-05:
      DOI: 10.1002/2017JA024452
       
  • Gravity wave-induced ionospheric irregularities in the postsunset
           equatorial valley region
    • Authors: D. L. Hysell; D. C. Fritts, B. Laughman, J. L. Chau
      Abstract: Plasma irregularities in the postsunset equatorial valley region ionosphere are investigated experimentally and through numerical simulation. Coherent radar backscatter observed at the Jicamarca Radio Observatory shows two classes of irregularities in different altitude bands – one mainly below about 125 km and the other mainly above. Irregularities in both bands are organized into wavefronts with wavelengths of a few km. However, only the irregularities in the high-altitude band exhibit consistent propagation speeds and directions. Some previous observations of irregularities in the nighttime electrojet suggest that gravity waves may sometimes influence their morphology. The possibility that the valley-region irregularities are also related to gravity waves (GWs) is therefore investigated numerically. A model of a GW packet propagating through a tidal wind field is used to drive another model which predicts the resulting ionospheric electrodynamics. The combined simulation shows that GWs can induce field-aligned currrents and excite resistive drift waves which could be responsible for the valley-region irregularities in the high-altitude band. The GWs also induce irregularities in the upper E region directly through simple dynamo action which subsequently deform under the influence of shear flow. This may explain the irregularities in the low-altitude band.
      PubDate: 2017-09-29T18:55:53.270649-05:
      DOI: 10.1002/2017JA024514
       
  • Pulsations in the Earth's Lower Ionosphere Synchronized with Solar Flare
           Emission
    • Authors: Laura A. Hayes; Peter T. Gallagher, Joseph McCauley, Brian R. Dennis, Jack Ireland, Andrew Inglis
      Abstract: Solar flare emission at X-ray and extreme ultraviolet (EUV) energies can cause substantial enhancements in the electron density in the Earth's lower ionosphere. It has now become clear that flares exhibit quasi-periodic pulsations with timescales of minutes at X-ray energies, but to date, it has not been known if the ionosphere is sensitive to this variability. Here, using a combination of Very Low Frequency (24 kHz) measurement together with space-based X-ray and EUV observations, we report pulsations of the ionospheric D-region, which are synchronized with a set of pulsating flare loops. Modeling of the ionosphere show that the D-region electron density varies by up to an order of magnitude over the timescale of the pulsations (∼ 20 mins). Our results reveal that the Earth's ionosphere is more sensitive to small-scale changes in solar soft X-ray flux than previously thought, and implies that planetary ionospheres are closely coupled to small-scale changes in solar/stellar activity.
      PubDate: 2017-09-29T18:55:27.154372-05:
      DOI: 10.1002/2017JA024647
       
  • Thermosphere global time response to geomagnetic storms caused by coronal
           mass ejections
    • Authors: D. M. Oliveira; E. Zesta, P. W. Schuck, E. K. Sutton
      Abstract: We investigate, for the first time with a spatial superposed epoch analysis study, the thermosphere global time response to 159 geomagnetic storms caused by CMEs observed in the solar wind at Earth's orbit during the period of September 2001 to September 2011. The thermosphere neutral mass density is obtained from the CHAMP and GRACE spacecraft. All density measurements are inter-calibrated against densities computed by the Jacchia-Bowman 2008 empirical model under the regime of very low geomagnetic activity. We explore both the effects of the pre-CME shock impact on the thermosphere and of the storm main phase onset by taking their times of occurrence as zero epoch times (CME impact and IMF Bz southward turning) for each storm. We find that the shock impact produces quick and transient responses at the two high latitude regions with minimal propagation toward lower latitudes. In both cases, thermosphere is heated in very high latitude regions within several minutes. The Bz southward turning of the storm onset has a fast heating manifestation at the two high latitude regions and it takes approximately 3 hours for that heating to propagate down to equatorial latitudes and to globalize in the thermosphere. This heating propagation is presumably accomplished, at least in part, with traveling atmospheric disturbances (TADs) and complex meridional wind structures. Current models use longer lag times in computing thermosphere density dynamics during storms. Our results suggest that the thermosphere response time scales are shorter, and should be accordingly adjusted in thermospheric empirical models.
      PubDate: 2017-09-28T17:42:10.363657-05:
      DOI: 10.1002/2017JA024006
       
  • Ion-Scale Wave Properties and Enhanced Ion Heating across the Low-Latitude
           Boundary Layer during Kelvin-Helmholtz Instability
    • Authors: T. W. Moore; K. Nykyri, A. P. Dimmock
      Abstract: In the Earth's magnetosphere, the magnetotail plasma sheet ions are much hotter than in the shocked solar wind. On the dawn-sector, the cold-component ions are more abundant and hotter by 30-40 percent when compared to the dusk sector. Recent statistical studies of the flank magnetopause and magnetosheath have shown that the level of temperature asymmetry of the magnetosheath is unable to account for this, so additional physical mechanisms must be at play, either at the magnetopause or plasma sheet that contribute to this asymmetry. In this study, we perform a statistical analysis on the ion-scale wave properties in the three main plasma regimes common to flank magnetopause boundary crossings when the boundary is unstable to KHI: hot and tenuous magnetospheric, cold and dense magnetosheath and mixed [Hasegawa et al., 2004]. These statistics of ion-scale wave properties are compared to observations of fast magnetosonic wave modes that have recently been linked to Kelvin-Helmholtz (KH) vortex centered ion heating [Moore et al., 2016]. The statistical analysis shows that during KH events there is enhanced non-adiabatic heating calculated during ion scale wave intervals when compared to non-KH events. This suggests that during KH events there is more free energy for ion-scale wave generation, which in turn can heat ions more effectively when compared to cases when KH waves are absent. This may contribute to the dawn favored temperature asymmetry of the plasma sheet, recent studies suggest KH waves favor the dawn flank during Parker-Spiral (PS) interplanetary magnetic field (IMF).
      PubDate: 2017-09-26T18:15:48.64994-05:0
      DOI: 10.1002/2017JA024591
       
  • High Latitude Neutral Mass Density Maxima
    • Authors: C. Y. Huang; Y. Huang, Y.-J. Su, T. Huang, E. K. Sutton
      Abstract: Recent studies have reported that thermospheric effects due to solar wind driving can be observed poleward of auroral latitudes. In these papers, the measured neutral mass density perturbations appear as narrow, localized maxima in the cusp and polar cap. They conclude that Joule heating below the spacecraft is the cause of the mass density increases which are sometimes associated with local field-aligned current structures, but not always.In this paper we investigate neutral mass densities measured by accelerometers on the CHAllenging Minisatellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) spacecraft from launch until years 2010 (CHAMP) and 2012 (GRACE), approximately 10 years of observations from each satellite. We extract local maxima in neutral mass densities over the background using a smoothing window with size of one quarter of the orbit. The maxima have been analyzed for each year, and also for the duration of each set of satellite observations. We show where they occur, under what solar wind conditions, and their relation to magnetic activity. The region with the highest frequency of occurrence coincides approximately with the cusp and mantle, with little direct evidence of an auroral zone source. Our conclusions agree with the “hot polar cap” observations which have been reported and studied in the past.
      PubDate: 2017-09-26T18:15:45.17038-05:0
      DOI: 10.1002/2017JA024334
       
  • Modeling Particle Acceleration and Transport at a 2D CME-Driven Shock
    • Authors: Junxiang Hu; Gang Li, Xianzhi Ao, Gary P. Zank, Olga Verkhoglyadova
      Abstract: We extend our earlier Particle Acceleration and Transport in the Heliosphere (PATH) model to study particle acceleration and transport at a CME-driven shock. We model the propagation of a CME-driven shock in the ecliptic plane using the ZEUS-3D code from 20 solar radii to 2 AU. As in the previous PATH model, the initiation of the CME-driven shock is simplified and modelled as a disturbance at the inner boundary. Different from the earlier PATH model, the disturbance is now longitudinally dependent. Particles are accelerated at the 2D shock via the diffusive shock acceleration (DSA) mechanism. The acceleration depends on both the parallel and perpendicular diffusion coefficients κ and κ⊥ and is therefore shock-obliquity dependent. Following the procedure used in Li et al. [2012a], we obtain the particle injection energy, the maximum energy and the accelerated particle spectra at the shock front. Once accelerated, particles diffuse and convect in the shock complex. The diffusion and convection of these particles are treated using a refined 2-D shell model in an approach similar to Zank et al. [2000], When particles escape from the shock, they propagate along and across the interplanetary magnetic field (IMF). The propagation is modelled using a focused transport equation with the addition of perpendicular diffusion. We solve the transport equation using a backward stochastic differential equation method where adiabatic cooling, focusing, pitch angle scattering, and cross field diffusion effects are all included. Time intensity profiles and instantaneous particle spectra as well as particle pitch angle distributions are shown for two example CME shocks.
      PubDate: 2017-09-26T18:15:42.018997-05:
      DOI: 10.1002/2017JA024077
       
  • Multipoint observations of energetic particle injections and substorm
           activity during a conjunction between Magnetospheric Multiscale (MMS) and
           Van Allen Probes
    • Authors: D. L. Turner; J. F. Fennell, J. B. Blake, S. G. Claudepierre, J. H. Clemmons, A. N. Jaynes, T. Leonard, D. N. Baker, I. J. Cohen, M. Gkioulidou, A. Y. Ukhorskiy, B. H. Mauk, C. Gabrielse, V. Angelopoulos, R. J. Strangeway, C. A. Kletzing, O. Le Contel, H. E. Spence, R. B. Torbert, J. L. Burch, G. D. Reeves
      Abstract: This study examines multipoint observations during a conjunction between MMS and Van Allen Probes on 07 April 2016 in which a series of energetic particle injections occurred. With complementary data from THEMIS, Geotail, and LANL-GEO (16 spacecraft in total), we develop new insights on the nature of energetic particle injections associated with substorm activity. Despite this case involving only weak substorm activity (max. AE < 300 nT) during quiet geomagnetic conditions in steady, below-average solar wind, a complex series of at least six different electron injections was observed throughout the system. Intriguingly, only one corresponding ion injection was clearly observed. All ion and electron injections were observed at < 600 keV only. MMS reveals detailed substructure within the largest electron injection. A relationship between injected electrons with energy < 60 keV and enhanced whistler-mode chorus wave activity is also established from Van Allen Probes and MMS. Drift mapping using a simplified magnetic field model provides estimates of the dispersionless injection boundary locations as a function of universal time, magnetic local time, and L-shell. The analysis reveals that at least five electron injections, which were localized in magnetic local time, preceded a larger injection of both electrons and ions across nearly the entire nightside of the magnetosphere near geosynchronous orbit. The larger, ion and electron injection did not penetrate to L < 6.6, but several of the smaller, electron injections penetrated to L < 6.6. Due to the discrepancy between the number, penetration depth, and complexity of electron vs. ion injections, this event presents challenges to the current conceptual models of energetic particle injections.
      PubDate: 2017-09-25T21:35:28.940644-05:
      DOI: 10.1002/2017JA024554
       
  • Mercury's solar wind interaction as characterized by magnetospheric plasma
           mantle observations with MESSENGER
    • Authors: Jamie M. Jasinski; James A. Slavin, Jim M. Raines, Gina A. DiBraccio
      Abstract: We analyze 94 traversals of Mercury's southern magnetospheric plasma mantle using data from the MESSENGER spacecraft. The mean and median proton number density in the mantle are 1.5 and 1.3 cm-3, respectively. For sodium number density these values are 0.004 and 0.002 cm-3. Moderately higher densities are observed on the magnetospheric dusk side. The mantle supplies up to 1.5x108 cm-2 s-1 and 0.8 x 108cm-2 s-1 of proton and sodium flux to the plasma sheet, respectively. We estimate the cross-electric magnetospheric potential from each observation and find a mean of ~19 kV (standard deviation of 16 kV) and a median of ~13 kV. This is an important result as it is lower than previous estimations and shows that Mercury's magnetosphere is at times not as highly driven by the solar wind as previously thought. Our values are comparable to the estimations for the ice giant planets, Uranus and Neptune, but lower than Earth. The estimated potentials do have a very large range of values (1 – 74 kV), showing that Mercury's magnetosphere is highly dynamic. A correlation of the potential is found to the interplanetary magnetic field (IMF) magnitude, supporting evidence that dayside magnetic reconnection can occur at all shear angles at Mercury. But we also see that Mercury has an Earth-like magnetospheric response, favoring –BZ IMF orientation. We find evidence that –BX orientations in the IMF favor the southern cusp and southern mantle. This is in agreement with telescopic observations of exospheric emission, but in disagreement with modeling.
      PubDate: 2017-09-21T20:08:16.945064-05:
      DOI: 10.1002/2017JA024594
       
  • Time-Integral Correlations of Multiple Variables with the
           Relativistic-Electron Flux at Geosynchronous Orbit: The Strong Roles of
           Substorm-Injected Electrons and the Ion Plasma Sheet
    • Authors: Joseph E. Borovsky
      Abstract: Time-integral correlations are examined between the geosynchronous relativistic-electron flux index Fe1.2 and 31 variables of the solar wind and magnetosphere. An “evolutionary algorithm” is used to maximize correlations. Time integrations (into the past) of the variables are found to be superior to time-lagged variables for maximizing correlations with the radiation belt. Physical arguments are given as to why. Dominant correlations are found for the substorm-injected electron flux at geosynchronous orbit and for the pressure of the ion plasma sheet. Different sets of variables are constructed and correlated with Fe1.2: some sets maximize the correlations, some sets are based on purely solar-wind variables. Examining known physical mechanisms that act on the radiation belt, sets of correlations are constructed (1) using magnetospheric variables that control those physical mechanisms and (2) using the solar-wind variables that control those magnetospheric variables. Fe1.2-increasing intervals are correlated separately from Fe1.2-decreasing intervals and the introduction of autoregression into the time-integral correlations is explored. A great impediment to discerning physical cause and effect from the correlations is the fact that all solar-wind variables are intercorrelated and carry much of the same information about the time sequence of the solar wind that drives the time sequence of the magnetosphere.
      PubDate: 2017-09-21T20:05:30.517528-05:
      DOI: 10.1002/2017JA024476
       
  • Cold ionospheric ions in the magnetic reconnection outflow region
    • Authors: W. Y. Li; M. André, Yu. V. Khotyaintsev, A. Vaivads, S. A. Fuselier, D. B. Graham, S. Toledo-Redondo, B. Lavraud, D. L. Turner, C. Norgren, B. B. Tang, C. Wang, P.-A. Lindqvist, D. T. Young, M. Chandler, B. Giles, C. Pollock, R. Ergun, C. T. Russell, R. Torbert, T. Moore, J. Burch
      Abstract: Magnetosheath plasma usually determines properties of asymmetric magnetic reconnection at the subsolar region of Earth's magnetopause. However, cold plasma that originated from the ionosphere can also reach the magnetopause, and modify the kinetic physics of asymmetric reconnection. We present a magnetopause crossing with high-density (10-60 cm−3) cold ions and ongoing reconnection from the observation of the Magnetospheric Multiscale (MMS) spacecraft. The magnetopause crossing is estimated to be 300 ion-inertial lengths south of the X line. Two distinct ion populations are observed on the magnetosheath edge of the ion jet. One population with large parallel velocities (200-300 km/s) is identified to be cold ion beams, and the other population is the magnetosheath ions. In the deHoffman-Teller frame, the field-aligned magnetosheath ions are Alfvénic and move towards the jet region, while the field-aligned cold ion beams move towards the magnetosheath boundary layer, with much small speeds. These cold ion beams are suggested to be from the cold ions entering the jet close to the X line. This is the first observation of the cold ionospheric ions in the reconnection outflow region, including the reconnection jet and the magnetosheath boundary layer.
      PubDate: 2017-09-20T17:16:04.617643-05:
      DOI: 10.1002/2017JA024287
       
  • Hybrid simulations of positively and negatively charged pickup ions and
           cyclotron wave generation at Europa
    • Authors: R. T. Desai; M. M. Cowee, H. Wei, X. Fu, S. P. Gary, M. Volwerk, A. J. Coates
      Abstract: In the vicinity of Europa, Galileo observed bursty Alfvén-cyclotron wave power at the gyrofrequencies of a number of species including K+, 2+, Na+, and Cl+, indicating the localised pickup of these species. Additional evidence for the presence of Chlorine was the occurrence of both left-hand (LH) and right-hand (RH) polarised transverse wave power near the Cl+ gyrofrequency, thought to be due to the pickup of both Cl+ and the easily formed Chlorine anion, Cl−. To test this hypothesis we use one-dimensional hybrid (kinetic ion, massless fluid electron) simulations for both positive and negative pickup ions and self-consistently reproduce the growth of both LH and RH Alfvén-cyclotron waves in agreement with linear theory. We show how the simultaneous generation of LH and RH waves can result in non-gyrotropic ion distributions and increased wave amplitudes, and how even trace quantities of negative pickup ions are able to generate an observable RH signal. Through comparing simulated and observed wave amplitudes, we are able to place the first constraints on the densities of Chlorine pickup ions in localised regions at Europa.
      PubDate: 2017-09-19T17:51:23.198718-05:
      DOI: 10.1002/2017JA024479
       
  • Constraining Balmer-alpha Fine-Structure Excitation Measured in Geocoronal
           Hydrogen Observations
    • Authors: D. D. Gardner; E. J. Mierkiewicz, F. L. Roesler, S. M. Nossal, L. M. Haffner
      Abstract: Cascade contributions to geocoronal Balmer α airglow line profiles are directly proportional to the Balmer β/α line ratio and can therefore be determined with near simultaneous Balmer β observations. Due to scattering differences for solar Lyman β and Lyman γ (responsible for the terrestrial Balmer α and Balmer β fluorescence, respectively) there is an expected trend for the cascade emission to become a smaller fraction of the Balmer α intensity at larger shadow altitudes. Near coincident Balmer α and Balmer β data sets, obtained from the Wisconsin Hα Mapper (WHAM) Fabry–Perot, are used to determine the cascade contribution to the Balmer α line profile, and to show, for the first time, the Balmer β/α line ratio, as a function of shadow altitude. We show that this result is in agreement with direct cascade determinations from Balmer α line profile fits obtained independently by high resolution Fabry–Perot at Pine Bluff, WI. We also demonstrate with radiative transport forward modeling that a solar cycle influence on cascade is expected, and that the Balmer β/α line ratio poses a tight constraint on retrieved aeronomical parameters (such as hydrogen's evaporative escape rate and exobase density). Index Terms: Hydrogen Geocorona, Balmer-alpha Line Profile, Fabry–Perot Interferometry
      PubDate: 2017-09-19T17:51:13.207338-05:
      DOI: 10.1002/2017JA024055
       
  • Ring Current He-Ion Control by Bounce Resonant ULF Waves
    • Authors: Hyomin Kim; Andrew J. Gerrard, Louis J. Lanzerotti, Rualdo Soto-Chavez, Ross J. Cohen, Jerry W. Manweiler
      Abstract: Ring current energy He-ion (∼65 keV to ∼520 keV) differential flux data from the Radiation Belt Storm Probe Ion Composition Experiment (RBSPICE) instrument aboard the Van Allan Probes spacecraft show considerable variability during quiet solar wind and geomagnetic time periods. Such variability is apparent from orbit to orbit (∼9 hours) of the spacecraft and is observed to be ∼50–100% of the nominal flux. Using data from the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) instrument, also aboard the Van Allen Probes spacecraft, we identify that a dominant source of this variability is from ULF waveforms with periods of 10's of sec. These periods correspond to the bounce resonant timescales of the ring current He-ions being measured by RBSPICE. A statistical survey using the particle and field data for one full spacecraft precession period (approximately two years) shows that the wave and He-ion flux variations are generally anti-correlated, suggesting the bounce resonant pitch-angle scattering process as a major component in the scattering of He-ions.
      PubDate: 2017-09-19T17:50:44.882328-05:
      DOI: 10.1002/2017JA023958
       
  • Global three-dimensional simulation of Earth's dayside reconnection using
           a two-way coupled magnetohydrodynamics with embedded particle-in-cell
           model: initial results
    • Authors: Yuxi Chen; Gábor Tóth, Paul Cassak, Xianzhe Jia, Tamas I. Gombosi, James A. Slavin, Stefano Markidis, Ivy Bo Peng, Vania K. Jordanova, Michael G. Henderson
      Abstract: We perform a three-dimensional (3D) global simulation of Earth's magnetosphere with kinetic reconnection physics to study the flux transfer events (FTEs) and dayside magnetic reconnection with the recently developed magnetohydrodynamics with embedded particle-in-cell model (MHD-EPIC). During the one-hour long simulation, the FTEs are generated quasi-periodically near the subsolar point and move toward the poles. We find the magnetic field signature of FTEs at their early formation stage is similar to a ‘crater FTE’, which is characterized by a magnetic field strength dip at the FTE center. After the FTE core field grows to a significant value, it becomes an FTE with typical flux rope structure. When an FTE moves across the cusp, reconnection between the FTE field lines and the cusp field lines can dissipate the FTE. The kinetic features are also captured by our model. A crescent electron phase space distribution is found near the reconnection site. A similar distribution is found for ions at the location where the Larmor electric field appears. The lower hybrid drift instability (LHDI) along the current sheet direction also arises at the interface of magnetosheath and magnetosphere plasma. The LHDI electric field is about 8 mV/m and its dominant wavelength relative to the electron gyroradius agrees reasonably with MMS observations.
      PubDate: 2017-09-18T18:56:26.034516-05:
      DOI: 10.1002/2017JA024186
       
  • Scaling the ion inertial length and its implications for modeling
           reconnection in global simulations
    • Authors: Gábor Tóth; Yuxi Chen, Tamas I. Gombosi, Paul Cassak, Stefano Markidis, Ivy Bo Peng
      Abstract: We investigate the use of artificially increased ion and electron kinetic scales in global plasma simulations. We argue that as long as the global and ion inertial scales remain well separated, 1) the overall global solution is not strongly sensitive to the value of the ion inertial scale, while 2) the ion inertial scale dynamics will also be similar to the original system, but it occurs at a larger spatial scale, and 3) structures at intermediate scales, such as magnetic islands, grow in a self-similar manner. To investigate the validity and limitations of our scaling hypotheses, we carry out many simulations of a two-dimensional magnetosphere with the magnetohydrodynamics with embedded particle-in-cell (MHD-EPIC) model. The PIC model covers the dayside reconnection site. The simulation results confirm that the hypotheses are true as long as the increased ion inertial length remains less than about 5% of the magnetopause standoff distance. Since the theoretical arguments are general, we expect these results to carry over to three dimensions. The computational cost is reduced by the third and fourth powers of the scaling factor in two- and three-dimensional simulations, respectively, which can be many orders of magnitude. The present results suggest that global simulations that resolve kinetic scales for reconnection are feasible. This is a crucial step for applications to the magnetospheres of Earth, Saturn and Jupiter and to the solar corona.
      PubDate: 2017-09-18T18:55:46.586477-05:
      DOI: 10.1002/2017JA024189
       
  • Imprints of quasi-adiabatic ion dynamics on the current sheet structures
           observed in the Martian magnetotail by MAVEN
    • Authors: E. E. Grigorenko; S. D. Shuvalov, H. V. Malova, E. Dubinin, V. Yu. Popov, L. M. Zelenyi, J. Espley, Mc Fadden J.P.
      Abstract: Numerous studies of the Current Sheets (CS) in the Earth magnetotail showed that quasi-adiabatic ion dynamics plays an important role in formation of complicated multilayered current structures. In order to check whether the similar mechanisms operate in the Martian magnetotail we analyzed 80 CS crossings using MAVEN measurements on the nightside of Mars at radial distances ~1.0–2.8RM. We found that CS structures experience the similar dependence on the value of the normal component of the magnetic field at the neutral plane (BN) and on the ratio of the ion drift velocity outside the CS to the thermal velocity (VT/VD) as it was observed for the CSs in the Earth’s magnetotail. For the small values of BN a thin and intense CS embedded in a thicker one is observed. The half-thickness of this layer is L~30–100km ≤ ρH+ (ρH+ is a gyroradius of thermal protons outside the CS). With the increase of BN the L also increases up to several hundred km (~ρO+, ρO2+), the current density decreases and the embedding feature disappears. Our statistical analysis showed a good agreement between L values observed by MAVEN and the CS scaling obtained from the quasi-adiabatic model, if the plasma characteristics in Martian CSs are used as input parameters. Thus, we may conclude that, in spite of the differences in magnetic topology, ion composition and plasma thermal characteristics observed in the Earth’s and Martian magnetotails, similar quasi-adiabatic mechanisms contribute to the formation of the CSs in the magnetotails of both planets.
      PubDate: 2017-09-18T18:55:24.317393-05:
      DOI: 10.1002/2017JA024216
       
  • Interaction of magnetic flux ropes via magnetic reconnection observed at
           the magnetopause
    • Authors: Rongsheng Wang; Quanming Lu, Rumi Nakamura, Wolfgang Baumjohann, C. T. Russell, J. L. Burch, R. E. Ergun, P. A. Lindqvist, Shui Wang, Barbara Giles, Dan Gershman
      Abstract: Using the high resolution field and plasma data obtained from the Magnetospheric Multiscale mission (MMS) at the magnetopause, a series of three flux transfer events was observed one after another inside southward ion flows, without time gap between any two successive flux ropes. Using the plasma measurements, the current densities within the flux ropes were studied in detail. The currents within the first two flux ropes, dubbed Fr1 and Fr2, was composed of a series of well-separated filamentary currents. The thickness of the filamentary currents and the gap between them were sub-ion scale, occasionally dropped down to electron-scale. In the third flux rope Fr3 which was closest to the expected reconnection X-line, the current displayed a singular compact current layer, was ion scale in width and concentrated on its center. Considering the location of the flux ropes relative to the reconnection X-line, we suggested the current density could be a singular structure when the flux rope was just created and then fragmented into a series of filamentary currents as time. By examining the inter-regions between Fr1 and Fr2, and between Fr2 and Fr3, reconnection was only confirmed to occur between Fr2 and Fr3 and no reconnection signature was found between Fr1 and Fr2. It seems that magnetic field compression resulted from collision of two neighboring flux ropes is one necessary condition for the occurrence of the coalescence.
      PubDate: 2017-09-14T19:00:53.407279-05:
      DOI: 10.1002/2017JA024482
       
  • High-latitude Thermosphere Neutral Density Response to Solar Wind Dynamic
           Pressure Enhancement
    • Authors: Y. Shi; E. Zesta, H. K. Connor, Y.-J. Su, E. K. Sutton, C. Y. Huang, D. M. Ober, C. Christodoulou, S. Delay, D. M. Oliveira
      Abstract: We examine the response of the thermosphere to the impact of solar wind dynamic pressure enhancements using observations and global magneto-hydrodynamics (MHD) simulations by the OpenGGCM model. Combining neutral density observations from the Challenging Mini-satellite Payload (CHAMP) and the Gravity Recovery and Climate Experiment (GRACE) satellites with simultaneous Poynting flux measurements from the Defense Meteorological Satellite Program (DMSP) F16 we find that thermospheric density as well as downward Poynting flux intensified shortly after a sudden enhancement of the solar wind dynamic pressure. The intensification manifested mostly on the dayside high-latitude region with peak intensity in the vicinity of the noon and pre-noon cusp. OpenGGCM modeling results show that the ionospheric Joule heating increased abruptly in response to the sudden enhancement of the dynamic pressure in the same region as the observed Poynting flux and neutral density enhancements. The modeling results show that the enhanced Joule heating coincides, both in time and location, with the appearance of a pair of high-latitude localized field-aligned currents (FACs) in the cusp region. The FACs intensified and extended azimuthally. Coincidental with the solar wind dynamic pressure enhancement the y-component of the interplanetary magnetic field (IMF) By became strongly positive and, in addition, had some large fluctuations. We explore the separate and combined effects of the dynamic pressure and IMF By perturbations, with specifically designed simulation experiments that isolate the effect of each solar wind parameter. We find that the dynamic pressure enhancement is the primary source for the Joule heating and neutral density enhancements, but the IMF By modulates the level of enhancement.
      PubDate: 2017-09-14T19:00:48.046019-05:
      DOI: 10.1002/2017JA023889
       
  • The first use of coordinated ionospheric radio and optical observations
           over Italy: Convergence of high and low latitude storm-induced
           effects'
    • Authors: C. Cesaroni; L. Alfonsi, M. Pezzopane, C. Martinis, J. Baumgardner, J. Wroten, M. Mendillo, E. Musicò, M. Lazzarin, G. Umbriaco
      Abstract: Ionospheric storm effects at mid latitudes were analyzed using different ground-based instruments distributed in Italy during the 13-15 November 2012 geomagnetic storm. These included an all-sky imager (ASI) in Asiago (45.8°N, 11.5°E), a network of dual-frequencies GNSS receivers (RING network), and ionosondes in Rome (41.8°N, 12.5°E) and San Vito (40.6°N, 17.8°E). GPS measurements showed an unusual enhancement of Total Electron Content (TEC) in southern Italy, during the nights of 14 and 15 November. The ASI observed co-located enhancements of 630 nm airglow at the same time, as did variations in NmF2 measured by the ionosondes. Moreover, wave-like perturbations were identified propagating from the north. The Ensemble Empirical Mode Decomposition, applied to TEC values revealed the presence of travelling ionospheric disturbances (TIDs) propagating southward between 01:30 UT and 03:00 UT on 15 November. These TIDs were characterized by weak TEC oscillations (~ ±0.5 TEC unit), period of 45 minutes and velocity of 500 m/s typical of Large Scale TIDs. Optical images showed enhanced airglow entering the field of view of the ASI from the N-NE at 02:00 UT and propagating to the S-SW, reaching the region covered by the GPS stations after 03:00 UT, when TEC fluctuations are very small (~ ±0.2 TEC unit). The enhancement of TEC and airglow observed in Southern Italy could be a consequence of a poleward expansion of the northern crest of the equatorial ionization anomaly. The enhanced airglow propagating from the north and the TEC waves resulted from energy injected at auroral latitudes as confirmed by magnetometer observations in Scandinavia.
      PubDate: 2017-09-11T19:02:52.265021-05:
      DOI: 10.1002/2017JA024325
       
  • The Warm Plasma Composition in the Inner Magnetosphere during
           2012–2015
    • Authors: J.-M. Jahn; J. Goldstein, G. D. Reeves, P. A. Fernandes, R. M. Skoug, B. A. Larsen, H. E. Spence
      Abstract: Ionospheric heavy ions play an important role in the dynamics of Earth's magnetosphere. The greater mass and gyro radius of ionospheric oxygen differentiates its behavior from protons at the same energies. Oxygen may have an impact on tail reconnection processes, and it can at least temporarily dominate the energy content of the ring current during geomagnetic storms. At sub-keV energies, multi-species ion populations in the inner magnetosphere form the warm plasma cloak, occupying the energy range between the plasmasphere and the ring current. Lastly, cold lighter ions from the mid-latitude ionosphere create the co-rotating plasmasphere whose outer regions can interact with the plasma cloak, plasma sheet, ring current, and outer electron belt. In this paper we present a statistical view of warm, cloak-like ion populations in the inner magnetosphere, contrasting in particular the warm plasma composition during quiet and active times. We study the relative abundances and absolute densities of warm plasma measured by the Van Allen Probes, whose two spacecraft cover the inner magnetosphere from plasmaspheric altitudes close to Earth to just inside geostationary orbit. We observe that warm (>30 eV) oxygen is most abundant closer to the plasmasphere boundary whereas warm hydrogen dominates closer to geostationary orbit. Warm helium is usually a minor constituent, but shows a noticeable enhancement in the near-Earth dusk sector.
      PubDate: 2017-09-11T19:02:49.288696-05:
      DOI: 10.1002/2017JA024183
       
  • MESSENGER Observations of Magnetotail Loading and Unloading: Implications
           for Substorms at Mercury
    • Authors: S. M. Imber; J. A. Slavin
      Abstract: We present the first statistical study of loading and unloading of magnetic flux in Mercury's magnetotail. These events describe the global circulation of magnetic flux through the magnetosphere, and provide strong evidence that terrestrial-type substorms take place at Mercury. 438 events were identified over the four years of the MESSENGER mission by a gradual, short-lived increase in the magnetotail lobe magnetic field strength, coincident with an outward flaring of the magnetotail. Substorm duration ranged from tens of seconds to several minutes, with a median of 195 seconds and a mean of 212 seconds. The median amplitude of lobe magnetic field increase was ~11.5 nT, which represents an increase of 23.4% on the background lobe field strength, compared with ~10% for terrestrial substorms. The magnetotail lobes were found to contain ~2-3 MWb of magnetic flux based on 1031 tail passes, with a mean of 2.52 MWb and a standard deviation of 0.48 MWb. An estimate of the change in open flux content during the loading phase of each substorm ranged from 0.08 to 3.7 MWb with a mean value of 0.69 MWb and a standard deviation of 0.38 MWb. These changes in open flux content are an underestimate as the change in magnetotail radius during the events was not accounted for. The maximum lobe flux content during each substorm (~3 MWb) represented ~40% of the total available magnetic flux in the system (~7.5 MWb). During terrestrial substorms, the maximum lobe magnetic flux content is ~10-12% of the total flux from the dipole. A typical substorm at Mercury therefore cycles through a significantly larger fraction of the available magnetic flux than all but the largest substorms at the Earth.
      PubDate: 2017-09-07T20:11:07.558988-05:
      DOI: 10.1002/2017JA024332
       
  • Global structure and sodium ion dynamics in Mercury's magnetosphere with
           the offset dipole
    • Authors: M. Yagi; K. Seki, Y. Matsumoto, D. C. Delcourt, F. Leblanc
      Abstract: We conducted global magnetohydrodynamics (MHD) simulation of Mercury's magnetosphere with the dipole offset, which was revealed by MESSENGER observations, in order to investigate its global structure under northward interplanetary magnetic field (IMF) conditions. Sodium ion dynamics originating from the Mercury's exosphere is also investigated based on statistical trajectory tracing in the electric and magnetic fields obtained from the MHD simulations. The results reveal a north-south asymmetry characterized by open field lines around southern polar region, and northward deflection of the plasma sheet in the far tail. The asymmetry of magnetic field structure near the planet drastically affects trajectories of sodium ion, and thus, their pressure distributions and precipitation pattern onto the planet. Weaker magnetic field strength in the southern hemisphere than in the north increases ion loss by precipitation onto the planetary surface in the southern hemisphere. The ‘sodium ring', which is formed by high-energy sodium ions drifting around the planet, is also found in the vicinity of the planet. The 'sodium ring' is almost circular under nominal solar wind conditions. The ring becomes partial under high solar wind density, because dayside magnetosphere is so compressed that there is no space for the sodium ions to drift around. In both cases, the 'sodium ring' is formed by sodium ions that are picked up and accelerated in the magnetosheath just outside the magnetopause and reentered into the magnetosphere due to combined effects of finite Larmor radius and convection electric field in the dawn-side magnetosphere.
      PubDate: 2017-09-05T17:36:38.457527-05:
      DOI: 10.1002/2017JA024082
       
  • The delayed time response of geomagnetic activity to the solar wind
    • Authors: R. Maggiolo; M. Hamrin, J. De Keyser, T. Pitkänen, G. Cessateur, H. Gunell, L. Maes
      Abstract: We investigate the lagged correlation between a selection of geomagnetic indices and solar wind parameters for a complete solar cycle, from 2000 to 2011. We first discuss the mathematical assumptions required for such a correlation analysis. The solar wind parameters and geomagnetic indices have inherent time scales that smooth the variations of the correlation coefficients with time lag. Furthermore, the solar wind structure associated with co-rotating interaction regions and coronal mass ejections, and the compression regions compression regions ahead of them, strongly impacts the lagged correlation analysis results. This work shows that such bias must be taken into account in a correct interpretation of correlations. We then evidence that the magnetospheric response time to solar wind parameters involves multiple time scales. The simultaneous and quick response of the PC and AE index to solar wind dynamic pressure with a delay of ~5 minutes suggests that magnetospheric compression by solar wind can trigger substorm activity. We find that the PC and AE indices respond to IMF BZ with a response time of respectively ~20 and ~35 minutes. The response of the SYM-H index takes longer (~80 minutes) and is less sharp, SYM-H being statistically significantly correlated to the IMF BZ observed up to more than ~10 hours before. Our results suggest that the solar wind velocity's dominant impact on geomagnetic activity is caused by the compression regions at the interface of fast/slow solar wind regimes, which are very geoeffective as they are associated with high solar wind pressure and strong interplanetary magnetic field.
      PubDate: 2017-09-05T17:36:24.995621-05:
      DOI: 10.1002/2016JA023793
       
  • Discovery of suprathermal ionospheric origin Fe+ in and near Earth's
           magnetosphere
    • Authors: S. P. Christon; D. C. Hamilton, J. M. C. Plane, D. G. Mitchell, J. M. Grebowsky, W. N. Spjeldvik, S. R. Nylund
      Abstract: Suprathermal (87-212 keV/e) singly charged iron, Fe+, has been discovered in and near Earth's ~9-30 Re equatorial magnetosphere using ~21 years of Geotail STICS (suprathermal ion composition spectrometer) data. Its detection is enhanced during higher geomagnetic and solar activity levels. Fe+, rare compared to dominant suprathermal solar wind and ionospheric origin heavy ions, might derive from one or all three candidate lower-energy sources: (A) ionospheric outflow of Fe+ escaped from ion layers near ~100 km altitude, or (B) charge exchange of nominal solar wind iron, Fe+≥7, in Earth's exosphere, or (C) inner source pickup Fe+ carried by the solar wind, likely formed by solar wind Fe interaction with near sun interplanetary dust particles. Earth's semi-permanent ionospheric Fe+ layers derive from tons of interplanetary dust particles entering Earth's atmosphere daily, and Fe+ scattered from these layers is observed up to ~1000 km altitude, likely escaping in strong ionospheric outflows. Using ~26% of STICS's magnetosphere-dominated data when possible Fe+2 ions are not masked by other ions, we demonstrate that solar wind Fe charge exchange secondaries are not an obvious Fe+ source. Contemporaneous Earth flyby and cruise data from CHEMS (charge-energy-mass spectrometer) on the Cassini spacecraft, a functionally identical instrument, show that inner source pickup Fe+ is likely not important at suprathermal energies. Consequently, we suggest ionospheric Fe+ constitutes at least a significant portion of Earth's suprathermal Fe+, comparable to the situation at Saturn where suprathermal Fe+ is also likely of ionospheric origin.
      PubDate: 2017-08-31T17:22:36.223634-05:
      DOI: 10.1002/2017JA024414
       
  • GOCE Gradiometer measurements response to ionospheric dynamics
    • Authors: E. Sinem Ince; Spiros D. Pagiatakis
      Abstract: With the launch of dedicated satellite gravity missions, starting with CHAMP in 2000, with GRACE in 2002 and GOCE in 2009, the accuracy and spatial resolution of the Earth's global gravity field models have been improved. Highly sensitive accelerometer measurements have not only been useful for gravity field modelling but have also been contributing to the studies of thermospheric dynamics. While improving the sensitivity of the accelerometer measurements, the new instrumentation used onboard GOCE brings different challenges in understanding the data and developing sophisticated data processing. Our analyses reveal that the GOCE gravitational gradient measurements were affected by highly variable ionospheric dynamics that did not only degrade the quality of the GOCE EGG measurements but also proved that some characteristics of ionospheric dynamics can be measured by GOCE accelerometers and other LEOs. In this paper, we show how GOCE-retrieved neutral winds respond to main ionospheric currents and we develop the impulse-response relation between intense ionospheric dynamics (plasma drift) represented by Poynting energy flux and the GGT trace disturbances observed over the north geomagnetic polar region.
      PubDate: 2017-08-30T19:26:06.850479-05:
      DOI: 10.1002/2017JA023890
       
  • Plasma Transport Driven by the Three-dimensional Kelvin-Helmholtz
           Instability
    • Authors: Xuanye Ma; Peter Delamere, Antonius Otto, Brandon Burkholder
      Abstract: It has been well demonstrated that the nonlinear Kelvin-Helmholtz (KH) instability plays a critical role for the solar wind interaction with the Earth's magnetosphere. Although, the two-dimensional (2-D) KH instability has been fully explored during the past decades, more and more studies show the fundamental difference between the two- and three-dimensional (3-D) KH instability. For northward interplanetary magnetic field (IMF) conditions, the nonlinear KH wave that is localized in the vicinity of the equatorial plane can dramatically bend the magnetic field line, generating strong anti-parallel magnetic field components at high latitudes in both north and south hemispheres, which satisfy the onset condition for magnetic reconnection. This high-latitude double reconnection process can exchange the portion of magnetosheath and magnetospheric flux tubes between those two reconnection sites. This study used a high-resolution 3-D magnetohydrodynamic (MHD) simulation to demonstrate that nonlinear KH waves can generate a large amount of double reconnected flux during the northward IMF condition, which can efficiently transport the plasma with a high diffusion coefficient of 1 × 1010m2s−1 for typical magnetopause conditions at Earth. The presence of the magnetic field component along the shear flow direction not only decreases the KH growth rate, but also causes north-south asymmetry, which generates more open flux and reduces the efficiency of the plasma transport process.
      PubDate: 2017-08-28T17:57:53.416319-05:
      DOI: 10.1002/2017JA024394
       
  • Induction signals from Callisto's ionosphere and their implications on a
           possible subsurface ocean
    • Authors: Oliver Hartkorn; Joachim Saur
      Abstract: We investigate whether induction within Callisto's electrically conductive ionosphere can explain observed magnetic fields which have previously been interpreted as evidence of induction in a saline, electrically conductive subsurface ocean. Callisto's ionosphere is subject to the flow of time-periodic magnetized plasma of Jupiter's magnetosphere, which induces electric fields and electric currents in Callisto's electrically conductive ionosphere. We develop a simple analytic model for a first quantitative understanding of the effects of induction in Callisto's ionosphere caused by the interaction with a time-variable magnetic field environment. With this model, we also investigate how the associated ionospheric currents close in the ambient magnetospheric plasma. Based on this model, we find that the anisotropic nature of Callisto's ionospheric conductivity generates an enhancement effect on ionospheric loop currents which are driven by the time-variable magnetic field. This effect is similar to the Cowling channel effect known from Earth's ionosphere. Subsequently, we numerically calculate the expected induced magnetic fields due to Jupiter's time-variable magnetic field in an anisotropic conductive ionosphere and compare our results with the Galileo C-3 and C-9 flybys. We find that induction within Callisto's ionosphere is responsible for a significant part of the observed magnetic fields. It creates induced magnetic fields similar as expected from a subsurface water ocean. Depending on currently unknown properties such as Callisto's nightside ionosphere, the existence of layers of 'dirty ice' and the details of the plasma interaction, a water ocean might be located much deeper than previously thought or might not exist at all.
      PubDate: 2017-08-01T20:56:24.027745-05:
      DOI: 10.1002/2017JA024269
       
  • Swarm Observation of Field-Aligned Currents Associated With Multiple
           Auroral Arc Systems
    • Authors: J. Wu; D. J. Knudsen, D. M. Gillies, E. F. Donovan, J. K. Burchill
      Pages: 10,145 - 10,156
      Abstract: Auroral arcs occur in regions of upward field-aligned currents (FACs); however, the relation is not one to one, since kinetic energy of the current-carrying electrons is also important in the production of auroral luminosity. Multiple auroral arc systems provide an opportunity to study the relation between FACs and auroral brightness in detail. In this study, we have identified two types of FAC configurations in multiple parallel arc systems using ground-based optical data from the Time History of Events and Macroscale Interactions during Substorms all-sky imagers, magnetometers and electric field instruments on board the Swarm satellites. In “unipolar FAC” events, each arc is an intensification within a broad, unipolar current sheet and downward return currents occur outside of this broad sheet. In “multipolar FAC” events, multiple arc systems represent a collection of multiple up/down current pairs. By collecting 17 events with unipolar FAC and 12 events with multipolar FACs, we find that (1) unipolar FAC events occur most frequently between 20 and 21 magnetic local time and multipolar FAC events tend to occur around local midnight and within 1 h after substorm onset. (2) Arcs in unipolar FAC systems have a typical width of 10–20 km and a spacing of 25–50 km. Arcs in multipolar FAC systems are wider and more separated. (3) Upward currents with more arcs embedded have larger intensities and widths. (4) Electric fields are strong and highly structured on the edges of multiple arc system with unipolar FAC. The fact that arcs with unipolar FAC are much more highly structured than the associated currents suggests that arc multiplicity is indicative not of a structured generator deep in the magnetosphere, but rather of the magnetosphere-ionosphere coupling process.
      PubDate: 2017-10-17T13:25:39.44833-05:0
      DOI: 10.1002/2017JA024439
       
  • Energetic Proton Spectra Measured by the Van Allen Probes
    • Authors: Danny Summers; Run Shi, Mark J. Engebretson, Kjellmar Oksavik, Jerry W. Manweiler, Donald G. Mitchell
      Pages: 10,129 - 10,144
      Abstract: We test the hypothesis that pitch angle scattering by electromagnetic ion cyclotron (EMIC) waves can limit ring current proton fluxes. For two chosen magnetic storms, during 17–20 March 2013 and 17–20 March 2015, we measure proton energy spectra in the region 3 ≤ L ≤ 6 using the RBSPICE-B instrument on the Van Allen Probes. The most intense proton spectra are observed to occur during the recovery periods of the respective storms. Using proton precipitation data from the POES (NOAA and MetOp) spacecraft, we deduce that EMIC wave action was prevalent at the times and L-shell locations of the most intense proton spectra. We calculate limiting ring current proton energy spectra from recently developed theory. Comparisons between the observed proton energy spectra and the theoretical limiting spectra show reasonable agreement. We conclude that the measurements of the most intense proton spectra are consistent with self-limiting by EMIC wave scattering.
      PubDate: 2017-10-17T13:21:45.421855-05:
      DOI: 10.1002/2017JA024484
       
  • Ultralow Frequency Waves Deep Inside the Inner Magnetosphere Driven by
           Dipolarizing Flux Bundles
    • Authors: Jiang Liu; V. Angelopoulos, X.-J. Zhang, A. Runov, A. Artemyev, F. Plaschke, Song Fu, San Lu, Yi-Hsin Liu, Xiangning Chu
      Pages: 10,112 - 10,128
      Abstract: Dipolarizing flux bundles (DFBs) are small flux tubes (typical cross-tail scale of 1–3 RE) in the nightside magnetosphere that have magnetic field more dipolar than the background. They are generated at or beyond 20 RE downtail and then travel earthward. Although DFBs usually stop before reaching the geosynchronous orbit (GEO), they may still transfer some portion of their energy into the inner magnetosphere by radiating ULF waves. We show the clearest evidence of this process, to date, using in situ data from a fleet of spacecraft and ground stations. We illustrate that a typical DFB stopped before reaching GEO but excited Pi2 waves that filled a volume of space extending from the plasma sheet to deep inside the plasmasphere. This event provides the first in situ, direct link between stopped DFBs and ULF waves deep inside the inner magnetosphere (as deep as L = 3). The waves were attenuated when traveling away from the DFB, but even at L = 3 (XGSM = −2.2), they were still traveling with a significant earthward Poynting flux. In addition, we observed evidence of interaction between these waves and electrons at GEO.
      PubDate: 2017-10-17T13:21:10.986923-05:
      DOI: 10.1002/2017JA024270
       
  • The Modulation of the Quasi-Two-Day Wave on Total Electron Content as
           Revealed by BeiDou GEO and Meteor Radar Observations Over Central China
    • Authors: Sheng-Yang Gu; Jiuhou Lei, Xiankang Dou, Xianghui Xue, Fuqing Huang, Mingjiao Jia
      Pages: 10,651 - 10,657
      Abstract: The ground-based total electron content (TEC) observations from BeiDou Geostationary Orbit (GEO) satellites provide more accurate temporal ionospheric variations nearly at the fixed ionospheric pierce points as compared with the non-GEO TEC. BeiDou GEO TEC measurements during 2016, along with the horizontal wind observations from a collocated meteor radar, are utilized to quantitatively study the ionospheric response to the quasi-two-day wave (QTDW) in the neutral atmosphere at Meng Cheng (116.6°E, 33.3°N) over central China. The QTDW peaks during July with amplitudes of ~25–30 m/s for both zonal and meridional winds. It is suggested that the nonlinear interactions between the QTDW and tides and their influences on TEC are insignificant for this QTDW event. Nevertheless, the diurnal tide decreases by ~40–60% during the QTDW episode, which is most likely related to the change of background wind induced by the QTDW. Correspondingly, the 24 h oscillations in TEC decrease by ~18%. In addition, the wind perturbations of the QTDW can also possibly modulate the midlatitude electric dynamo and result in a quasi-two-day oscillation (QTDO) of ~1.2 total electron content unit (TECU), about ~10% of the background TEC. Our analysis also shows that the background TEC decreases by ~1 TECU (~8%) when the QTDW reaches maximum amplitude. This is probably related to the thermospheric composition changes (e.g., O and N2) induced by the QTDW dissipation.
      PubDate: 2017-10-17T13:20:44.868199-05:
      DOI: 10.1002/2017JA024349
       
  • Acceleration of radiation belt electrons and the role of the average
           interplanetary magnetic field Bz component in high-speed streams
    • Authors: V. M. Souza; R. E. Lopez, P. R. Jauer, D. G. Sibeck, K. Pham, L. A. Da Silva, J. P. Marchezi, L. R. Alves, D. Koga, C. Medeiros, M. Rockenbach, W. D. Gonzalez
      Pages: 10,084 - 10,101
      Abstract: In this study we examine the recovery of relativistic radiation belt electrons on 15–16 November 2014, after a previous reduction in the electron flux resulting from the passage of a corotating interaction region (CIR). Following the CIR, there was a period of high-speed streams characterized by large, nonlinear fluctuations in the interplanetary magnetic field (IMF) components. However, the outer radiation belt electron flux remained at a low level for several days before it increased in two major steps. The first increase is associated with the IMF background field turning from slightly northward on average to slightly southward on average. The second major increase is associated with an increase in the solar wind velocity during a period of southward average IMF background field. We present evidence that when the IMF Bz is negative on average, the whistler mode chorus wave power is enhanced in the outer radiation belt, and the amplification of magnetic integrated power spectral density in the ULF frequency range, in the nightside magnetosphere, is more efficient as compared to cases in which the mean IMF Bz is positive. Preliminary analysis of the time evolution of phase space density radial profiles did not provide conclusive evidence on which electron acceleration mechanism is the dominant. We argue that the acceleration of radiation belt electrons requires (i) a seed population of keV electrons injected into the inner magnetosphere by substorms and both (ii) enhanced whistler mode chorus waves activity as well as (iii) large-amplitude MHD waves.
      PubDate: 2017-10-17T11:22:39.730085-05:
      DOI: 10.1002/2017JA024187
       
  • Signatures of Ultrarelativistic Electron Loss in the Heart of the Outer
           Radiation Belt Measured by Van Allen Probes
    • Authors: N. A. Aseev; Y. Y. Shprits, A. Y. Drozdov, A. C. Kellerman, M. E. Usanova, D. Wang, I. S. Zhelavskaya
      Pages: 10,102 - 10,111
      Abstract: Up until recently, signatures of the ultrarelativistic electron loss driven by electromagnetic ion cyclotron (EMIC) waves in the Earth's outer radiation belt have been limited to direct or indirect measurements of electron precipitation or the narrowing of normalized pitch angle distributions in the heart of the belt. In this study, we demonstrate additional observational evidence of ultrarelativistic electron loss that can be driven by resonant interaction with EMIC waves. We analyzed the profiles derived from Van Allen Probe particle data as a function of time and three adiabatic invariants between 9 October and 29 November 2012. New local minimums in the profiles are accompanied by the narrowing of normalized pitch angle distributions and ground-based detection of EMIC waves. Such a correlation may be indicative of ultrarelativistic electron precipitation into the Earth's atmosphere caused by resonance with EMIC waves.
      PubDate: 2017-10-17T11:21:28.907095-05:
      DOI: 10.1002/2017JA024485
       
  • Whistler Mode Waves Below Lower Hybrid Resonance Frequency: Generation and
           Spectral Features
    • Authors: D. R. Shklyar; M. A. Balikhin
      Pages: 10,072 - 10,083
      Abstract: Equatorial noise in the frequency range below the lower hybrid resonance frequency, whose structure is shaped by high proton cyclotron harmonics, has been observed by the Cluster spacecraft. We develop a model of this wave phenomenon which assumes (as, in general, has been suggested long ago) that the observed spectrum is excited due to loss cone instability of energetic ions in the equatorial region of the magnetosphere. The wavefield is represented as a sum of constant frequency wave packets which cross a number of cyclotron resonances while propagating in a highly oblique mode along quite specific trajectories. The growth (damping) rate of these wave packets varies both in sign and magnitude along the raypath, making the wave net amplification, but not the growth rate, the main characteristic of the wave generation process. The growth rates and the wave amplitudes along the ray paths, determined by the equations of geometrical optics, have been calculated for a 3-D set of wave packets with various frequencies, initial L shells, and initial wave normal angles at the equator. It is shown that the dynamical spectrum resulting from the proposed model qualitatively matches observations.
      PubDate: 2017-10-16T15:00:44.256262-05:
      DOI: 10.1002/2017JA024416
       
  • Two-Dimensional Vlasov Simulations of Fast Stochastic Electron Heating in
           Ionospheric Modification Experiments
    • Authors: David Carruthers Speirs; Bengt Eliasson, Lars K. S. Daldorff
      Pages: 10,638 - 10,650
      Abstract: Ionospheric heating experiments using high-frequency ordinary (O)-mode electromagnetic waves have shown the induced formation of magnetic field-aligned density striations in the ionospheric F region, in association with lower hybrid (LH) and upper hybrid (UH) turbulence. In recent experiments using high-power transmitters, the creation of new plasma regions and the formation of descending artificial ionospheric layers (DAILs) have been observed. These are attributed to suprathermal electrons ionizing the neutral gas, so that the O-mode reflection point and associated turbulence is moving to a progressively lower altitude. We present the results of two-dimensional (2-D) Vlasov simulations used to study the mode conversion of an O-mode pump wave to trapped UH waves in a small-scale density striation of circular cross section. Subsequent multiwave parametric decays lead to UH and LH turbulence and to the excitation of electron Bernstein (EB) waves. Large-amplitude EB waves result in rapid stochastic electron heating when the wave amplitude exceeds a threshold value. For typical experimental parameters, the electron temperature is observed to rise from 1,500 K to about 8,000 K in a fraction of a millisecond, much faster than Ohmic heating due to collisions which occurs on a timescale of an order of a second. This initial heating could then lead to further acceleration due to Langmuir turbulence near the critical layer. Stochastic electron heating therefore represents an important potential mechanism for the formation of DAILs.
      PubDate: 2017-10-16T14:56:12.015235-05:
      DOI: 10.1002/2017JA024665
       
  • The Characteristic Response of Whistler Mode Waves to Interplanetary
           Shocks
    • Authors: Chao Yue; Lunjin Chen, Jacob Bortnik, Qianli Ma, Richard M. Thorne, Vassilis Angelopoulos, Jinxing Li, Xin An, Chen Zhou, Craig Kletzing, Geoffrey D. Reeves, Harlan E. Spence
      Pages: 10,047 - 10,057
      Abstract: Magnetospheric whistler mode waves play a key role in regulating the dynamics of the electron radiation belts. Recent satellite observations indicate a significant influence of interplanetary (IP) shocks on whistler mode wave power in the inner magnetosphere. In this study, we statistically investigate the response of whistler mode chorus and plasmaspheric hiss to IP shocks based on Van Allen Probes and THEMIS satellite observations. Immediately after the IP shock arrival, chorus wave power is usually intensified, often at postmidnight to prenoon sector, while plasmaspheric hiss wave power predominantly decreases near the dayside but intensifies near the nightside. We conclude that chorus wave intensification outside the plasmasphere is probably associated with the suprathermal electron flux enhancement caused by the IP shock. Through a simple ray tracing modeling assuming the scenario that plasmaspheric hiss is originated from chorus, we find that the solar wind dynamic pressure increase changes the magnetic field configuration to favor ray penetration in the nightside and promote ray refraction away from the dayside, potentially explaining the magnetic local time-dependent responses of plasmaspheric hiss waves following IP shock arrivals.
      PubDate: 2017-10-14T16:11:02.904694-05:
      DOI: 10.1002/2017JA024574
       
  • Seasonal Variation of High-Latitude Geomagnetic Activity in Individual
           Years
    • Authors: E. I. Tanskanen; R. Hynönen, K. Mursula
      Pages: 10,058 - 10,071
      Abstract: We study the seasonal variation of high-latitude geomagnetic activity in individual years in 1966–2014 (solar cycles 20–24) by identifying the most active and the second most active season based on westward electrojet indices AL (1966–2014) and IL (1995–2014). The annual maximum is found at either equinox in two thirds and at either solstice in one third of the years examined. The traditional two-equinox maximum pattern is found in roughly one fourth of the years. We found that the seasonal variation of high-latitude geomagnetic activity closely follows the solar wind speed. While the mechanisms leading to the two-equinox maxima pattern are in operation, the long-term change of solar wind speed tends to mask the effect of these mechanisms for individual years. Large cycle-to-cycle variation is found in the seasonal pattern: equinox maxima are more common during cycles 21 and 22 than in cycles 23 or 24. Exceptionally long winter dominance in high-latitude activity and solar wind speed is seen in the declining phase of cycle 23, after the appearance of the long-lasting low-latitude coronal hole.
      PubDate: 2017-10-14T16:10:47.242228-05:
      DOI: 10.1002/2017JA024276
       
  • Estimates of Ionospheric Transport and Ion Loss at Mars
    • Authors: T. E. Cravens; O. Hamil, S. Houston, S. Bougher, Y. Ma, D. Brain, S. Ledvina
      Pages: 10,626 - 10,637
      Abstract: Ion loss from the topside ionosphere of Mars associated with the solar wind interaction makes an important contribution to the loss of volatiles from this planet. Data from NASA's Mars Atmosphere and Volatile Evolution mission combined with theoretical modeling are now helping us to understand the processes involved in the ion loss process. Given the complexity of the solar wind interaction, motivation exists for considering a simple approach to this problem and for understanding how the loss rates might scale with solar wind conditions and solar extreme ultraviolet irradiance. This paper reviews the processes involved in the ionospheric dynamics. Simple analytical and semiempirical expressions for ion flow speeds and ion loss are derived. In agreement with more sophisticated models and with purely empirical studies, it is found that the oxygen loss rate from ion transport is about 5% (i.e., global O ion loss rate of Qion ≈ 4 × 1024 s−1) of the total oxygen loss rate. The ion loss is found to approximately scale as the square root of the solar ionizing photon flux and also as the square root of the solar wind dynamic pressure. Typical ion flow speeds are found to be about 1 km/s in the topside ionosphere near an altitude of 300 km on the dayside. Not surprisingly, the plasma flow speed is found to increase with altitude due to the decreasing ion-neutral collision frequency.
      PubDate: 2017-10-14T16:10:33.832856-05:
      DOI: 10.1002/2017JA024582
       
  • Ion Heating in the Martian Ionosphere
    • Authors: C. M. Fowler; R. E. Ergun, L. Andersson, W. K. Peterson, T. Hara, J. Mcfadden, J. Espley, J. Halekas, D. L. Mitchell, C. Mazelle, B. M. Jakosky
      Pages: 10,612 - 10,625
      Abstract: Energetic O+ and O2+ ions with energies of up to a few hundred eV are observed in the Martian ionosphere. Corresponding ion velocity distributions show ion conics, suggesting that the observed ion populations have been heated perpendicular to the local magnetic field before experiencing a magnetic mirror force. Magnetic field observations support these interpretations: wave power at the local O+ and O2+ gyrofrequencies in the spacecraft frame is observed coincident with the energetic ions, within an apparent magnetic field bottle-like topology. Analysis of the observed ion conics leads to estimates of ion temperatures of 10–30 eV. We suggest that the ion populations are initially heated perpendicular to the local magnetic field by wave power propagating inward from the Mars-solar wind interaction. The local magnetic field “balloons out” in response to these enhanced ion temperatures and pressures. The resultant magnetic field topology is bottle like; the transversely heated ions would subsequently experience a magnetic mirror force in the converging field regions, agreeing with the reported observations. Such strong heating events that significantly increase the ion temperature and pressure, thereby decreasing the net magnetic field, are rare and seem to occur under specific interplanetary magnetic field orientations. Events were observed to span the upper exobase region and just above, a region characterized by significant ion densities in an increasingly collisionless domain. Ion heating in this region has the potential to drive significant ion outflows, thus contributing to atmospheric loss from the planet.
      PubDate: 2017-10-14T16:06:03.601792-05:
      DOI: 10.1002/2017JA024578
       
  • Simulated Prompt Acceleration of Multi-MeV Electrons by the 17 March 2015
           Interplanetary Shock
    • Authors: Mary Hudson; Allison Jaynes, Brian Kress, Zhao Li, Maulik Patel, Xiao-Chen Shen, Scott Thaller, Michael Wiltberger, John Wygant
      Pages: 10,036 - 10,046
      Abstract: Prompt enhancement of relativistic electron flux at L = 3–5 has been reported from Van Allen Probes Relativistic Electron Proton Telescope (REPT) measurements associated with the 17 March 2015 interplanetary shock compression of the dayside magnetosphere. Acceleration by ∼1 MeV is inferred on less than a drift timescale as seen in prior shock compression events, which launch a magnetosonic azimuthal electric field impulse tailward. This impulse propagates from the dayside around the flanks accelerating electrons in drift resonance at the dusk flank. Such longitudinally localized acceleration events produce a drift echo signature which was seen at>1 MeV energy on both Van Allen Probe spacecraft, with sustained observations by Probe B outbound at L = 5 at 2100 MLT at the time of impulse arrival, measured by the Electric Fields and Waves instrument. MHD test particle simulations are presented which reproduce drift echo features observed in the REPT measurements at Probe B, including the energy and pitch angle dependence of drift echoes observed. While the flux enhancement was short lived for this event due to subsequent inward motion of the magnetopause, stronger events with larger electric field impulses, as observed in March 1991 and the Halloween 2003 storm, produce enhancements which can be quantified by the inward radial transport and energization determined by the induction electric field resulting from dayside compression.
      PubDate: 2017-10-14T16:03:03.539035-05:
      DOI: 10.1002/2017JA024445
       
  • Interesting Equatorial Plasma Bubbles Observed by All-Sky Imagers in the
           Equatorial Region of China
    • Authors: Kun Wu; Jiyao Xu, Wenbin Wang, Longchang Sun, Xiao Liu, Wei Yuan
      Pages: 10,596 - 10,611
      Abstract: This paper reports some interesting equatorial plasma bubbles (EPBs) on 4 and 5 October 2013 and 29 and 30 September 2013 observed by two all-sky imagers located in the magnetically equatorial region of Hainan (19.5°N; 9.5° geomagnetic latitude) and Guiping (23.8°N; 13.9° geomagnetic latitude), China. The case of 4 and 5 October 2013 shows the following: (1) EPBs had planar wave-like structures and their wavefronts were parallel to each other before midnight. (2) The angle between the wavefronts of those EPBs and the geomagnetic meridian were ~30°. (3) Near midnight, the higher-latitude part of one EPB suddenly began to rotate with a large eastward zonal drift, resulting in an unusual C-shaped EPB that was observed for the first time by an all-sky imager. (4) After midnight, one EPB merged into another EPB and formed an integrated EPB. The other EPB case on 29 and 30 September 2013 shows the following: (1) The images captured the full evolution processes of an “I” shaped EPB with an angle of ~30° with respect to the geomagnetic meridian. The imagers also observed “S” and “Y” shaped EPBs. (2) The change of EPBs' shape was directly related to the change of EPBs' zonal drift velocities. (3) The angle between the I-shaped EPBs and the geomagnetic meridian was likely caused by the change of the latitudinal gradient of zonal neutral wind velocities and ionospheric conductivity. (4) The zonal velocity of each branch of the “Y” shape EPB was different, which could be related to the polarization electric fields within each branch.
      PubDate: 2017-10-14T16:02:29.800821-05:
      DOI: 10.1002/2017JA024561
       
  • A Numerical Investigation on Tidal and Gravity Wave Contributions to the
           Summer Time Na Variations in the Midlatitude E Region
    • Authors: Xuguang Cai; Tao Yuan, J. Vincent Eccles
      Pages: 10,577 - 10,595
      Abstract: The Na density variations in the E region have been studied over the past few decades. Although considerable progress in understanding and in modeling the metal layer observations has been made, Na density features above 100 km have yet to be explained. Various studies have linked them to the Na+ variations, a major reservoir for Na in E region. But the lack of comprehensive modeling investigations and of wind and temperature observations prevents further understanding on this important ion-neutral coupling topic. In this study, we conduct a numerical simulation on the summer time Na density behavior in the midlatitude E region, where both the ion density and the neutral atmosphere are modulated by tidal and gravity waves. Simulation results show good agreement with Na lidar measurements and reveal that atmospheric waves can transport Na upward to generate Na layers and variations in E region considerably. The vertical wind component of the large amplitude tidal wave can extend the Na layer above 120 km into the thermosphere. The simulation also demonstrates that the modulation of large amplitude gravity (GW) wave can generate small-scale sporadic Na layers (Nas) in the E region. Finally, eddy diffusion enhancement in the GW saturation process can significantly alter the Nas spatial and temporal structures.
      PubDate: 2017-10-14T16:00:53.653369-05:
      DOI: 10.1002/2016JA023764
       
  • Elves Accompanying Terrestrial Gamma Ray Flashes
    • Authors: Ningyu Liu; Joseph R. Dwyer, Steven A. Cummer
      Pages: 10,563 - 10,576
      Abstract: This paper reports a modeling study of the optical phenomenon in the lower ionosphere known as elves that may accompany terrestrial gamma ray flashes (TGFs). Recent research has indicated that the in-cloud (IC) discharge processes, termed energetic in-cloud pulses (EIPs), associated with some TGFs can produce a current moment waveform with a peak of hundreds of kA km and a duration of 10 μs. Simulations using the source current moment waveform associated with a Fermi TGF indicate that the radiated electric field at ionospheric altitudes reaches a few times the threshold electric field to excite the optical emissions. A bright elve is therefore induced, with the intensity reaching tens of Megarayleigh, comparable to the brightest elves caused by cloud-to-ground lightning. Because of the strong electromagnetic field radiated, significant blue emissions from the second positive band system of N2 and the first negative band system of N2+ are excited, besides the dominant red emissions from the first positive band system of N2. The elves caused by EIPs with durations of ∼10 μs are elve doublets. For EIPs of longer durations, for example, 30–40 μs, elve multiplets greater than two can be produced. We conclude that elves can be produced by an IC lightning process previously unconnected to elves and that at least some TGFs should have accompanying optical signatures in the lower ionosphere. In addition, TGFs of short durations are more likely to have accompanying elves, because their source currents vary more rapidly.
      PubDate: 2017-10-14T15:56:28.3019-05:00
      DOI: 10.1002/2017JA024344
       
  • Scaling Features of High-Latitude Geomagnetic Field Fluctuations at Swarm
           Altitude: Impact of IMF Orientation
    • Authors: Paola De Michelis; Giuseppe Consolini, Roberta Tozzi, Maria Federica Marcucci
      Pages: 10,548 - 10,562
      Abstract: This paper attempts to explore the statistical scaling features of high-latitude geomagnetic field fluctuations at Swarm altitude. Data for this study are low-resolution (1 Hz) magnetic data recorded by the vector field magnetometer on board Swarm A satellite over 1 year (from 15 April 2014 to 15 April 2015). The first- and second-order structure function scaling exponents and the degree of intermittency of the fluctuations of the intensity of the horizontal component of the magnetic field at high northern latitudes have been evaluated for different interplanetary magnetic field orientations in the GSM Y-Z plane and seasons. In the case of the first-order structure function scaling exponent, a comparison between the average spatial distributions of the obtained values and the statistical convection patterns obtained using a Super Dual Auroral Radar Network dynamic model (CS10 model) has been also considered. The obtained results support the idea that the knowledge of the scaling features of the geomagnetic field fluctuations can help in the characterization of the different ionospheric turbulence regimes of the medium crossed by Swarm A satellite. This study shows that different turbulent regimes of the geomagnetic field fluctuations exist in the regions characterized by a double-cell convection pattern and in those regions near the border of the convective structures.
      PubDate: 2017-10-14T15:55:52.46802-05:0
      DOI: 10.1002/2017JA024156
       
  • Systems science of the magnetosphere: Creating indices of substorm
           activity, of the substorm-injected electron population, and of the
           electron radiation belt
    • Authors: Joseph E. Borovsky; Kateryna Yakymenko
      Pages: 10,012 - 10,035
      Abstract: To more fully monitor the elements of the magnetosphere-ionosphere system and its activity, indices are created to quantify the rate of substorm onset occurrence, the rate of substorm activations, the intensity of the 130 keV substorm-injected electrons and the intensity of the 1.2 MeV radiation-belt electrons. The reactions of these various indices to the onset of isolated substorms are examined, and the behaviors of the indices are explored before and during high-speed-stream-driven storms and during solar wind rarefactions after high-speed streams. Autocorrelation functions of the indices are analyzed and compared with autocorrelation functions of solar wind parameters and geomagnetic indices. Lagged-time correlations between the indices and solar wind variables and geomagnetic indices are surveyed. These 1 h resolution indices are available to the research community.
      PubDate: 2017-10-14T12:16:45.495124-05:
      DOI: 10.1002/2017JA024250
       
  • Simultaneous Measurements of Substorm-Related Electron Energization in the
           Ionosphere and the Plasma Sheet
    • Authors: N. Sivadas; J. Semeter, Y. Nishimura, A. Kero
      Pages: 10,528 - 10,547
      Abstract: On 26 March 2008, simultaneous measurements of a large substorm were made using the Poker Flat Incoherent Scatter Radar, Time History of Events and Macroscale Interactions during Substorm (THEMIS) spacecraft, and all sky cameras. After the onset, electron precipitation reached energies ≳100 keV leading to intense D region ionization. Identifying the source of energetic precipitation has been a challenge because of lack of quantitative and magnetically conjugate measurements of loss cone electrons. In this study, we use the maximum entropy inversion technique to invert altitude profiles of ionization measured by the radar to estimate the loss cone energy spectra of primary electrons. By comparing them with magnetically conjugate measurements from THEMIS-D spacecraft in the nightside plasma sheet, we constrain the source location and acceleration mechanism of precipitating electrons of different energy ranges. Our analysis suggests that the observed electrons ≳100 keV are a result of pitch angle scattering of electrons originating from or tailward of the inner plasma sheet at ~9RE, possibly through interaction with electromagnetic ion cyclotron waves. The electrons of energy 10–100 keV are produced by pitch angle scattering due to a potential drop of ≲10 kV in the auroral acceleration region (AAR) as well as wave–particle interactions in and tailward of the AAR. This work demonstrates the utility of magnetically conjugate ground- and space-based measurements in constraining the source of energetic electron precipitation. Unlike in situ spacecraft measurements, ground-based incoherent scatter radars combined with an appropriate inversion technique can be used to provide remote and continuous-time estimates of loss cone electrons in the plasma sheet.
      PubDate: 2017-10-13T11:51:42.797057-05:
      DOI: 10.1002/2017JA023995
       
  • Equatorial Ionospheric Response to Different Estimated Disturbed Electric
           Fields as Investigated Using Sheffield University Plasmasphere Ionosphere
           Model at INPE
    • Authors: M. A. Bravo; I. S. Batista, J. R. Souza, A. J. Foppiano
      Pages: 10,511 - 10,527
      Abstract: Good ionospheric modeling is important to understand anomalous effects, mainly during geomagnetic storm events. Ionospheric electric fields, thermospheric winds, and neutral composition are affected at different degrees, depending on the intensity of the magnetic disturbance which, in turns, affects the electron density distribution at all latitudes. The most important disturbed parameter for the equatorial ionosphere is the electric field, which is responsible for the equatorial ionization anomaly. Here various electric field measurements and models are analyzed: (1) measured by the Jicamarca incoherent scatter radar (ISR), (2) from Jicamarca Unattended Long-Term studies of the Ionosphere and Atmosphere (JULIA) radar, (3) deduced from magnetometers, (4) calculated from the time variations of the F layer height (dh′F/dt), and (5) deduced from interplanetary electric field determinations. The response of ionospheric parameters foF2 and hmF2 to the electric fields simulated using the Sheffield University Plasmasphere Ionosphere Model version available at Instituto Nacional de Pesquisas Espaciais is compared with observations for two locations, during the geomagnetic storm events of 17–18 April 2002 and 7–10 November 2004. Results are found to be consistent with the observations in such a way that a hierarchy among the different types of drifts used can be established. When no ISR measurements are available, the drifts deduced from magnetometers or measured by the JULIA are best when including the contribution derived from dh′F/dt for the 18–24 LT time interval. However, when none of these drifts are available, drifts inferred from the interplanetary electric field seem to be a good alternative for some purposes.
      PubDate: 2017-10-13T11:50:57.839559-05:
      DOI: 10.1002/2017JA024265
       
  • Observations and Simulations of Eddy Diffusion and Tidal Effects on the
           Semiannual Oscillation in the Ionosphere
    • Authors: Qian Wu; W. S. Schreiner, S.-P. Ho, H.-L. Liu, Liying Qian
      Pages: 10,502 - 10,510
      Abstract: We use the National Center for Atmospheric Research TIEGCM (Thermosphere Ionosphere Electrodynamics General Circulation Model) model to investigate the eddy diffusion and tidal effects on the ionosphere SAO (semiannual oscillation). We also use the COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) satellite GPS radio occultation observations to validate the simulation results. The TIEGCM is driven at the 97 km lower boundary by tidal and gravity wave (eddy diffusion coefficient) inputs. The eddy diffusion input can be constant or with a SAO modulation, and the tidal input has on and off options. The TIEGCM simulation with a SAO modulated eddy diffusion (with tidal input) agrees better with the COSMIC observation than that without the SAO. Turning off the tides at the lower boundary makes the TIEGCM-simulated ionospheric density much higher than the COSMIC observation. The simulations showed two results: (1) the need to add the SAO modulation to the eddy diffusion and (2) how tides reduce the ionospheric density and SAO. As to how much of the SAO should be added to the eddy diffusion is dependent on the amplitudes of the tides since both can have effects on the ionospheric density. The TIEGCM results also demonstrate that the ionospheric density diurnal signal is mostly in situ excited, while the semidiurnal signal comes from lower atmosphere.
      PubDate: 2017-10-05T04:31:31.655074-05:
      DOI: 10.1002/2017JA024341
       
  • Impact of Sudden Stratospheric Warming of 2009 on the Equatorial and
           Low-Latitude Ionosphere of the Indian Longitudes: A Case Study
    • Authors: Sneha Yadav; Tarun K. Pant, R. K. Choudhary, C. Vineeth, Surendra Sunda, K. K. Kumar, P. R. Shreedevi, S. Mukherjee
      Pages: 10,486 - 10,501
      Abstract: Using the equatorial electrojet (EEJ)-induced surface magnetic field and total electron content (TEC) measurements, we investigated the impact of the sudden stratospheric warming (SSW) of January 2009 on the equatorial electrodynamics and low-latitude ionosphere over the Indian longitudes. Results indicate that the intensity of EEJ and the TEC over low latitudes (extending up to 30°N) exhibit significant perturbations during and after the SSW peak. One of the interesting features is the deviation of EEJ and TEC from the normal quiet time behavior well before the onset of the SSW. This is found to coincide with the beginning of enhanced planetary wave (PW) activity over high latitudes. The substantial amplification of the semidiurnal perturbation after the SSW peak is seen to be coinciding with the onset of new and full moons. The response of TEC to SSW is found to be latitude dependent as the near-equatorial (NE) stations show the semidiurnal perturbation only after the SSW peak. Another notable feature is the presence of reduced ionization in the night sector over the NE and low-latitude regions, appearing as an “ionization hole,” well after the SSW peak. The investigation revealed the existence of a quasi 16 day wave in the TEC over low latitudes similar to the one present in the EEJ strength. These results have been discussed in the light of changes in the dynamical background because of enhanced PW activity during SSW, which creates favorable conditions for the amplification of lunar tides, and their subsequent interaction with the lower thermospheric tidal fields.
      PubDate: 2017-10-05T04:30:58.51098-05:0
      DOI: 10.1002/2017JA024392
       
  • The Martian Photoelectron Boundary as Seen by MAVEN
    • Authors: P. Garnier; M. Steckiewicz, C. Mazelle, S. Xu, D. Mitchell, M. K. G. Holmberg, J. S. Halekas, L. Andersson, D. A. Brain, J. E. P. Connerney, J. R. Espley, R. J. Lillis, J. G. Luhmann, J.-A. Sauvaud, B. M. Jakosky
      Pages: 10,472 - 10,485
      Abstract: Photoelectron peaks in the 20–30 eV energy range are commonly observed in the planetary atmospheres, produced by the intense photoionization from solar 30.4 nm photons. At Mars, these photoelectrons are known to escape the planet down its tail, making them tracers for the atmospheric escape. Furthermore, their presence or absence allow to define the so-called photoelectron boundary (PEB), which separates the photoelectron dominated ionosphere from the external environment. We provide here a detailed statistical analysis of the location and properties of the PEB based on the Mars Atmosphere and Volatile EvolutioN (MAVEN) electron and magnetic field data obtained from September 2014 to May 2016 (including 1696 PEB crossings). The PEB appears as mostly sensitive to the solar wind dynamic and crustal fields pressures. Its variable altitude thus leads to a variable wake cross section for escape (up to ∼+50%), which is important for deriving escape rates. The PEB is not always sharp and is characterized on average by the following: a magnetic field topology typical for the end of magnetic pileup region above it, more field-aligned fluxes above than below, and a clear change of the altitude slopes of both electron fluxes and total density (that appears different from the ionopause). The PEB thus appears as a transition region between two plasma and fields configurations determined by the draping topology of the interplanetary magnetic field around Mars and much influenced by the crustal field sources below, whose dynamics also impacts the estimated escape rate of ionospheric plasma.
      PubDate: 2017-10-05T04:26:13.958961-05:
      DOI: 10.1002/2017JA024497
       
  • Issue Information
    • Pages: 9763 - 9767
      Abstract: No abstract is available for this article.
      PubDate: 2017-11-21T02:42:11.758265-05:
      DOI: 10.1002/jgra.52922
       
  • A Study on Sunward Propagating Alfvénic Fluctuations With a Power Law
           Spectrum Observed by the Wind Spacecraft
    • Authors: Honghong Wu; Xin Wang, Chuanyi Tu, Linghua Wang, Jiansen He, Hui Tian
      Pages: 9768 - 9776
      Abstract: Sunward propagating Alfvénic fluctuations with a power law spectrum (SAFP) have been recently observed in the upstream region of the Earth's bow shock. However, some physical properties of these fluctuations such as anisotropy remain unclear. Here we develop a new method for identifying SAFPs, and present for the first time the anisotropy of SAFPs power and spectral index. In this method, the propagation direction determination of SAFPs does not rely on a radial magnetic geometry but the pitch angle distribution of strahl electron outflow. Therefore, the SAFPs with any value of θRB (angle between the global mean magnetic field and the Sun-to-Earth radial direction) can be identified, so that enables the study of the spectral anisotropy. We find 508 SAFPs using the Wind spacecraft measurements from 1995 to 2014. We show that the SAFP has an averaged spectral index of −1.77 ± 0.28 and the index changes continuously from −2.18 ± 0.21 when θRB=0°–10° to −1.71 ± 0.03 when θRB=80°–90°. These SAFPs are observed more frequently in the slow solar wind especially at solar minimum. We also select antisunward propagating Alfvénic fluctuations with a power law spectrum using the same method for comparison. The results indicate that the power spectrum of SAFP is steeper, and the spectral intensity as well as the power anisotropy of SAFP is weaker. These new findings may provide information on the generation of turbulence in the upstream region.
      PubDate: 2017-10-06T14:46:07.996172-05:
      DOI: 10.1002/2017JA024422
       
  • Characterization of Low-Altitude Nightside Martian Magnetic Topology Using
           Electron Pitch Angle Distributions
    • Authors: Tristan Weber; David Brain, David Mitchell, Shaosui Xu, Jack Connerney, Jasper Halekas
      Pages: 9777 - 9789
      Abstract: Magnetic field lines at Mars act as direct pathways for both energy inflow and ion escape. Local variations in magnetic field topology can therefore directly impact the interaction between the solar wind and the Martian ionosphere. One method of analyzing magnetic topology is through the use of electron pitch angle distributions (PADs). Previous PAD investigations have characterized magnetic topology in the Martian system using data from the Mars Global Surveyor spacecraft, but these studies were orbitally constrained to ∼400 km altitude and 2 a.m./2 p.m. local time. With the Mars Atmosphere and Volatile Evolution (MAVEN) mission, we are now able to extend this analysis to a larger range of altitudes and local times. Here we use electron PADs measured using the Solar Wind Electrostatic Analyzer and Magnetometer instruments on MAVEN to analyze the magnetic topology of the nightside Martian environment. We use several characteristic PAD shapes to determine where Martian magnetic field lines are open or closed to the solar wind and present frequency maps of how these PAD shapes vary both geographically and with altitude. Finally, we present an initial analysis of the variation of the PAD shapes with local time, finding that trapped electron distributions become increasingly frequent as crustal fields rotate from dusk to dawn across the nightside of Mars.
      PubDate: 2017-10-06T14:45:34.649648-05:
      DOI: 10.1002/2017JA024491
       
  • Neutron Monitors and Cosmogenic Isotopes as Cosmic Ray Energy-Integration
           Detectors: Effective Yield Functions, Effective Energy, and Its Dependence
           on the Local Interstellar Spectrum
    • Authors: Eleanna Asvestari; Agnieszka Gil, Gennady A. Kovaltsov, Ilya G. Usoskin
      Pages: 9790 - 9802
      Abstract: The method of assessment of galactic cosmic rays (GCR) variability over different timescales, using energy-integrating ground-based detectors such as a neutron monitor and cosmogenic isotopes 10Be and 14C stored in natural archives is revisited here. The effective yield functions for cosmogenic 14C (globally mixed in the atmosphere) and 10Be (realistically deposited in the polar region) are calculated and provided, in a tabulated form, in the supporting information. The effective energy of a detector is redefined so that the variability of the flux of GCR particles at this energy is equal to that of the detector's count rate. The effective energy is found as 11–12 GeV/nucleon for the standard polar neutron monitor, and 6–7 GeV/nucleon and 5.5–6 GeV/nucleon for 14C and 10Be, respectively. New “calibration” relations between the force-field modulation potentials, based on different models of local interstellar spectra (LIS) are provided. While such relations are typically based on refitting the modeled cosmic ray spectra with a prescribed LIS model, the method introduced here straightforwardly accounts for the exact type of the detector used to assess the spectrum. The relations are given separately for ground-based neutron monitors and cosmogenic isotopes. This work allows for harmonization of different works related to variability of galactic cosmic ray flux in the vicinity of Earth, on long-term scale.
      PubDate: 2017-10-13T11:45:27.511118-05:
      DOI: 10.1002/2017JA024469
       
  • In Situ Analysis of Heliospheric Current Sheet Propagation
    • Authors: Jun Peng; Yong C.-M. Liu, Jia Huang, Hui Li, Berndt Klecker, Antoinette B. Galvin, Kristin Simunac, Charles Farrugia, L. K. Jian, Yang Liu, Jie Zhang
      Pages: 9803 - 9814
      Abstract: The heliospheric current sheet (HCS) is an important structure not only for understanding the physics of interplanetary space but also for space weather prediction. We investigate the differences of the HCS arrival time between three spacecraft separated in heliolongitude, heliolatitude and radial distance from the Sun (STEREO A, STEREO B, and ACE) to understand the key factors controlling the HCS propagation. By assuming that the source of the solar wind does not evolve except for the effects of solar rotation, we first test the first-order approach method (ignoring latitudinal differences), using STEREO observation during the year 2007, when the Sun was quiet and the two STEREO spacecraft were separated in heliolongitude by less than 44°. The first-order approach method matches well with observations for many events except for those events when the HCS has a small inclination angle to the ecliptic plane. The latitudinal effect is suggested to account for such discrepancies. The predictions are not improved much by considering the HCS inclination angle obtained from the potential field source surface (PFSS) model. However, the predictions match well with the observations when the HCS inclination angle at 1 AU is obtained from the time differences of HCS arrival times between the STEREO B and ACE spacecraft. An improved model of calculating the inclination of the heliospheric current sheet other than PFSS is needed.
      PubDate: 2017-10-13T11:50:35.224386-05:
      DOI: 10.1002/2017JA024194
       
  • Applying Nyquist's method for stability determination to solar wind
           observations
    • Authors: Kristopher G. Klein; Justin C. Kasper, K. E. Korreck, Michael L. Stevens
      Pages: 9815 - 9823
      Abstract: The role instabilities play in governing the evolution of solar and astrophysical plasmas is a matter of considerable scientific interest. The large number of sources of free energy accessible to such nearly collisionless plasmas makes general modeling of unstable behavior, accounting for the temperatures, densities, anisotropies, and relative drifts of a large number of populations, analytically difficult. We therefore seek a general method of stability determination that may be automated for future analysis of solar wind observations. This work describes an efficient application of the Nyquist instability method to the Vlasov dispersion relation appropriate for hot, collisionless, magnetized plasmas, including the solar wind. The algorithm recovers the familiar proton temperature anisotropy instabilities, as well as instabilities that had been previously identified using fits extracted from in situ observations in Gary et al. (2016). Future proposed applications of this method are discussed.
      PubDate: 2017-10-16T14:55:29.667365-05:
      DOI: 10.1002/2017JA024486
       
  • Solar Wind Plasma Parameter Variability Across Solar Cycles 23 and 24:
           From Turbulence to Extremes
    • Authors: E. Tindale; S. C. Chapman
      Pages: 9824 - 9840
      Abstract: Solar wind variability spans a wide range of amplitudes and timescales, from turbulent fluctuations to the 11 year solar cycle. We apply the data quantile-quantile (DQQ) method to NASA/Wind observations spanning solar cycles 23 and 24, to study how the uniqueness of each cycle maximum and minimum manifests in the changing statistical distribution of plasma parameters in fast and slow solar wind. The DQQ method allows us to discriminate between two distinct components of the distribution: the core region simply tracks the solar cycle in its moments but shows little sensitivity to solar wind state or the specific activity of each cycle. This would be consistent with an underlying in situ process such as turbulence driving the evolution of fluctuations up to an outer scale. In contrast, the tail component of the distribution is sensitive both to the differences between the maxima and minima of cycles 23 and 24, and the fast or slow state of the solar wind. The tail component varies over the solar cycle in such a way as to maintain a constant functional form, with only its moments varying with solar activity. Finally, after isolating the core region of the distribution, we test its lognormality over the solar cycle in each solar wind state and find the lognormal provides a more robust description of the statistics in slow wind than fast; however, in both states the goodness of fit is significantly reduced at solar maximum.
      PubDate: 2017-10-17T11:23:10.568119-05:
      DOI: 10.1002/2017JA024412
       
  • Solar Cycle Occurrence of Alfvénic Fluctuations and Related
           Geo-Efficiency
    • Authors: E. I. Tanskanen; K. Snekvik, J. A. Slavin, D. Pérez-Suárez, A. Viljanen, M. L. Goldstein, M. J. Käpylä, R. Hynönen, L. V. T. Häkkinen, K. Mursula
      Pages: 9848 - 9857
      Abstract: We examine solar wind intervals with Alfvénic fluctuations (ALFs) in 1995–2011. The annual number, the total annual duration, and the average length of ALFs vary over the solar cycle, having a maximum in 2003 and a minimum in 2009. ALFs are most frequent in the declining phase of solar cycle, when the number of high-speed streams at the Earth's vicinity is increased. There is a rapid transition after the maximum of solar cycle 23 from ALFs being mainly embedded in slow solar wind (600 km/s) since 2003. Cross helicity increased by 30% from 2002 to 2003 and maximized typically 4–6 h before solar wind speed maximum. Cross helicity remained elevated for several days for highly Alfvénic non-ICME streams, but only for a few hours for ICMEs. The number of substorms increased by about 40% from 2002 to 2003, and the annual number of substorms closely follows the annual cross helicity. This further emphasizes the role of Alfvénic fluctuations in modulating substorm activity. The predictability of substorm frequency and size would be greatly improved by monitoring solar wind Alfvénic fluctuations in addition to the mean values of the important solar wind parameters.
      PubDate: 2017-10-17T13:30:38.824657-05:
      DOI: 10.1002/2017JA024385
       
  • Understanding the Mechanisms of Radiation Belt Dropouts Observed by Van
           Allen Probes
    • Authors: Zheng Xiang; Weichao Tu, Xinlin Li, Binbin Ni, S. K. Morley, D. N. Baker
      Pages: 9858 - 9879
      Abstract: To achieve a better understanding of the dominant loss mechanisms for the rapid dropouts of radiation belt electrons, three distinct radiation belt dropout events observed by Van Allen Probes are comprehensively investigated. For each event, observations of the pitch angle distribution of electron fluxes and electromagnetic ion cyclotron (EMIC) waves are analyzed to determine the effects of atmospheric precipitation loss due to pitch angle scattering induced by EMIC waves. Last closed drift shells (LCDS) and magnetopause standoff position are obtained to evaluate the effects of magnetopause shadowing loss. Evolution of electron phase space density (PSD) versus L* profiles and the μ and K (first and second adiabatic invariants) dependence of the electron PSD drops are calculated to further analyze the dominant loss mechanisms at different L*. Our findings suggest that these radiation belt dropouts can be classified into distinct classes in terms of dominant loss mechanisms: magnetopause shadowing dominant, EMIC wave scattering dominant, and combination of both mechanisms. Different from previous understanding, our results show that magnetopause shadowing can deplete electrons at L*  4. Compared to the magnetopause standoff position, it is more reliable to use LCDS to evaluate the impact of magnetopause shadowing. The evolution of electron PSD versus L* profile and the μ, K dependence of electron PSD drops can provide critical and credible clues regarding the mechanisms responsible for electron losses at different L* over the outer radiation belt.
      PubDate: 2017-10-03T09:01:36.065674-05:
      DOI: 10.1002/2017JA024487
       
  • Rapid Loss of Radiation Belt Relativistic Electrons by EMIC Waves
    • Authors: Zhenpeng Su; Zhonglei Gao, Huinan Zheng, Yuming Wang, Shui Wang, H. E. Spence, G. D. Reeves, D. N. Baker, J. R. Wygant
      Pages: 9880 - 9897
      Abstract: How relativistic electrons are lost is an important question surrounding the complex dynamics of the Earth's outer radiation belt. Radial loss to the magnetopause and local loss to the atmosphere are two main competing paradigms. Here on the basis of the analysis of a radiation belt storm event on 27 February 2014, we present new evidence for the electromagnetic ion cyclotron (EMIC) wave-driven local precipitation loss of relativistic electrons in the heart of the outer radiation belt. During the main phase of this storm, the radial profile of relativistic electron phase space density was quasi-monotonic, qualitatively inconsistent with the prediction of radial loss theory. The local loss at low L shells was required to prevent the development of phase space density peak resulting from the radial loss process at high L shells. The rapid loss of relativistic electrons in the heart of outer radiation belt was observed as a dip structure of the electron flux temporal profile closely related to intense EMIC waves. Our simulations further confirm that the observed EMIC waves within a quite limited longitudinal region were able to reduce the off-equatorially mirroring relativistic electron fluxes by up to 2 orders of magnitude within about 1.5 h.
      PubDate: 2017-10-05T04:27:47.388504-05:
      DOI: 10.1002/2017JA024169
       
  • Hot Ion Flows in the Distant Magnetotail: ARTEMIS Observations From Lunar
           Orbit to ∼−200 RE
    • Authors: A. V. Artemyev; V. Angelopoulos, A. Runov, I. Y. Vasko
      Pages: 9898 - 9909
      Abstract: Plasma energization in Earth's magnetotail is supported by acceleration processes in (and around) magnetic reconnection regions. Hot plasma flows and strong electromagnetic waves, generated by magnetic energy release during reconnection, transport energy necessary for current system intensification and particle acceleration in the inner magnetosphere. Earth's magnetotail configuration includes two main reconnection regions (X lines): the near-Earth X line, which has been well studied by several multispacecraft missions, and the distant X line, which has been much less investigated. In this paper, we utilize the unique data set gathered by two ARTEMIS spacecraft in 2010 at radial distances between lunar orbit and ∼200 RE (Earth radii). We identify an X line at around ∼80 RE and collect statistics on hot plasma flows observed around and beyond this distance. Ion spectra within these flows are well fitted by a power law with the exponential tail starting above an energy ϵ0∼ 2–5 keV. Assuming that these spectra are originated at the distant X line, we examine the characteristics of the acceleration at the distant tail reconnection region.
      PubDate: 2017-10-05T04:25:54.61325-05:0
      DOI: 10.1002/2017JA024433
       
  • Spontaneous hot flow anomalies at Mars and Venus
    • Authors: Glyn Collinson; David Sibeck, Nick Omidi, Joseph Grebowsky, Jasper Halekas, David Mitchell, Jared Espley, Tielong Zhang, Moa Persson, Yoshifumi Futaana, Bruce Jakosky
      Pages: 9910 - 9923
      Abstract: We report the first observations of Spontaneous Hot Flow Anomalies (SHFAs) at Venus and Mars, demonstrating their existence in the foreshocks of other planets beyond Earth. Using data from the ESA Venus Express and the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we present magnetic and plasma observations from events at both planets, exhibiting properties similar to “classical” Hot Flow Anomalies, with bounding shock-like compressive regions and a hot and diffuse core. However, these explosive foreshock transients were observed without any attendant interplanetary magnetic field discontinuity, consistent with SHFAs observed at Earth and our hybrid simulations.
      PubDate: 2017-10-06T13:11:03.920512-05:
      DOI: 10.1002/2017JA024196
       
  • Nonlinear Generation Mechanism of EMIC Falling Tone Emissions
    • Authors: Masafumi Shoji; Yoshiharu Omura
      Pages: 9924 - 9933
      Abstract: We have conducted a self-consistent hybrid simulation, successfully reproducing electromagnetic ion cyclotron (EMIC) emissions with falling tone frequencies. The hybrid simulation is implemented with a parabolic ambient magnetic field. In the simulation, strong oxygen band EMIC emissions are generated through nonlinear wave growth. The cold ion density is modulated by electrostatic structures which are induced by the forward and backward propagating oxygen band EMIC waves. Along with the growth of the oxygen band, the helium band waves also grow because of the initial linear growth followed by the nonlinear growth. The nonlinear growth of the helium band waves is affected by the cold plasma density modulation, and there appear short wave packets of helium band emissions. The short wave packets entrap energetic protons efficiently, resulting in electromagnetic proton hills in the velocity phase space. The proton hill forms a nonlinear resonant current causing the falling frequency of the EMIC waves. We find strong deformation of the velocity distribution function of the energetic protons due to the proton hill being guided by the increasing resonance velocity.
      PubDate: 2017-10-09T11:11:14.125963-05:
      DOI: 10.1002/2017JA023883
       
  • MMS Observations of Reconnection at Dayside Magnetopause Crossings During
           Transitions of the Solar Wind to Sub-Alfvénic Flow
    • Authors: C. J. Farrugia; N. Lugaz, L. Alm, B. Vasquez, M. R. Argall, H. Kucharek, H. Matsui, R. B. Torbert, B. Lavraud, O. Le Contel, I. J. Cohen, J. L. Burch, C. T. Russell, R. J. Strangeway, J. Shuster, J. C. Dorelli, J. P. Eastwood, R. E. Ergun, S. A. Fuselier, D. J. Gershman, B. L. Giles, Y. V. Khotyaintsev, P. A. Lindqvist, G. T. Marklund, K. W. Paulson, S. M. Petrinec, T. D. Phan, C. J. Pollock
      Pages: 9934 - 9951
      Abstract: We present MMS observations during two dayside magnetopause crossings under hitherto unexamined conditions: (i) when the bow shock is weakening and the solar wind transitioning to sub-Alfvénic flow and (ii) when it is reforming. Interplanetary conditions consist of a magnetic cloud with (i) a strong B (∼20 nT) pointing south and (ii) a density profile with episodic decreases to values of ∼0.3 cm−3 followed by moderate recovery. During the crossings the magnetosheath magnetic field is stronger than the magnetosphere field by a factor of ∼2.2. As a result, during the outbound crossing through the ion diffusion region, MMS observed an inversion of the relative positions of the X and stagnation (S) lines from that typically the case: the S line was closer to the magnetosheath side. The S line appears in the form of a slow expansion fan near which most of the energy dissipation is taking place. While in the magnetosphere between the crossings, MMS observed strong field and flow perturbations, which we argue to be due to kinetic Alfvén waves. During the reconnection interval, whistler mode waves generated by an electron temperature anisotropy (Te⊥>Te∥) were observed. Another aim of the paper is to distinguish bow shock-induced field and flow perturbations from reconnection-related signatures. The high-resolution MMS data together with 2-D hybrid simulations of bow shock dynamics helped us to distinguish between the two sources. We show examples of bow shock-related effects (such as heating) and reconnection effects such as accelerated flows satisfying the Walén relation.
      PubDate: 2017-10-09T11:41:43.435593-05:
      DOI: 10.1002/2017JA024563
       
  • The effects of magnetospheric processes on relativistic electron dynamics
           in the Earth's outer radiation belt
    • Authors: C. L. Tang; Y. X. Wang, B. Ni, Z. P. Su, G. D. Reeves, J.-C. Zhang, D. N. Baker, H. E. Spence, H. O. Funsten, J. B. Blake
      Pages: 9952 - 9968
      Abstract: Using the electron phase space density (PSD) data measured by Van Allen Probe A from January 2013 to April 2015, we investigate the effects of magnetospheric processes on relativistic electron dynamics in the Earth's outer radiation belt during 50 geomagnetic storms. A statistical study shows that the maximum electron PSDs for various μ (μ = 630, 1096, 2290, and 3311 MeV/G) at L*~4.0 after the storm peak have good correlations with storm intensity (cc~0.70). This suggests that the occurrence and magnitude of geomagnetic storms are necessary for relativistic electron enhancements at the inner edge of the outer radiation belt (L* = 4.0). For moderate or weak storm events (SYM-Hmin > ~−100 nT) with weak substorm activity (AEmax  0.77). For storm events with intense substorms after the storm peak, relativistic electron enhancements at L* = 4.5 and 5.0 are observed. This shows that intense substorms during the storm recovery phase are crucial to relativistic electron enhancements in the heart of the outer radiation belt. Our statistics study suggests that magnetospheric processes during geomagnetic storms have a significant effect on relativistic electron dynamics.
      PubDate: 2017-10-10T11:20:55.008522-05:
      DOI: 10.1002/2017JA024407
       
  • Low-Energy (
    • Authors: Jie Ren; Q.-G. Zong, Y. Miyoshi, X. Z. Zhou, Y. F. Wang, R. Rankin, C. Yue, H. E. Spence, H. O. Funsten, J. R. Wygant, C. A. Kletzing
      Pages: 9969 - 9982
      Abstract: We report observational evidence of cold plasmaspheric electron (
      PubDate: 2017-10-13T11:26:26.222911-05:
      DOI: 10.1002/2017JA024316
       
  • Identifying Ultra Low Frequency Waves in the Lunar Plasma Environment
           Using Trajectory Analysis and Resonance Conditions
    • Authors: S. K. Howard; J. S. Halekas, W. M. Farrell, J. P. McFadden, K.-H. Glassmeier
      Pages: 9983 - 9993
      Abstract: Recent studies show that localized crustal magnetic fields on the lunar surface can reflect a significant portion of the incoming solar wind protons. These reflected ions can drive a wide range of plasma waves. It is difficult to determine the intrinsic properties of low-frequency waves with single-spacecraft observations, which can be heavily Doppler shifted. We describe a technique to combine trajectory analysis of reflected protons with the Doppler shift and resonance conditions to identify ultralow-frequency waves at the Moon. On 31 January 2014 plasma waves were detected by one of the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) probes as it approached the lunar wake; these waves were not detected by the second ARTEMIS probe located upstream in the undisturbed solar wind. The observed waves had a frequency below the local ion cyclotron frequency and had right-hand circular polarization in the reference frame of the Moon. By solving the Doppler shift and the cyclotron resonance equations, we determined the conditions for reflected ions to excite the observed waves. Simulated trajectories of reflected ions correspond to ARTEMIS ion observations and support the hypothesis that reflected ions are the primary driver of the waves. By combining trajectory analysis with the resonance conditions, we identify scenarios where ions that satisfy the resonance conditions are present in the right location to generate the observed waves. Using this method, we can uniquely identify the observed waves as upstream propagating right-hand polarized waves, subject to the assumption that they are generated by cyclotron resonance with ions.
      PubDate: 2017-10-13T11:25:50.50447-05:0
      DOI: 10.1002/2017JA024018
       
  • Storm and Substorm Causes and Effects at Midlatitude Location for the St.
           Patrick's 2013 and 2015 Events
    • Authors: A. Guerrero; J. Palacios, M. Rodríguez-Bouza, I. Rodríguez-Bilbao, A. Aran, C. Cid, M. Herraiz, E. Saiz, G. Rodríguez-Caderot, Y. Cerrato
      Pages: 9994 - 10,011
      Abstract: Midlatitude locations are unique regions exposed to both geomagnetic storm and substorm effects, which may be superposed on specific events imposing an extra handicap for the analysis and identification of the sources and triggers. We study space weather effects at the midlatitude location of the Iberian Peninsula for the St. Patrick's day events in 2013 and 2015. We have been able to identify and separate storm and substorm effects on ground magnetometer data from San Pablo-Toledo observatory during storm time revealing important contributions of the Substorm Current Wedge on both events. The analysis of these substorm local signatures have shown to be related to the production of effective geomagnetically induced currents and ionospheric disturbances as measured from Global Navigation Satellite Systems data at MAD2 IGS permanent station and not directly related to the storm main phase. The whole Sun-to-Earth chain has been analyzed in order to identify the solar and interplanetary triggers. In both events a high-speed stream (HSS) and a coronal mass ejections (CME) are involved, though for 2015 event, the HSS has merged with the CME, increasing the storm geoeffectiveness. The enhancement of substorm geoeffectiveness is justified by the effects of the inclined magnetic axes of the Sun and of the Earth during equinox period.
      PubDate: 2017-10-13T11:31:06.969677-05:
      DOI: 10.1002/2017JA024224
       
 
 
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