Annales Geophysicae (ANGEO)
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Open Access journal
ISSN (Print) 0992-7689 - ISSN (Online) 1432-0576
Published by European Geosciences Union [8 journals] [SJR: 1.151] [H-I: 57]
[5 followers] Follow
Open Access journal
ISSN (Print) 0992-7689 - ISSN (Online) 1432-0576
Published by European Geosciences Union [8 journals] [SJR: 1.151] [H-I: 57]
- The structure of fast sausage waves in current-carrying coronal loops
Abstract: The structure of fast sausage waves in current-carrying coronal loops
Annales Geophysicae, 32, 1189-1193, 2014
Author(s): D. B. Bembitov, B. B. Mikhalyaev, and M. S. Ruderman
We study fast sausage waves in a model coronal loop that consists of a cylindrical core with axial magnetic field and coaxial annulus with purely azimuthal magnetic field. The magnetic field is discontinuous at the tube and core boundaries, and there are surface currents with the opposite directions on these boundaries. The principal mode of fast sausage waves in which the magnetic pressure perturbation has no nodes in the radial direction can exist for arbitrary wavelength. The results for the fundamental radial mode of sausage waves are applied to the interpretation of observed periodic pulsations of microwave emission in flaring loops with periods of a few tens of seconds. Radial plasma motion has opposite directions at the tube and core boundaries. This leads to the periodic contraction and expansion of the annulus. We assume that the principal mode of fast sausage waves in the current-carrying coronal loops is able to produce a current sheet. However, the nonlinear analysis is needed to confirm this conjecture.
- Observations of the generation of eastward equatorial electric fields near
Abstract: Observations of the generation of eastward equatorial electric fields near dawn
Annales Geophysicae, 32, 1169-1175, 2014
Author(s): M. C. Kelley, F. S. Rodrigues, R. F. Pfaff, and J. Klenzing
We report and discuss interesting observations of the variability of electric fields and ionospheric densities near sunrise in the equatorial ionosphere made by instruments onboard the Communications/Navigation Outage Forecasting System (C/NOFS) satellite over six consecutive orbits. Electric field measurements were made by the Vector Electric Field Instrument (VEFI), and ionospheric plasma densities were measured by Planar Langmuir Probe (PLP). The data were obtained on 17 June 2008, a period of solar minimum conditions. Deep depletions in the equatorial plasma density were observed just before sunrise on three orbits, for which one of these depletions was accompanied by a very large eastward electric field associated with the density depletion, as previously described by de La Beaujardière et al. (2009), Su et al. (2009) and Burke et al. (2009). The origin of this large eastward field (positive upward/meridional drift), which occurred when that component of the field is usually small and westward, is thought to be due to a large-scale Rayleigh–Taylor process. On three subsequent orbits, however, a distinctly different, second type of relationship between the electric field and plasma density near dawn was observed. Enhancements of the eastward electric field were also detected, one of them peaking around 3 mV m−1, but they were found to the east (later local time) of pre-dawn density perturbations. These observations represent sunrise enhancements of vertical drifts accompanied by eastward drifts such as those observed by the San Marco satellite (Aggson et al., 1995). Like the San Marco measurements, the enhancements occurred during winter solstice and low solar flux conditions in the Pacific longitude sector. While the evening equatorial ionosphere is believed to present the most dramatic examples of variability, our observations exemplify that the dawn sector can be highly variable as well.
- Comparison of methods for modelling geomagnetically induced currents
Abstract: Comparison of methods for modelling geomagnetically induced currents
Annales Geophysicae, 32, 1177-1187, 2014
Author(s): D. H. Boteler and R. J. Pirjola
Assessing the geomagnetic hazard to power systems requires reliable modelling of the geomagnetically induced currents (GIC) produced in the power network. This paper compares the Nodal Admittance Matrix method with the Lehtinen–Pirjola method and shows them to be mathematically equivalent. GIC calculation using the Nodal Admittance Matrix method involves three steps: (1) using the voltage sources in the lines representing the induced geoelectric field to calculate equivalent current sources and summing these to obtain the nodal current sources, (2) performing the inversion of the admittance matrix and multiplying by the nodal current sources to obtain the nodal voltages, (3) using the nodal voltages to determine the currents in the lines and in the ground connections. In the Lehtinen–Pirjola method, steps 2 and 3 of the Nodal Admittance Matrix calculation are combined into one matrix expression. This involves inversion of a more complicated matrix but yields the currents to ground directly from the nodal current sources. To calculate GIC in multiple voltage levels of a power system, it is necessary to model the connections between voltage levels, not just the transmission lines and ground connections considered in traditional GIC modelling. Where GIC flow to ground through both the high-voltage and low-voltage windings of a transformer, they share a common path through the substation grounding resistance. This has been modelled previously by including non-zero, off-diagonal elements in the earthing impedance matrix of the Lehtinen–Pirjola method. However, this situation is more easily handled in both the Nodal Admittance Matrix method and the Lehtinen–Pirjola method by introducing a node at the neutral point.
- New results of structured VLF emissions observed simultaneously at two
closely located stations near L ~ 5.5
Abstract: New results of structured VLF emissions observed simultaneously at two closely located stations near L ~ 5.5
Annales Geophysicae, 32, 1163-1167, 2014
Author(s): J. Manninen, N. G. Kleimenova, Yu. V. Fedorenko, P. A. Bespalov, and T. Turunen
Simultaneous records of VLF (very low frequencies) emissions have been carried out at two ground-based stations located at similar geomagnetic latitudes near L ~ 5.5 and spaced in the longitude by ~ 400 km, Kannuslehto (KAN) in Finland and Lovozero (LOZ) in Russia, using quite similar VLF receivers with two calibrated orthogonal air-core loop antennas. We found that the general spectral properties of the VLF chorus emissions at these two stations were similar and typically have right-hand polarization. Contrary to VLF chorus, the short-period VLF emissions (periodic emissions, PE) in which separated spectral elements are repeated with the periodicity of 3–4 s were mostly left-hand polarized. Usually, these waves propagated in the north–south direction. We suppose that PEs are generated inside of the plasmasphere by the cyclotron instability under a quasi-linear relaxation of the energetic electron distribution function. However, sometimes PE occurred only at an individual station. We speculated that this could be due to the influence of the local inhomogeneities to the VLF waves during the propagation through the ionospheric trough to the ground. Unusual series of short-duration (10–100 s) bursts of VLF emissions, lasting several hours, were also found in the morning under very quiet geomagnetic conditions (Kp ~ 0–1). Generally, these emissions were observed simultaneously at KAN and LOZ showing both right-hand and left-hand polarization, and different arrival directions provided the rather extended ionospheric exit area.
- Ionospheric shock waves triggered by rockets
Abstract: Ionospheric shock waves triggered by rockets
Annales Geophysicae, 32, 1145-1152, 2014
Author(s): C. H. Lin, J. T. Lin, C. H. Chen, J. Y. Liu, Y. Y. Sun, Y. Kakinami, M. Matsumura, W. H. Chen, H. Liu, and R. J. Rau
This paper presents a two-dimensional structure of the shock wave signatures in ionospheric electron density resulting from a rocket transit using the rate of change of the total electron content (TEC) derived from ground-based GPS receivers around Japan and Taiwan for the first time. From the TEC maps constructed for the 2009 North Korea (NK) Taepodong-2 and 2013 South Korea (SK) Korea Space Launch Vehicle-II (KSLV-II) rocket launches, features of the V-shaped shock wave fronts in TEC perturbations are prominently seen. These fronts, with periods of 100–600 s, produced by the propulsive blasts of the rockets appear immediately and then propagate perpendicularly outward from the rocket trajectory with supersonic velocities between 800–1200 m s−1 for both events. Additionally, clear rocket exhaust depletions of TECs are seen along the trajectory and are deflected by the background thermospheric neutral wind. Twenty minutes after the rocket transits, delayed electron density perturbation waves propagating along the bow wave direction appear with phase velocities of 800–1200 m s−1. According to their propagation character, these delayed waves may be generated by rocket exhaust plumes at earlier rocket locations at lower altitudes.
- On the man-made contamination on ULF measurements: evidence for
disturbances related to an electrified DC railway
Abstract: On the man-made contamination on ULF measurements: evidence for disturbances related to an electrified DC railway
Annales Geophysicae, 32, 1153-1161, 2014
Author(s): U. Villante, A. Piancatelli, and P. Palangio
An analysis of measurements performed at L'Aquila (Italy) during a deep minimum of solar and magnetospheric activity (2008–2010) allowed for the evaluation of possible contamination of the ultralow-frequency (ULF) spectrum (f ≈ 1–500 mHz) from artificial disturbances, practically in absence of natural signals. In addition, the city evacuation and the interruption of all industrial and social activities after the strong earthquake of 6 April 2009 allowed also for the examination of possible changes of the contamination level under remarkably changed environmental conditions. Our analysis reveals a persistent, season-independent, artificial signal, with the same characteristics in the H and Z components, that affects during daytime hours the entire spectrum; such contamination persists after the city evacuation. We speculate that the DC electrified railway (located ≈ 33 km from the Geomagnetic Observatory of L'Aquila, it maintained the same train traffic after the earthquake) is responsible for the observed disturbances.
- Spatial and seasonal variability of medium- and high-frequency gravity
waves in the lower atmosphere revealed by US radiosonde data
Abstract: Spatial and seasonal variability of medium- and high-frequency gravity waves in the lower atmosphere revealed by US radiosonde data
Annales Geophysicae, 32, 1129-1143, 2014
Author(s): S. D. Zhang, C. M. Huang, K. M. Huang, F. Yi, Y. H. Zhang, Y. Gong, and Q. Gan
We extended the broad spectral method proposed by Zhang et al. (2013) for the extraction of medium- and high-frequency gravity waves (MHGWs). This method was applied to 11 years (1998–2008) of radiosonde data from 92 stations in the Northern Hemisphere to investigate latitudinal, continuous vertical and seasonal variability of MHGW parameters in the lower atmosphere (2–25 km). The latitudinal and vertical distributions of the wave energy density and horizontal momentum fluxes as well as their seasonal variations exhibit considerable consistency with those of inertial gravity waves. Despite the consistency, the MHGWs have much larger energy density, horizontal momentum fluxes and wave force, indicating the more important role of MHGWs in energy and momentum transportation and acceleration of the background. For the observed MHGWs, the vertical wavelengths are usually larger than 8 km; the horizontal wavelengths peak in the middle troposphere at middle–high latitudes. These characteristics are obviously different from inertial gravity waves. The energy density and horizontal momentum fluxes have similar latitude-dependent seasonality: both of them are dominated by a semiannual variation at low latitudes and an annual variation at middle latitudes; however at high latitudes, they often exhibit more than two peaks per year in the troposphere. Compared with the inertial GWs, the derived intrinsic frequencies are more sensitive to the spatiotemporal variation of the buoyancy frequency, and at all latitudinal regions they are higher in summer. The wavelengths have a weaker seasonal variation; an evident annual cycle can be observed only at middle latitudes.
- Remote sensing of the Io torus plasma ribbon using natural radio
occultation of the Jovian radio emissions
Abstract: Remote sensing of the Io torus plasma ribbon using natural radio occultation of the Jovian radio emissions
Annales Geophysicae, 32, 1119-1128, 2014
Author(s): M. Y. Boudjada, P. H. M. Galopeau, S. Sawas, and H. Lammer
We study the Jovian hectometric (HOM) emissions recorded by the RPWS (Radio and Plasma Wave Science) experiment onboard the Cassini spacecraft during its Jupiter flyby. We analyze the attenuation band associated with the intensity extinction of HOM radiation. This phenomenon is interpreted as a refraction effect of the Jovian hectometric emission inside the Io plasma torus. This attenuation band was regularly observed during periods of more than 5 months, from the beginning of October 2000 to the end of March 2001. We estimate for this period the variation of the electron density versus the central meridian longitude (CML). We find a clear local time dependence. Hence the electron density was not higher than 5.0 × 104 cm−3 during 2 months, when the spacecraft approached the planet on the dayside. In the late afternoon and evening sectors, the electron density increases to 1.5 × 105 cm−3 and reach a higher value at some specific occasions. Additionally, we show that ultraviolet and hectometric wavelength observations have common features related to the morphology of the Io plasma torus. The maxima of enhancements/attenuations of UV/HOM observations occur close to the longitudes of the tip of the magnetic dipole in the southern hemisphere (20° CML) and in the northern hemisphere (200° CML), respectively. This is a significant indication about the importance of the Jovian magnetic field as a physical parameter in the coupling process between Jupiter and the Io satellite.
- Cluster observations of the substructure of a flux transfer event:
analysis of high-time-resolution particle data
Abstract: Cluster observations of the substructure of a flux transfer event: analysis of high-time-resolution particle data
Annales Geophysicae, 32, 1093-1117, 2014
Author(s): A. Varsani, C. J. Owen, A. N. Fazakerley, C. Forsyth, A. P. Walsh, M. André, I. Dandouras, and C. M. Carr
Flux transfer events (FTEs) are signatures of transient reconnection at the dayside magnetopause, transporting flux from the dayside of the magnetosphere into the magnetotail lobes. They have previously been observed to contain a combination of magnetosheath and magnetospheric plasma. On 12 February 2007, the four Cluster spacecraft were widely separated across the magnetopause and observed a crater-like FTE as they crossed the Earth's dayside magnetopause through its low-latitude boundary layer. The particle instruments on the Cluster spacecraft were in burst mode and returning data providing 3-D velocity distribution functions (VDFs) at 4 s resolution during the observation of this FTE. Moreover, the magnetic field observed during the event remained closely aligned with the spacecraft spin axis and thus we have been able to use these 3-D data to reconstruct nearly full pitch angle distributions of electrons and ions at high time resolution (up to 32 times faster than available from the normal mode data stream). These observations within the boundary layer and inside the core of the FTE show that both the interior and the surrounding structure of the FTE consist of multiple individual layers of plasma, in greater number than previously identified. Our observations show a cold plasma inside the core, a thin layer of antiparallel-moving electrons at the edge of FTE itself, and field-aligned ions with Alfvénic speeds at the trailing edge of the FTE. We discuss the plasma characteristics in these FTE layers, their possible relevance to the magnetopause reconnection processes and attempt to distinguish which of the various different FTE models may be relevant in this case. These data are particularly relevant given the impending launch of NASA's MMS mission, for which similar observations are expected to be more routine.
- Analysis of cloud-to-ground lightning and its relation with surface
pollutants over Taipei, Taiwan
Abstract: Analysis of cloud-to-ground lightning and its relation with surface pollutants over Taipei, Taiwan
Annales Geophysicae, 32, 1085-1092, 2014
Author(s): S. K. Kar and Y. A. Liou
Premonsoon (March–April) cloud-to-ground (CG) lightning activity over Taipei, Taiwan, is analyzed in relation to surface pollutants like particulate matter (PM10), sulfur dioxide (SO2), nitrogen oxides (NOx) and ozone (O3) concentration for a period of 6 years (2005–2010). Other surface parameters like aerosol optical depth and cloud top temperature are also investigated taking data from Moderate Resolution Imaging Spectroradiometer satellite products. Results reveal that SO2 is more strongly associated with CG lightning activity compared to PM10 concentration. Other surface pollutants like NOx and O3 also show strong linear association with CG lightning flashes. Additional investigations have also been performed to extreme lightning events, particularly to a few long-lasting lightning episodes considering the concentrations of NOx and O3 found on days with no lightning activity as representative of the background concentration levels of the said two parameters. Results indicate that the NOx concentration on days with lightning activity is more than 2-fold compared to the non-lightning days while the O3 concentration is increased by 1.5-fold. Such increase in NOx and O3 concentration on days with lightning strongly supports the transport phenomena of NOx and O3 from the upper or middle troposphere to the lower troposphere by downdraft of the thunderstorm during its dissipation stage. Overall, studies suggest that enhanced surface pollution in a near-storm environment is strongly related to the increased lightning activity, which in turn increases the surface NOx level and surface O3 concentration over the area under study.
- Waves off Gopalpur, northern Bay of Bengal during Cyclone Phailin
Abstract: Waves off Gopalpur, northern Bay of Bengal during Cyclone Phailin
Annales Geophysicae, 32, 1073-1083, 2014
Author(s): M. M. Amrutha, V. Sanil Kumar, T. R. Anoop, T. M. Balakrishnan Nair, A. Nherakkol, and C. Jeyakumar
The wave statistical parameters during Cyclone Phailin which crossed the northern Bay of Bengal are described based on the Directional Waverider buoy-measured wave data from 8 to 13 October 2013. On 12 October 2013, the cyclone passed within 70 km of the Waverider buoy location with a wind speed of 59.2 m s−1 (115 knots), and during this period, a maximum significant wave height of 7.3 m and a maximum wave height of 13.5 m were measured at 50 m water depth. Eight freak wave events are observed during the study period. The ratio of the maximum wave height to significant wave height recorded is found to be higher than the theoretical value and the ratio of the crest height to wave height during the cyclone was 0.6 to 0.7. The characteristics of the wave spectra before and after the cyclone is studied and found that the high-frequency face of the wave spectrum is proportional to f−3 before the cyclone and is between f−4 and f−5 during the cyclone period.
- Effect of plasma density on diffusion rates due to wave particle
interactions with chorus and plasmaspheric hiss: extreme event analysis
Abstract: Effect of plasma density on diffusion rates due to wave particle interactions with chorus and plasmaspheric hiss: extreme event analysis
Annales Geophysicae, 32, 1059-1071, 2014
Author(s): A. Sicard-Piet, D. Boscher, R. B. Horne, N. P. Meredith, and V. Maget
Wave particle interactions play an important role in controlling the dynamics of the radiation belts. The purpose of this study is to estimate how variations in the plasma density can affect diffusion rates resulting from interactions between chorus waves and plasmaspheric hiss with energetic particles and the resulting evolution of the energetic electron population. We perform a statistical analysis of the electron density derived from the plasma wave experiment on the CRRES satellite for two magnetic local time sectors corresponding to near midnight and near noon. We present the cumulative probability distribution of the electron plasma density for three levels of magnetic activity as measured by Kp. The largest densities are seen near L* = 2.5 while the smallest occur near L* = 6. The broadest distribution, corresponding to the greatest variability, occurs near L* = 4. We calculate diffusion coefficients for plasmaspheric hiss and whistler mode chorus for extreme values of the electron density and estimate the effects on the radiation belts using the Salammbô model. At L* = 4 and L* = 6, in the low density case, using the density from the 5th percentile of the cumulative distribution function, electron energy diffusion by chorus waves is strongest at 2 MeV and increases the flux by up to 3 orders of magnitude over a period of 24 h. In contrast, in the high density case, using the density from the 95th percentile, there is little acceleration at energies above 800 keV at L* = 6, and virtually no acceleration at L* = 4. In this case the strongest energy diffusion occurs at lower energies around 400 keV where the flux at L* = 6 increases 3 orders of magnitude.
- O+ and H+ ion heat fluxes at
high altitudes and high latitudes
Abstract: O+ and H+ ion heat fluxes at high altitudes and high latitudes
Annales Geophysicae, 32, 1043-1057, 2014
Author(s): I. A. Barghouthi, H. Nilsson, and S. H. Ghithan
Higher order moments, e.g., perpendicular and parallel heat fluxes, are related to non-Maxwellian plasma distributions. Such distributions are common when the plasma environment is not collision dominated. In the polar wind and auroral regions, the ion outflow is collisionless at altitudes above about 1.2 RE geocentric. In these regions wave–particle interaction is the primary acceleration mechanism of outflowing ionospheric origin ions. We present the altitude profiles of actual and "thermalized" heat fluxes for major ion species in the collisionless region by using the Barghouthi model. By comparing the actual and "thermalized" heat fluxes, we can see whether the heat flux corresponds to a small perturbation of an approximately bi-Maxwellian distribution (actual heat flux is small compared to "thermalized" heat flux), or whether it represents a significant deviation (actual heat flux equal or larger than "thermalized" heat flux). The model takes into account ion heating due to wave–particle interactions as well as the effects of gravity, ambipolar electric field, and divergence of geomagnetic field lines. In the discussion of the ion heat fluxes, we find that (1) the role of the ions located in the energetic tail of the ion velocity distribution function is very significant and has to be taken into consideration when modeling the ion heat flux at high altitudes and high latitudes; (2) at times the parallel and perpendicular heat fluxes have different signs at the same altitude. This indicates that the parallel and perpendicular parts of the ion energy are being transported in opposite directions. This behavior is the result of many competing processes; (3) we identify altitude regions where the actual heat flux is small as compared to the "thermalized" heat flux. In such regions we expect transport equation solutions based on perturbations of bi-Maxwellian distributions to be applicable. This is true for large altitude intervals for protons, but only the lowest altitudes for oxygen.
- Difference between even- and odd-numbered cycles in the predictability of
solar activity and prediction of the amplitude of cycle 25
Abstract: Difference between even- and odd-numbered cycles in the predictability of solar activity and prediction of the amplitude of cycle 25
Annales Geophysicae, 32, 1035-1042, 2014
Author(s): A. Yoshida
It was shown previously that the sunspot number (SSN) at a point 3 years before the minimum is well correlated with the maximum SSN of the succeeding cycle, and a better correlation is obtained when the maximum SSN is replaced by the average SSN over a cycle for which the average SSN is calculated by dividing cycles at a point 3 years before the minimum (Yoshida and Yamagishi, 2010; Yoshida and Sayre, 2012). Following these findings, we demonstrate in this paper that the correlation between the SSN 3 years before the minimum and the amplitude of the coming cycle differs significantly between even-numbered and odd-numbered cycles: the correlation is much better for even-numbered cycles. Further, it is shown that the amplitude of even-numbered cycles is strongly correlated with that of the succeeding odd-numbered cycles, while the correlation between amplitudes of odd-numbered cycles and those of succeeding even-numbered cycles is very poor. Using the excellent correlations, we estimate the maximum SSN of the current cycle 24 at 81.3 and predict the maximum SSN of cycle 25 to be 115.4 ± 11.9. It is of note, however, that a peak of the SSN has been observed in February 2012 and the peak value 66.9 is considerably smaller than the estimated maximum SSN of cycle 24. We conjecture that the second higher peak of the SSN may appear.
- Plasma wave mediated attractive potentials: a prerequisite for electron
Abstract: Plasma wave mediated attractive potentials: a prerequisite for electron compound formation
Annales Geophysicae, 32, 975-989, 2014
Author(s): R. A. Treumann and W. Baumjohann
Coagulation of electrons to form macro-electrons or compounds in high temperature plasma is not generally expected to occur. Here we investigate, based on earlier work, the possibility for such electron compound formation (non-quantum "pairing") mediated in the presence of various kinds of plasma waves via the generation of attractive electrostatic potentials, the necessary condition for coagulation. We confirm the possibility of production of attractive potential forces in ion- and electron-acoustic waves, pointing out the importance of the former and expected consequences. While electron-acoustic waves presumably do not play any role, ion-acoustic waves may potentially contribute to formation of heavy electron compounds. Lower-hybrid waves also mediate compound formation but under different conditions. Buneman modes which evolve from strong currents may also potentially cause non-quantum "pairing" among cavity-/hole-trapped electrons constituting a heavy electron component that populates electron holes. The number densities are, however, expected to be very small and thus not viable for justification of macro-particles. All these processes are found to potentially generate cold compound populations. If such electron compounds are produced by the attractive forces, the forces provide a mechanism of cooling a small group of resonant electrons, loosely spoken, corresponding to classical condensation.
- Ion dynamics in electron beam–plasma interaction: particle-in-cell
Abstract: Ion dynamics in electron beam–plasma interaction: particle-in-cell simulations
Annales Geophysicae, 32, 1025-1033, 2014
Author(s): K. Baumgärtel
Electron beam–plasma interaction including ions is studied by particle-in-cell (PIC) simulations using a one-dimensional, electrostatic code. Evidence for Langmuir wave decay is given for sufficiently energetic beams, as in previous Vlasov–Maxwell simulations. The mechanism for the generation of localized finite-amplitude ion density fluctuations is analyzed. Amplitude modulation due to interference between the beam-generated Langmuir waves causes random wave localization including strong transient spikes in field intensity which create bursty ion density structures via ponderomotive forces. More dense beams may quench the decay instability and generate low-frequency variations dominated by the wave number of the fastest growing Langmuir mode.
- Waves in high-speed plasmoids in the magnetosheath and at the magnetopause
Abstract: Waves in high-speed plasmoids in the magnetosheath and at the magnetopause
Annales Geophysicae, 32, 991-1009, 2014
Author(s): H. Gunell, G. Stenberg Wieser, M. Mella, R. Maggiolo, H. Nilsson, F. Darrouzet, M. Hamrin, T. Karlsson, N. Brenning, J. De Keyser, M. André, and I. Dandouras
Plasmoids, defined here as plasma entities with a higher anti-sunward velocity component than the surrounding plasma, have been observed in the magnetosheath in recent years. During the month of March 2007 the Cluster spacecraft crossed the magnetopause near the subsolar point 13 times. Plasmoids with larger velocities than the surrounding magnetosheath were found on seven of these 13 occasions. The plasmoids approach the magnetopause and interact with it. Both whistler mode waves and waves in the lower hybrid frequency range appear in these plasmoids, and the energy density of the waves inside the plasmoids is higher than the average wave energy density in the magnetosheath. When the spacecraft are in the magnetosphere, Alfvénic waves are observed. Cold ions of ionospheric origin are seen in connection with these waves, when the wave electric and magnetic fields combine with the Earth's dc magnetic field to yield an E × B/B2 drift speed that is large enough to give the ions energies above the detection threshold.
- Pre-onset auroral signatures and subsequent development of substorm
auroras: a development of ionospheric loop currents at the onset latitudes
Abstract: Pre-onset auroral signatures and subsequent development of substorm auroras: a development of ionospheric loop currents at the onset latitudes
Annales Geophysicae, 32, 1011-1023, 2014
Author(s): O. Saka, K. Hayashi, and M. Thomsen
Substorm auroras observed on 17 January 1994 were localized within the field of view of an all-sky imager installed at Dawson City (DWS, 65.7° ILAT). In association with the enhancement of the anti-sunward convection in the polar cap and the ion flux enhancement in 1–6 keV at geosynchronous altitudes, a wave-like structure propagating equatorward to the onset latitudes with a high wave number in azimuth (m ~ 76, T ~ 120 s) was observed 30 min prior to the activation in the equatorward latitudes. The activation of the auroras in the equatorward latitudes and the subsequent poleward expansion lasted for approximately 6 min until a diffuse aurora formed. The auroras in the last 6 min were isolated and localized within the field of view of DWS, from 400 km west to 400 km east, and accompanied the magnetic pulse at the optical station. The magnetic pulse is interpreted by the propagating ionospheric current loop with a size comparable to the isolated auroras (~ 1000 km). We conclude that the wave-like structures in the pre-onset interval relate to the intrusion of the plasma-sheet plasmas from the tail by the convection. The plasmas from the tail eventually developed the ionospheric loop currents at the onset latitudes, in association with the triggering of the bead-like rippling of auroras and subsequent breaking out from the onset latitudes.
- The occurrence altitudes of middle atmospheric temperature inversions and
mesopause over low-latitude Indian sector
Abstract: The occurrence altitudes of middle atmospheric temperature inversions and mesopause over low-latitude Indian sector
Annales Geophysicae, 32, 967-974, 2014
Author(s): M. Sivakandan, D. Kapasi, and A. Taori
We study the occurrence characteristics of mesospheric inversion layers (MILs) in the 60–105 km altitude region over the low-latitude Indian sector. We note that lower inversions in the mesospheric temperatures occur in the 70–75 km altitude regions while the upper inversions occur in 90–95 km altitude regions. The mesopause altitude is mostly noted to be ~ 98 km with the night-time mesopause (particularly in the year 2002) showing a small peak in the mesopause occurrence at ~ 75 km altitude. We note higher occurrence rate of MILs during high solar activity year compared to low solar activity year. It is also observed that night time MILs show a systematic seasonal variability, with higher occurrence of single and double temperature inversions during equinoxes.
- Topside signature of medium-scale traveling ionospheric disturbances
Abstract: Topside signature of medium-scale traveling ionospheric disturbances
Annales Geophysicae, 32, 959-965, 2014
Author(s): E. S. Miller, H. Kil, J. J. Makela, R. A. Heelis, E. R. Talaat, and A. Gross
Plasma blobs, localized plasma density enhancements that occur singularly or in periodic groups, have been observed by in situ sensors in the lower- and middle-latitude nighttime ionosphere. Traditionally, creation of blobs has been thought to be connected to equatorial plasma bubbles, which are localized plasma depletions. Here, we report the association of blobs with medium-scale traveling ionospheric disturbances (MSTIDs). On 17 January 2010, an all-sky imager on the Caribbean island of Bonaire (geographic: 12.190° N, 68.244° W; geomagnetic 22.46° N, 7.099° E) observed a nighttime electrified MSTID propagating to the southwest. At the time of the MSTID's transit, the Coupled Ion-Neutral Dynamics Investigation instrument onboard the Communication/Navigation Outage Forecasting System satellite detected a group of blobs along the same geomagnetic flux tubes. The electron density variations measured at the satellite altitude, indicating the blobs, are anticorrelated with the intensity variations of the 630.0 nm dissociative recombination emission measured on the same magnetic field lines. This relationship is explained by a modulation of the O+ profile altitude due to electric fields generated within the MSTID. This idea is supported by in situ measurements of the vertical ion velocity. We argue that common climatology between blobs and MSTIDs reported in the literature, as well as this coincident observation, suggest that blobs may be the in situ signature of MSTIDs in the topside ionosphere.