Annales Geophysicae (ANGEO)
[SJR: 1.176] [H-I: 63] [4 followers] Follow
Open Access journal
ISSN (Print) 0992-7689 - ISSN (Online) 1432-0576
Published by European Geosciences Union [8 journals]
- Multi-year observations of gravity wave momentum fluxes at low and middle
latitudes inferred by all-sky meteor radar
Abstract: Multi-year observations of gravity wave momentum fluxes at low and middle latitudes inferred by all-sky meteor radar
Annales Geophysicae, 33, 1183-1193, 2015
Author(s): V. F. Andrioli, P. P. Batista, B. R. Clemesha, N. J. Schuch, and R. A. Buriti
We have applied a modified composite day analysis to the Hocking (2005) technique to study gravity wave (GW) momentum fluxes in the mesosphere and lower thermosphere (MLT). Wind measurements from almost continuous meteor radar observations during June 2004–December 2008 over São João do Cariri (Cariri; 7° S, 36° W), April 1999–November 2008 over Cachoeira Paulista (CP; 23° S, 45° W), and February 2005–December 2009 over Santa Maria (SM; 30° S, 54° W) were used to estimate the GW momentum fluxes and variances in the MLT region. Our analysis can provide monthly mean altitude profiles of vertical fluxes of horizontal momentum for short-period (less than 2–3 h) GWs. The averages for each month throughout the entire data series have shown different behavior for the momentum fluxes depending on latitude and component. The meridional component has almost the same behavior at the three sites, being positive (northward), for most part of the year. On the other hand, the zonal component shows different behavior at each location: it is positive for almost half the year at Cariri and SM but predominantly negative over CP. Annual variation in the GW momentum fluxes is present at all sites in the zonal component and also in SM at 89 km in the meridional component. The seasonal analysis has also shown a 4-month oscillation at 92.5 km over SM in the zonal component and over CP at the same altitudes but for the meridional component.
- The latitudinal structure of the nightside outer magnetosphere of Saturn
as revealed by velocity moments of thermal ions
Abstract: The latitudinal structure of the nightside outer magnetosphere of Saturn as revealed by velocity moments of thermal ions
Annales Geophysicae, 33, 1195-1202, 2015
Author(s): Z. Nemeth, K. Szego, L. Foldy, M. G. Kivelson, X. Jia, K. M. Ramer, S. W. H. Cowley, G. Provan, and M. Thomsen
In this study we investigate the latitudinal behavior of the azimuthal plasma velocities in the outer magnetosphere of Saturn using the numerical ion moments derived from the measurements of the Cassini Plasma Spectrometer. One of the new results presented is that although these moments display some scatter, a significant positive correlation is found to exist between the azimuthal velocity and the plasma density, such that on average, the higher the density the higher the rotation speed. We also found that both the azimuthal velocity and the density anticorrelate with the magnitude of the radial component of the magnetic field and drop rapidly with increasing distance from the magnetic equator. The azimuthal velocities show periodic behavior with a period near the planetary rotation period, which can also be explained by the strong dependence on magnetic latitude, taking into account the flapping of the magnetodisk. It is thus found that the dense plasma near the magnetic equator rotates around the planet at high speed, while the dilute plasma at higher latitudes in the northern and southern hemispheres rotates significantly slower. The latitudinal gradient observed in the azimuthal speed is suggested to be a direct consequence of the sub-corotation of the plasma in the outer magnetosphere, with highest speeds occurring on field lines at lowest latitudes mapping to the rapidly rotating inner regions of the plasma sheet, and the speed falling as one approaches the lobe, where the field lines are connected to strongly sub-corotating plasma.
- Combined effects of concurrent Pc5 and chorus waves on relativistic
Abstract: Combined effects of concurrent Pc5 and chorus waves on relativistic electron dynamics
Annales Geophysicae, 33, 1173-1181, 2015
Author(s): C. Katsavrias, I. A. Daglis, W. Li, S. Dimitrakoudis, M. Georgiou, D. L. Turner, and C. Papadimitriou
We present electron phase space density (PSD) calculations as well as concurrent Pc5 and chorus wave activity observations during two intense geomagnetic storms caused by interplanetary coronal mass ejections (ICMEs) resulting in contradicting net effect. We show that, during the 17 March 2013 storm, the coincident observation of chorus and relativistic electron enhancements suggests that the prolonged chorus wave activity seems to be responsible for the enhancement of the electron population in the outer radiation belt even in the presence of pronounced outward diffusion. On the other hand, the significant depletion of electrons, during the 12 September 2014 storm, coincides with long-lasting outward diffusion driven by the continuous enhanced Pc5 activity since chorus wave activity was limited both in space and time.
- Convective gravity wave propagation and breaking in the stratosphere:
comparison between WRF model simulations and lidar data
Abstract: Convective gravity wave propagation and breaking in the stratosphere: comparison between WRF model simulations and lidar data
Annales Geophysicae, 33, 1155-1171, 2015
Author(s): L. Costantino, P. Heinrich, N. Mzé, and A. Hauchecorne
In this work we perform numerical simulations of convective gravity waves (GWs), using the WRF (Weather Research and Forecasting) model. We first run an idealized, simplified and highly resolved simulation with model top at 80 km. Below 60 km of altitude, a vertical grid spacing smaller than 1 km is supposed to reliably resolve the effects of GW breaking. An eastward linear wind shear interacts with the GW field generated by a single convective thunderstorm. After 70 min of integration time, averaging within a radius of 300 km from the storm centre, results show that wave breaking in the upper stratosphere is largely dominated by saturation effects, driving an average drag force up to −41 m s−1 day−1. In the lower stratosphere, mean wave drag is positive and equal to 4.4 m s−1 day−1.
In a second step, realistic WRF simulations are compared with lidar measurements from the NDACC network (Network for the Detection of Atmospheric Composition Changes) of gravity wave potential energy (Ep) over OHP (Haute-Provence Observatory, southern France). Using a vertical grid spacing smaller than 1 km below 50 km of altitude, WRF seems to reliably reproduce the effect of GW dynamics and capture qualitative aspects of wave momentum and energy propagation and transfer to background mean flow. Averaging within a radius of 120 km from the storm centre, the resulting drag force for the study case (2 h storm) is negative in the higher (−1 m s−1 day−1) and positive in the lower stratosphere (0.23 m s−1 day−1).
Vertical structures of simulated potential energy profiles are found to be in good agreement with those measured by lidar. Ep is mostly conserved with altitude in August while, in October, Ep decreases in the upper stratosphere to grow again in the lower mesosphere. On the other hand, the magnitude of simulated wave energy is clearly underestimated with respect to lidar data by about 3–4 times.
- In situ evidence of breaking the ion frozen-in condition via the
non-gyrotropic pressure effect in magnetic reconnection
Abstract: In situ evidence of breaking the ion frozen-in condition via the non-gyrotropic pressure effect in magnetic reconnection
Annales Geophysicae, 33, 1147-1153, 2015
Author(s): L. Dai, C. Wang, V. Angelopoulos, and K.-H. Glassmeier
For magnetic reconnection to proceed, the frozen-in condition for both ion fluid and electron fluid in a localized diffusion region must be violated by inertial effects, thermal pressure effects, or inter-species collisions. It has been unclear which underlying effects unfreeze ion fluid in the diffusion region. By analyzing in situ THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft measurements at the dayside magnetopause, we present clear evidence that the off-diagonal components of the ion pressure tensor is mainly responsible for breaking the ion frozen-in condition in reconnection. The off-diagonal pressure tensor, which corresponds to a non-gyrotropic pressure effect in this event, is a fluid manifestation of ion demagnetization in the diffusion region. From the perspective of the ion momentum equation, the reported non-gyrotropic ion pressure tensor is a fundamental aspect in specifying the reconnection electric field that controls how quickly reconnection proceeds.
- Comparison of total column ozone obtained by the IASI-MetOp satellite with
ground-based and OMI satellite observations in the southern tropics and
Abstract: Comparison of total column ozone obtained by the IASI-MetOp satellite with ground-based and OMI satellite observations in the southern tropics and subtropics
Annales Geophysicae, 33, 1135-1146, 2015
Author(s): A. M. Toihir, H. Bencherif, V. Sivakumar, L. El Amraoui, T. Portafaix, and N. Mbatha
This paper presents comparison results of the total column ozone (TCO) data product over 13 southern tropical and subtropical sites recorded from the Infrared Atmospheric Sounder Interferometer (IASI) onboard the EUMETSAT (European organization for the exploitation of METeorological SATellite) MetOp (Meteorological Operational satellite program) satellite. TCO monthly averages obtained from IASI between June 2008 and December 2012 are compared with collocated TCO measurements from the Ozone Monitoring Instrument (OMI) on the OMI/Aura satellite and the Dobson and SAOZ (Système d'Analyse par Observation Zénithale) ground-based instruments. The results show that IASI displays a positive bias with an average less than 2 % with respect to OMI and Dobson observations, but exhibits a negative bias compared to SAOZ over Bauru with a bias around 2.63 %. There is a good agreement between IASI and the other instruments, especially from 15° S southward where a correlation coefficient higher than 0.87 is found. IASI exhibits a seasonal dependence, with an upward trend in autumn and a downward trend during spring, especially before September 2010. After September 2010, the autumn seasonal bias is considerably reduced due to changes made to the retrieval algorithm of the IASI level 2 (L2) product. The L2 product released after August (L2 O3 version 5 (v5)) matches TCO from the other instruments better compared to version 4 (v4), which was released between June 2008 and August 2010. IASI bias error recorded from September 2010 is estimated to be at 1.5 % with respect to OMI and less than ±1 % with respect to the other ground-based instruments. Thus, the improvement made by O3 L2 version 5 (v5) product compared with version 4 (v4), allows IASI TCO products to be used with confidence to study the distribution and interannual variability of total ozone in the southern tropics and subtropics.
- Polar cap patches observed during the magnetic storm of November 2003:
observations and modeling
Abstract: Polar cap patches observed during the magnetic storm of November 2003: observations and modeling
Annales Geophysicae, 33, 1117-1133, 2015
Author(s): C. E. Valladares, T. Pedersen, and R. Sheehan
We present multi-instrumented measurements and multi-technique analysis of polar cap patches observed early during the recovery phase of the major magnetic storm of 20 November 2003 to investigate the origin of the polar cap patches. During this event, the Qaanaaq imager observed elongated polar cap patches, some of which containing variable brightness; the Qaanaaq digisonde detected abrupt NmF2 fluctuations; the Sondrestrom incoherent scatter radar (ISR) measured patches placed close to but poleward of the auroral oval–polar cap boundary; and the DMSP-F13 satellite intersected topside density enhancements, corroborating the presence of the patches seen by the imager, the digisonde, and the Sondrestrom ISR. A 2-D cross-correlation analysis was applied to series of two consecutive red-line images, indicating that the magnitude and direction of the patch velocities were in good agreement with the SuperDARN convection patterns. We applied a back-tracing analysis to the patch locations and found that most of the patches seen between 20:41 and 21:29 UT were likely transiting the throat region near 19:41 UT. Inspection of the SuperDARN velocities at this time indicates spatial and temporal collocation of a gap region between patches and large (1.7 km s−1) line-of-sight velocities. The variable airglow brightness of the patches observed between 20:33 and 20:43 UT was investigated using the numerical Global Theoretical Ionospheric Model (GTIM) driven by the SuperDARN convection patterns and a variable upward/downward neutral wind. Our numerical results indicate that variations in the airglow intensity up to 265 R can be produced by a constant 70 m s−1 downward vertical wind.
- Simulation of the Indian summer monsoon onset-phase rainfall using a
Abstract: Simulation of the Indian summer monsoon onset-phase rainfall using a regional model
Annales Geophysicae, 33, 1097-1115, 2015
Author(s): C. V. Srinivas, D. Hari Prasad, D. V. Bhaskar Rao, R. Baskaran, and B. Venkatraman
This study examines the ability of the Advanced Research WRF (ARW) regional model to simulate Indian summer monsoon (ISM) rainfall climatology in different climate zones during the monsoon onset phase in the decade 2000–2009. The initial and boundary conditions for ARW are provided from the NCEP/NCAR Reanalysis Project (NNRP) global reanalysis. Seasonal onset-phase rainfall is compared with corresponding values from 0.25° IMD (India Meteorological Department) rainfall and NNRP precipitation data over seven climate zones (perhumid, humid, dry/moist, subhumid, dry/moist, semiarid and arid) of India to see whether dynamical downscaling using a regional model yields advantages over just using large-scale model predictions. Results show that the model could simulate the onset phase in terms of progression and distribution of rainfall in most zones (except over the northeast) with good correlations and low error metrics. The observed mean onset dates and their variability over different zones are well reproduced by the regional model over most climate zones. It has been found that the ARW performed similarly to the reanalysis in most zones and improves the onset time by 1 to 3 days in zones 4 and 7, in which the NNRP shows a delayed onset compared to the actual IMD onset times. The variations in the onset-phase rainfall during the below-normal onset (June negative) and above-normal onset (June positive) phases are well simulated. The slight underestimation of onset-phase rainfall in the northeast zone could be due to failure in resolving the wide extent of topographic variations and the associated multiscale interactions in that zone. Spatial comparisons showed improvement of pentad rainfall in both space and quantity in ARW simulations over NNRP data, as evident from a wider eastward distribution of pentad rainfall over the Western Ghats, central and eastern India, as in IMD observations. While NNRP under-represented the high pentad rainfall over northeast, east and west coast areas, the ARW captured these regional features showing improvement upon NNRP reanalysis, which may be due to the high resolution (30 km) employed. The onset-phase rainfall characteristics during the contrasting ISM of 2003 and 2009 are well simulated in terms of the variations in the strength of low-level jet (LLJ) and outgoing long-wave radiation (OLR).
- First experimental verification of summertime mesospheric momentum balance
based on radar wind measurements at 69° N
Abstract: First experimental verification of summertime mesospheric momentum balance based on radar wind measurements at 69° N
Annales Geophysicae, 33, 1091-1096, 2015
Author(s): M. Placke, P. Hoffmann, and M. Rapp
Gravity waves (GWs) greatly influence the background state of the middle atmosphere by imposing their momentum on the mean flow upon breaking and by thus driving, e.g., the upper mesospheric summer zonal wind reversal. In this situation momentum is conserved by a balance between the vertical divergence of GW momentum flux (the so-called GW drag) and the Coriolis acceleration of the mean meridional wind. In this study, we present first quantitative mean annual cycles of these two balancing quantities from the medium frequency Doppler radar at the polar site Saura (SMF radar, 69° N, 16° E). Three-year means for 2009 through 2011 clearly show that the observed zonal momentum balance between 70 and 100 km with contributions from GWs only is fulfilled during summer when GW activity is strongest and more stable than in winter. During winter, the balance between GW drag and Coriolis acceleration of the mean meridional wind is not existent, which is likely due to the additional contribution from planetary waves, which are not considered by the present investigation. The differences in the momentum balance between summer and winter conditions are additionally clarified by 3-month mean vertical profiles for summer 2010 and winter 2010/2011.
- Tropical upper tropospheric humidity variations due to potential vorticity
Abstract: Tropical upper tropospheric humidity variations due to potential vorticity intrusions
Annales Geophysicae, 33, 1081-1089, 2015
Author(s): M. Sandhya, S. Sridharan, and M. Indira Devi
Four cases (March 2009, May 2009, April 2010 and February 2012) are presented in which the ERA-interim relative humidity (RH) shows consistent increase by more than 50 % in the upper troposphere (200–250 hPa) over tropics at the eastward side of the potential vorticity (PV) intrusion region. The increase in RH is confirmed with the spaceborne microwave limb sounder observations and radiosonde observations over Gadanki (13.5° N, 79.2° E) and is observed irrespective of whether the PV intrusions are accompanied by deep convection or not. It is demonstrated that the increase in RH is due to poleward advection induced by the PV intrusions in their eastward side at the upper tropospheric heights. It is suggested that the low-latitude convection, which is not necessarily triggered by the PV intrusion, might have transported water vapour to the upper tropospheric heights.
- Tomography of the ionospheric electron density with geostatistical
Abstract: Tomography of the ionospheric electron density with geostatistical inversion
Annales Geophysicae, 33, 1071-1079, 2015
Author(s): D. Minkwitz, K. G. van den Boogaart, T. Gerzen, and M. Hoque
In relation to satellite applications like global navigation satellite systems (GNSS) and remote sensing, the electron density distribution of the ionosphere has significant influence on trans-ionospheric radio signal propagation. In this paper, we develop a novel ionospheric tomography approach providing the estimation of the electron density's spatial covariance and based on a best linear unbiased estimator of the 3-D electron density. Therefore a non-stationary and anisotropic covariance model is set up and its parameters are determined within a maximum-likelihood approach incorporating GNSS total electron content measurements and the NeQuick model as background. As a first assessment this 3-D simple kriging approach is applied to a part of Europe. We illustrate the estimated covariance model revealing the different correlation lengths in latitude and longitude direction and its non-stationarity. Furthermore, we show promising improvements of the reconstructed electron densities compared to the background model through the validation of the ionosondes Rome, Italy (RO041), and Dourbes, Belgium (DB049), with electron density profiles for 1 day.
- Energy–latitude dispersion patterns near the isotropy boundaries of
Abstract: Energy–latitude dispersion patterns near the isotropy boundaries of energetic protons
Annales Geophysicae, 33, 1059-1070, 2015
Author(s): V. A. Sergeev, S. A. Chernyaeva, S. V. Apatenkov, N. Y. Ganushkina, and S. V. Dubyagin
Non-adiabatic motion of plasma sheet protons causes pitch-angle scattering and isotropic precipitation to the ionosphere, which forms the proton auroral oval. This mechanism related to current sheet scattering (CSS) provides a specific energy–latitude dispersion pattern near the equatorward boundary of proton isotropic precipitation (isotropy boundary, IB), with precipitation sharply decreasing at higher (lower) latitude for protons with lower (higher) energy. However, this boundary maps to the inner magnetosphere, where wave-induced scattering may provide different dispersion patterns as recently demonstrated by Liang et al. (2014). Motivated by the potential usage of the IBs for the magnetotail monitoring as well as by the need to better understand the mechanisms forming the proton IB, we investigate statistically the details of particle flux patterns near the proton IB using NOAA-POES polar spacecraft observations made during September 2009. By comparing precipitated-to-trapped flux ratio (J0/J90) at >30 and >80 keV proton energies, we found a relatively small number of simple CSS-type dispersion events (only 31 %). The clear reversed (wave-induced) dispersion patterns were very rare (5 %). The most frequent pattern had nearly coinciding IBs at two energies (63 %). The structured precipitation with multiple IBs was very frequent (60 %), that is, with two or more significant J0/J90 dropouts. The average latitudinal width of multiple IB structures was about 1°. Investigation of dozens of paired auroral zone crossings of POES satellites showed that the IB pattern is stable on a timescale of less than 2 min (a few proton bounce periods) but can evolve on a longer (several minutes) scale, suggesting temporal changes in some mesoscale structures in the equatorial magnetosphere.
We discuss the possible role of CSS-related and wave-induced mechanisms and their possible coupling to interpret the emerging complicated patterns of proton isotropy boundaries.
- Association of the pre-monsoon thermal field over north India and the
western Tibetan Plateau with summer monsoon rainfall over India
Abstract: Association of the pre-monsoon thermal field over north India and the western Tibetan Plateau with summer monsoon rainfall over India
Annales Geophysicae, 33, 1051-1058, 2015
Author(s): S. D. Bansod, S. Fadnavis, and S. P. Ghanekar
In this paper, interannual variability of tropospheric air temperatures over the Asian summer monsoon region during the pre-monsoon months is examined in relation to Indian summer monsoon rainfall (ISMR; June to September total rainfall). For this purpose, monthly grid-point temperatures in the entire troposphere over the Asian summer monsoon region and ISMR data for the period 1949–2012 have been used. Spatial correlation patterns are investigated between the temperature field in the lower tropospheric levels during May over the Asian summer monsoon region and ISMR. The results indicate a strong and significant northwest–southeast dipole structure in the spatial correlations over the Indian region, with highly significant positive (negative) correlations over the regions of north India and the western Tibetan Plateau region – region R1 (north Bay of Bengal: region R2). The observed dipole is seen significantly up to a level of 850 hPa and eventually disappears at 700 hPa. Thermal indices evaluated at 850 hPa level, based on average air temperatures over the north India and western Tibetan Plateau region (TI1) and the north Bay of Bengal region (TI2) during May, show a strong, significant relationship with the ISMR. The results are found to be consistent and robust, especially in the case of TI1 during the period of analysis. A physical mechanism for the relationship between these indices and ISMR is proposed. Finally the composite annual cycle of tropospheric air temperature over R1 during flood/drought years of ISMR is examined. The study brings out the importance of the TI1 in the prediction of flood/drought conditions over the Indian subcontinent.
- Magnetohydrodynamic modeling of three Van Allen Probes storms in 2012 and
Abstract: Magnetohydrodynamic modeling of three Van Allen Probes storms in 2012 and 2013
Annales Geophysicae, 33, 1037-1050, 2015
Author(s): J. Paral, M. K. Hudson, B. T. Kress, M. J. Wiltberger, J. R. Wygant, and H. J. Singer
Coronal mass ejection (CME)-shock compression of the dayside magnetopause has been observed to cause both prompt enhancement of radiation belt electron flux due to inward radial transport of electrons conserving their first adiabatic invariant and prompt losses which at times entirely eliminate the outer zone. Recent numerical studies suggest that enhanced ultra-low frequency (ULF) wave activity is necessary to explain electron losses deeper inside the magnetosphere than magnetopause incursion following CME-shock arrival. A combination of radial transport and magnetopause shadowing can account for losses observed at radial distances into L = 4.5, well within the computed magnetopause location. We compare ULF wave power from the Electric Field and Waves (EFW) electric field instrument on the Van Allen Probes for the 8 October 2013 storm with ULF wave power simulated using the Lyon–Fedder–Mobarry (LFM) global magnetohydrodynamic (MHD) magnetospheric simulation code coupled to the Rice Convection Model (RCM). Two other storms with strong magnetopause compression, 8–9 October 2012 and 17–18 March 2013, are also examined. We show that the global MHD model captures the azimuthal magnetosonic impulse propagation speed and amplitude observed by the Van Allen Probes which is responsible for prompt acceleration at MeV energies reported for the 8 October 2013 storm. The simulation also captures the ULF wave power in the azimuthal component of the electric field, responsible for acceleration and radial transport of electrons, at frequencies comparable to the electron drift period. This electric field impulse has been shown to explain observations in related studies (Foster et al., 2015) of electron acceleration and drift phase bunching by the Energetic Particle, Composition, and Thermal Plasma Suite (ECT) instrument on the Van Allen Probes.
- Observation of a new type of low-frequency waves at comet
Abstract: Observation of a new type of low-frequency waves at comet 67P/Churyumov-Gerasimenko
Annales Geophysicae, 33, 1031-1036, 2015
Author(s): I. Richter, C. Koenders, H.-U. Auster, D. Frühauff, C. Götz, P. Heinisch, C. Perschke, U. Motschmann, B. Stoll, K. Altwegg, J. Burch, C. Carr, E. Cupido, A. Eriksson, P. Henri, R. Goldstein, J.-P. Lebreton, P. Mokashi, Z. Nemeth, H. Nilsson, M. Rubin, K. Szegö, B. T. Tsurutani, C. Vallat, M. Volwerk, and K.-H. Glassmeier
We report on magnetic field measurements made in the innermost coma of 67P/Churyumov-Gerasimenko in its low-activity state. Quasi-coherent, large-amplitude (δ B/B ~ 1), compressional magnetic field oscillations at ~ 40 mHz dominate the immediate plasma environment of the nucleus. This differs from previously studied cometary interaction regions where waves at the cometary ion gyro-frequencies are the main feature. Thus classical pickup-ion-driven instabilities are unable to explain the observations. We propose a cross-field current instability associated with newborn cometary ion currents as a possible source mechanism.
- Numerical study of upper hybrid to Z-mode leakage
during electromagnetic pumping of groups of striations in the ionosphere
Abstract: Numerical study of upper hybrid to Z-mode leakage during electromagnetic pumping of groups of striations in the ionosphere
Annales Geophysicae, 33, 1019-1030, 2015
Author(s): B. Eliasson and T. B. Leyser
We investigate numerically the interaction between ionospheric magnetic field-aligned density striations and a left-hand circularly polarized (L)-mode wave. The L-mode wave is scattered into upper hybrid (UH) waves which are partially trapped in the striations, but leak energy to electromagnetic waves in the Z-mode branch. For small-amplitude (1 %) striations, this loss mechanism leads to a significant reduction in amplitude of the UH waves. For several striations organized in a lattice, the leaking of Z-mode waves is compensated by influx of Z-mode radiation from neighboring striations, leading to an increased amplitude of the weakly trapped UH waves. For large-amplitude (10 %) striations the trapped UH waves rapidly increase in amplitude far beyond the threshold for parametric instabilities, and the Z-mode leakage is less important. The results have relevance for the growth of striations and the onset of UH and lower hybrid turbulence during electromagnetic high-frequency pumping of ionospheric plasma, which require large-amplitude UH waves.
- Profile of a low-Mach-number shock in two-fluid plasma theory
Abstract: Profile of a low-Mach-number shock in two-fluid plasma theory
Annales Geophysicae, 33, 1011-1017, 2015
Author(s): M. Gedalin, Y. Kushinsky, and M. Balikhin
Magnetic profiles of low-Mach-number collisionless shocks in space plasmas are studied within the two-fluid plasma theory. Particular attention is given to the upstream magnetic oscillations generated at the ramp. By including weak resistive dissipation in the equations of motion for electrons and protons, the dependence of the upstream wave train features on the ratio of the dispersion length to the dissipative length is established quantitatively. The dependence of the oscillation amplitude and spatial damping scale on the shock normal angle θ is found.
- Nonlinear wave interactions of kinetic sound waves
Abstract: Nonlinear wave interactions of kinetic sound waves
Annales Geophysicae, 33, 1007-1010, 2015
Author(s): G. Brodin and L. Stenflo
We reconsider the nonlinear resonant interaction between three electrostatic waves in a magnetized plasma. The general coupling coefficients derived from kinetic theory are reduced here to the low-frequency limit. The main contribution to the coupling coefficient we find in this way agrees with the coefficient recently presented in Annales Geophysicae. But we also deduce another contribution which sometimes can be important, and which qualitatively agrees with that of an even more recent paper. We have thus demonstrated how results derived from fluid theory can be improved and generalized by means of kinetic theory. Possible extensions of our results are outlined.
- Ionospheric variations over Indian low latitudes close to the equator and
comparison with IRI-2012
Abstract: Ionospheric variations over Indian low latitudes close to the equator and comparison with IRI-2012
Annales Geophysicae, 33, 997-1006, 2015
Author(s): P. Pavan Chaitanya, A. K. Patra, N. Balan, and S. V. B. Rao
In this paper, we analyze daytime observations of the critical frequencies of the F2 (foF2) and F3 (foF3) layers based on ionosonde observations made from Indian low latitudes close to the magnetic equator and study their local time, seasonal, planetary-scale variations (including the solar rotation effect), and solar activity dependence. Given the occurrence of the F3 layer, which has remarkable local time, seasonal and solar activity dependences, variations in foF2 have been evaluated. Local time variations in foF2 and foF3 show noon "bite-out" in all seasons and in all solar activity conditions, which are attributed to vertically upward plasma transport by the zonal electric field and meridional neutral wind. Comparison of observed foF2 with those of the IRI-2012 model clearly shows that the model values are always higher than observed values and the largest difference is observed during noontime owing to the noon bite-out phenomenon. Peak frequency of the F layer (foF2 / foF3), however, is found to have better agreement with IRI-2012 model. Seasonal variations of foF2 and foF3 show stronger asymmetry at the solstices than at the equinoxes. The strong asymmetry at the solstice is attributed to the asymmetry in the meridional neutral wind with a secondary contribution from E × B drifts, and the relatively weak asymmetry observed at the equinox is attributed to the asymmetry in E × B drifts. Variations in foF2 and foF3 with solar flux clearly show the saturation effect when F10.7 exceeds ~ 120 sfu, which is different from that of the mid-latitudes. Irrespective of solar flux, both foF2 and foF3 in summer, however, are found to be remarkably lower than those observed in other seasons. Variations in foF2 show dominant periods of ~ 27, ~ 16 and ~ 6 days. Intriguingly, amplitudes of ~ 27-day variations in foF2 are found to be maximum in low solar activity (LSA), moderate in medium solar activity (MSA) and minimum in high solar activity (HSA), while the amplitudes of ~ 27-day variations in F10.7 are minimum in LSA, moderate in MSA and maximum in HSA. These results are presented and discussed in light of current observational and model-based knowledge on the variations of low-latitude foF2 and foF3.
- Strange VLF bursts in northern Scandinavia: case study of the afternoon
"mushroom-like" hiss on 8 December 2013
Abstract: Strange VLF bursts in northern Scandinavia: case study of the afternoon "mushroom-like" hiss on 8 December 2013
Annales Geophysicae, 33, 991-995, 2015
Author(s): J. Manninen, N. G. Kleimenova, A. Kozlovsky, I. A. Kornilov, L. I. Gromova, Y. V. Fedorenko, and T. Turunen
We investigate a non-typical very low frequency (VLF) 1–4 kHz hiss representing a sequence of separated noise bursts with a strange "mushroom-like" shape in the frequency–time domain, each one lasting several minutes. These strange afternoon VLF emissions were recorded at Kannuslehto (KAN, ϕ = 67.74° N, λ = 26.27° E; L ∼ 5.5) in northern Finland during the late recovery phase of the small magnetic storm on 8 December 2013. The left-hand (LH) polarized 2–3 kHz "mushroom caps" were clearly separated from the right-hand (RH) polarized "mushroom stems" at the frequency of about 1.8–1.9 kHz, which could match the lower ionosphere waveguide cutoff (the first transverse resonance of the Earth–ionosphere cavity). We hypothesize that this VLF burst sequence could be a result of the modulation of the VLF hiss electron–cyclotron instability from the strong Pc5 geomagnetic pulsations observed simultaneously at ground-based stations as well as in the inner magnetosphere by the Time History of Events and Macroscale Interactions during Substorms mission probe (THEMIS-E; ThE). This assumption is confirmed by a similar modulation of the intensity of the energetic (1–10 keV) electrons simultaneously observed by the same ThE spacecraft. In addition, the data of the European Incoherent Scatter Scientific Association (EISCAT) radar at Tromsø show a similar quasi-periodicity in the ratio of the Hall-to-Pedersen conductance, which may be used as a proxy for the energetic particle precipitation enhancement. Our findings suggest that this strange mushroom-like shape of the considered VLF hiss could be a combined mutual effect of the magnetospheric ULF–VLF (ultra low frequency–very low frequency) wave interaction and the ionosphere waveguide propagation.