ALBERT

Description: Stimulated Brillouin scattering during electron gyro-harmonic heating at EISCAT Annales Geophysicae, 33, 983-990, 2015 Author(s): H. Y. Fu, W. A. Scales, P. A. Bernhardt, S. J. Briczinski, M. J. Kosch, A. Senior, M. T. Rietveld, T. K. Yeoman, and J. M. Ruohoniemi Observations of secondary radiation, stimulated electromagnetic emission (SEE), produced during ionospheric modification experiments using ground-based, high-power, high-frequency (HF) radio waves are considered. The High Frequency Active Auroral Research Program (HAARP) facility is capable of generating narrowband SEE in the form of stimulated Brillouin scatter (SBS) and stimulated ion Bernstein scatter (SIBS) in the SEE spectrum. Such narrowband SEE spectral lines have not been reported using the European Incoherent Scatter (EISCAT) heater facility before. This work reports the first EISCAT results of narrowband SEE spectra and compares them to SEE previously observed at HAARP during electron gyro-harmonic heating. An analysis of experimental SEE data shows observations of emission lines within 100 Hz of the pump frequency, interpreted as SBS, during the 2012 July EISCAT campaign. Experimental results indicate that SBS strengthens as the pump frequency approaches the third electron gyro-harmonic. Also, for different heater antenna beam angles, the CUTLASS radar backscatter induced by HF radio pumping is suppressed near electron gyro-harmonics, whereas electron temperature enhancement weakens as measured by EISCAT/UHF radar. The main features of these new narrowband EISCAT observations are generally consistent with previous SBS measurements at HAARP.

Description: 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.

Description: A case study on generation mechanisms of a sporadic sodium layer above Tromsø (69.6° N) during a night of high auroral activity Annales Geophysicae, 33, 941-953, 2015 Author(s): T. Takahashi, S. Nozawa, T. T. Tsuda, Y. Ogawa, N. Saito, T. Hidemori, T. D. Kawahara, C. Hall, H. Fujiwara, N. Matuura, A. Brekke, M. Tsutsumi, S. Wada, T. Kawabata, S. Oyama, and R. Fujii We have quantitatively evaluated generation mechanisms of a sporadic sodium layer (SSL) based on observational data obtained by multiple instruments at a high-latitude station: Ramfjordmoen, Tromsø, Norway (69.6° N, 19.2° E). The sodium lidar observed an SSL at 21:18 UT on 22 January 2012. The SSL was observed for 18 min, with a maximum sodium density of about 1.9 × 10 10 m −3 at 93 km with a 1.1 km thickness. The European Incoherent Scatter (EISCAT) UHF radar observed a sporadic E layer (Es layer) above 90 km from 20:00 to 23:00 UT. After 20:00 UT, the Es layer gradually descended and reached 94 km at 21:18 UT when the SSL appeared at the same altitude. In this event, considering the abundance of sodium ions (10 % or less), the Es layer could provide only about 37 % or less of the sodium atoms to the SSL. We have investigated a temporal development of the normal sodium ion layer with a consideration of chemical reactions and the effect of the (southwestward) electric field using observational values of the neutral temperature, electron density, horizontal neutral wind, and electric field. This calculation has shown that those processes, including contributions of the Es layer, would provide about 88 % of sodium atoms of the SSL. The effects of meteor absorption and auroral particle sputtering appear to be less important. Therefore, we have concluded that the major source of the SSL was sodium ions in a normal sodium ion layer. Two processes – namely the downward transportation of sodium ions from a normal sodium ion layer due to the electric field and the additional supply of sodium ions from the Es layer under relatively high electron density conditions (i.e., in the Es layer) – played a major role in generating the SSL in this event. Furthermore, we have found that the SSL was located in a lower-temperature region and that the temperature inside the SSL did not show any remarkable temperature enhancements.

Description: 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.

Description: 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.

Description: 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 (TI 1 ) and the north Bay of Bengal region (TI 2 ) during May, show a strong, significant relationship with the ISMR. The results are found to be consistent and robust, especially in the case of TI 1 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 TI 1 in the prediction of flood/drought conditions over the Indian subcontinent.

Description: Statistical analysis of storm-time near-Earth current systems Annales Geophysicae, 33, 965-982, 2015 Author(s): M. W. Liemohn, R. M. Katus, and R. Ilie Currents from the Hot Electron and Ion Drift Integrator (HEIDI) inner magnetospheric model results for all of the 90 intense storms (disturbance storm-time (Dst) minimum 〈 −100 nT) from solar cycle 23 (1996–2005) are calculated, presented, and analyzed. We have categorized these currents into the various systems that exist in near-Earth space, specifically the eastward and westward symmetric ring current, the partial ring current, the banana current, and the tail current. The current results from each run set are combined by a normalized superposed epoch analysis technique that scales the timeline of each phase of each storm before summing the results. It is found that there is a systematic ordering to the current systems, with the asymmetric current systems peaking during storm main phase (tail current rising first, then the banana current, followed by the partial ring current) and the symmetric current systems peaking during the early recovery phase (westward and eastward symmetric ring current having simultaneous maxima). The median and mean peak amplitudes for the current systems ranged from 1 to 3 MA, depending on the setup configuration used in HEIDI, except for the eastward symmetric ring current, for which the mean never exceeded 0.3 MA for any HEIDI setup. The self-consistent electric field description in HEIDI yielded larger tail and banana currents than the Volland–Stern electric field, while the partial and symmetric ring currents had similar peak values between the two applied electric field models.

Description: 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.

Description: 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 ( fo F2) and F3 ( fo F3) 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 fo F2 have been evaluated. Local time variations in fo F2 and fo F3 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 fo F2 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 ( fo F2 / fo F3), however, is found to have better agreement with IRI-2012 model. Seasonal variations of fo F2 and fo F3 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 fo F2 and fo F3 with solar flux clearly show the saturation effect when F 10.7 exceeds ~ 120 sfu, which is different from that of the mid-latitudes. Irrespective of solar flux, both fo F2 and fo F3 in summer, however, are found to be remarkably lower than those observed in other seasons. Variations in fo F2 show dominant periods of ~ 27, ~ 16 and ~ 6 days. Intriguingly, amplitudes of ~ 27-day variations in fo F2 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 F 10.7 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 fo F2 and fo F3.

Description: 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.

Description: 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 N m F 2 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.

Description: 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 O 3 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 O 3 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.

Description: 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.

Description: 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.

Description: A comparative study of GPS ionospheric scintillations and ionogram spread F over Sanya Annales Geophysicae, 33, 1421-1430, 2015 Author(s): Y. Zhang, W. Wan, G. Li, L. Liu, L. Hu, and B. Ning We analyze the data recorded during December 2011–November 2012 by a digital ionosonde and a GPS (Global Positioning System) scintillation and (total electron content) TEC receiver collocated at Sanya (109.6° E, 18.3° N; dip lat. 12.8° N), a low-latitude station in the Chinese longitude sector, to carry out a comparative study of ionospheric scintillations and spread F. A good consistency between the temporal variations of GPS scintillation (represented by the S4 index) and of ionogram spread F (represented by the QF index) is found in the pre-midnight period during equinox. However in the post-midnight period during equinox and in the period from post-sunset to pre-sunrise during June solstice, moderate spread F is seen without concurrent GPS scintillation. The possible cause responsible for the difference between post-midnight GPS scintillation and spread F during equinox could be due to the decaying of 400 m scale irregularities associated with equatorial spread F. Regarding the irregularities producing moderate QF and low S4 indices during June solstice, we suggest that the frequently observed sporadic E (Es) layer and the medium-scale traveling ionospheric disturbances (MSTIDs) over Sanya could play important roles in triggering the June solstitial spread-F events.

Description: Association of radiation belt electron enhancements with earthward penetration of Pc5 ULF waves: a case study of intense 2001 magnetic storms Annales Geophysicae, 33, 1431-1442, 2015 Author(s): M. Georgiou, I. A. Daglis, E. Zesta, G. Balasis, I. R. Mann, C. Katsavrias, and K. Tsinganos Geospace magnetic storms, driven by the solar wind, are associated with increases or decreases in the fluxes of relativistic electrons in the outer radiation belt. We examine the response of relativistic electrons to four intense magnetic storms, during which the minimum of the Dst index ranged from −105 to −387 nT, and compare these with concurrent observations of ultra-low-frequency (ULF) waves from the trans-Scandinavian IMAGE magnetometer network and stations from multiple magnetometer arrays available through the worldwide SuperMAG collaboration. The latitudinal and global distribution of Pc5 wave power is examined to determine how deep into the magnetosphere these waves penetrate. We then investigate the role of Pc5 wave activity deep in the magnetosphere in enhancements of radiation belt electrons population observed in the recovery phase of the magnetic storms. We show that, during magnetic storms characterized by increased post-storm electron fluxes as compared to their pre-storm values, the earthward shift of peak and inner boundary of the outer electron radiation belt follows the Pc5 wave activity, reaching L shells as low as 3–4. In contrast, the one magnetic storm characterized by irreversible loss of electrons was related to limited Pc5 wave activity that was not intensified at low L shells. These observations demonstrate that enhanced Pc5 ULF wave activity penetrating deep into the magnetosphere during the main and recovery phase of magnetic storms can, for the cases examined, distinguish storms that resulted in increases in relativistic electron fluxes in the outer radiation belts from those that did not.

Description: A case study of formation and maintenance of a lower stratospheric cirrus cloud over the tropics Annales Geophysicae, 33, 599-608, 2015 Author(s): M. Sandhya, S. Sridharan, M. Indira Devi, K. Niranjan, and A. Jayaraman A rare occurrence of stratospheric cirrus at 18.6 km height persisting for about 5 days during 3–7 March 2014 is inferred from the ground-based Mie lidar observations over Gadanki (13.5° N, 79.2° E) and spaceborne observations. Due to the vertical transport by large updrafts on 3 March in the troposphere, triggered by a potential vorticity intrusion, the water vapour mixing ratio shows an increase around the height of 18.6 km. Relative humidity with respect to ice is ~ 150%, indicating that the cirrus cloud may be formed though homogeneous nucleation of sulfuric acid. The cirrus cloud persists due to the cold anomaly associated with the presence of a 4-day wave.

Description: Field-aligned chorus wave spectral power in Earth's outer radiation belt Annales Geophysicae, 33, 583-597, 2015 Author(s): H. Breuillard, O. Agapitov, A. Artemyev, E. A. Kronberg, S. E. Haaland, P. W. Daly, V. V. Krasnoselskikh, D. Boscher, S. Bourdarie, Y. Zaliznyak, and G. Rolland Chorus-type whistler waves are one of the most intense electromagnetic waves generated naturally in the magnetosphere. These waves have a substantial impact on the radiation belt dynamics as they are thought to contribute to electron acceleration and losses into the ionosphere through resonant wave–particle interaction. Our study is devoted to the determination of chorus wave power distribution on frequency in a wide range of magnetic latitudes, from 0 to 40°. We use 10 years of magnetic and electric field wave power measured by STAFF-SA onboard Cluster spacecraft to model the initial (equatorial) chorus wave spectral power, as well as PEACE and RAPID measurements to model the properties of energetic electrons (~ 0.1–100 keV) in the outer radiation belt. The dependence of this distribution upon latitude obtained from Cluster STAFF-SA is then consistently reproduced along a certain L -shell range (4 ≤ L ≤ 6.5), employing WHAMP-based ray tracing simulations in hot plasma within a realistic inner magnetospheric model. We show here that, as latitude increases, the chorus peak frequency is globally shifted towards lower frequencies. Making use of our simulations, the peak frequency variations can be explained mostly in terms of wave damping and amplification, but also cross- L propagation. These results are in good agreement with previous studies of chorus wave spectral extent using data from different spacecraft (Cluster, POLAR and THEMIS). The chorus peak frequency variations are then employed to calculate the pitch angle and energy diffusion rates, resulting in more effective pitch angle electron scattering (electron lifetime is halved) but less effective acceleration. These peak frequency parameters can thus be used to improve the accuracy of diffusion coefficient calculations.

Description: High-speed solar wind streams and polar mesosphere winter echoes at Troll, Antarctica Annales Geophysicae, 33, 609-622, 2015 Author(s): S. Kirkwood, A. Osepian, E. Belova, and Y.-S. Lee A small, 54 MHz wind-profiler radar, MARA, was operated at Troll, Antarctica (72° S, 2.5° E), continuously from November 2011 to January 2014, covering two complete Antarctic winters. Despite very low power, MARA observed echoes from heights of 55–80 km (polar mesosphere winter echoes, PMWE) on 60% of all winter days (from March to October). This contrasts with previous reports from radars at high northern latitudes, where PWME have been reported only by very high power radars or during rare periods of unusually high electron density at PMWE heights, such as during solar proton events. Analysis shows that PWME at Troll were not related to solar proton events but were often closely related to the arrival of high-speed solar wind streams (HSS) at the Earth, with PWME appearing at heights as low as 56 km and persisting for up to 15 days following HSS arrival. This demonstrates that HSS effects penetrate directly to below 60 km height in the polar atmosphere. Using local observations of cosmic-noise absorption (CNA), a theoretical ionization/ion-chemistry model and a statistical model of precipitating energetic electrons associated with HSS, the electron density conditions during the HSS events are estimated. We find that PMWE detectability cannot be explained by these variations in electron density and molecular-ion chemistry alone. PWME become detectable at different thresholds depending on solar illumination and height. In darkness, PWME are detected only when the modelled electron density is above a threshold of about 1000 cm −3 , and only above 75 km height, where negative ions are few. In daylight, the electron density threshold falls by at least 2 orders of magnitude and PWME are found primarily below 75 km height, even in conditions when a large proportion of negative ions is expected. There is also a strong dawn–dusk asymmetry with PWME detected very rarely during morning twilight but often during evening twilight. This behaviour cannot be explained if PMWE are caused by small-scale structure in the neutral/molecular-ion gas alone but may be explained by the presence of charged meteoric dust.

Description: Investigation of energy transport and thermospheric upwelling during quiet magnetospheric and ionospheric conditions from the studies of low- and middle-altitude cusp Annales Geophysicae, 33, 623-635, 2015 Author(s): T. Živković, S. Buchert, P. Ritter, L. Palin, and H. Opgenoorth We investigate energy fluxes and small, kilometre-scale Birkeland currents in the magnetospheric cusp at a 1–3 Earth radii altitude and in the ionosphere using satellites when they were, according to the Tsyganenko model, in magnetic conjunction within 50–60 km and up to 15 min apart. We use Cluster and CHAMP satellites, and study three conjunction events that occurred in 2008 and 2009, when the Cluster spacecraft were crossing the cusps at only a few Earth radii altitude. Our goal is to understand better the influence of processes in the magnetospheric cusp on the upper thermosphere and its upwelling which was usually observed by the CHAMP satellite passing the cusp. Three studied events occurred under relatively quiet and steady magnetospheric and ionospheric conditions, which explains why observed thermospheric density enhancements were rather low. Our findings point out that for each studied event soft electron precipitation influences thermospheric density enhancements in a way that stronger electron precipitation produces stronger thermospheric upwelling. Therefore, in the case of these weak events, soft electron precipitation seems to be more important cause of the observed, thermospheric density enhancements than is the Joule heating.

Description: Latitude dependence of long-term geomagnetic activity and its solar wind drivers Annales Geophysicae, 33, 573-581, 2015 Author(s): M. Myllys, N. Partamies, and L. Juusola To validate the usage of global indices in studies of geomagnetic activity, we have examined the latitude dependence of geomagnetic variations in Fennoscandia and Svalbard from 1994 to 2010. Daily standard deviation (SD) values of the horizontal magnetic field have been used as a measure of the ground magnetic disturbance level. We found that the timing of the geomagnetic minimum depends on the latitude region: corresponding to the minimum of sunspot cycle 22 (in 1996), the geomagnetic minimum occurred between the geomagnetic latitudes 57–61° in 1996 and at the latitudes 64–67° in 1997, which are the average auroral oval latitudes. During sunspot cycle 23, all latitude regions experienced the minimum in 2009, a year after the sunspot minimum. These timing differences are due to the latitude dependence of the 10 s daily SD on the different solar wind drivers. In the latitude region of 64–67°, the impact of the high-speed solar wind streams (HSSs) on the geomagnetic activity is the most pronounced compared to the other latitude groups, while in the latitude region of 57–61°, the importance of the coronal mass ejections (CMEs) dominates. The geomagnetic activity maxima during ascending solar cycle phases are typically caused by CME activity and occur especially in the oval and sub-auroral regions. The strongest geomagnetic activity occurs during the descending solar cycle phases due to a mixture of CME and HSS activity. Closer to the solar minimum, less severe geomagnetic activity is driven by HSSs and mainly visible in the poleward part of the auroral region. According to our study, however, the timing of the geomagnetic activity minima (and maxima) in different latitude bands is different, due to the relative importance of different solar wind drivers at different latitudes.

Description: Electron-scale nested quadrupole Hall field in Cluster observations of magnetic reconnection Annales Geophysicae, 33, 719-724, 2015 Author(s): N. Jain and A. S. Sharma This paper presents the first evidence of a new and unique feature of spontaneous reconnection at multiple sites in electron current sheet, viz. a "nested quadrupole" structure of the Hall field at electron scales, in Cluster observations. The new nested quadrupole is a consequence of electron-scale processes in reconnection. Whistler response of the upstream plasma to the interaction of electron flows from neighboring reconnection sites produces a large-scale quadrupole Hall field enclosing the quadrupole fields of the multiple sites, thus forming a nested structure. Electron-magnetohydrodynamic simulations of an electron current sheet yields a mechanism of the formation of a nested quadrupole.

Description: The MAGIC of CINEMA: first in-flight science results from a miniaturised anisotropic magnetoresistive magnetometer Annales Geophysicae, 33, 725-735, 2015 Author(s): M. O. Archer, T. S. Horbury, P. Brown, J. P. Eastwood, T. M. Oddy, B. J. Whiteside, and J. G. Sample We present the first in-flight results from a novel miniaturised anisotropic magnetoresistive space magnetometer, MAGIC (MAGnetometer from Imperial College), aboard the first CINEMA (CubeSat for Ions, Neutrals, Electrons and MAgnetic fields) spacecraft in low Earth orbit. An attitude-independent calibration technique is detailed using the International Geomagnetic Reference Field (IGRF), which is temperature dependent in the case of the outboard sensor. We show that the sensors accurately measure the expected absolute field to within 2% in attitude mode and 1% in science mode. Using a simple method we are able to estimate the spacecraft's attitude using the magnetometer only, thus characterising CINEMA's spin, precession and nutation. Finally, we show that the outboard sensor is capable of detecting transient physical signals with amplitudes of ~ 20–60 nT. These include field-aligned currents at the auroral oval, qualitatively similar to previous observations, which agree in location with measurements from the DMSP (Defense Meteorological Satellite Program) and POES (Polar-orbiting Operational Environmental Satellites) spacecraft. Thus, we demonstrate and discuss the potential science capabilities of the MAGIC instrument onboard a CubeSat platform.

Description: Optimization of Saturn paraboloid magnetospheric field model parameters using Cassini equatorial magnetic field data Elena S. Belenkaya, Vladimir V. Kalegaev, Stanley W. H. Cowley, Gabrielle Provan, Marina S. Blokhina, Oleg G. Barinov, Alexander A. Kirillov, and Maria S. Grigoryan Ann. Geophys., 34, 641-656, doi:10.5194/angeo-34-641-2016, 2016 The paraboloid model of Saturn’s magnetosphere describes the magnetic field of the planet, the ring current, magnetopause current, and the tail current. The model parameters are determined by comparison with the Cassini magnetic field data from 18 near-equatorial passes that span wide ranges of LT. The best-fit model parameters are employed to determine how the parameters vary with the subsolar distance of the magnetopause, governed by pressure balance at the magnetospheric boundary.

Description: Solar energetic particle interactions with the Venusian atmosphere Christina Plainaki, Pavlos Paschalis, Davide Grassi, Helen Mavromichalaki, and Maria Andriopoulou Ann. Geophys., 34, 595-608, doi:10.5194/angeo-34-595-2016, 2016 In the context of planetary space weather, we estimate the ion production rates in the Venusian atmosphere due to the interactions of solar energetic particles (SEPs) with gas. The assumed concept for our estimations is based on two cases of SEP events, previously observed in near-Earth space: the event in October 1989 and the event in May 2012. For both cases, we assume that the directional properties of the flux and the interplanetary magnetic field configuration would have allowed the SEPs' arrival at Venus and their penetration to the planet's atmosphere. For the event in May 2012, we consider the solar particle properties (integrated flux and rigidity spectrum) obtained by the Neutron Monitor Based Anisotropic GLE Pure Power Law (NMBANGLE PPOLA) model (Plainaki et al., 2010, 2014) applied previously for the Earth case and scaled to the distance of Venus from the Sun. For the simulation of the actual cascade in the Venusian atmosphere initiated by the incoming particle fluxes, we apply the DYASTIMA code, a Monte Carlo (MC) application based on the Geant4 software (Paschalis et al., 2014). Our predictions are afterwards compared to other estimations derived from previous studies and discussed. Finally, we discuss the differences between the nominal ionization profile due to galactic cosmic-ray–atmosphere interactions and the profile during periods of intense solar activity, and we show the importance of understanding space weather conditions on Venus in the context of future mission preparation and data interpretation.

Description: Two-point observations of low-frequency waves at 67P/Churyumov-Gerasimenko during the descent of PHILAE: comparison of RPCMAG and ROMAP Ingo Richter, Hans-Ulrich Auster, Gerhard Berghofer, Chris Carr, Emanuele Cupido, Karl-Heinz Fornaçon, Charlotte Goetz, Philip Heinisch, Christoph Koenders, Bernd Stoll, Bruce T. Tsurutani, Claire Vallat, Martin Volwerk, and Karl-Heinz Glassmeier Ann. Geophys., 34, 609-622, doi:10.5194/angeo-34-609-2016, 2016 We have analysed the magnetic field measurements performed on the ROSETTA orbiter and the lander PHILAE during PHILAE's descent to comet 67P/Churyumov-Gerasimenko on 12 November 2014. We observed a new type of low-frequency wave with amplitudes of ~ 3 nT, frequencies of 20–50 mHz, wavelengths of ~ 300 km, and propagation velocities of ~ 6 km s −1 . The waves are generated in a ~ 100 km region around the comet a show a highly correlated behaviour, which could only be determined by two-point observations.

Description: Quantitative assessment of AOD from 17 CMIP5 models based on satellite-derived AOD over India Amit Misra, Vijay P. Kanawade, and Sachchida Nand Tripathi Ann. Geophys., 34, 657-671, doi:10.5194/angeo-34-657-2016, 2016 For an accurate understanding of earth climate system, it is necessary to evaluate the performance of the climate models used to perform these simulations. In this work we have examined aerosol optical depths simulated by 17 models by comparing them with satellite-derived aerosol optical depth. Our results indicate the role of dust aerosols and biogeochemistry in the simulation of aerosols by models.

Description: Secondary electron emission from meteoric smoke particles inside the polar ionosphere Carsten Baumann, Markus Rapp, and Antti Kero Ann. Geophys., 34, 573-580, doi:10.5194/angeo-34-573-2016, 2016 Meteor smoke particles (MSPs), originating from evaporated meteoric matter at 60–110 km altitude, are present in the whole atmosphere including polar regions. As electron precipitation is present at high latitudes, these MSPs are bombarded by energetic electrons. The energetic electrons can enter the MSPs and excite secondary electrons. That can lead to a change of the charge state of these MSPs. The study finds that other charging processes, e.g., electron attachment, are more important.

Description: The dawn–dusk asymmetry of ion density in the dayside magnetosheath and its annual variability measured by THEMIS Andrew P. Dimmock, Tuija I. Pulkkinen, Adnane Osmane, and Katariina Nykyri Ann. Geophys., 34, 511-528, doi:10.5194/angeo-34-511-2016, 2016 Ion densities measured on the dawn-side magnetosheath flank were higher than the dusk-flank. The asymmetry was measured close to the magnetopause, but it became more ambiguous in the central magnetosheath. We show that the asymmetry was at its maximum at the 2009 solar minimum (~ 20 %), but then decreased in the rising stage of the next solar cycle (

Description: Properties of inertia-gravity waves in the lowermost stratosphere as observed by the PANSY radar over Syowa Station in the Antarctic Maria Mihalikova, Kaoru Sato, Masaki Tsutsumi, and Toru Sato Ann. Geophys., 34, 543-555, doi:10.5194/angeo-34-543-2016, 2016 Inertia-gravity waves (IGWs) are an important component for the dynamics of the middle atmosphere. However, observational studies needed to constrain their forcing are still insufficient especially in the remote areas of the Antarctic region. One year of observational data (January to December 2013) by the PANSY radar of the wind components (vertical resolution of 150 m and temporal resolution of 30 min) are used to derive statistical analysis of the properties of IGWs with short vertical wavelengths ( ≤ 4 km) and ground-based periods longer than 4 h in the lowermost stratosphere (height range 10 to 12 km) with the help of the hodograph method. The annual change of the IGWs parameters are inspected but no pronounced year cycle is found. The year is divided into two seasons (summer and winter) based on the most prominent difference in the ratio of Coriolis parameter ( f ) to intrinsic frequency ( ω ^ ) distribution. Average of f ∕ ω ^   for the winter season is 0.40 and for the summer season 0.45 and the average horizontal wavelengths are 140 and 160 km respectively. Vertical wavelengths have an average of 1.85 km through the year. For both seasons the properties of IGWs with upward and downward propagation of the energy are also derived and compared. The percentage of downward propagating waves is 10.7 and 18.4 % in the summer and winter season respectively. This seasonal change is more than the one previously reported in the studies from mid-latitudes and model-based studies. It is in agreement with the findings of past radiosonde data-based studies from the Antarctic region. In addition, using the so-called dual-beam technique, vertical momentum flux and the variance of the horizontal perturbation velocities of IGWs are examined. Tropospheric disturbances of synoptic-scale are suggested as a source of episodes of IGWs with large variance of horizontal perturbation velocities, and this is shown in a number of cases.

Description: A multi-platform investigation of midlatitude sporadic E and its ties to E – F coupling and meteor activity Joseph Helmboldt Ann. Geophys., 34, 529-541, doi:10.5194/angeo-34-529-2016, 2016 This paper details observed connections between dense, transient layers within the lower ionosphere (sporadic E ) and both meteor activity and irregularities higher in the ionosphere. The coupling between these layers and wavelike disturbances in the middle ionosphere seems to be shorted out when extremely dense “clouds” are present, which possible disrupt the stability of the layers.

Description: Extreme value analysis of the time derivative of the horizontal magnetic field and computed electric field Peter Wintoft, Ari Viljanen, and Magnus Wik Ann. Geophys., 34, 485-491, doi:10.5194/angeo-34-485-2016, 2016 Extreme value analysis has been applied to 1-minute-resolution magnetic fields and computed electric fields over Europe. We find that on average the largest disturbances of the fields are observed close to the auroral oval, as expected. However, the analysis indicates that as we move south from Scandinavia to northern continental Europe the distribution becomes more extreme. This could be due to that strong storms regularly occur at high latitudes, while the extreme storms push the oval south.

Description: Hydromagnetic waves in a compressed-dipole field via field-aligned Klein–Gordon equations Jinlei Zheng, Qiang Hu, Gary M. Webb, and James F. McKenzie Ann. Geophys., 34, 473-484, doi:10.5194/angeo-34-473-2016, 2016 This work provides an alternative approach to examine one commonly occurring type of electromagnetic waves in Earth's inner magnetosphere. In certain wave modes, the waves propagate along individual field lines of Earth's magnetic field. Governed by the coupled system between the magnetic field and ionized gas of low temperature, the waves show certain characteristics similar to standing waves along an elastic string. We present solutions in agreement with spacecraft observations.

Description: Critical pitch angle for electron acceleration in a collisionless shock layer Y. Narita, H. Comişel, and U. Motschmann Ann. Geophys., 34, 591-593, doi:10.5194/angeo-34-591-2016, 2016 Collisionless shock waves in space and astrophysical plasmas can accelerate electrons along the shock layer by an electrostatic potential, and scatter or reflect electrons back to the upstream region by the amplified magnetic field or turbulent fluctuations. The notion of the critical pitch angle is introduced for non-adiabatic electron acceleration by balancing the two timescales under a quasi-perpendicular shock wave geometry in which the upstream magnetic field is nearly perpendicular to the shock layer normal direction. An analytic expression of the critical pitch angle is obtained as a function of the electron velocity parallel to the magnetic field, the ratio of the electron gyro- to plasma frequency, the cross-shock potential, the width of the shock transition layer, and the shock angle (which is the angle between the upstream magnetic field and the shock normal direction). For typical non-relativistic solar system applications, the critical pitch angle is predicted to be about 10°. An efficient acceleration is expected below the critical pitch angle.

Description: Daytime twin-peak structures observed at southern African and European middle latitudes on 8–13 April 2012 Zama T. Katamzi, John Bosco Habarulema, and Nigussie M. Giday Ann. Geophys., 34, 581-590, doi:10.5194/angeo-34-581-2016, 2016 Daytime twin-peak structures, also known as bite-out or diurnal double-maxima structures, are ionospheric phenomena in which the diurnal ionospheric trend shows two peaks (instead of the normal one) during the daytime. This study reports on first simultaneous observations of these structures in the Global Positioning System and ionosonde measurements from the southern African and European middle-latitude stations during a mostly quiet geomagnetic condition period of 8–13 April 2012.

Description: Multi-year GNSS monitoring of atmospheric IWV over Central and South America for climate studies Clara Eugenia Bianchi, Luciano Pedro Oscar Mendoza, Laura Isabel Fernández, María Paula Natali, Amalia Margarita Meza, and Juan Francisco Moirano Ann. Geophys., 34, 623-639, doi:10.5194/angeo-34-623-2016, 2016 Long-term monitoring of the amount of water vapour in the lower atmosphere is essential for climate studies. We analysed satellite observations, at hundreds of locations in Central and South America, to look for changes in this parameter over several years. We found evidence of drying of the troposphere in temperate regions, and also evidence of slow moistening over the tropics. Moreover, we openly provide the complete data collection to the scientific community.

Description: Extremely intense (SML ≤–2500 nT) substorms: isolated events that are externally triggered? Annales Geophysicae, 33, 519-524, 2015 Author(s): B. T. Tsurutani, R. Hajra, E. Echer, and J. W. Gjerloev We examine particularly intense substorms (SML ≤–2500 nT), hereafter called "supersubstorms" or SSS events, to identify their nature and their magnetic storm dependences. It is found that these intense substorms are typically isolated events and are only loosely related to magnetic storms. SSS events can occur during super (Dst ≤–250 nT) and intense (−100 nT ≥ Dst 〉–250) magnetic storms. SSS events can also occur during nonstorm (Dst ≥–50 nT) intervals. SSSs are important because the strongest ionospheric currents will flow during these events, potentially causing power outages on Earth. Several SSS examples are shown. SSS events appear to be externally triggered by small regions of very high density (~30 to 50 cm −3 ) solar wind plasma parcels (PPs) impinging upon the magnetosphere. Precursor southward interplanetary magnetic fields are detected prior to the PPs hitting the magnetosphere. Our hypothesis is that these southward fields input energy into the magnetosphere/magnetotail and the PPs trigger the release of the stored energy.

Description: Direct observations of blob deformation during a substorm Annales Geophysicae, 33, 525-530, 2015 Author(s): T. Ishida, Y. Ogawa, A. Kadokura, K. Hosokawa, and Y. Otsuka Ionospheric blobs are localized plasma density enhancements, which are mainly produced by the transportation process of plasma. To understand the deformation process of a blob, observations of plasma parameters with good spatial–temporal resolution are desirable. Thus, we conducted the European Incoherent Scatter radar observations with high-speed meridional scans (60–80 s) during October and December 2013, and observed the temporal evolution of a blob during a substorm on 4 December 2013. This paper is the first report of direct observations of blob deformation during a substorm. The blob deformation arose from an enhanced plasma flow shear during the substorm expansion phase, and then the blob split into two smaller-scale blobs, whose scale sizes were more than ~100 km in latitude. Our analysis indicates that the Kelvin–Helmholtz instability and dissociative recombination could have deformed the blob structure.

Description: Climatology of GPS phase scintillation at northern high latitudes for the period from 2008 to 2013 Annales Geophysicae, 33, 531-545, 2015 Author(s): P. Prikryl, P. T. Jayachandran, R. Chadwick, and T. D. Kelly Global positioning system scintillation and total electron content (TEC) data have been collected by ten specialized GPS Ionospheric Scintillation and TEC Monitors (GISTMs) of the Canadian High Arctic Ionospheric Network (CHAIN). The phase scintillation index σ Φ is obtained from the phase of the L1 signal sampled at 50 Hz. Maps of phase scintillation occurrence as a function of the altitude-adjusted corrected geomagnetic (AACGM) latitude and magnetic local time (MLT) are computed for the period from 2008 to 2013. Enhanced phase scintillation is collocated with regions that are known as ionospheric signatures of the coupling between the solar wind and magnetosphere. The phase scintillation mainly occurs on the dayside in the cusp where ionospheric irregularities convect at high speed, in the nightside auroral oval where energetic particle precipitation causes field-aligned irregularities with steep electron density gradients and in the polar cap where electron density patches that are formed from a tongue of ionization. Dependences of scintillation occurrence on season, solar and geomagnetic activity, and the interplanetary magnetic field (IMF) orientation are investigated. The auroral phase scintillation shows semiannual variation with equinoctial maxima known to be associated with auroras, while in the cusp and polar cap the scintillation occurrence is highest in the autumn and winter months and lowest in summer. With rising solar and geomagnetic activity from the solar minimum to solar maximum, yearly maps of mean phase scintillation occurrence show gradual increase and expansion of enhanced scintillation regions both poleward and equatorward from the statistical auroral oval. The dependence of scintillation occurrence on the IMF orientation is dominated by increased scintillation in the cusp, expanded auroral oval and at subauroral latitudes for strongly southward IMF. In the polar cap, the IMF B Y polarity controls dawn–dusk asymmetries in scintillation occurrence collocated with a tongue of ionization for southward IMF and with sun-aligned arcs for northward IMF. In investigating the shape of scintillation-causing irregularities, the distributions of scintillation occurrence as a function of "off-meridian" and "off-shell" angles that are computed for the receiver–satellite ray at the ionospheric pierce point are found to suggest predominantly field-aligned irregularities in the auroral oval and L-shell-aligned irregularities in the cusp.

Description: Electrostatic double layers as auroral particle accelerators – a problem Annales Geophysicae, 33, 481-482, 2015 Author(s): D. A. Bryant and G. M. Courtier A search of the Annales Geophysicae database shows that double layers and other quasi-static electric potential structures have been invoked hundreds of times since the year 2000 as being the agents of auroral electron acceleration. This is despite the fact that energy transfer by conservative fields has been known for some 200 years to be impossible. Attention is drawn to a long-standing interpretation of the acceleration process in terms of the dynamic fields of electrostatic waves.

Description: Driving of the SAO by gravity waves as observed from satellite Annales Geophysicae, 33, 483-504, 2015 Author(s): M. Ern, P. Preusse, and M. Riese It is known that atmospheric dynamics in the tropical stratosphere have an influence on higher altitudes and latitudes as well as on surface weather and climate. In the tropics, the dynamics are governed by an interplay of the quasi-biennial oscillation (QBO) and semiannual oscillation (SAO) of the zonal wind. The QBO is dominant in the lower and middle stratosphere, and the SAO in the upper stratosphere/lower mesosphere. For both QBO and SAO the driving by atmospheric waves plays an important role. In particular, the role of gravity waves is still not well understood. In our study we use observations of the High Resolution Dynamics Limb Sounder (HIRDLS) satellite instrument to derive gravity wave momentum fluxes and gravity wave drag in order to investigate the interaction of gravity waves with the SAO. These observations are compared with the ERA-Interim reanalysis. Usually, QBO westward winds are much stronger than QBO eastward winds. Therefore, mainly gravity waves with westward-directed phase speeds are filtered out through critical-level filtering already below the stratopause region. Accordingly, HIRDLS observations show that gravity waves contribute to the SAO momentum budget mainly during eastward wind shear, and not much during westward wind shear. These findings confirm theoretical expectations and are qualitatively in good agreement with ERA-Interim and other modeling studies. In ERA-Interim most of the westward SAO driving is due to planetary waves, likely of extratropical origin. Still, we find in both observations and ERA-Interim that sometimes westward-propagating gravity waves may contribute to the westward driving of the SAO. Four characteristic cases of atmospheric background conditions are identified. The forcings of the SAO in these cases are discussed in detail, supported by gravity wave spectra observed by HIRDLS. In particular, we find that the gravity wave forcing of the SAO cannot be explained by critical-level filtering alone; gravity wave saturation without critical levels being reached is also important.

Description: Online NARMAX model for electron fluxes at GEO Annales Geophysicae, 33, 405-411, 2015 Author(s): R. J. Boynton, M. A. Balikhin, and S. A. Billings Multi-input single-output (MISO) nonlinear autoregressive moving average with exogenous inputs (NARMAX) models have been derived to forecast the 〉 0.8 MeV and 〉 2 MeV electron fluxes at geostationary Earth orbit (GEO). The NARMAX algorithm is able to identify mathematical model for a wide class of nonlinear systems from input–output data. The models employ solar wind parameters as inputs to provide an estimate of the average electron flux for the following day, i.e. the 1-day forecast. The identified models are shown to provide a reliable forecast for both 〉 0.8 and 〉 2 MeV electron fluxes and are capable of providing real-time warnings of when the electron fluxes will be dangerously high for satellite systems. These models, named SNB 3 GEO 〉 0.8 and 〉 2 MeV electron flux models, have been implemented online at http://www.ssg.group.shef.ac.uk/USSW/UOSSW.html .

Description: A quantitative study of magnetospheric magnetic field line deformation by a two-loop substorm current wedge Annales Geophysicae, 33, 505-517, 2015 Author(s): A. V. Nikolaev, V. A. Sergeev, N. A. Tsyganenko, M. V. Kubyshkina, H. Opgenoorth, H. Singer, and V. Angelopoulos Substorm current wedge (SCW) formation is associated with global magnetic field reconfiguration during substorm expansion. We combine a two-loop model SCW (SCW2L) with a background magnetic field model to investigate distortion of the ionospheric footpoint pattern in response to changes of different SCW2L parameters. The SCW-related plasma sheet footprint shift results in formation of a pattern resembling an auroral bulge, the poleward expansion of which is controlled primarily by the total current in the region 1 sense current loop ( I 1 ). The magnitude of the footprint latitudinal shift may reach ∼ 10° corrected geomagnetic latitude (CGLat) during strong substorms ( I 1 = 2 MA). A strong helical magnetic field around the field-aligned current generates a surge-like region with embedded spiral structures, associated with a westward traveling surge (WTS) at the western end of the SCW. The helical field may also contribute to rotation of the ionospheric projection of narrow plasma streams (auroral streamers). Other parameters, including the total current in the second (region 2 sense) loop, were found to be of secondary importance. Analyzing two consecutive dipolarizations on 17 March 2010, we used magnetic variation data obtained from a dense midlatitude ground network and several magnetospheric spacecraft, as well as the adaptive AM03 model, to specify SCW2L parameters, which allowed us to predict the magnitude of poleward auroral expansion. Auroral observations made during the two substorm activations demonstrate that the SCW2L combined with the AM03 model nicely describes the azimuthal progression and the observed magnitude of the auroral expansion. This finding indicates that the SCW-related distortions are responsible for much of the observed global development of bright auroras.

Description: Analysis of the enhanced negative correlation between electron density and electron temperature related to earthquakes Annales Geophysicae, 33, 471-479, 2015 Author(s): X. H. Shen, X. Zhang, J. Liu, S. F. Zhao, and G. P. Yuan Ionospheric perturbations in plasma parameters have been observed before large earthquakes, but the correlation between different parameters has been less studied in previous research. The present study is focused on the relationship between electron density ( N e ) and temperature ( T e ) observed by the DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) satellite during local nighttime, in which a positive correlation has been revealed near the equator and a weak correlation at mid- and low latitudes over both hemispheres. Based on this normal background analysis, the negative correlation with the lowest percent in all N e and T e points is studied before and after large earthquakes at mid- and low latitudes. The multiparameter observations exhibited typical synchronous disturbances before the Chile M8.8 earthquake in 2010 and the Pu'er M6.4 in 2007, and T e varied inversely with N e over the epicentral areas. Moreover, statistical analysis has been done by selecting the orbits at a distance of 1000 km and ±7 days before and after the global earthquakes. Enhanced negative correlation coefficients lower than −0.5 between N e and T e are found in 42% of points to be connected with earthquakes. The correlation median values at different seismic levels show a clear decrease with earthquakes larger than 7. Finally, the electric-field-coupling model is discussed; furthermore, a digital simulation has been carried out by SAMI2 (Sami2 is Another Model of the Ionosphere), which illustrates that the external electric field in the ionosphere can strengthen the negative correlation in N e and T e at a lower latitude relative to the disturbed source due to the effects of the geomagnetic field. Although seismic activity is not the only source to cause the inverse N e – T e variations, the present results demonstrate one possibly useful tool in seismo-electromagnetic anomaly differentiation, and a comprehensive analysis with multiple parameters helps to further understand the seismo–ionospheric coupling mechanism. \keywords{Ionosphere (plasma temperature and density)}

Description: Latitudinal variability of the quasi-16-day wave in the middle atmosphere over Brazilian stations Amitava Guharay, Paulo Prado Batista, Barclay Robert Clemesha, Ricardo Arlen Buriti, and Nelson Jorge Schuch Ann. Geophys., 34, 411-419, doi:10.5194/angeo-34-411-2016, 2016 A comparative study of the quasi-16-day wave in the middle from three Brazilian stations, indicates multiple modes of the concerned wave component. The wave amplitude shows maxima in summer and winter. A potential coupling of the concerned wave with other short period planetary waves is found. The dominant wave components vary from the westward to eastward from the tropical to mid-latitude in the stratosphere. The prevailing westerly wind may favor the wave filtering of westward waves.

Description: Solar wind reconstruction from magnetosheath data using an adjoint approach C. Nabert, C. Othmer, and K.-H. Glassmeier Ann. Geophys., 33, 1513-1524, doi:10.5194/angeo-33-1513-2015, 2015 The solar wind plasma interacts with a planetary magnetic field. A magnetohydrodynamic model is used to simulate the interaction and resulting plasma flow. The model uses solar wind inflow parameters as boundary condition. Spacecraft data of the interaction region are compared to the flow model. The solar wind boundary parameters are varied until the model matches the data. With a time-resolution of about 10min, the time-dependent solar wind boundary parameters were reconstructed from the data.

Description: Boltzmann electron PIC simulation of the E-sail effect P. Janhunen Ann. Geophys., 33, 1507-1512, doi:10.5194/angeo-33-1507-2015, 2015 The solar wind electric sail (E-sail) is a new way to move in the solar system efficiently without consuming propellant. The E-sail taps momentum from the solar wind by long, charged and centrifugally stretched tethers. Here we develop a new type of simulation for predicting E-sail thrust. The new simulation involves some approximations, but is robust and executes fast on a computer. The results are in good agreement with an earlier theoretical prediction. The E-sail thrust seems strong.

Description: Magnetospheric conditions near the equatorial footpoints of proton isotropy boundaries V. A. Sergeev, I. A. Chernyaev, V. Angelopoulos, and N. Y. Ganushkina Ann. Geophys., 33, 1485-1493, doi:10.5194/angeo-33-1485-2015, 2015 Adaptive magnetospheric models based on THEMIS magnetic observations made at 6-9Re in the nightside magnetosphere are used to map the magnetically conjugate 30 and 80keV proton isotropy boundaries (IBs) to investigate the value of Kib=Rc/rc (magnetic curvature radius to particle gyroradius) in the neutral sheet at the IB generation place. For the most accurate mapping, the group Kib spread spans from 4 to 32; its median value is ~13, slightly larger than Kib8 expected for current sheet scatter.

Description: Gravity wave transmission diagram Y. Tomikawa Ann. Geophys., 33, 1479-1484, doi:10.5194/angeo-33-1479-2015, 2015 A possibility of gravity wave propagation from a source region to the airglow layer around the mesopause has been discussed based on the gravity wave blocking diagram taking into account the critical level filtering alone. This paper proposes a new gravity wave transmission diagram in which both the critical level filtering and turning level reflection of gravity waves are considered. It shows a significantly different distribution of gravity wave transmissivity from the blocking diagram.

Description: A comparative study of GPS ionospheric scintillations and ionogram spread F over Sanya Y. Zhang, W. Wan, G. Li, L. Liu, L. Hu, and B. Ning Ann. Geophys., 33, 1421-1430, doi:10.5194/angeo-33-1421-2015, 2015 We analyze the data recorded during December 2011–November 2012 by a digital ionosonde and a GPS (Global Positioning System) scintillation and (total electron content) TEC receiver collocated at Sanya (109.6° E, 18.3° N; dip lat. 12.8° N), a low-latitude station in the Chinese longitude sector, to carry out a comparative study of ionospheric scintillations and spread F. A good consistency between the temporal variations of GPS scintillation (represented by the S4 index) and of ionogram spread F (represented by the QF index) is found in the pre-midnight period during equinox. However in the post-midnight period during equinox and in the period from post-sunset to pre-sunrise during June solstice, moderate spread F is seen without concurrent GPS scintillation. The possible cause responsible for the difference between post-midnight GPS scintillation and spread F during equinox could be due to the decaying of 400 m scale irregularities associated with equatorial spread F. Regarding the irregularities producing moderate QF and low S4 indices during June solstice, we suggest that the frequently observed sporadic E (Es) layer and the medium-scale traveling ionospheric disturbances (MSTIDs) over Sanya could play important roles in triggering the June solstitial spread-F events.

Description: A critical note on the IAGA-endorsed Polar Cap index procedure: effects of solar wind sector structure and reverse polar convection P. Stauning Ann. Geophys., 33, 1443-1455, doi:10.5194/angeo-33-1443-2015, 2015 Polar Cap (PC) index values are derived from polar magnetic variations and are important for the scaling of solar-terrestrial relations. The International Association of Geomagnetism and Aeronomy (IAGA) has recently endorsed a new PC index version to supersede the many previous index versions. However, the new PC index procedure has some adverse features which should be known and taken into account by users of the index. These features are discussed and alternative procedures are suggested.

Description: Statistical modelling of wildfire size and intensity: a step toward meteorological forecasting of summer extreme fire risk C. Hernandez, C. Keribin, P. Drobinski, and S. Turquety Ann. Geophys., 33, 1495-1506, doi:10.5194/angeo-33-1495-2015, 2015 In this article we investigate the use of statistical methods for wildfire risk assessment in the Mediterranean Basin using three meteorological covariates, the 2 m temperature anomaly, the 10 m wind speed and the January–June rainfall occurrence anomaly. We focus on two remotely sensed characteristic fire variables, the burnt area (BA) and the fire radiative power (FRP), which are good proxies for fire size and intensity respectively. Using the fire data we determine an adequate parametric distribution function which fits best the logarithm of BA and FRP. We reconstruct the conditional density function of both variables with respect to the chosen meteorological covariates. These conditional density functions for the size and intensity of a single event give information on fire risk and can be used for the estimation of conditional probabilities of exceeding certain thresholds. By analysing these probabilities we find two fire risk regimes different from each other at the 90 % confidence level: a "background" summer fire risk regime and an "extreme" additional fire risk regime, which corresponds to higher probability of occurrence of larger fire size or intensity associated with specific weather conditions. Such a statistical approach may be the ground for a future fire risk alert system.

Description: Comparisons of electron acceleration efficiency among different structures during magnetic reconnection: a Cluster multicase study M. Zhou, T. Li, X. Deng, S. Huang, and H. Li Ann. Geophys., 33, 1469-1478, doi:10.5194/angeo-33-1469-2015, 2015 Electron energization is an outstanding issue in magnetic reconnection, which is a hot topic in plasma physics and astrophysics. Four different structures have been proposed for electron energization during reconnection. We find that there is no specific sequence of electron energization through the four structures; there is no specific order of electron acceleration efficiency among the four structures, and electron energization occurs in a wide region around X-line.

Description: Superimposed disturbance in the ionosphere triggered by spacecraft launches in China L. M. He, L. X. Wu, S. J. Liu, and S. N. Liu Ann. Geophys., 33, 1361-1368, doi:10.5194/angeo-33-1361-2015, 2015 Using GPS dual-frequency observations collected by continuously operating GPS tracking stations in China, superimposed disturbances caused by the integrated action of spacecraft's physical effect and chemical effect on ionosphere during the launches of the spacecrafts Tiangong-1 and Shenzhou-8 in China were firstly determined. The results show that the superimposed disturbance was composed of remarkable ionospheric waves and significant ionospheric depletion emerged after both launches. Meanwhile, we found for the first time that the ionospheric waves were made up of two periods of wave by wavelet analysis. The first period of ∼ 4 min shows one event in the near stations and two sub-events in the few far stations. The second period of ∼ 9 min shows only one event in all the observed stations. Finally, the time characteristics for ionospheric waves and depletions were examined.

Description: Defining and resolving current systems in geospace N. Y. Ganushkina, M. W. Liemohn, S. Dubyagin, I. A. Daglis, I. Dandouras, D. L. De Zeeuw, Y. Ebihara, R. Ilie, R. Katus, M. Kubyshkina, S. E. Milan, S. Ohtani, N. Ostgaard, J. P. Reistad, P. Tenfjord, F. Toffoletto, S. Zaharia, and O. Amariutei Ann. Geophys., 33, 1369-1402, doi:10.5194/angeo-33-1369-2015, 2015 A number of current systems exist in the Earth's magnetosphere. It is very difficult to identify local measurements as belonging to a specific current system. Therefore, there are different definitions of supposedly the same current, leading to unnecessary controversy. This study presents a robust collection of these definitions of current systems in geospace, particularly in the near-Earth nightside magnetosphere, as viewed from a variety of observational and computational analysis techniques.

Description: A physical explanation for the magnetic decrease ahead of dipolarization fronts Z. H. Yao, J. Liu, C. J. Owen, C. Forsyth, I. J. Rae, Z. Y. Pu, H. S. Fu, X.-Z. Zhou, Q. Q. Shi, A. M. Du, R. L. Guo, and X. N. Chu Ann. Geophys., 33, 1301-1309, doi:10.5194/angeo-33-1301-2015, 2015 We use THEMIS large data set of dipolarization front events to build a 2-D pressure distribution in XZ plane, and thus derive the current system around the dipolarization front. Our results show that a banana current loop is formed around the dipolarization front. This current is also suggested to be the reason for the magnetic dip observed ahead of the dipolarization front. In addition, the current density is too small to contribute a substorm current wedge.

Description: Association of radiation belt electron enhancements with earthward penetration of Pc5 ULF waves: a case study of intense 2001 magnetic storms M. Georgiou, I. A. Daglis, E. Zesta, G. Balasis, I. R. Mann, C. Katsavrias, and K. Tsinganos Ann. Geophys., 33, 1431-1442, doi:10.5194/angeo-33-1431-2015, 2015 Our study demonstrates a remarkable association between the earthward penetration of ULF waves and radiation belt electron enhancements during four magnetic storms that occurred in 2001. In the past, ULF waves had been observed at unusual depths during rare superstorms. But ULF wave activity, reaching magnetic shells as low as 2, was also observed during relatively intense storms when it played a key role in diffusing electrons radially inward and thereby accelerating them to higher energies.

Description: Interhemispheric structure and variability of the 5-day planetary wave from meteor radar wind measurements H. Iimura, D. C. Fritts, D. Janches, W. Singer, and N. J. Mitchell Ann. Geophys., 33, 1349-1359, doi:10.5194/angeo-33-1349-2015, 2015 The quasi-5-day wave at mid- and high-latitudes in the mesosphere and lower-thermosphere was compared between the hemispheres using meteor radar horizontal wind measurements, spanning June 2010 to December 2012. Variances of the quasi-5-day wave showed a wave activity from July to August in both hemispheres and in April 2012 in the Northern Hemisphere and November 2012 in the Southern Hemisphere with unique characteristics at each site.

Description: Sub-ionospheric VLF signal anomaly due to geomagnetic storms: a statistical study K. Tatsuta, Y. Hobara, S. Pal, and M. Balikhin Ann. Geophys., 33, 1457-1467, doi:10.5194/angeo-33-1457-2015, 2015 We study the effect of geomagnetic storms on the sub-ionospheric VLF/LF propagations based on 2-year data from the network observations in Japan. Three statistical parameters such as average signal amplitude, variability of the signal amplitude, and nighttime fluctuation were calculated. These statistical parameters indicate significant anomalies during the geomagnetic storm in the high-latitude paths. While mid-latitude and low-mid latitude paths have smaller influence from the storm activity.

Description: Radar and satellite investigations of equatorial evening vertical drifts and spread F J. M. Smith, F. S. Rodrigues, and E. R. de Paula Ann. Geophys., 33, 1403-1412, doi:10.5194/angeo-33-1403-2015, 2015 We analyzed pre-midnight equatorial F region observations made by the 30 MHz coherent backscatter radar of São Luis, Brazil between August 2010 and February 2012. These measurements were processed, and used to create monthly maps of the echo occurrence as a function of local time and height. The maps show the inter-annual variability associated with equatorial spread F (ESF) occurrence in the Brazilian longitude sector. We also constructed monthly curves of the evening vertical drifts, for the Brazilian sector, using measurements by the ion velocity meter (IVM) onboard the C/NOFS satellite. The IVM evening drifts show a good overall agreement with the Scherliess and Fejer (1999) empirical model. Measured and model drifts show the development of the pre-reversal enhancement (PRE) of the vertical plasma drifts during ESF season. Using joint radar and satellite measurements, we found that evening (18:00–18:30 LT) mean non-negative drifts provide a necessary but not sufficient condition for the occurrence of topside ESF echoes. Evening downward (negative) drifts preceded the absence of topside ESF irregularities.

Description: Dust devil vortex generation from convective cells O. Onishchenko, O. Pokhotelov, W. Horton, and V. Fedun Ann. Geophys., 33, 1343-1347, doi:10.5194/angeo-33-1343-2015, 2015 We have developed a hydrodynamic theory of the nonlinear stage of dust devil generation in a convectively unstable atmosphere with large-scale seed vertical vorticity. It is shown that convective motion in such an atmosphere transforms into dust devils extremely fast. The strong vortical structure of the dust devils can be formed in a few minutes or even in a fraction of a minute. The formation process strongly depends on the convective instability growth rate and horizontal vorticity. The work is supported by the US Department of Energy under Contract DEFG02-04ER-54742.

Description: Non-elliptic wavevector anisotropy for magnetohydrodynamic turbulence Y. Narita Ann. Geophys., 33, 1413-1419, doi:10.5194/angeo-33-1413-2015, 2015 A lot of efforts have been put into understanding the turbulence structure in space and astrophysical plasmas, in particular how the filamentary structure develops as the length scale of the turbulent fluctuations changes from large to smaller ones. Motivated by the recent spacecraft observations in the solar wind, an analytic model is proposed to explain the nature of filament-formation processes in space plasma turbulence with a successful test against the spacecraft observations.

Description: Climatology of the ionospheric slab thickness along the longitude of 120° E in China and its adjacent region during the solar minimum years of 2007–2009 Z. Huang and H. Yuan Ann. Geophys., 33, 1311-1319, doi:10.5194/angeo-33-1311-2015, 2015 The climatology of ionospheric slab thickness is analysed using dual-frequency GPS and COSMIC measurements in China during the solar minimum. It is found that a pronounced peak of slab thickness occurs during the post-midnight period. A large diurnal ratio exists at the EIA, and a large night-to-day ratio occurs near the equatorial latitudes and mid- to high latitudes. The behaviours of the slab thickness and the mF2 are well correlated at the middle latitudes and during the daytime.

Description: Observational evidence of quasi-27-day oscillation propagating from the lower atmosphere to the mesosphere over 20° N K. M. Huang, A. Z. Liu, S. D. Zhang, F. Yi, C. M. Huang, Q. Gan, Y. Gong, Y. H. Zhang, and R. Wang Ann. Geophys., 33, 1321-1330, doi:10.5194/angeo-33-1321-2015, 2015 By using meteor radar, radiosonde and satellite observations over 20° N and NCEP/NCAR reanalysis data during 81 days from 22 December 2004 to 12 March 2005, a quasi-27-day oscillation propagating from the troposphere to the mesosphere is reported. A pronounced 27-day periodicity is observed in the raw zonal wind from meteor radar. Spectral analysis shows that the oscillation also occurs in the meridional wind and temperature and propagates westward with wavenumber s = 1; thus the oscillation is of Rossby wave type. The oscillation attains a large amplitude of about 12 m s −1 in the eastward wind shear region of the troposphere. When the wind shear reverses, its amplitude rapidly decays, and the background wind gradually evolves to be westward. However, the oscillation can penetrate through the weak westward wind field due to its relatively large phase speed. After this, the oscillation restrengthens with its upward propagation and reaches about 20 m s −1 in the mesosphere. Reanalysis data show that the oscillation can propagate to the mid and high latitudes from the low latitudes and has large amplitudes over there. There is another interesting phenomenon that a quasi-46-day oscillation appears simultaneously in the troposphere, but it cannot penetrate through the westward wind field because of its smaller phase speed. In the observational interval, a quasi-27-day periodicity in outgoing long-wave radiation (OLR) and specific humidity is found in a latitudinal zone of 5–20° N. Thus the quasi-27-day oscillation may be an atmospheric response to forcing due to the convective activity with a period of about 27 days in the tropical region.

Description: Measurement of momentum flux using two meteor radars in Indonesia Naoki Matsumoto, Atsuki Shinbori, Dennis M. Riggin, and Toshitaka Tsuda Ann. Geophys., 34, 369-377, doi:10.5194/angeo-34-369-2016, 2016 We investigated the zonal and meridional momentum flux at 86–94 km using two nearly identical sets of meteor radar observation data at Koto Tabang and Biak in Indonesia (both at the Equator) by applying a method proposed by Hocking (2005). The observed zonal momentum flux at the two sites agreed reasonably well at 86, 90, and 94 km during the observation periods when the data acquisition rate was large enough. Our results suggest the usefulness of the Hocking method.

Description: Spatial dimensions of the electron diffusion region in anti-parallel magnetic reconnection Takuma Nakamura, Rumi Nakamura, and Hiroshi Haseagwa Ann. Geophys., 34, 357-367, doi:10.5194/angeo-34-357-2016, 2016 Magnetic reconnection is a key process in space and laboratory plasmas which transfers energies through the magnetic field topology change. The topology change in this process takes place in a small scale region called the electron diffusion region (EDR). In this paper, using high-resolution fully kinetic simulations, we successfully obtained the firm scaling laws of spatial dimensions of the EDR. The obtained scalings allow us to precisely predict observable dimensions of the EDR in real space.

Description: Wave–particle resonance condition test for ion-kinetic waves in the solar wind Y. Narita, E. Marsch, C. Perschke, K.-H. Glassmeier, U. Motschmann, and H. Comişel Ann. Geophys., 34, 393-398, doi:10.5194/angeo-34-393-2016, 2016 Conditions for the Landau and cyclotron resonances are tested for 543 waves (identified as local peaks in the energy spectra) in the magnetic field fluctuations of the solar wind measured by the Cluster spacecraft on a tetrahedral scale of 100 km. The resonance parameters are evaluated using the frequencies in the plasma rest frame, the parallel components of the wavevectors, the ion cyclotron frequency, and the ion thermal speed. The observed waves show a character of the sideband waves associated with the ion Bernstein mode, and are in a weak agreement with the fundamental electron cyclotron resonance in spite of the ion-kinetic scales. The electron cyclotron resonance is likely taking place in solar wind turbulence near 1 AU (astronomical unit).

Description: 〈b〉Global sounding of 〈i〉F〈/i〉 region irregularities by COSMIC during a geomagnetic storm〈/b〉〈br〉 Klemens Hocke, Huixin Liu, Nicholas Pedatella, and Guanyi Ma〈br〉 Ann. Geophys. Discuss., https//doi.org/10.5194/angeo-2018-117,2018〈br〉 〈b〉Manuscript under review for ANGEO〈/b〉 (discussion: open, 0 comments)〈br〉 The GPS radio occultation data of the COSMIC-FORMOSAT-3 mission are used to visualize the global distribution of ionospheric irregularities in the F2 region during a geomagnetic storm, at solar minimum, and at solar maximum.

Description: 〈b〉On the radiation belt location in the 23–24 solar cycles〈/b〉〈br〉 Alexei V. Dmitriev〈br〉 Ann. Geophys. Discuss., https//doi.org/10.5194/angeo-2018-118,2018〈br〉 〈b〉Manuscript under review for ANGEO〈/b〉 (discussion: open, 1 comment)〈br〉 The Earth’s radiation belt (ERB) is formed by energetic particles caught in the geomagnetic trap. Within the last two solar cycles (from 2001 to 2018), observation of the ERB by a fleet of low-altitude POES satellites allowed to find an abnormal equatorward displacement of the outer part of ERB in the Siberian sector. The displacement can partially explain an increase in the occurrence rate of mid-latitude aurora borealis observed in the Eastern Hemisphere.

Description: 〈b〉Can an interplanetary magnetic field reach the surface of Venus?〈/b〉〈br〉 Yasuhito Narita and Uwe Motschmann〈br〉 Ann. Geophys., 36, 1537-1543, https://doi.org/10.5194/angeo-36-1537-2018, 2018〈br〉 Venus has no intrinsic magnetic field. On the other hand, we discover that an interplanetary magnetic field may nevertheless penetrate the planetary ionosphere by the diffusion process and reach the planetary surface when the solar wind condition remains for a sufficiently long time, between 12 and 54 h, depending on the condition of ionosphere.

Description: 〈b〉Non-linear Effects in Electromagnetic Wave Activity Observed in the RELEC Experiment on-board Vernov Mission〈/b〉〈br〉 Mikhail I. Panasyuk, Sergey I. Svertilov, Stanislav I. Klimov, Valery A. Grushin, Denis I. Novikov, Yuri Y. Ruzhin, Yuri M. Mikhailov, Csaba Ferencz, Peter Szegedi, Valery E. Korepanov, Vitaly V. Bogomolov, Gali K. Garipov, Serhiy V. Belyayev, and Olexander N. Demidov〈br〉 Ann. Geophys. Discuss., https//doi.org/10.5194/angeo-2018-119,2018〈br〉 〈b〉Manuscript under review for ANGEO〈/b〉 (discussion: open, 0 comments)〈br〉 During observations of the electromagnetic waves onboard Vernov satellite in 2014 some interesting phenomena were discovered. They are connected to non-linear effects in wave activity of the type of two or three wave decays as well as splitting into two wave structures. Whistlers with specific unusual temporal structure of swallowtail type were observed. It was shown that such signals can be caused by seismic activity.

Description: 〈b〉On heating of solar wind protons by the parametric decay of large-amplitude Alfvén waves〈/b〉〈br〉 Horia Comişel, Yasuhiro Nariyuki, Yasuhito Narita, and Uwe Motschmann〈br〉 Ann. Geophys., 36, 1647-1655, https://doi.org/10.5194/angeo-36-1647-2018, 2018〈br〉 Space plasmas are assumed to be highly active and dynamic systems including waves and turbulence. Electromagnetic waves such as Alfven waves interact with one another, producing daughter waves. In our study based on three-dimensional hybrid simulations, we emphasize the role of obliquely propagating daughter waves in particle heating in low-temperature (or low-beta) plasmas. The evolutions of plasma turbulence, wave dissipation, and heating are essential problems in astrophysics.

Description: 〈b〉Comparison of gravity wave propagation directions observed by mesospheric airglow imaging at three different latitudes using the M-transform〈/b〉〈br〉 Septi Perwitasari, Takuji Nakamura, Masaru Kogure, Yoshihiro Tomikawa, Mitsumu K. Ejiri, and Kazuo Shiokawa〈br〉 Ann. Geophys., 36, 1597-1605, https://doi.org/10.5194/angeo-36-1597-2018, 2018〈br〉 We have developed a user-friendly program that can efficiently deal with extensive amounts of airglow data. We have applied this new program to airglow data obtained at different latitudes in polar, midlatitude, and equatorial regions and demonstrated distinct differences in atmospheric gravity wave (AGW) propagation characteristics and energy distribution. We aim to encourage other AGW research groups to use the program and do comparisons to reveal AGW characteristics on a more global scale.

Description: 〈b〉Regional Ionosphere Mapping Using Zero Difference GPS Carrier Phase〈/b〉〈br〉 Heba Tawfeek, Ahmed Sedeek, Mostafa Rabah, and Gamal El-Fiky〈br〉 Ann. Geophys. Discuss., https//doi.org/10.5194/angeo-2018-121,2018〈br〉 〈b〉Manuscript under review for ANGEO〈/b〉 (discussion: open, 1 comment)〈br〉 〈p〉Ionospheric delay, can be derived from dual frequency GNSS signals, and then converted into the Vertical Total Electron Contents (VTEC) along the signal path. Various models were devised to calculate VTEC. Examples of such models are the polynomial function model and spherical harmonics model. A common hypothesis of these models is that they are constructed based on the assumption that the entire electron content in the ionosphere is concentrated in a single thin shell at a selected height above Earth.〈/p〉 〈p〉The primary goal of the current research to develop an algorithm capable of producing VTEC maps on an hourly basis, using carrier phase observations from dual frequency GPS receiver. The developed algorithm uses a single GPS station (Zero-difference) to map VTEC over a regional area.〈/p〉 〈p〉The carrier phase measurements are much more precise than the code pseudorange measurements, but they contain an ambiguous term. If such ambiguities are fixed, thence the carrier phase measurements become as unambiguous pseudoranges, but accurate at the level of few millimeters.〈/p〉 〈p〉In current research Sequential Least Square Adjustment (SLSA) was considered to fix ambiguity term in carrier phase observations. The proposed algorithm was written using MATLAB and Called (ZDPID). Two GPS stations (ANKR and BSHM) were used from IGS network to evaluate the developed code, VTEC values were estimated over these two stations. Results of the proposed algorithm were compared with the Global Ionosphere Maps (GIMs), which is generally used as a reference. The results show that the mean difference between VTEC from GIM and estimated VTEC at ANKR station is ranging from −2.1 to 3.67 TECU and its RMS is 0.44. The mean difference between VTEC from GIM and estimated VTEC at BSHM station is ranging from −0.29 to 3.65 TECU and its RMS is 0.38. Another three GPS stations in Egypt were used to generate regional ionosphere maps over Nile Delta, Egypt. The mean differences between VTEC from GIM and estimated VTEC at SAID station is ranging from −1.1 to 3.69 TECU and its RMS is 0.37, from −1.29 to 3.27 TECU for HELW station with RMS equal 0.39, and from 0.2 to 4.2 TECU for BORG station with RMS equal 0.46. Therefore, the proposed algorithm can be used to estimate VTEC efficiently.〈/p〉

Description: 〈b〉Validation of Clyde River SuperDARN radar velocity measurements with the RISR-C incoherent scatter radar〈/b〉〈br〉 Alexander Koustov, Robert Gillies, and Peter Bankole〈br〉 Ann. Geophys., 36, 1657-1666, https://doi.org/10.5194/angeo-36-1657-2018, 2018〈br〉 Clyde River (CLY) SuperDARN radar velocities reflecting plasma flows in the ionosphere are consistent with measurements by the incoherent scatter radar RISR. While agreement is good in the range of RISR velocity magnitudes of 0–700 m s〈sup〉−1〈/sup〉, CLY velocities become progressively smaller at faster flows. In one example of strong disagreements between the instruments, by 200 m s〈sup〉−1〈/sup〉, the radars monitored strongly sheared flows. Validation of the CLY radar confirms the reliability of SuperDARN operation.

Description: 〈b〉Spatial gradient of total electron content (TEC) between two nearby stations as indicator of occurrence of ionospheric irregularity〈/b〉〈br〉 Teshome Dugassa, John Bosco Habarulema, and Melessew Nigussie〈br〉 Ann. Geophys. Discuss., https//doi.org/10.5194/angeo-2018-131,2018〈br〉 〈b〉Manuscript under review for ANGEO〈/b〉 (discussion: open, 1 comment)〈br〉 〈p〉The relation between the occurrence of ionospheric irregularity and spatial gradient of total electron content (TEC) during the post-sunset hours over the equatorial region is studied. The ionospheric irregularities could pose serious challenges to satellite-based navigation and positioning applications when trans-ionospheric signals pass through them. Different instruments and techniques have been applied to study the behavior of these ionospheric irregularities. In this study, the Global positioning system (GPS) based derived total electron content (TEC) was used to investigate the spatial gradient of TEC between two nearby stations as an indicator of the occurrence of ionospheric irregularity over the East African sector. The gradient of TEC between the two stations (ASAB: 4:34° N, 114:39° E and DEBK: 3:71° N, 109:34° E, geomagnetic) located within the equatorial region of Africa were considered in this study during the year 2014. The rate of change of TEC based derived index (〈i〉ROTI〈/i〉〈sub〉ave〈/sub〉) is also used to observe the correlation between the spatial gradient of TEC and the occurrence of ionospheric irregularities. The result obtained shows that most of the maximum positive/depletions in the spatial gradient of TEC observed in March and September equinoxes are more pronounced between 19:00 LT–24:00 LT as the large-scale ionospheric irregularities do. Moreover, the observed spatial gradient of TEC shows two peaks (in March and September) and they exhibit equinoctial asymmetry where the March equinox is greater than September equinox. The enhancement in the spatial gradient of TEC and 〈i〉ROTI〈/i〉〈sub〉ave〈/sub〉 during the 15 evening time period also show similar trends but lag 1–2 hrs from the equatorial electric field (EEF). The spatial gradient of TEC between the two nearby stations could be used as an indicator of the occurrence of ionospheric irregularities.〈/p〉

Description: 〈b〉The current sheet flapping motions induced by non-adiabatic ions: case study〈/b〉〈br〉 Xinhua Wei, Chunlin Cai, Henri Rème, Iannis Dandouras, and George Parks〈br〉 Ann. Geophys. Discuss., https//doi.org/10.5194/angeo-2018-124,2018〈br〉 〈b〉Manuscript under review for ANGEO〈/b〉 (discussion: final response, 6 comments)〈br〉 Observations of flapping current sheet in the magnetotail are presented to reveal their intrinsic excitation mechanism induced by alternating north-south asymmetric ion populations in the sheet center. The results suggest that nonadiabatic ions play a substantial role to determine current sheet dynamics, both its bulk mechanical instability and current profiles.

Description: 〈b〉Influence of gravity waves on the climatology of high-altitude Martian carbon dioxide ice clouds〈/b〉〈br〉 Erdal Yiğit, Alexander S. Medvedev, and Paul Hartogh〈br〉 Ann. Geophys., 36, 1631-1646, https://doi.org/10.5194/angeo-36-1631-2018, 2018〈br〉 Carbon dioxide (CO〈sub〉2〈/sub〉) clouds have been frequently observed in the Martian middle atmosphere. There are still uncertainties concerning the formation of the clouds. Using an atmospheric model for Mars, including a gravity wave parameterization, we assess the role of gravity waves in cloud formation. Simulations suggest that gravity wave processes constitute a necessary physical mechanism for CO〈sub〉2〈/sub〉 cloud formation in the Martian upper atmosphere during all seasons.

Description: 〈b〉On the ion-inertial range density power spectra in solar wind turbulence〈/b〉〈br〉 Rudolf A. Treumann, Wolfgang Baumjohann, and Yasuhito Narita〈br〉 Ann. Geophys. Discuss., https//doi.org/10.5194/angeo-2018-129,2018〈br〉 〈b〉Manuscript under review for ANGEO〈/b〉 (discussion: final response, 2 comments)〈br〉 Occasional deviations in density and magnetic power spectral densities in an intermediate frequency range are interpreted as ion response to the Kolmogorov turbulent velocity spectrum.

Description: In situ magnetotail magnetic flux calculation Annales Geophysicae, 33, 769-781, 2015 Author(s): M. A. Shukhtina and E. Gordeev We explore two new modifications of the magnetotail magnetic flux ( F ) calculation algorithm based on the Petrinec and Russell (1996) (PR96) approach of the tail radius determination. Unlike in the PR96 model, the tail radius value is calculated at each time step based on simultaneous magnetotail and solar wind observations. Our former algorithm, described in Shukhtina et al. (2009), required that the "tail approximation" requirement were fulfilled, i.e., it could be applied only tailward x ∼ −15 R E . The new modifications take into account the approximate uniformity of the magnetic field of external sources in the near and middle tail. Tests, based on magnetohydrodynamics (MHD) simulations, show that this approach may be applied at smaller distances, up to x ∼ −3 R E . The tests also show that the algorithm fails during long periods of strong positive interplanetary magnetic field (IMF) B z . A new empirical formula has also been obtained for the tail radius at the terminator (at x = 0) which improves the calculations.

Description: Van Allen Probe observations of drift-bounce resonances with Pc 4 pulsations and wave–particle interactions in the pre-midnight inner magnetosphere Annales Geophysicae, 33, 955-964, 2015 Author(s): G. I. Korotova, D. G. Sibeck, K. Tahakashi, L. Dai, H. E. Spence, C. A. Kletzing, J. R. Wygant, J. W. Manweiler, P. S. Moya, K.-J. Hwang, and R. J. Redmon We present Van Allen Probe B observations of azimuthally limited, antisymmetric, poloidal Pc 4 electric and magnetic field pulsations in the pre-midnight sector of the magnetosphere from 05:40 to 06:00 UT on 1 May 2013. Oscillation periods were similar for the magnetic and electric fields and proton fluxes. The flux of energetic protons exhibited an energy-dependent response to the pulsations. Energetic proton variations were anticorrelated at medium and low energies. Although we attribute the pulsations to a drift-bounce resonance, we demonstrate that the energy-dependent response of the ion fluxes results from pulsation-associated velocities sweeping energy-dependent radial ion flux gradients back and forth past the spacecraft.

Description: 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.

Description: 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 ( J 0 / J 90 ) 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 J 0 / J 90 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.

Description: How much does weather control fire size and intensity in the Mediterranean region? Annales Geophysicae, 33, 931-939, 2015 Author(s): C. Hernandez, P. Drobinski, and S. Turquety This study investigates the synoptic conditions favorable to wildfires in the Mediterranean region, in terms of fire intensity and burnt area. As reported in the literature, Mediterranean large wildfires are associated with a blocking situation. However, this study shows the existence of two types of wildfires controlled by the blocking high intensity: (1) fast build-up of a weak blocking produces intense wildfires associated with strong winds which allow propagation over long distances; (2) longer build-up of strong blocking situation produces less intense wildfires associated with weaker winds which also propagate over long distances. Another major step forward of this study in the understanding of the drivers of those wildfires is the evidence of a perfect match between the period of wildfire activity and the persistence of the favorable synoptic conditions: the wildfire activity starts at the onset of the blocking situation and ends with the transition to a less favorable synoptic weather pattern. Such strong control of the wildfire activity by the concomitant weather is a very promising result regarding fire risk management, especially considering the accidental nature of the Mediterranean wildfires.

Description: Wide-banded NTC radiation: local to remote observations by the four Cluster satellites Annales Geophysicae, 33, 1285-1300, 2015 Author(s): P. M. E. Décréau, S. Aoutou, A. Denazelle, I. Galkina, J.-L. Rauch, X. Vallières, P. Canu, S. Rochel Grimald, F. El-Lemdani Mazouz, and F. Darrouzet The Cluster multi-point mission offers a unique collection of non-thermal continuum (NTC) radio waves observed in the 2–80 kHz frequency range over almost 15 years, from various view points over the radiating plasmasphere. Here we present rather infrequent case events, such as when primary electrostatic sources of such waves are embedded within the plasmapause boundary far from the magnetic equatorial plane. The spectral signature of the emitted electromagnetic waves is structured as a series of wide harmonic bands within the range covered by the step in plasma frequency encountered at the boundary. Developing the concept that the frequency distance df between harmonic bands measures the magnetic field magnitude B at the source ( df = F c e , electron gyrofrequency), we analyse three selected events. The first one (studied in Grimald et al., 2008) presents electric field signatures observed by a Cluster constellation of small size (~ 200 to 1000 km spacecraft separation) placed in the vicinity of sources. The electric field frequency spectra display frequency peaks placed at frequencies fs = n df ( n being an integer), with df of the order of F c e values encountered at the plasmapause by the spacecraft. The second event, taken from the Cluster tilt campaign, leads to a 3-D view of NTC waves ray path orientations and to a localization of a global source region at several Earth radii ( R E ) from Cluster (Décréau et al., 2013). The measured spectra present successive peaks placed at fs ~ (n+ 1/2) df . Next, considering if both situations might be two facets of the same phenomenon, we analyze a third event. The Cluster fleet, configured into a constellation of large size (~ 8000 to 25 000 km spacecraft separation), allows us to observe wide-banded NTC waves at different distances from their sources. Two new findings can be derived from our analysis. First, we point out that a large portion of the plasmasphere boundary layer, covering a large range of magnetic latitudes, is radiating radio waves. The radio waves are issued from multiple sources of small size, each related to a given fs series and radiating inside a beam of narrow cone angle, referred to as a beamlet. The beamlets illuminate different satellites simultaneously, at different characteristic fs values, according to the latitude at which the satellite is placed. Second, when an observing satellite moves away from its assumed source region (the plasmapause surface), it is illuminated by several beamlets, issued from nearby sources with characteristic fs values close to each other. The addition of radio waves blurs the spectra of the overall received electric field. It can move the signal peaks such that their position fs satisfies fs = (n+α) df , with 0 〈 α 〈 1. These findings open new perspectives for the interpretation of NTC events displaying harmonic signatures.

Description: Terrestrial exospheric hydrogen density distributions under solar minimum and solar maximum conditions observed by the TWINS stereo mission Annales Geophysicae, 33, 413-426, 2015 Author(s): J. H. Zoennchen, U. Nass, and H. J. Fahr Circumterrestrial Lyman-α column brightness observations above 3 Earth radii ( R e ) have been used to derive separate 3-D neutral hydrogen density models of the Earth's exosphere for solar minimum (2008, 2010) and near-solar-maximum (2012) conditions. The data used were measured by Lyman-α detectors (LAD1/2) onboard each of the TWINS satellites from very different orbital positions with respect to the exosphere. Exospheric H atoms resonantly scatter the near-line-center solar Lyman-α flux at 121.6 nm. Assuming optically thin conditions above 3 R e along a line of sight (LOS), the scattered LOS-column intensity is proportional to the LOS H-column density. We found significant differences in the density distribution of the terrestrial exosphere under different solar conditions. Under solar maximum conditions we found higher H densities and a larger spatial extension compared to solar minimum. After a continuous, 2-month decrease in (27 day averaged) solar activity, significantly lower densities were found. Differences in shape and orientation of the exosphere under different solar conditions exist. Above 3 R e , independent of solar activity, increased H densities appear on the Earth's nightside shifted towards dawn. With increasing distance (as measured at 8 R e ) this feature is shifted westward/duskward by between −4 and −5° with respect to midnight. Thus, at larger geocentric distance the exosphere seems to be aligned with the aberrated Earth–solar-wind line, defined by the solar wind velocity and the orbital velocity of the Earth. The results presented in this paper are valid for geocentric distances between 3 and 8 R e .

Description: GPS phase scintillation at high latitudes during geomagnetic storms of 7–17 March 2012 – Part 1: The North American sector Annales Geophysicae, 33, 637-656, 2015 Author(s): P. Prikryl, R. Ghoddousi-Fard, E. G. Thomas, J. M. Ruohoniemi, S. G. Shepherd, P. T. Jayachandran, D. W. Danskin, E. Spanswick, Y. Zhang, Y. Jiao, and Y. T. Morton The interval of geomagnetic storms of 7–17 March 2012 was selected at the Climate and Weather of the Sun-Earth System (CAWSES) II Workshop for group study of space weather effects during the ascending phase of solar cycle 24 (Tsurutani et al., 2014). The high-latitude ionospheric response to a series of storms is studied using arrays of GPS receivers, HF radars, ionosondes, riometers, magnetometers, and auroral imagers focusing on GPS phase scintillation. Four geomagnetic storms showed varied responses to solar wind conditions characterized by the interplanetary magnetic field (IMF) and solar wind dynamic pressure. As a function of magnetic latitude and magnetic local time, regions of enhanced scintillation are identified in the context of coupling processes between the solar wind and the magnetosphere–ionosphere system. Large southward IMF and high solar wind dynamic pressure resulted in the strongest scintillation in the nightside auroral oval. Scintillation occurrence was correlated with ground magnetic field perturbations and riometer absorption enhancements, and collocated with mapped auroral emission. During periods of southward IMF, scintillation was also collocated with ionospheric convection in the expanded dawn and dusk cells, with the antisunward convection in the polar cap and with a tongue of ionization fractured into patches. In contrast, large northward IMF combined with a strong solar wind dynamic pressure pulse was followed by scintillation caused by transpolar arcs in the polar cap.

Description: GPS phase scintillation at high latitudes during geomagnetic storms of 7–17 March 2012 – Part 2: Interhemispheric comparison Annales Geophysicae, 33, 657-670, 2015 Author(s): P. Prikryl, R. Ghoddousi-Fard, L. Spogli, C. N. Mitchell, G. Li, B. Ning, P. J. Cilliers, V. Sreeja, M. Aquino, M. Terkildsen, P. T. Jayachandran, Y. Jiao, Y. T. Morton, J. M. Ruohoniemi, E. G. Thomas, Y. Zhang, A. T. Weatherwax, L. Alfonsi, G. De Franceschi, and V. Romano During the ascending phase of solar cycle 24, a series of interplanetary coronal mass ejections (ICMEs) in the period 7–17 March 2012 caused geomagnetic storms that strongly affected high-latitude ionosphere in the Northern and Southern Hemisphere. GPS phase scintillation was observed at northern and southern high latitudes by arrays of GPS ionospheric scintillation and TEC monitors (GISTMs) and geodetic-quality GPS receivers sampling at 1 Hz. Mapped as a function of magnetic latitude and magnetic local time (MLT), the scintillation was observed in the ionospheric cusp, the tongue of ionization fragmented into patches, sun-aligned arcs in the polar cap, and nightside auroral oval and subauroral latitudes. Complementing a companion paper (Prikryl et al., 2015a) that focuses on the high-latitude ionospheric response to variable solar wind in the North American sector, interhemispheric comparison reveals commonalities as well as differences and asymmetries between the northern and southern high latitudes, as a consequence of the coupling between the solar wind and magnetosphere. The interhemispheric asymmetries are caused by the dawn–dusk component of the interplanetary magnetic field controlling the MLT of the cusp entry of the storm-enhanced density plasma into the polar cap and the orientation relative to the noon–midnight meridian of the tongue of ionization.

Description: Inverse scattering problem in turbulent magnetic fluctuations Rudolf A. Treumann, Wolfgang Baumjohann, and Yasuhito Narita Ann. Geophys., 34, 673-689, doi:10.5194/angeo-34-673-2016, 2016 In support of low-frequency electromagnetic turbulence we formulate the inverse scattering theory of electromagnetic fluctuations in plasma. Its solution provides the turbulent response function which contains all information of the dynamical causes of the electromagnetic fluctuations. This is of basic interest in any electromagnetic turbulence. It requires measurement of magnetic and electric fluctuations but makes no direct use of the turbulent power spectral density.

Description: Relating field-aligned beams to inverted-V structures and visible auroras Annales Geophysicae, 33, 1263-1269, 2015 Author(s): E. Lee, G. K. Parks, S. Y. Fu, M. Fillingim, Y. B. Cui, J. Hong, I. Dandouras, and H. Rème The ion composition experiment on Cluster measures 3-D distributions in one spin of the spacecraft (4 s). These distributions often measure field-aligned ion beams (H + , He + and O + ) accelerated out of the ionosphere. The standard model of these beams relies on a quasi-static U-shaped potential model. The beams contain important information about the structure and distribution of the U-shaped potential structures. For example, a simple beam with a narrow velocity range tells us that the particles are accelerated going through a quasi-static U-shaped potential structure localized in space. A more complex beam with a large range of velocities varying smoothly (a few tens of kilometers per second to 〉 100 km s −1 ) tells us that the potential structure is extended and distributed along the magnetic field. The Cluster experiment has now revealed new features about the beams. Some beams are broken into many individual structures each with their own velocity. The U-shaped potential model would interpret the new features in terms of particles accelerated by narrow isolated potential structures maintained over an extended region of the magnetic field. Another interpretation is that these features arise as Cluster traverses toward the center of a small-scale U-shaped potential region detecting particles accelerated on different equipotential contours. The estimate of the distance of the adjacent contours is ~ 590–610 m at a Cluster height of ~ 3.5 R E . The observed dimensions map to ~ 295–305 m in the ionosphere, suggesting Cluster has measured the potential structure of an auroral arc.

Description: The relationship between plasmapause, solar wind and geomagnetic activity between 2007 and 2011 Annales Geophysicae, 33, 1271-1283, 2015 Author(s): G. Verbanac, V. Pierrard, M. Bandić, F. Darrouzet, J.-L. Rauch, and P. Décréau Taking advantage of the Cluster satellite mission and especially the observations made by the instrument WHISPER to deduce the electron number density along the orbit of the satellites, we studied the relationships between the plasmapause positions ( L PP ) and the following L PP indicators: (a) solar wind coupling functions B z ( Z component of the interplanetary magnetic field vector, B , in GSM system), BV (related to the interplanetary electric field; B is the magnitude of the interplanetary magnetic field vector, V is solar wind velocity), and dΦ mp /d t (which combines different physical processes responsible for the magnetospheric activity) and (b) geomagnetic indices Dst, Ap and AE. The analysis is performed separately for three magnetic local time (MLT) sectors (Sector1 – night sector (01:00–07:00 MLT); Sector2 – day sector (07:00–16:00 MLT); Sector3 – evening sector (16:00–01:00 MLT)) and for all MLTs taken together. All L PP indicators suggest the faster plasmapause response in the postmidnight sector. Delays in the plasmapause responses (hereafter time lags) are approximately 2–27 h, always increasing from Sector1 to Sector3. The obtained fits clearly resolve the MLT structures. The variability in the plasmapause is the largest for low values of L PP indicators, especially in Sector2. At low activity levels, L PP exhibits the largest values on the dayside (in Sector2) and the smallest on the postmidnight side (Sector1). Displacements towards larger values on the evening side (Sector3) and towards lower values on the dayside (Sector2) are identified for enhanced magnetic activity. Our results contribute to constraining the physical mechanisms involved in the plasmapause formation and to further study the still not well understood related issues.

Description: A physical explanation for the magnetic decrease ahead of dipolarization fronts Annales Geophysicae, 33, 1301-1309, 2015 Author(s): Z. H. Yao, J. Liu, C. J. Owen, C. Forsyth, I. J. Rae, Z. Y. Pu, H. S. Fu, X.-Z. Zhou, Q. Q. Shi, A. M. Du, R. L. Guo, and X. N. Chu Recent studies have shown that the ambient plasma in the near-Earth magnetotail can be compressed by the arrival of a dipolarization front (DF). In this paper we study the variations in the characteristics of currents flowing in this compressed region ahead of the DF, particularly the changes in the cross-tail current, using observations from the THEMIS satellites. Since we do not know whether the changes in the cross-tail current lead to a field-aligned current formation or just form a current loop in the magnetosphere, we thus use redistribution to represent these changes of local current density. We found that (1) the redistribution of the cross-tail current is a common feature preceding DFs; (2) the redistribution of cross-tail current is caused by plasma pressure gradient ahead of the DF and (3) the resultant net current redistributed by a DF is an order of magnitude smaller than the typical total current associated with a moderate substorm current wedge (SCW). Moreover, our results also suggest that the redistributed current ahead of the DF is closed by currents on the DF itself, forming a closed current loop around peaks in plasma pressure, what is traditionally referred to as a banana current.

Description: Boltzmann electron PIC simulation of the E-sail effect Annales Geophysicae, 33, 1507-1512, 2015 Author(s): P. Janhunen The solar wind electric sail (E-sail) is a planned in-space propulsion device that uses the natural solar wind momentum flux for spacecraft propulsion with the help of long, charged, centrifugally stretched tethers. The problem of accurately predicting the E-sail thrust is still somewhat open, however, due to a possible electron population trapped by the tether. Here we develop a new type of particle-in-cell (PIC) simulation for predicting E-sail thrust. In the new simulation, electrons are modelled as a fluid, hence resembling hybrid simulation, but in contrast to normal hybrid simulation, the Poisson equation is used as in normal PIC to calculate the self-consistent electrostatic field. For electron-repulsive parts of the potential, the Boltzmann relation is used. For electron-attractive parts of the potential we employ a power law which contains a parameter that can be used to control the number of trapped electrons. We perform a set of runs varying the parameter and select the one with the smallest number of trapped electrons which still behaves in a physically meaningful way in the sense of producing not more than one solar wind ion deflection shock upstream of the tether. By this prescription we obtain thrust per tether length values that are in line with earlier estimates, although somewhat smaller. We conclude that the Boltzmann PIC simulation is a new tool for simulating the E-sail thrust. This tool enables us to calculate solutions rapidly and allows to easily study different scenarios for trapped electrons.

Description: Calculating ultra-low-frequency wave power of the compressional magnetic field vs. L and time: multi-spacecraft analysis using the Van Allen probes, THEMIS and GOES Theodore E. Sarris and Xinlin Li Ann. Geophys., 34, 565-571, doi:10.5194/angeo-34-565-2016, 2016 Ultra-low-frequency (ULF) pulsations are critical in radial diffusion processes of energetic particles, and the power spectral density (PSD) of these fluctuations is an integral part of the radial diffusion coefficients and of assimilative models of the radiation belts. Using simultaneous measurements from two Geostationary Operational Environmental Satellites (GOES) geosynchronous satellites, three satellites of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft constellation and the two Van Allen probes during a 10-day period of intense geomagnetic activity and ULF pulsations of October 2012, we calculate the PSDs of ULF pulsations at different L  shells. By following the time history of measurements at different L it is shown that, during this time, ULF wave power is not enhanced uniformly throughout the magnetosphere but instead is mostly enhanced in the outer L  shells, close to the magnetopause, and to a lesser extent in the inner magnetosphere, closer to the plasmapause. Furthermore, by using phase differences between two GOES geosynchronous satellite pairs, we estimate the daily-averaged distribution of power at different azimuthal wave numbers. These results can have significant implications in better defining the effect of radial diffusion in the phase space density of energetic particles for different wave numbers or L  shell distributions of ULF power.

Description: Generalised partition functions: inferences on phase space distributions Rudolf A. Treumann and Wolfgang Baumjohann Ann. Geophys., 34, 557-564, doi:10.5194/angeo-34-557-2016, 2016 It is demonstrated that the statistical mechanical partition function can be used to construct various different forms of phase space distributions. This indicates that its structure is not restricted to the Gibbs–Boltzmann factor prescription based on counting statistics. Consequences concerning generalised Lorentzians and more general distribution functions are discussed.

Description: An evaluation of International Reference Ionosphere electron density in the polar cap and cusp using EISCAT Svalbard radar measurements Lindis Merete Bjoland, Vasyl Belyey, Unni Pia Løvhaug, and Cesar La Hoz Ann. Geophys., 34, 751-758, doi:10.5194/angeo-34-751-2016, 2016 ​The international reference ionosphere (IRI) model is a widely used model of the ionosphere. We compared this model with 16 years of radar measurements of electron density from Svalbard at altitudes between 200 and 500 km. Our results show that the model is biased towards an underestimation of the electron density in the high-latitude ionosphere and that this underestimation is most severe at nighttime. The model performs best at altitudes around the peak height of the electron density.​

Description: Simultaneous observations of structure function parameter of refractive index using a high-resolution radar and the DataHawk small airborne measurement system Danny E. Scipión, Dale A. Lawrence, Marco A. Milla, Ronald F. Woodman, Diego A. Lume, and Ben B. Balsley Ann. Geophys., 34, 767-780, doi:10.5194/angeo-34-767-2016, 2016 The paper presents simultaneous observations made with a radar (SOUSY) and an unmanned aerial system (DataHawk) with the propose of studying the lower troposphere with high resolution. Through the comparison of both measurements, it was possible to compute the radar calibration constant, which will help to obtain calibrated measurements of turbulent parameters of the atmosphere.

Description: Atmospheric inertia-gravity waves retrieved from level-2 data of the satellite microwave limb sounder Aura/MLS Klemens Hocke, Martin Lainer, Lorena Moreira, Jonas Hagen, Susana Fernandez Vidal, and Franziska Schranz Ann. Geophys., 34, 781-788, doi:10.5194/angeo-34-781-2016, 2016 The dense horizontal sampling of atmospheric temperature profiles by the microwave limb sounder MLS on the NASA satellite AURA permit the estimation of global distributions of inertia-gravity waves (IGWs) in the middle atmosphere. We present and discuss the estimated global distributions of IGWs for July 2015 and January 2016. A dependence on the zonal wind distribution is obvious. The distributions of IGWs are a bit similar to the global distributions of small-scale gravity waves.

Description: Determination of errors in derived magnetic field directions in geosynchronous orbit: results from a statistical approach Yue Chen, Gregory Cunningham, and Michael Henderson Ann. Geophys., 34, 831-843, doi:10.5194/angeo-34-831-2016, 2016 This study statistically quantifies the errors in local magnetic field directions that are derived from electron directional distributions measured by Los Alamos National Laboratory geosynchronous (LANL GEO) satellites. We demonstrate for the first time that derived magnetic field directions in GEO orbit should statistically match the real magnetic directions ~2 times better than those from a list of empirical models and have no significant dependence on magnetospheric activities.

Description: Numerical study of the generation and propagation of ultralow-frequency waves by artificial ionospheric F region modulation at different latitudes Xiang Xu, Chen Zhou, Run Shi, Binbin Ni, Zhengyu Zhao, and Yuannong Zhang Ann. Geophys., 34, 815-829, doi:10.5194/angeo-34-815-2016, 2016 ULF waves can be generated by modulated HF heating in the ionospheric F region, which has long been considered for secure communication with submarines. In this paper we study the effects of background parameters on the process of ULF wave generation and propagation by using a numerical simulation. We find that wave radiation efficiency is higher in the daytime ionosphere at lower latitudes, while ground wave intensity is larger in the nighttime ionosphere with lower modulation frequency.