ALBERT

Description: Spatio-temporal structure of Alfvén waves excited by a sudden impulse localized on an L-shell Annales Geophysicae, 30, 1099-1106, 2012 Author(s): D. Yu. Klimushkin, P. N. Mager, and K.-H. Glassmeier This paper is concerned with the spatial structure and temporal evolution of the azimuthally small scale Alfvén wave generated by a sudden impulse concentrated on a given magnetic shell. At the outset, both poloidal and toroidal components are present in the wave's magnetic field. The oscillation in the poloidal component on a given magnetic shell is a superposition of two monochromatic oscillations, one with the local resonance frequency on this shell, and the other with the frequency corresponding to the resonance frequency on the source surface. The superposition of these two oscillations leads to beating. Due to phase mixing, the poloidal component of the oscillation decreases with time down to zero, transferring its energy to the toroidal component. Beating in the toroidal component is less pronounced. As time elapses, energy concentration near the source magnetic shell occurs with the frequency of the oscillation corresponding to the Alfvénic resonance frequency on this surface. Outside this thin region wave amplitudes become rather small at oscillation frequencies corresponding to the local resonance frequency of the respective magnetic shell.

Description: Predictive model of magnetosheath plasma flow and its validation against Cluster and THEMIS data Annales Geophysicae, 30, 973-982, 2012 Author(s): J. Soucek and C. P. Escoubet An analytical model of magnetosheath plasma flow is described and compared with a large dataset of magnetosheath ion flow velocity measurements from Cluster and THEMIS spacecraft. The model is based on previous works by Kobel and Flückiger (1994) and Génot et al. (2011) and has been modified to overcome the restrictions of these models on the shape of model magnetopause and bow shock. Our model is compatible with any parabolic bow shock model and arbitrary magnetopause model. The model is relatively simple to implement and computationally inexpensive, and its only inputs are upstream solar wind parameters. Comparison with observed data yields a good correspondence: median error in the direction of flow velocity is comparable with the instrumental error, and flow magnitude is predicted with a reasonable accuracy (relative error in flow speed was less than 25% for 86.5% of observations).

Description: Solar wind plasma interaction with solar probe plus spacecraft Annales Geophysicae, 30, 1075-1092, 2012 Author(s): S. Guillemant, V. Génot, J.-C. Matéo-Vélez, R. Ergun, and P. Louarn 3-D PIC (Particle In Cell) simulations of spacecraft-plasma interactions in the solar wind context of the Solar Probe Plus mission are presented. The SPIS software is used to simulate a simplified probe in the near-Sun environment (at a distance of 0.044 AU or 9.5 R S from the Sun surface). We begin this study with a cross comparison of SPIS with another PIC code, aiming at providing the static potential structure surrounding a spacecraft in a high photoelectron environment. This paper presents then a sensitivity study using generic SPIS capabilities, investigating the role of some physical phenomena and numerical models. It confirms that in the near- sun environment, the Solar Probe Plus spacecraft would rather be negatively charged, despite the high yield of photoemission. This negative potential is explained through the dense sheath of photoelectrons and secondary electrons both emitted with low energies (2–3 eV). Due to this low energy of emission, these particles are not ejected at an infinite distance of the spacecraft and would rather surround it. As involved densities of photoelectrons can reach 10 6 cm −3 (compared to ambient ions and electrons densities of about 7 × 10 3 cm −3 ), those populations affect the surrounding plasma potential generating potential barriers for low energy electrons, leading to high recollection. This charging could interfere with the low energy (up to a few tens of eV) plasma sensors and particle detectors, by biasing the particle distribution functions measured by the instruments. Moreover, if the spacecraft charges to large negative potentials, the problem will be more severe as low energy electrons will not be seen at all. The importance of the modelling requirements in terms of precise prediction of spacecraft potential is also discussed.

Description: Auroral kilometric radiation from a nonstationary thin plasma cavity Annales Geophysicae, 30, 1093-1097, 2012 Author(s): T. M. Burinskaya and J.-L. Rauch Results obtained using a waveguide model of the AKR generation in thin plasma cavities are presented. Taking into account the occurrence of low frequency plasma motion in the regions of the AKR generation, we have considered a wave escape from a thin plasma cavity with adiabatically slowly varying width, and show that there can exist localized regions of instability from which the extraordinary X-mode waves, growing in time, can be radiated outwards. It has been found that waves, propagating quasi-tangentially to the source frontiers, have the maximum growth rate and escape outward most efficiently, which is in accordance with experimental observations.

Description: On the problem of Plasma Sheet Boundary Layer identification from plasma moments in Earth's magnetotail Annales Geophysicae, 30, 1331-1343, 2012 Author(s): E. E. Grigorenko, R. Koleva, and J.-A. Sauvaud The problem of identification of the interface region between the lobe and the Plasma Sheet (PS) – the Plasma Sheet Boundary Layer (PSBL) – using ion moments and magnetic field data often arises in works devoted to statistical studies of various PSBL phenomena. Our experience in the identification of this region based on the analysis of ion velocity distribution functions demonstrated that plasma parameters, such as the ion density and bulk velocity, the plasma beta or the dynamic pressure vary widely depending on the state of magnetotail activity. For example, while field-aligned beams of accelerated ions are often observed propagating along the lobeward edge of the PSBL there are times when no signatures of these beams could be observed. In the last case, a spacecraft moving from the lobe region to the PS registers almost isotropic PS-like ion velocity distribution. Such events may be classified as observations of the outer PS region. In this paper, we attempt to identify ion parameter ranges or their combinations that result in a clear distinction between the lobe, the PSBL and the adjacent PS or the outer PS regions. For this we used 100 crossings of the lobe-PSBL-PS regions by Cluster spacecraft (s/c) made in different periods of magnetotail activity. By eye inspection of the ion distribution functions we first identify and separate the lobe, the PSBL and the adjacent PS or outer PS regions and then perform a statistical study of plasma and magnetic field parameters in these regions. We found that the best results in the identification of the lobe-PSBL boundary are reached when one uses plasma moments, namely the ion bulk velocity and density calculated not for the entire energy range, but for the energies higher than 2 keV. In addition, we demonstrate that in many cases the plasma beta fails to correctly identify and separate the PSBL and the adjacent PS or the outer PS regions.

Description: The effect of a gamma ray flare on Schumann resonances Annales Geophysicae, 30, 1321-1329, 2012 Author(s): A. P. Nickolaenko, I. G. Kudintseva, O. Pechony, M. Hayakawa, Y. Hobara, and Y. T. Tanaka We describe the ionospheric modification by the SGR 1806-20 gamma flare (27 December 2004) seen in the global electromagnetic (Schumann) resonance. The gamma rays lowered the ionosphere over the dayside of the globe and modified the Schumann resonance spectra. We present the extremely low frequency (ELF) data monitored at the Moshiri observatory, Japan (44.365° N, 142.24° E). Records are compared with the expected modifications, which facilitate detection of the simultaneous abrupt change in the dynamic resonance pattern of the experimental record. The gamma flare modified the current of the global electric circuit and thus caused the "parametric" ELF transient. Model results are compared with observations enabling evaluation of changes in the global electric circuit.

Description: Electron density profiles in the quiet lower ionosphere based on the results of modeling and experimental data Annales Geophysicae, 30, 1345-1360, 2012 Author(s): V. Barabash, A. Osepian, P. Dalin, and S. Kirkwood The theoretical PGI (Polar Geophysical Institute) model for the quiet lower ionosphere has been applied for computing the ionization rate and electron density profiles in the summer and winter D-region at solar zenith angles less than 80° and larger than 99° under steady state conditions. In order to minimize possible errors in estimation of ionization rates provided by solar electromagnetic radiation and to obtain the most exact values of electron density, each wavelength range of the solar spectrum has been divided into several intervals and the relations between the solar radiation intensity at these wavelengths and the solar activity index F 10.7 have been incorporated into the model. Influence of minor neutral species (NO, H 2 O, O, O 3 ) concentrations on the electron number density at different altitudes of the sunlit quiet D-region has been examined. The results demonstrate that at altitudes above 70 km, the modeled electron density is most sensitive to variations of nitric oxide concentration. Changes of water vapor concentration in the whole altitude range of the mesosphere influence the electron density only in the narrow height interval 73–85 km. The effect of the change of atomic oxygen and ozone concentration is the least significant and takes place only below 70 km. Model responses to changes of the solar zenith angle, solar activity (low–high) and season (summer–winter) have been considered. Modeled electron density profiles have been evaluated by comparison with experimental profiles available from the rocket measurements for the same conditions. It is demonstrated that the theoretical model for the quiet lower ionosphere is quite effective in describing variations in ionization rate, electron number density and effective recombination coefficient as functions of solar zenith angle, solar activity and season. The model may be used for solving inverse tasks, in particular, for estimations of nitric oxide concentration in the mesosphere.

Description: Simulation study of the large-scale modification of the mid-latitude F-layer by HF radio waves with different powers Annales Geophysicae, 30, 1213-1222, 2012 Author(s): G. I. Mingaleva, V. S. Mingalev, and O. V. Mingalev A mathematical model of the ionosphere, developed earlier, is applied to investigate the large-scale mid-latitude F-layer modification by HF radio waves with different powers. Simulations are performed for the point with geographic coordinates of the "Sura" heating facility (Nizhny Novgorod, Russia) for autumn conditions. The calculations are made for distinct cases, in which the effective absorbed power has different values belonging to the 5–100 MW range, both for nocturnal and daytime conditions. The frequency of powerful HF waves is chosen to be close to the most effective frequency for the large-scale F2-layer modification. The results of modeling indicate that the effective absorbed power can influence considerably the F-layer response to high-power radio waves in the mid-latitude ionosphere.

Description: Underlying mechanisms of transient luminous events: a review Annales Geophysicae, 30, 1185-1212, 2012 Author(s): V. V. Surkov and M. Hayakawa Transient luminous events (TLEs) occasionally observed above a strong thunderstorm system have been the subject of a great deal of research during recent years. The main goal of this review is to introduce readers to recent theories of electrodynamics processes associated with TLEs. We examine the simplest versions of these theories in order to make their physics as transparent as possible. The study is begun with the conventional mechanism for air breakdown at stratospheric and mesospheric altitudes. An electron impact ionization and dissociative attachment to neutrals are discussed. A streamer size and mobility of electrons as a function of altitude in the atmosphere are estimated on the basis of similarity law. An alternative mechanism of air breakdown, runaway electron mechanism, is discussed. In this section we focus on a runaway breakdown field, characteristic length to increase avalanche of runaway electrons and on the role played by fast seed electrons in generation of the runaway breakdown. An effect of thunderclouds charge distribution on initiation of blue jets and gigantic jets is examined. A model in which the blue jet is treated as upward-propagating positive leader with a streamer zone/corona on the top is discussed. Sprite models based on streamer-like mechanism of air breakdown in the presence of atmospheric conductivity are reviewed. To analyze conditions for sprite generation, thunderstorm electric field arising just after positive cloud-to-ground stroke is compared with the thresholds for propagation of positively/negatively charged streamers and with runway breakdown. Our own estimate of tendril's length at the bottom of sprite is obtained to demonstrate that the runaway breakdown can trigger the streamer formation. In conclusion we discuss physical mechanisms of VLF (very low frequency) and ELF (extremely low frequency) phenomena associated with sprites.

Description: Estimating the geoeffectiveness of halo CMEs from associated solar and IP parameters using neural networks Annales Geophysicae, 30, 963-972, 2012 Author(s): J. Uwamahoro, L. A. McKinnell, and J. B. Habarulema Estimating the geoeffectiveness of solar events is of significant importance for space weather modelling and prediction. This paper describes the development of a neural network-based model for estimating the probability occurrence of geomagnetic storms following halo coronal mass ejection (CME) and related interplanetary (IP) events. This model incorporates both solar and IP variable inputs that characterize geoeffective halo CMEs. Solar inputs include numeric values of the halo CME angular width (AW), the CME speed ( V cme ), and the comprehensive flare index (cfi), which represents the flaring activity associated with halo CMEs. IP parameters used as inputs are the numeric peak values of the solar wind speed ( V sw ) and the southward Z-component of the interplanetary magnetic field (IMF) or B s . IP inputs were considered within a 5-day time window after a halo CME eruption. The neural network (NN) model training and testing data sets were constructed based on 1202 halo CMEs (both full and partial halo and their properties) observed between 1997 and 2006. The performance of the developed NN model was tested using a validation data set (not part of the training data set) covering the years 2000 and 2005. Under the condition of halo CME occurrence, this model could capture 100% of the subsequent intense geomagnetic storms (Dst ≤ −100 nT). For moderate storms (−100 〈 Dst ≤ −50), the model is successful up to 75%. This model's estimate of the storm occurrence rate from halo CMEs is estimated at a probability of 86%.

Description: Spectral characteristics and scatter cross-section of low latitude mesospheric echoes measured by the Indian MST radar at Gadanki Annales Geophysicae, 30, 983-990, 2012 Author(s): E. Belova, S. Kirkwood, T. Narayana Rao, S. Satheesh Kumar, and T. Sergienko In November 2008 and in March and April 2009 the Indian MST radar (53 MHz) at Gadanki was operated during the daytime in a special experiment, with 600 m altitude resolution, for understanding the characteristics of low-latitude mesospheric echoes (LLME). The data of three days when the echoes were strongest have been analysed in terms of spectral widths and radar volume reflectivities. Spectral widths of LLME show some decrease with altitude, with median values of 4–6 m s −1 at 69–72 km and of 2–4 m s −1 at 73–78 km. This corresponds to 20–200 mW kg −1 turbulent energy dissipation rates. It has been shown that stronger echoes have broader spectra consistent with a turbulent scattering mechanism. For the first time, the volume reflectivities for the strong LLME for Gadanki have also been calculated. They are in the range of 10 −17 –10 −15 m −1 , so LLME at Gadanki are somewhat stronger than those reported so far from Jicamarca, Peru (Lehmacher et al., 2009).

Description: Variability of MLT winds and waves over mid-latitude during the 2000/2001 and 2009/2010 winter stratospheric sudden warming Annales Geophysicae, 30, 991-1001, 2012 Author(s): X. Chen, X. Hu, and C. Xiao The mesosphere and lower thermosphere (MLT) wind structure over Wuhan (30° N, 114° E) in 2000/2001 winter and over Langfang (39.4° N, 116.6° E) in 2009/2010 winter are examined to reveal the effects of stratospheric sudden warming (SSW) in mid-low-latitude MLT region. The result shows that the MLT daily zonal wind over these two sites reversed from eastward wind to westward wind for several days during the SSW events. The reversals were almost coincident with the polar stratospheric temperature reaching its maximum at 10 hPa, 90° N and were about ten days prior to the reversal of high latitude stratospheric zonal wind at 10 hPa, 60° N. The temporal variations of tides, gravity waves and 2-day planetary waves in the mid-latitude MLT showed different behavior during the two SSW events. During the 2001 SSW event, MLT diurnal tide reached its maximum when the MLT zonal wind decreased rapidly and SSW event began in polar stratosphere; the activity of 2-day waves decreased after the onset of the 2001 SSW, while the gravity wave increased when the 2001 SSW developed into a major warming. However, in the 2009/2010 winter, the semidiurnal tide and 2-day wave in MLT over Langfang reached a peak about two days earlier than zonal wind reversal at 10 hPa, 60° N; no significant features were found in diurnal tides, terdiurnal tides and gravity waves related to the 2010 SSW event.

Description: Energetic electrons along the high-latitude magnetopause Annales Geophysicae, 30, 1003-1013, 2012 Author(s): B. M. Walsh, S. E. Haaland, P. W. Daly, E. A. Kronberg, and T. A. Fritz A case study is presented to determine the source of the energetic electron layer frequently observed along the high-latitude magnetopause. Measurements by the Cluster spacecraft show bursts of field-aligned electrons occurring during time periods with high potential for dayside reconnection. These properties are compared with the expected signatures from several sources including escape from the exterior cusp, acceleration in a reconnection region, and release from the dayside trapping region through reconnection. The observed properties are most consistent with the electrons being released from the magnetosphere due to reconnection. In this model the electrons would flow along the newly reconnected IMF draped along the magnetopause and propagate along the high-latitude magnetopause. These observations demonstrate an active source for populating the energetic particle layer frequently observed along and just outside the high-latitude magnetopause.

Description: Behavior of substorm auroral arcs and Pi2 waves: implication for the kinetic ballooning instability Annales Geophysicae, 30, 911-926, 2012 Author(s): T. F. Chang, C. Z. Cheng, C. Y. Chiang, and A. B. Chen We present synoptic observations of the 21 December 2006 substorm event by the THEMIS ground-based All-Sky-Imagers, the ISUAL CCD Imager aboard the FORMOSAT-2 satellite, the geosynchronous satellites and the ground-based magnetometers, and discuss the implication of the observations. There are three subsequent arc breakups with time separation of

Description: Study of the microphysical properties associated with the Monsoon Intraseasonal Oscillation as seen from the TRMM observations Annales Geophysicae, 30, 897-910, 2012 Author(s): M. Halder, P. Mukhopadhyay, and S. Halder The spatio-temporal variability of Indian Summer Monsoon is well studied based on different types of rainfall data. However, very few attempts have been made to study the underlying role of clouds and its hydrometeors on Monsoon Intraseasonal Oscillations. The northward propagating Monsoon Intraseasonal Oscillations and its characteristics remain a challenge for the numerical modelers even today. In view of this, we have set out to analyze the role of cloud hydrometeors and their linkage with northward propagating Monsoon Intraseasonal Oscillations. The science question that we intend to address here is whether the different phases of the cloud hydrometeors show similar propagation characteristics as that of rainfall, and what are the relations of their phases with the convection centre using Tropical Rainfall Measuring Mission data. In answering the question, we have analyzed ten years of Tropical Rainfall Measuring Mission 2A12 hydrometeor data over Indian region. Our analyses show that the cloud water and cloud ice do show a large scale organization during the Indian Summer Monsoon regime of June–September, and systematically progress northward getting initiated over equatorial Indian Ocean. On further analyses, we found that cloud water actually leads the rainfall and cloud ice lags the rainfall. We have further demonstrated the process by analyzing dynamical parameters from Modern Era-Retrospective Analysis for Research and Applications. The presence of cloud water in the lower troposphere in the leading edge of rainfall indicates the lower level moistening and preconditioning of the convective instability due to enhanced moisture convergence. Subsequently, deep convection is triggered, which generates hydrometeor above freezing level and cloud ice in the upper troposphere. To quantify objectively the relation among cloud liquid water, cloud ice and rainfall, the lag correlation is computed with respect to convection center, where the above hypothesis is established that cloud liquid water leads the rainfall and cloud ice lag. This relation among hydrometeors may help the numerical modelers to incorporate such processes for capturing the characteristics of Monsoon Intraseasonal Oscillations.

Description: Observations of magnetic flux ropes during magnetic reconnection in the Earth's magnetotail Annales Geophysicae, 30, 761-773, 2012 Author(s): A. L. Borg, M. G. G. T. Taylor, and J. P. Eastwood We present an investigation of magnetic flux ropes observed by the four Cluster spacecraft during periods of magnetic reconnection in the Earth's magnetotail. Using a list of 21 Cluster encounters with the reconnection process in the period 2001–2006 identified in Borg et al. (2012), we present the distribution and characteristics of the flux ropes. We find 27 flux ropes embedded in the reconnection outflows of only 11 of the 21 reconnection encounters. Reconnection processes associated with no flux rope observations were not distinguishable from those where flux ropes were observed. Only 7 of the 27 flux ropes show evidence of enhanced energetic electron flux above 50 keV, and there was no clear signature of the flux rope in the thermal particle measurements. We found no clear correlation between the flux rope core field and the prevailing IMF B y direction.

Description: Magnetic field amplification in electron phase-space holes and related effects Annales Geophysicae, 30, 711-724, 2012 Author(s): R. A. Treumann and W. Baumjohann Three-dimensional electron phase-space holes are shown to have positive charges on the plasma background, which produce a radial electric field and force the trapped electron component into an azimuthal drift. In this way electron holes generate magnetic fields in the hole. We solve the cylindrical hole model exactly for the hole charge, electric potential and magnetic field. In electron holes, the magnetic field is amplified on the flux tube of the hole; equivalently, in ion holes the field would be decreased. The flux tube adjacent to the electron hole is magnetically depleted by the external hole dipole field. This causes magnetic filamentation. It is also shown that holes are massive objects, each carrying a finite magnetic moment. Binary magnetic dipole interaction of these moments will cause alignment of the holes into chains along the magnetic field or, in the three-dimensional case, produce a magnetic fabric in the volume of hole formation. Since holes, in addition to being carriers of charges and magnetic moments, also have finite masses, they behave like quasi-particles, performing E × B , magnetic field, and diamagnetic drifts. In an inhomogeneous magnetic field, their magnetic moments experience torque, which causes nutation of the hole around the direction of the magnetic field, presumably giving rise to low frequency magnetic modulations like pulsations. A gas of many such holes may allow for a kinetic description, in which holes undergo binary dipole interactions. This resembles the polymeric behaviour. Both magnetic field generation and magnetic structure formation are of interest in auroral, solar coronal and shock physics, in particular in the problem of magnetic field filamentation in relativistic foreshocks and cosmic ray acceleration.

Description: A remarkable correlation between short period gravity waves and semiannual oscillation of the zonal wind in the equatorial mesopause region Annales Geophysicae, 30, 703-710, 2012 Author(s): N. Venkateswara Rao, T. Tsuda, and Y. Kawatani The variability of zonal winds and the horizontal wind velocity variance of short period (20–120 min) gravity waves (GWs) in the equatorial mesopause region are studied using medium frequency (MF) radar observations from Pameungpeuk (7.4° S, 107.4° E) during 2004–2010. The zonal winds display a distinct semiannual oscillation (called mesospheric semiannual oscillation, MSAO), with westward winds during equinoxes and eastward winds during solstices. Furthermore, the westward winds during March equinox are larger during 2008 and 2009. The short period GW variance also shows a semiannual oscillation with enhanced activity during equinoxes. A good correlation is observed between the zonal winds and the short period GW variance from 2008–2010, with the winds being westward during the times of enhanced GW activity. Such a correlation, however, is less obvious during 2004–2006. The long period (10–20 h) GW variance, on the other hand, does not show such a correlation throughout the observation period.

Description: The emission of oxygen green line and density of O atom determined by using ISUAL and SABER measurements Annales Geophysicae, 30, 695-701, 2012 Author(s): H. Gao, J.-B. Nee, and J. Xu Emissions of the 557.7 nm green line airglow observed by the ISUAL (Imager of Sprites and Upper Atmospheric Lightning) instrument on board the FORMOSAT-2 satellite in May and November 2008 are studied here to derive the density distributions of the atomic oxygen by using atmospheric parameters from MSISE-00 model and TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics)/SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) measurements. The May observations were made in 10 days from a fixed orbit of longitude (100° E) with the results showing emission rate and O atom density both peaked at heights of about 90 km over 10° to 20° latitudes in the Northern Hemisphere (NH). In the Southern Hemisphere (SH), the emission rate and density of O atom are both low compared with those in NH. In November, the observations were made as the satellite traveled over all 14 orbits around the earth, covering all longitudes and latitudes of 25° S–45° N. Strong peaks of emission rates and O atoms are found at heights of about 95 km in the mid-latitudes in both hemispheres. In the equator, the airglow layer has a weaker emission rate but with higher altitude compared with those of mid-latitudes. In the lower and upper mesosphere at heights below 85 km and above 105 km, there are more O atoms in the equatorial regions than in the mid-latitudes. And there is a good correlation between the O atom and the temperature structure. A comparison with O atom distribution derived from OH airglow observed by TIMED/SABER at about the same time shows similar results.

Description: Modelling of optical emissions enhanced by the HF pumping of the ionospheric F-region Annales Geophysicae, 30, 885-895, 2012 Author(s): T. Sergienko, B. Gustavsson, U. Brändström, and K. Axelsson Strong enhancement of the optical emissions with excitation threshold from 1.96 eV (630.0 nm from O( 1 D)) up to 18.75 eV (427.8 nm from N 2 + (1NG)) have been observed during experiments of the ionosphere modification by high power HF radio waves. Analysis of the optical emission ratios showed clearly that a significant part of the ionospheric electrons have to be accelerated to energies above 30 eV and more in the region where the HF radio wave effectively interacts with the ionospheric plasma. The Monte-Carlo model of electron transport and the optical emission model were used to study the dependence of the optical emission intensity on the acceleration electron parameters. We obtained the following results from analysis of the enhanced intensities of the four optical emissions (630.0, 557.7, 844.6 and 427.8 nm) observed in the EISCAT heating experiment on 10 March 2002. The 630.0 emission with an excitation threshold of 1.96 eV is formed predominately by the thermal electrons, where the accelerated electrons play a minor role in the excitation of this emission. In order to explain the experimentally observed intensity ratios, the accelerated electrons must gain energies of more than 60 eV. For accelerated electrons with a power law energy dependence, the efficiency of the optical emission excitation depends on the exponent defining the shape of the electron spectra. However, an agreement with the observed emission intensities is achieved for exponent values not less than zero. Moreover, increasing the exponent to higher values does not affect the emission intensity ratios.

Description: On the relaxation of magnetospheric convection when B z turns northward Annales Geophysicae, 30, 927-928, 2012 Author(s): M. C. Kelley The solar wind inputs considerable energy into the upper atmosphere, particularly when the interplanetary magnetic field (IMF) is southward. According to Poynting's theorem (Kelley, 2009), this energy becomes stored as magnetic fields and then is dissipated by Joule heat and by energizing the plasmasheet plasma. If the IMF turns suddenly northward, very little energy is transferred into the system while Joule dissipation continues. In this process, the polar cap potential (PCP) decreases. Experimentally, it was shown many years ago that the energy stored in the magnetosphere begins to decay with a time constant of two hours. Here we use Poynting's theorem to calculate this time constant and find a result that is consistent with the data.

Description: Electron radiation belt data assimilation with an ensemble Kalman filter relying on the Salammbô code Annales Geophysicae, 30, 929-943, 2012 Author(s): S. A. Bourdarie and V. F. Maget In this study we implement a data assimilation tool using a 3-D radiation belt model and an ensemble Kalman filter approach. High time and space reanalysis of the electron radiation belt fluxes is obtained over the time period 5 October to 25 October 1990 by combining sparse observations with the Salammbô 3-D model in an optimal way. The convergence of the ensemble Kalman filter is analyzed carefully. The risk of using a biased physical model is discussed and relative consequences are highlighted. Finally, a validation against CRRES data and major improvements compared to pure physics based model are presented.

Description: Evolution of the plasma sheet electron pitch angle distribution by whistler-mode chorus waves in non-dipole magnetic fields Annales Geophysicae, 30, 751-760, 2012 Author(s): Q. Ma, B. Ni, X. Tao, and R. M. Thorne We present a detailed numerical study on the effects of a non-dipole magnetic field on the Earth's plasma sheet electron distribution and its implication for diffuse auroral precipitation. Use of the modified bounce-averaged Fokker-Planck equation developed in the companion paper by Ni et al. (2012) for 2-D non-dipole magnetic fields suggests that we can adopt a numerical scheme similar to that used for a dipole field, but should evaluate bounce-averaged diffusion coefficients and bounce period related terms in non-dipole magnetic fields. Focusing on nightside whistler-mode chorus waves at L = 6, and using various Dungey magnetic models, we calculate and compare of the bounce-averaged diffusion coefficients in each case. Using the Alternative Direction Implicit (ADI) scheme to numerically solve the 2-D Fokker-Planck diffusion equation, we demonstrate that chorus driven resonant scattering causes plasma sheet electrons to be scattered much faster into loss cone in a non-dipole field than a dipole. The electrons subject to such scattering extends to lower energies and higher equatorial pitch angles when the southward interplanetary magnetic field (IMF) increases in the Dungey magnetic model. Furthermore, we find that changes in the diffusion coefficients are the dominant factor responsible for variations in the modeled temporal evolution of plasma sheet electron distribution. Our study demonstrates that the effects of realistic ambient magnetic fields need to be incorporated into both the evaluation of resonant diffusion coefficients and the calculation of Fokker-Planck diffusion equation to understand quantitatively the evolution of plasma sheet electron distribution and the occurrence of diffuse aurora, in particular at L 〉 5 during geomagnetically disturbed periods when the ambient magnetic field considerably deviates from a magnetic dipole.

Description: Bounce-averaged Fokker-Planck diffusion equation in non-dipolar magnetic fields with applications to the Dungey magnetosphere Annales Geophysicae, 30, 733-750, 2012 Author(s): B. Ni, R. M. Thorne, and Q. Ma We perform a detailed derivation of the bounce-averaged relativistic Fokker-Planck diffusion equation applicable to arbitrary magnetic field at a constant Roederer L . The form of the bounce-averaged diffusion equation is found regardless of details of the mirror geometry, suggesting that the numerical schemes developed for solving the modified two-dimensional (2-D) Fokker-Planck equation in a magnetic dipole should be feasible for similar computation efforts on modeling wave-induced particle diffusion processes in any non-dipolar magnetic field. However, bounce period related terms and bounce-averaged diffusion coefficients are required to be computed in realistic magnetic fields. With the application to the Dungey magnetosphere that is controlled by the intensity of southward interplanetary magnetic field (IMF), we show that with enhanced southward IMF the normalized bounce period related term decreases accordingly, and bounce-averaged diffusion coefficients cover a broader range of electron energy and equatorial pitch angle with a tendency of increased magnitude and peaking at lower energies. The compression of the Dungey magnetosphere can generally produce scattering loss of plasma sheet electrons ~100 keV on a timescale shorter than that in a dipolar field, and induce momentum diffusion at high pitch angles closer to 90°. Correspondingly, the strong diffusion rate drops considerably as a product of changes in both the equatorial loss cone and the bounce period. The extent of differences in all the parameters introduced by the southward IMF intensification also becomes larger for a field line with higher equatorial crossing. With the derived general formulism of bounce-averaged diffusion equation for arbitrary 2-D magnetic field, our results confirm the need for the adoption of realistic magnetic fields to perform accurate determination of electron resonant scattering rates and precise multi-dimensional diffusion simulations of magnetospheric electron dynamics.

Description: Experimental evidence of the simultaneous occurrence of VLF chorus on the ground in the global azimuthal scale – from pre-midnight to the late morning Annales Geophysicae, 30, 725-732, 2012 Author(s): J. Manninen, N. G. Kleimenova, O. V. Kozyreva, M. Parrot, T. Raita, and T. Turunen Night-time VLF (very low frequency) chorus bursts lasting about one hour have been recorded at Finnish temporal station Kannuslehto (CGM: 64.2°; 107.9°, L = 5.3) during two VLF campaigns (on 25 February–4 March 2008 and 27 March–17 April 2011). The chorus bursts were associated with substorm development. They were accompanied by riometer absorption enhancements, which occurred simultaneously within as large longitude areas as from pre-midnight (Sodankylä, ~22:00 MLT) to the late morning (Tixie, ~03:00 MLT and Gakona, ~08:00 MLT) longitudes. It was found that the pre-midnight chorus observed on the ground occurred simultaneously with VLF chorus emissions recorded in the late morning on the low-altitude DEMETER satellite crossing the similar geomagnetic latitudes on the opposite local time sector. For the first time some evidence of simultaneous chorus burst generation in the global longitudinal scale was found (from pre-midnight to the late morning) by using direct comparison with satellite data as well as using non-direct indicator–azimuthally extended riometer absorption enhancements.

Description: On the prediction of the auroral westward electrojet index Annales Geophysicae, 30, 841-847, 2012 Author(s): O. A. Amariutei and N. Yu. Ganushkina An ARMAX based model, to forecast the evolution of the of AL index, is developed. The model has been trained and validated using neural networks with the half wave rectifier ( VB s ) as input. It is shown that the model posses a good, reliable forecasting ability, including periods of intense geomagnetic activity. Prediction efficiency of the model is discussed in the context of 1 min resolution output smoothed over 7 min.

Description: Propagation properties of Rossby waves for latitudinal β-plane variations of f and zonal variations of the shallow water speed Annales Geophysicae, 30, 849-855, 2012 Author(s): C. T. Duba and J. F. McKenzie Using the shallow water equations for a rotating layer of fluid, the wave and dispersion equations for Rossby waves are developed for the cases of both the standard β-plane approximation for the latitudinal variation of the Coriolis parameter f and a zonal variation of the shallow water speed. It is well known that the wave normal diagram for the standard (mid-latitude) Rossby wave on a β-plane is a circle in wave number ( k y , k x ) space, whose centre is displaced −β/2 ω units along the negative k x axis, and whose radius is less than this displacement, which means that phase propagation is entirely westward. This form of anisotropy (arising from the latitudinal y variation of f ), combined with the highly dispersive nature of the wave, gives rise to a group velocity diagram which permits eastward as well as westward propagation. It is shown that the group velocity diagram is an ellipse, whose centre is displaced westward, and whose major and minor axes give the maximum westward, eastward and northward (southward) group speeds as functions of the frequency and a parameter m which measures the ratio of the low frequency-long wavelength Rossby wave speed to the shallow water speed. We believe these properties of group velocity diagram have not been elucidated in this way before. We present a similar derivation of the wave normal diagram and its associated group velocity curve for the case of a zonal ( x ) variation of the shallow water speed, which may arise when the depth of an ocean varies zonally from a continental shelf.

Description: A new global model for the ionospheric F2 peak height for radio wave propagation Annales Geophysicae, 30, 797-809, 2012 Author(s): M. M. Hoque and N. Jakowski The F2-layer peak density height hm F2 is one of the most important ionospheric parameters characterizing HF propagation conditions. Therefore, the ability to model and predict the spatial and temporal variations of the peak electron density height is of great use for both ionospheric research and radio frequency planning and operation. For global hm F2 modelling we present a nonlinear model approach with 13 model coefficients and a few empirically fixed parameters. The model approach describes the temporal and spatial dependencies of hm F2 on global scale. For determining the 13 model coefficients, we apply this model approach to a large quantity of global hm F2 observational data obtained from GNSS radio occultation measurements onboard CHAMP, GRACE and COSMIC satellites and data from 69 worldwide ionosonde stations. We have found that the model fits to these input data with the same root mean squared (RMS) and standard deviations of 10%. In comparison with the electron density NeQuick model, the proposed Neustrelitz global hm F2 model (Neustrelitz Peak Height Model – NPHM) shows percentage RMS deviations of about 13% and 12% from the observational data during high and low solar activity conditions, respectively, whereas the corresponding deviations for the NeQuick model are found 18% and 16%, respectively.

Description: On the role of ozone in long-term trends in the upper atmosphere-ionosphere system Annales Geophysicae, 30, 811-816, 2012 Author(s): J. Laštovička Origin of long-term trends in the thermosphere-ionosphere system has been discussed since the beginning of trend studies. The two most prioritized explanations have been those via long-term increase of atmospheric concentration of greenhouse gases and long-term increase of geomagnetic activity throughout the 20th century. Secular changes of the Earth's main magnetic field play an important role in trends in a limited region. Recently, Walsh and Oliver (2011) suggested that the long-term cooling of the upper thermosphere (above 200 km) may be due largely to the stratospheric ozone depletion. Here, we show that the role of ozone is very important in the mesosphere and lower thermosphere but not in the upper thermosphere. The suggestion of Walsh and Oliver (2011) is based on historical (before 1988) data from Saint-Santin radar, whereas more recent data do not support their conclusion.

Description: Positive ionospheric storm effects at Latin America longitude during the superstorm of 20–22 November 2003: revisit Annales Geophysicae, 30, 831-840, 2012 Author(s): B. Zhao, W. Wan, J. Lei, Y. Wei, Y. Sahai, and B. Reinisch Positive ionospheric storm effects that occurred during the superstorm on 20 November 2003 are investigated using a combination of ground-based Global Positioning System (GPS) total electron content (TEC), and the meridian chain of ionosondes distributed along the Latin America longitude of ~280° E. Both the ground-based GPS TEC and ionosonde electron density profile data reveal significant enhancements at mid-low latitudes over the 280° E region during the main phase of the November 2003 superstorm. The maximum enhancement of the topside ionospheric electron content is 3.2–7.7 times of the bottomside ionosphere at the locations of the ionosondes distributed around the mid- and low latitudes. Moreover, the height of maximum electron density exceeds 400 km and increases by 100 km compared with the quiet day over the South American area from middle to low latitudes, which might have resulted from a continuous eastward penetration electric field and storm-generated equatorward winds. Our results do not support the conclusions of Yizengaw et al. (2006), who suggested that the observed positive storm over the South American sector was mainly the consequence of the changes of the bottomside ionosphere. The so-called "unusual" responses of the topside ionosphere for the November 2003 storm in Yizengaw et al. (2006) are likely associated with the erroneous usage of magnetometer and incomplete data.

Description: Impact of cloud parameterization on the numerical simulation of a super cyclone Annales Geophysicae, 30, 775-795, 2012 Author(s): M. S. Deshpande, S. Pattnaik, and P. S. Salvekar This study examines the role of parameterization of convection and explicit moisture processes on the simulated track, intensity and inner core structure of Orissa super cyclone (1999) in Bay of Bengal (north Indian Ocean). Sensitivity experiments are carried out to examine the impact of cumulus parameterization schemes (CPS) using MM5 model (Version 3.7) in a two-way nested domain (D1 and D2) configuration at horizontal resolutions (45–15 km). Three different cumulus parameterization schemes, namely Grell (Gr), Betts-Miller (BM) and updated Kain Fritsch (KF2), are tested. It is noted that track and intensity both are very sensitive to CPS and comparatively, KF2 predicts them reasonably well. Particularly, the rapid intensification phase of the super cyclone is best simulated by KF2 compared to other CPS. To examine the effect of the cumulus parameterization scheme at high resolution (5 km), the three-domain configuration (45-15-5 km resolution) is utilized. Based on initial results, KF2 scheme is used for both the domains (D1 and D2). Two experiments are conducted: one in which KF2 is used as CPS and another in which no CPS is used in the third domain. The intensity is well predicted when no CPS is used in the innermost domain. The sensitivity experiments are also carried out to examine the impact from microphysics parameterization schemes (MPS). Four cloud microphysics parameterization schemes, namely mixed phase (MP), Goddard microphysics with Graupel (GG), Reisner Graupel (RG) and Schultz (Sc), are tested in these experiments. It is noted that the tropical cyclone tracks and intensity variation have considerable sensitivity to the varying cloud microphysical parameterization schemes. The MPS of MP and Sc could very well capture the rapid intensification phase. The final intensity is well predicted by MP, which is overestimated by Sc. The MPS of GG and RG underestimates the intensity.

Description: Plasma flows, Birkeland currents and auroral forms in relation to the Svalgaard-Mansurov effect Annales Geophysicae, 30, 817-830, 2012 Author(s): P. E. Sandholt and C. J. Farrugia The traditional explanation of the polar cap magnetic deflections, referred to as the Svalgaard-Mansurov effect, is in terms of currents associated with ionospheric flow resulting from the release of magnetic tension on newly open magnetic field lines. In this study, we aim at an updated description of the sources of the Svalgaard-Mansurov effect based on recent observations of configurations of plasma flow channels, Birkeland current systems and aurorae in the magnetosphere-ionosphere system. Central to our description is the distinction between two different flow channels (FC 1 and FC 2) corresponding to two consecutive stages in the evolution of open field lines in Dungey cell convection, with FC 1 on newly open, and FC 2 on old open, field lines. Flow channel FC 1 is the result of ionospheric Pedersen current closure of Birkeland currents flowing along newly open field lines. During intervals of nonzero interplanetary magnetic field B y component FC 1 is observed on either side of noon and it is accompanied by poleward moving auroral forms (PMAFs/prenoon and PMAFs/postnoon). In such cases the next convection stage, in the form of flow channel FC 2 on the periphery of the polar cap, is particularly important for establishing an IMF B y -related convection asymmetry along the dawn-dusk meridian, which is a central element causing the Svalgaard-Mansurov effect. FC 2 flows are excited by the ionospheric Pedersen current closure of the northernmost pair of Birkeland currents in the four-sheet current system, which is coupled to the tail magnetopause and flank low-latitude boundary layer. This study is based on a review of recent statistical and event studies of central parameters relating to the magnetosphere-ionosphere current systems mentioned above. Temporal-spatial structure in the current systems is obtained by ground-satellite conjunction studies. On this point we emphasize the important information derived from the continuous ground monitoring of the dynamical behaviour of aurora and plasma convection during intervals of well-organised solar wind plasma and magnetic field conditions in interplanetary coronal mass ejections (ICMEs) during their Earth passage.

Description: Dayside and nightside contributions to cross-polar cap potential variations: the 20 March 2001 ICME case Annales Geophysicae, 29, 2189-2201, 2011 Author(s): Y. L. Andalsvik, P. E. Sandholt, and C. J. Farrugia We investigate the association between temporal-spatial structure of polar cap convection and auroral electrojet intensifications during a 5-h-long interval of strong forcing of the magnetosphere by an ICME/Magnetic cloud on 20 March 2001. We use data from coordinated ground-satellite observations in the 15:00–20:00 MLT sector. We take advantage of the good latitudinal coverage in the polar cap and in the auroral zone of the IMAGE chain of ground magnetometers in Svalbard – Scandinavia – Russia and the stable magnetic field conditions in ICMEs. The electrojet events are characterized by a sequence of 10 min-long AL excursions to −1000/−1500 nT followed by poleward expansions and auroral streamers. These events are superimposed on a high disturbance level when the AL index remains around −500 nT for several hours. These signatures are different from those appearing in classical substorms, most notably the absence of a complete recovery phase when AL usually reaches above −100 nT. We concentrate on polar cap convection in both hemispheres (DMSP F13 data) in relation to the ICME B y conditions, electrojet intensifications, and the global UV auroral configuration obtained from the IMAGE spacecraft. The temporal evolution of convection properties such as the cross-polar cap potential (CPCP) drop and flow channels at the dawn/dusk polar cap (PC) boundaries around the time of the electrojet events are investigated. This approach allows us to distinguish between dayside (magnetopause reconnection) and nightside (magnetotail reconnection) sources of the PC convection events within the context of the expanding-contracting model of high-latitude convection in the Dungey cycle. Inter-hemispheric symmetries/asymmetries in the presence of newly-discovered convection channels at the dawn or dusk side PC boundaries are determined.

Description: Response of internal solitary waves to tropical storm Washi in the northwestern South China Sea Annales Geophysicae, 29, 2181-2187, 2011 Author(s): Z. H. Xu, B. S. Yin, and Y. J. Hou Based on in-situ time series data from an array of temperature sensors and an acoustic Doppler current profiler on the continental shelf of the northwestern South China Sea, a sequence of internal solitary waves (ISWs) were observed during the passage of tropical storm Washi in the summer of 2005, which provided a unique opportunity to investigate the ISW response to the tropical cyclone. The passing tropical storm is found to play an important role in affecting the stratification structure of the water column, and consequently leading to significant variability in the propagating features of the ISWs, such as the polarity reversal and amplitude variations of the waves. The response of the ISWs to Washi can be divided into two stages, direct forcing by the strong wind (during the arrival of Washi) and remote forcing via the near-inertial internal waves induced by the tropical storm (after the passage of Washi). The field observations as well as a theoretical analysis suggest that the variations of the ISWs closely coincide with the changing stratification structure and shear currents in accompanied by the typhoon wind and near-inertial waves. This study presents the first observations and analysis of the ISW response to the tropical cyclone in the South China Sea.

Description: Dusty space plasma diagnosis using temporal behavior of polar mesospheric summer echoes during active modification Annales Geophysicae, 29, 2169-2179, 2011 Author(s): A. Mahmoudian, W. A. Scales, M. J. Kosch, A. Senior, and M. Rietveld The objective of this paper is to study the effect of different plasma and dust parameters on Polar Mesospheric Summer Echoes (PMSE) temporal behavior after turn-on and turn-off of radio wave heating and to use these responses to diagnose the properties of the dust layer. The threshold radar frequency and dust parameters for the enhancement or suppression of radar echoes after radio wave heating turn-on are investigated for measured mesospheric plasma parameters. The effect of parameters such as the electron temperature enhancement during heating, dust density, dust charge polarity, ion-neutral collision frequency, electron density and dust radius on the temporal evolution of electron irregularities associated with PMSE are investigated. The possible diagnostic information for various charged dust and background plasma quantities using the temporal behavior of backscattered radar power in active experiments is discussed. The computational results are used to make predictions for PMSE active modification experiments at 7.9, 56, 139, 224 and 930 MHz corresponding to existing radar facilities. Data from a 2009 VHF (224 MHz) experiment at EISCAT is compared with the computational model to obtain dust parameters in the PMSE.

Description: Where to see climate change best in radio occultation variables – study using GCMs and ECMWF reanalyses Annales Geophysicae, 29, 2147-2167, 2011 Author(s): B. C. Lackner, A. K. Steiner, and G. Kirchengast Radio occultation (RO) is a new technique to observe the upper troposphere and lower stratosphere (UTLS), a region that reacts particularly sensitive to climate change. Featuring characteristics such as long-term stability, SI traceability, all-weather capability, global coverage, and high accuracy and vertical resolution, RO data fulfill the requirements for climate monitoring in the UTLS. However, while a range of studies has shown the climate utility of RO it has not yet been explored sytematically where to see climate change best in RO variables. Therefore we perform here a systematic trend study for the RO variables refractivity, pressure, and temperature (bending angle, not depending on height but impact parameter, is left for separate study). The trends, given at geopotential height levels and for layer gradients, are explored to determine seasons, geographic regions, and height domains, which show a significant trend signal. Because continuous RO data are available since 2001 only, reanalyses (ERA-40 and ERA-Interim) and global circulation model simulations of the Intergovernmental Panel on Climate Change Assessment Report 4 (CCSM3, ECHAM5, HadCM3) are used as proxy data for RO. It is shown that RO data are sensitive at different height ranges and that thus several indicators of climate change can be retrieved. Refractivity emerges as indicator in the lower stratosphere (LS) and tropopause region at about 14 km to 24 km, pressure over the whole UTLS, and both in all large-scale regions except the polar caps. Temperature qualifies as indicator in the upper troposphere below about 16 km and in the LS above about 21 km. Overall, refractivity and pressure alone are adequate indicators for the UTLS, but temperature as commonly used variable facilitates easy interpretation of results. Layer gradients were found to be further sensitive indicators providing additional information. Besides large-scale global and hemispheric means the tropics and the mid-latitudes appear as regions suitable to track climate change with RO data. The results also point to the value of utilizing in addition to annual means specific seasons, such as northern hemispheric fall and summer, for early climate signal detection. Since RO data feature much better vertical resolution than the proxy data of this study, more detailed insights can be expected when a longer RO record will be available.

Description: Estimates of vertical eddy diffusivity in the upper mesosphere in the presence of a mesospheric inversion layer Annales Geophysicae, 29, 2019-2029, 2011 Author(s): R. L. Collins, G. A. Lehmacher, M. F. Larsen, and K. Mizutani Rayleigh and resonance lidar observations were made during the Turbopause experiment at Poker Flat Research Range, Chatanika Alaska (65° N, 147° W) over a 10 h period on the night of 17–18 February 2009. The lidar observations revealed the presence of a strong mesospheric inversion layer (MIL) at 74 km that formed during the observations and was present for over 6 h. The MIL had a maximum temperature of 251 K, amplitude of 27 ± 7 K, a depth of 3.0 km, and overlying lapse rate of 9.4 ± 0.3 K km −1 . The MIL was located at the lower edge of the mesospheric sodium layer. During this coincidence the lower edge of the sodium layer was lowered by 2 km to 74 km and the bottomside scale height of the sodium increased from 1 km to 15 km. The structure of the MIL and sodium are analyzed in terms of vertical diffusive transport. The analysis yields a lower bound for the eddy diffusion coefficient of 430 m 2 s −1 and the energy dissipation rate of 2.2 mW kg −1 at 76–77 km. This value of the eddy diffusion coefficient, determined from naturally occurring variations in mesospheric temperatures and the sodium layer, is significantly larger than those reported for mean winter values in the Arctic but similar to individual values reported in regions of convective instability by other techniques.

Description: A comparison study between observations and simulation results of Barghouthi model for O + and H + outflows in the polar wind Annales Geophysicae, 29, 2061-2079, 2011 Author(s): I. A. Barghouthi, S. H. Ghithan, and H. Nilsson To advance our understanding of the effect of wave-particle interactions on ion outflows in the polar wind region and the resulting ion heating and escape from low altitudes to higher altitudes, we carried out a comparison between polar wind simulations obtained using Barghouthi model with corresponding observations obtained from different satellites. The Barghouthi model describes O + and H + outflows in the polar wind region in the range 1.7 R E to 13.7 R E , including the effects of gravity, polarization electrostatic field, diverging geomagnetic field lines, and wave-particle interactions. Wave-particle interactions were included into the model by using a particle diffusion equation, which depends on diffusion coefficients determined from estimates of the typical electric field spectral density at relevant altitudes and frequencies. We provide a formula for the velocity diffusion coefficient that depends on altitude and velocity, in which the velocity part depends on the perpendicular wavelength of the electromagnetic turbulence λ ⊥ . Because of the shortage of information about λ ⊥ , it was included into the model as a parameter. We produce different simulations (i.e. ion velocity distributions, ions density, ion drift velocity, ion parallel and perpendicular temperatures) for O + and H + ions, and for different λ ⊥ . We discuss the simulations in terms of wave-particle interactions, perpendicular adiabatic cooling, parallel adiabatic cooling, mirror force, and ion potential energy. The main findings of the simulations are as follows: (1) O + ions are highly energized at all altitudes in the simulation tube due to wave-particle interactions that heat the ions in the perpendicular direction, and part of this gained energy transfer to the parallel direction by mirror force, resulting in accelerating O + ions along geomagnetic field lines from lower altitudes to higher altitudes. (2) The effect of wave-particle interactions is negligible for H + ions at altitudes below ~7 R E , while it is important for altitudes above 7 R E . For O + wave particle interaction is very significant at all altitudes. (3) For certain λ ⊥ and at points, altitudes, where the ion gyroradius is equal to or less than λ ⊥ , the effect of wave-particle interactions is independent of the velocity and it depends only on the altitude part of the velocity diffusion coefficient; however, the effect of wave-particle interactions reduce above that point, called saturation point, and the heating process turns to be self-limiting heating. (4) The most interesting result is the appearance of O + conics and toroids at low altitudes and continue to appear at high altitudes; however, they appear at very high altitudes for H + ions. We compare quantitatively and qualitatively between the simulation results and the corresponding observations. As a result of many comparisons, we find that the best agreement occurs when λ ⊥ equals to 8 km. The quantitative comparisons show that many characteristics of the observations are very close to the simulation results, and the qualitative comparisons between the simulation results for ion outflows and the observations produce very similar behaviors. To our knowledge, most of the comparisons between observations (ion velocity distribution, density, drift velocity, parallel and perpendicular temperatures, anisotropy, etc.) and simulations obtained from different models produce few agreements and fail to explain many observations (see Yau et al., 2007; Lemaire et al., 2007; Tam et al., 2007; Su et al., 1998; Engwall et al., 2009). This paper presents many close agreements between observations and simulations obtained by Barghouthi model, for O + and H + ions at different altitudes i.e. from 1.7 R E to 13.7 R E .

Description: A study of Traveling Ionospheric Disturbances and Atmospheric Gravity Waves using EISCAT Svalbard Radar IPY-data Annales Geophysicae, 29, 2101-2116, 2011 Author(s): A. Vlasov, K. Kauristie, M. van de Kamp, J.-P. Luntama, and A. Pogoreltsev We present a statistical study of Traveling Ionospheric Disturbances (TIDs) as observed by the EISCAT Svalbard Radar (ESR) during the continuous IPY-run (March 2007–February 2008) with field-aligned measurements. We have developed a semi-automatic routine for searching and extracting Atmospheric Gravity Wave (AGW) activity. The collected data shows that AGW-TID signatures are common in the high-latitude ionosphere especially in the field-aligned ion velocity data (244 cases of AGW-TID signatures in daily records), but they can be observed also in electron density (26 cases), electron temperature (12 cases) and ion temperature (26 cases). During the IPY campaign (in solar minimum conditions) AGW-TID events appear more frequently during summer months than during the winter months. It remains still as a topic for future studies whether the observed seasonal variation is natural or caused by seasonal variation in the performance of the observational method that we use (AGW-TID signature may be more pronounced in a dense ionosphere). In our AGW-TID dataset the distribution of the oscillation periods has two peaks, one around 0.5–0.7 h and the other around 1.1–1.3 h. The diurnal occurrence rate has a deep minimum in the region of magnetic midnight, which might be partly explained by irregular auroral activity obscuring the TID signatures from our detection routines. As both the period and horizontal phase speed estimates (as derived from the classical AGW dispersion relation) show values typical both for large scale TIDs and mesoscale TIDs it is difficult to distinguish whether the generator for high-latitude AGW-TIDs resides typically in the troposphere or in the near-Earth space. The results of our statistical analysis give anyway some valuable reference information for the future efforts to learn more about the dominating TID source mechanisms in polar cap conditions, and to improve AGW simulations.

Description: Velocity-space diffusion of solar wind protons in oblique waves and weak turbulence Annales Geophysicae, 29, 2089-2099, 2011 Author(s): E. Marsch and S. Bourouaine The fast solar wind is permeated by all kinds of plasma waves which have a broad range of wavelengths and occur on many different scales. Kinetically, a plasma wave induces ion-wave interactions which can within the quasi-linear theory be described as a diffusion process. The impact this diffusion may have on the shape of the proton velocity distribution function (VDF) is studied. We first analyse theoretically some of the possible kinetic effects of the waves on the ions. Then the model predictions are compared with the detailed in-situ plasma measurements made by the Helios spacecraft on 14 April 1976 at 0.3 AU and found to comply favourably with resonant diffusion of protons in obliquely propagating magnetohydrodynamic waves. In particular, the shape at the edges of the VDFs at positive proton velocities in the wind frame can be well explained by cyclotron-resonant diffusion of the protons in oblique fast magnetoacoustic and Alfvén waves propagating away from the Sun.

Description: THEMIS observations of earthward convected flux ropes triggering field dipolarization/substorm expansion and associated particle energization Annales Geophysicae, 29, 2117-2130, 2011 Author(s): I. I. Vogiatzis, O. E. Malandraki, Q.-G. Zong, X.-Z. Zhou, T. E. Sarris, E. T. Sarris, H. Zhang, and T. A. Fritz We investigated a number of substorm events during major conjunctions of the THEMIS spacecraft for the tail seasons of the mission. We present simultaneous observations from various instruments onboard the THEMIS spacecraft during the events. We focus particularly on events when at least one of the THEMIS spacecraft is adjacent to the neutral sheet where convectional plasma flows are observed. The events demonstrate clear dipolarization signatures accompanied by high-speed earthward plasma flows and intense wave activity. We present evidence that flux ropes are embedded within the high-speed earthward convective plasma sheet flows. This fact has important implications since the leading edge of the flux rope having south polarity can impulsively merge with the north polarity field of the stretched magnetotail leading to mutual erosion of both magnetic structures. The merging of the vertically oriented oppositely-directed field lines can lead to local cross-tail current reduction and flux rope dissipation. These observations are very important in explaining the mechanism triggering near-Earth dipolarization and particle acceleration to supra-thermal energies, since they can be associated with non-adiabatic conditions and breakdown of the frozen-in condition in the near-Earth magnetotail similar to that taking place during magnetic reconnection in the mid-tail.

Description: Magnetopause displacements: the possible role of dust Annales Geophysicae, 29, 2219-2223, 2011 Author(s): R. A. Treumann and W. Baumjohann Large compressions of the magnetopause are proposed to occasionally result from temporary encounters of the magnetosphere with dust streams in interplanetary space. Such streams may have their origin in cometary dust tails or asteroids which cross the inner heliosphere or in meteoroids in Earth's vicinity. Dust ejected from such objects when embedding the magnetosphere for their limited transition time should cause substantial global deformations of the magnetopause/magnetosphere due to the very large dust grain mass and momentum which compensates for the low dust density when contributing to the upstream pressure variation.

Description: The TWINS exospheric neutral H-density distribution under solar minimum conditions Annales Geophysicae, 29, 2211-2217, 2011 Author(s): J. H. Zoennchen, J. J. Bailey, U. Nass, M. Gruntman, H. J. Fahr, and J. Goldstein Terrestrial exospheric atomic hydrogen (H) resonantly scatters solar Lyman-α (121.567 nm) radiation, observed as the glow of the H-geocorona. The Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) satellites are equiped with two Lyman-α line-of-sight Detectors (LADs) each. Since during the past solar minimum conditions the relevant solar control parameters practically did not vary, we are using LAD data between June and September 2008 to create a time averaged hydrogen geocorona model representative for these solar minimum conditions. In this averaged model we assume that the H-geocorona is longitudinally symmetric with respect to the earth-sun line. We find a 3-dimensional H-density distribution in the range from 3 to 8 earth radii which with some caution can also be extrapolated to larger distances. For lower geocentric distances than 3 earth radii a best fitting r-dependent Chamberlain (1963)-like model is adapted. Main findings are larger than conventionally expected H-densities at heights above 5 R E and a pronounced day-to-night side H-density asymmetry. The H-geocorona presented here should serve as a reference H-atmosphere for the earth during solar minimum conditions.

Description: Some aspects of large-scale travelling ionospheric disturbances which originate at conjugate locations in auroral zones, cross the equator and sometimes encircle the earth Annales Geophysicae, 29, 2203-2210, 2011 Author(s): G. G. Bowman and I. K. Mortimer The occurrence of large-scale travelling ionospheric disturbances (LS-TIDs) has been examined. Initially some literature on their generation is considered. Travel during daylight hours and also paths which involve propagation paths towards the poles are illustrated by a few examples from the literature. A daytime ionogram recording of an LS-TID is presented and discussed as are nighttime ionogram recordings for a poleward path of propagation. The tabulations of Moscow h'F recordings around midnight are examined for significant height increases which along with geomagnetic bays some hours earlier confirm the existence of LS-TIDs. A sunspot-maximum interval is involved. Some of the Moscow events were related to bays which occurred 32 h earlier thus indicating earth encirclements. Also additional encirclements are recorded by using superposed-epoch analyses for some other events.

Description: Electron-cylotron maser radiation from electron holes: upward current region Annales Geophysicae, 29, 1885-1904, 2011 Author(s): R. A. Treumann, W. Baumjohann, and R. Pottelette Electron holes are suggested to be an important source for generation of electron-cyclotron maser radiation. We demonstrate that electron holes generated in a ring-horseshoe distribution in the auroral-kilometric radiation source region have the capacity to emit band-limited radiation. The radiation is calculated in the proper frame of a circular model hole and shown to be strictly perpendicular in this frame. Its bandwidth under auroral conditions is of the order of ~1 kHz, which is a reasonable value. It is also shown that much of the drift of fine structure in the radiation can be interpreted as Doppler shift. Estimates based on data are in good agreement with theory. Growth and absorption rates have been obtained for the emitted radiation. However, the growth rate of a single hole obtained under conservative conditions is small, too small for reproducing the observed fine structure flux. Trapping of radiation inside the hole for the hole's lifetime helps amplifying the radiation additionally but introduces other problems. This entire set of questions is discussed at length and compared to radiation from the global horseshoe distribution. The interior of the hole produces a weak absorption at slightly higher frequency than emission. The absorptivity is roughly two orders of magnitude below the growth rate of the radiation thus being weak even when the emission and absorption bands overlap. Transforming to the stationary observer's frame it is found that the radiation becomes oblique against the magnetic field. For approaching holes the radiated frequencies may even exceed the local electron cyclotron frequency.

Description: Observations of OI 557.7 nm nightglow at Kolhapur (17° N), India Annales Geophysicae, 29, 1873-1884, 2011 Author(s): N. Parihar, S. Gurubaran, and G. K. Mukherjee Ground-based nightglow observations of the atomic oxygen green line at 557.7 nm have been carried out at a low latitude station Kolhapur (17° N), India, during November 2003–April 2004 and December 2004–May 2005. The nocturnal behaviour of OI 557.7 nm intensity and a comparative study with simultaneous OH Meinel band temperature measurements has been presented. OI 557.7 nm intensity and OH temperature variations covary on many occasions. It was found that an 8 h tide characterizes the variation of intensity and temperature on most nights, and especially during the month of January. This is the first report of prolonged measurements of OI 557.7 nm emission from India.

Description: Substorms during different storm phases Annales Geophysicae, 29, 2031-2043, 2011 Author(s): N. Partamies, L. Juusola, E. Tanskanen, K. Kauristie, J. M. Weygand, and Y. Ogawa After the deep solar minimum at the end of the solar cycle 23, a small magnetic storm occurred on 20–26 January 2010. The Dst (disturbance storm time) index reached the minimum of −38 nT on 20 January and the prolonged recovery that followed the main phase that lasted for about 6 days. In this study, we concentrate on three substorms that took place (1) just prior to the storm, (2) during the main phase of the storm, and (3) at the end of the recovery of the storm. We analyse the solar wind conditions from the solar wind monitoring spacecraft, the duration and intensity of the substorm events as well as the behaviour of the electrojet currents from the ground magnetometer measurements. We compare the precipitation characteristics of the three substorms. The results show that the F-region electron density enhancements and dominant green and red auroral emission of the substorm activity during the storm recovery resembles average isolated substorm precipitation. However, the energy dissipated, even at the very end of a prolonged storm recovery, is very large compared to the typical energy content of isolated substorms. In the case studied here, the dissipation of the excess energy is observed over a 3-h long period of several consecutive substorm intensifications. Our findings suggest that the substorm energy dissipation varies between the storm phases.

Description: Brightening of onset arc precedes the dipolarization onset: THEMIS observations of two events on 1 March 2008 Annales Geophysicae, 29, 2045-2059, 2011 Author(s): J. R. Kan, H. Li, C. Wang, H. U. Frey, M. V. Kubyshkina, A. Runov, C. J. Xiao, L. H. Lyu, and W. Sun We present a new M-I coupling model of substorm during southward IMF based on the THEMIS observations of two events on 1 March 2008. The first event (E-1) was classified as a pseudo-breakup: brightening of the onset arc preceded the first dipolarization onset by ∼71 ± 3 s, but the breakup arcs faded within ∼5 min without substantial poleward expansion and the dipolarization stopped and reversed to thinning. The second event (E-2) was identified as a substorm: brightening of the second onset arc preceded the second dipolarization onset by ∼80 ± 3 s, leading to a full-scale expanding auroral bulge during the substorm expansion phase for ∼20 min. The Alfvén travel time from the ionosphere to the dipolarization onset region is estimated at ∼69.3 s in E-1; at ∼80.3 s in E-2, which matched well with the observed time delay of the dipolarization onset after the brightening of the onset arc, respectively in E-1 and E-2. Brightening of the onset arc precedes the dipolarization onset suggest that the onset arc brightening is caused by the intense upward field-aligned currents originating from the divergence of the Cowling electrojet in the ionosphere. The Cowling electrojet current loop (CECL) is formed to close the field-aligned currents at all times. The closure current in the Alfvén wavefront is anti-parallel to the cross-tail current. Dipolarization onset occurs when the Alfvén wavefront incident on the near-Earth plasma sheet to disrupt the cross-tail current in the dipolarization region. Slow MHD waves dominate the disruption of the cross-tail current in the dipolarization region.

Description: Spatial scales of the magnetic ramp at the Venusian bow shock Annales Geophysicae, 29, 2081-2088, 2011 Author(s): A. P. Dimmock, S. N. Walker, T. L. Zhang, and S. A. Pope Typically multi-spacecraft missions are ideally suited to the study of shock spatial scales due to the separation of temporal and spatial variations. These missions are not possible at all locations and therefore in-situ multi-spacecraft measurements are not available beyond the Earth. The present paper presents a study of shock spatial scales using single spacecraft measurements made by the Venus Express spacecraft. The scales are determined based on previous knowledge of shock overshoot scales measured by the ISEE and Cluster missions. The study encompasses around 60 crossings of the Venusian bow shock from 2006 to 2009. The statistical relationship between the shock ramp spatial scales, overshoot and upstream shock parameters are investigated. We find that despite somewhat different solar wind conditions our results are comparable with those based on multi-spacecraft missions at the terrestrial bow shock.

Description: Cluster observations of a transient signature in the magnetotail: implications for the mode of reconnection Annales Geophysicae, 29, 2131-2146, 2011 Author(s): S. Beyene, C. J. Owen, A. P. Walsh, C. Forsyth, A. N. Fazakerley, S. Kiehas, I. Dandouras, and E. Lucek Travelling compression regions (TCRs) are perturbations in the magnetotail lobe magnetic field caused by structures moving Earthward or tailward within the plasma sheet. Previous works have suggested that these structures are created by either time-dependant reconnection occurring at a single X-line, forming a flux-bulge-type structure, or space-variant reconnection at multiple X-lines, forming flux-rope-type structures. In this study we examine an event in which Cluster 2 observed a TCR while the 3 remaining Cluster spacecraft observed the underlying magnetic structure at a range of distances from the neutral sheet. The magnetic structure has a velocity of (99, 154, −31) km s −1 in GSM (| V | = 186 km s −1 ), an estimated size of 1.19 R E along the direction of travel and a size between 1.94 and 2.86 R E in the direction perpendicular to the current sheet. As the structure passes the spacecraft, Cluster 1 and Cluster 4 observed a bipolar signature in B Z , plasma-sheet-like plasma and field-aligned electron flows. Cluster 3 passed closest to the centre of the structure and observed two separate reductions in the plasma density (with field-aligned electron flows); these drop-outs in the plasma sheet were possibly created by the actions of X-lines. The second drop-out in the plasma sheet also includes a reversal of the ion flow, a signature consistent with the passage of a reconnecting X-line past the spacecraft. Between the X-lines, the plasma outflow from the X-lines caused an increase in pressure which led to a localised expansion of the plasma and also the observations at Cluster 1 and Cluster 4 and the TCR. Our observations do not uniquely match either of the flux rope or the flux bulge predictions although the observation of two plasma sheet drop-outs (interpreted as X-lines, one active, one dormant) with plasma-sheet-like between them and only one TCR is a situation expected in multiple X-line reconnection.

Description: A note on the Weibel instability and thermal fluctuations Annales Geophysicae, 30, 427-431, 2012 Author(s): R. A. Treumann and W. Baumjohann The thermal fluctuation level of the Weibel instability is recalculated. It is shown that the divergence of the fluctuations at long wavelengths, i.e. the Weibel infrared catastrophe, never occurs. At large wavelengths the thermal fluctuation level is terminated by the presence of even the smallest available stable thermal anisotropy. Weibel fields penetrate only one skin depth into the plasma. When excited inside, they cause layers of antiparallel fields of skin depth width and vortices which may be subject to reconnection.

Description: Aspect sensitivity of polar mesosphere summer echoes based on ESRAD MST radar measurements in Kiruna, Sweden in 1997–2010 Annales Geophysicae, 30, 457-465, 2012 Author(s): M. Smirnova, E. Belova, and S. Kirkwood Aspect sensitivities of polar mesosphere summer echoes (PMSE) measured with the ESRAD 50 MHz radar in 1997–2010 are studied using the full correlation analysis technique. Half of PMSE detected each year are found to be highly aspect sensitive. Yearly median values of the aspect sensitivity parameter θ s , characterising the half-width of the scatterers' polar diagram, are 2.9–3.7° depending on the year. The other half of the PMSE have θ s values larger than 9–11° and cannot be evaluated using the ESRAD vertical beam only. PMSE aspect sensitivity reveals an altitude dependence, namely, the scatter becomes more isotropic with increasing height. This result is consistent with that reported in other studies. No dependence of PMSE aspect sensitivity on backscattered power for any year was identified. In the paper the limitations of the in-beam and off-vertical beam methods for estimation of PMSE aspect sensitivity are discussed. We conclude that both methods should be combined in order to get complete information about PMSE aspect sensitivity and to estimate correctly PMSE absolute strength.

Description: Signatures of moving polar cap arcs in the F-region PolarDARN echoes Annales Geophysicae, 30, 441-455, 2012 Author(s): A. V. Koustov, K. Hosokawa, N. Nishitani, K. Shiokawa, and H. Liu Joint observations of the all-sky camera at Resolute Bay (Nunavut, Canada) and the Polar Dual Auroral Radar Network (PolarDARN) HF radars at Rankin Inlet and Inuvik (Canada) are considered to establish radar signatures of poleward moving polar cap arcs "detaching" from the auroral oval. Common features of the events considered are enhanced power or echo occurrence in the wake of the arcs and enhanced spectral width of these echoes. When the arcs were oriented along some of the radar beams, velocity reversals at the arc location were observed with the directions of the arc-associated flows corresponding to a converging electric field. For the event of 9 December 2007, two arcs were poleward progressing almost along the central beams of the Inuvik radar at the speed close to the E × B drift of the bulk of the F-region plasma as inferred from HF Doppler velocities and from independent measurements by the Resolute Bay ionosonde. In global-scale convection maps inferred from all Super Dual Auroral Radar Network (SuperDARN) radar measurements, the polar cap arcs were often seen close to the reversal line of additional mesoscale convection cells located poleward of the normal cells related to the auroral oval.

Description: Electromagnetic ELF wave intensification associated with fast earthward flows in mid-tail plasma sheet Annales Geophysicae, 30, 467-488, 2012 Author(s): J. Liang, B. Ni, C. M. Cully, E. F. Donovan, R. M. Thorne, and V. Angelopoulos In this study we perform a statistical survey of the extremely-low-frequency wave activities associated with fast earthward flows in the mid-tail central plasma sheet (CPS) based upon THEMIS measurements. We reveal clear trends of increasing wave intensity with flow enhancement over a broad frequency range, from below f LH (lower-hybrid resonant frequency) to above f ce (electron gyrofrequency). We mainly investigate two electromagnetic wave modes, the lower-hybrid waves at frequencies below f LH , and the whistler-mode waves in the frequency range f LH 〈 f 〈 f ce . The waves at f 〈 f LH dramatically intensify during fast flow intervals, and tend to contain strong electromagnetic components in the high-plasma-beta CPS region, consistent with the theoretical expectation of the lower-hybrid drift instability in the center region of the tail current sheet. ULF waves with very large perpendicular wavenumber might be Doppler-shifted by the flows and also partly contribute to the observed waves in the lower-hybrid frequency range. The fast flow activity substantially increases the occurrence rate and peak magnitude of the electromagnetic waves in the frequency range f LH 〈 f 〈 f ce , though they still tend to be short-lived and sporadic in occurrence. We also find that the electron pitch-angle distribution in the mid-tail CPS undergoes a variation from negative anisotropy (perpendicular temperature smaller than parallel temperature) during weak flow intervals, to more or less positive anisotropy (perpendicular temperature larger than parallel temperature) during fast flow intervals. The flow-related electromagnetic whistler-mode wave tends to occur in conjunction with positive electron anisotropy.

Description: Magnetopause energy transfer dependence on the interplanetary magnetic field and the Earth's magnetic dipole axis orientation Annales Geophysicae, 30, 515-526, 2012 Author(s): M. Palmroth, R. C. Fear, and I. Honkonen We examine the spatial variation of magnetospheric energy transfer using a global magnetohydrodynamic (MHD) simulation (GUMICS-4) and a large data set of flux transfer events (FTEs) observed by the Cluster spacecraft. Our main purpose is to investigate whether it is possible to validate previous results on the spatial energy transfer variation from the GUMICS-4 simulation using the statistical occurrence of FTEs, which are manifestations of magnetospheric energy transfer. Previous simulation results have suggested that the energy transfer pattern at the magnetopause rotates according to the interplanetary magnetic field (IMF) orientation, and here we investigate whether a similar rotation is seen in the locations at which FTE signatures are observed. We find that there is qualitative agreement between the simulation and observed statistics, as the peaks in both distributions rotate as a function of the IMF clock angle. However, it is necessary to take into account the modulation of the statistical distribution that is caused by a bias towards in situ FTE signatures being observed in the winter hemisphere (an effect that has previously been predicted and observed in this data set). Taking this seasonal effect into account, the FTE locations support the previous simulation results and confirm the earlier prediction that the energy transfers in the plane of the IMF. In addition, we investigate the effect of the dipole orientation (both the dipole tilt angle and its orientation in the plane perpendicular to the solar wind flow) on the energy transfer spatial distribution. We find that the energy transfer occurs mainly in the summer hemisphere, and that the dayside reconnection region is located asymmetrically about the subsolar position. Finally, we find that the energy transfer is 10% larger at equinox conditions than at solstice, contributing to the discussion concerning the semiannual variation of magnetospheric dynamics (known as "the Russell-McPherron effect").

Description: Secondary charging effects due to icy dust particle impacts on rocket payloads Annales Geophysicae, 30, 433-439, 2012 Author(s): M. Kassa, M. Rapp, T. W. Hartquist, and O. Havnes We report measurements of dust currents obtained with a small probe and a larger probe during the flight of the ECOMA-4 rocket through the summer polar mesosphere. The payload included two small dust probes behind a larger dust probe located centrally at the front. For certain phases of the payload rotation, the current registered by one of the small dust probes was up to 2 times the current measured with the larger probe, even though the effective collection area of the larger probe was 4 times that of the small one. We analyze the phase dependence of the currents and their difference with a model based on the assumption that the small probe was hit by charged dust fragments produced in collisions of mesospheric dust with the payload body. Our results confirm earlier findings that secondary charge production in the collision of a noctilucent cloud/Polar Summer Mesospheric Echo (NLC/PMSE) dust particle with the payload body must be several orders of magnitude larger than might be expected from laboratory studies of collisions of pure ice particles with a variety of clean surfaces. An important consequence is that for some payload configurations, one should not assume that the current measured with a detector used to study mesospheric dust is simply proportional to the number density of ambient dust particles. The higher secondary charge production may be due to the NLC/PMSE particles containing multiple meteoric smoke particles.

Description: Overlapping ion structures in the mid-altitude cusp under northward IMF: signature of dual lobe reconnection? Annales Geophysicae, 30, 489-501, 2012 Author(s): F. Pitout, C. P. Escoubet, M. G. G. T. Taylor, J. Berchem, and A. P. Walsh On some rare occasions, data from the Cluster Ion Spectrometer (CIS) in the mid-altitude cusp reveal overlapping ion populations under northward interplanetary magnetic field (IMF). While the poleward part of the cusp exhibits the expected reverse dispersion due to lobe reconnection, its equatorward part shows a second ion population at higher-energy that coexists with the low energy tail of the dispersion. This second population is either dispersionless or slightly dispersed with energies increasing with increasing latitudes, indicative of lobe reconnection as well. Our analysis of a case that occurred 3 September 2002 when the IMF stayed northward for more than two hours suggests that the second population comes from the opposite hemisphere and is very likely on newly-closed field lines. We interpret this overlap of cusp populations as a clear mid-altitude signature of re-closed magnetic field lines by double lobe reconnection (reconnection in both hemispheres) under northward IMF. This interpretation is supported by modelling performed with the Cooling model and an MHD model.

Description: Global magnetospheric response to an interplanetary shock: THEMIS observations Annales Geophysicae, 30, 379-387, 2012 Author(s): H. Zhang, D. G. Sibeck, Q.-G. Zong, J. P. McFadden, D. Larson, K.-H. Glassmeier, and V. Angelopoulos We investigate the global response of the geospace plasma environment to an interplanetary (IP) shock at ~02:24 UT on 28 May 2008 from multiple THEMIS spacecraft observations in the magnetosheath (THEMIS B and C), the mid-afternoon magnetosphere (THEMIS A), and the dusk magnetosphere (THEMIS D and E). The interaction of the transmitted IP shock with the magnetosphere has global effects. Consequently, it can affect geospace plasma significantly. After interacting with the bow shock, the IP shock transmitted a fast shock and a discontinuity which propagated through the magnetosheath toward the Earth at speeds of 301 km s −1 and 137 km s −1 , respectively. THEMIS A observations indicate that the IP shock changed the properties of a plasmaspheric plume significantly. The plasmaspheric plume density increased rapidly from 10 to 100 cm −3 in 4 min and the ion distribution changed from an isotropic to a strongly anisotropic distribution. Electromagnetic ion cyclotron (EMIC) waves observed by THEMIS A are most likely excited by the anisotropic ion distributions caused by the IP shock impact. THEMIS A, but not D or E, observed a plasmaspheric plume in the dayside magnetosphere. Multiple spacecraft observations indicate that the dawn-side edge of the plasmaspheric plume was located between THEMIS A and D (or E).

Description: Revisiting "Narrow Bipolar Event" intracloud lightning using the FORTE satellite Annales Geophysicae, 30, 389-404, 2012 Author(s): A. R. Jacobson and T. E. L. Light The lightning stroke called a "Narrow Bipolar Event", or NBE, is an intracloud discharge responsible for significant charge redistribution. The NBE occurs within 10–20 μs, and some associated process emits irregular bursts of intense radio noise, fading at shorter timescales, sporadically during the charge transfer. In previous reports, the NBE has been inferred to be quite different from other forms of lightning strokes, in two ways: First, the NBE has been inferred to be relatively dark (non-luminous) compared to other lightning strokes. Second, the NBE has been inferred to be isolated within the storm, usually not participating in flashes, but when it is in a flash, the NBE has been inferred to be the flash initiator. These two inferences have sufficiently stark implications for NBE physics that they should be subjected to further independent test, with improved statistics. We attempt such a test with both optical and radio data from the FORTE satellite, and with lightning-stroke data from the Los Alamos Sferic Array. We show rigorously that by the metric of triggering the PDD optical photometer aboard the FORTE satellite, NBE discharges are indeed less luminous than ordinary lightning. Referred to an effective isotropic emitter at the cloud top, NBE light output is inferred to be less than ~3 × 10 8 W. To address isolation of NBEs, we first expand the pool of geolocated intracloud radio recordings, by borrowing geolocations from either the same flash's or the same storm's other recordings. In this manner we generate a pool of ~2 × 10 5 unique and independent FORTE intracloud radio recordings, whose slant range from the satellite can be inferred. We then use this slant range to calculate the Effective Radiated Power (ERP) at the radio source, in the passband 26–49 MHz. Stratifying the radio recordings by ERP into eight bins, from a lowest bin ( 140 kW), we document a trend for the radio recordings to become more isolated in time as the ERP increases. The highest ERP bin corresponds to the intracloud emissions associated with NBEs. At the highest ERP, the only significant probability of temporal neighbors is during times following the high-ERP events. In other words, when participating in a flash, the high-ERP emissions occur at the apparent flash initiation.

Description: Electron cross talk and asymmetric electron distributions near the Earth's bowshock Annales Geophysicae, 30, 503-513, 2012 Author(s): J. J. Mitchell, S. J. Schwartz, and U. Auster Electron distributions in the magnetosheath display a number of far from equilibrium features. It has been suggested that one factor influencing these distributions may be the large distances separating locations at which electrons with different energies and pitch angles must cross the bowshock in order to reach a given point in the magnetosheath. The overall heating requirements at these distant locations depends strongly on the shock geometry. In the absence of collisions or other isotropization processes this suggests that the convolution of electrons arriving from different locations should give rise to asymmetries in the distribution functions. Moreover, such cross-talk could influence the relative electron to ion heating, rendering the shock heating problem intrinsically non-local in contrast to classic shock physics. Here, we study electron distributions measured simultaneously by the Plasma Electron and Current Experiment (PEACE) on board the Cluster spacecraft and the Electrostatic Analyser (ESA) on board THEMIS b during a time interval in which both the Cluster spacecraft and THEMIS b are in the magnetosheath, close to the bowshock, and during which the local magnetic field orientation makes it likely that electron trajectories may connect both spacecraft. We find that the relevant portions of the velocity distributions of such electrons measured by each spacecraft display remarkable similarities. We map trajectories of electrons arriving at each spacecraft back to the locations at which they crossed the bowshock, as a function of pitch angle and energy. We then use the Rankine-Hugoniot relations to estimate the heating of electrons and compare this with temperature asymmetries actually observed. We conclude that the electron distributions and temperatures in the magnetosheath depend heavily on non-local shock properties.

Description: Global characteristics of the lunar tidal modulation of the equatorial electrojet derived from CHAMP observations Annales Geophysicae, 30, 527-536, 2012 Author(s): H. Lühr, T. A. Siddiqui, and S. Maus It has been known since many decades that lunar tide has an influence on the strength of the equatorial electrojet (EEJ). There has, however, never been a comprehensive study of the tidal effect on a global scale. Based on the continuous magnetic field measurements by the CHAMP satellite over 10 years it is possible to investigate the various aspects of lunar effects on the EEJ. The EEJ intensity is enhanced around times when the moon is overhead or at the antipode. This effect is particularly strong around noon, shortly after new and full moon. The lunar tide manifests itself as a semi-diurnal wave that precesses through all local times within one lunar month. The largest tidal amplitudes are observed around December solstice and smallest around June solstice. The tidal wave crest lags behind the moon phase. During December this amounts to about 4 days while it is around 2 days during other times of the year. We have not found significant longitudinal variations of the lunar influence on the EEJ. When comparing the average EEJ amplitude at high solar activity with that during periods of solar minimum conditions a solar cycle dependence can be found, but the ratio between tidal amplitude and EEJ intensity stays the same. Actually, tidal signatures standout clearer during times of low solar activity. We suggest that the tidal variations are caused by a current system added to the EEJ rather than by modulating the EEJ. Gravitational forcing of the lower atmosphere by the moon and the sun is assumed to be the driver of an upward propagating tidal wave. The larger tidal amplitudes around December solstice can be related to stratospheric warming events which seem to improve the conditions for upward propagation. The results described here have to large extent been presented as a Julius-Bartels Medal Lecture during the General Assembly 2011 of the European Geosciences Union.

Description: Vertical electron density and topside effective scale height ( H T ) variations over the Indian equatorial and low latitude stations Annales Geophysicae, 29, 1861-1872, 2011 Author(s): K. Venkatesh, P. V. S. Rama Rao, P. L. Saranya, D. S. V. V. D. Prasad, and K. Niranjan Understanding the vertical electron density profile, which is the altitudinal variation of ionospheric electron density distribution is an important aspect for the ionospheric investigations. In this paper, the bottom-side electron density profiles derived from ground based ionosonde data and the ROCSAT-1 in-situ electron density data were used to determine the estimates of the topside electron density profiles using α-Chapman function over an equatorial station Trivandrum (8.47° N, 76.91° E) and a low latitude station Waltair (17.7° N, 83.3° E) in the Indian region. The reconstructed electron density profiles are compared with IRI (2007) model derived vertical electron density profiles which resulted in significant deviations between the two different profiles. Both the reconstructed electron density profiles and the IRI model derived profiles are integrated independently to derive the Total Electron Content (TEC) values which are compared with GPS derived TEC values. TEC values derived from the reconstructed electron density profiles give better estimates with the GPS-TEC compared to those of IRI model derived TEC values. Compared to the GPS-TEC, the IRI model is underestimating the TEC values during day-time and is overestimating during night-time at both the stations. The percentage deviations of IRI derived TEC from GPS-TEC are larger compared to those between reconstructed profile derived TEC and GPS-TEC. F2-layer peak electron density, peak height and electron density at ROCSAT altitudes (≈600 km) are used to derive the effective scale heights ( H T ) of the topside ionosphere during the period from July 2003 to June 2004. The diurnal and seasonal variations of H T and E × B drift velocities are presented in this paper. The diurnal variation of the effective scale height ( H T ) shows peak values around noon hours with higher values during day-time and lower values during night-time both at Trivandrum and Waltair. The E × B drift velocities at both the places also have shown a clear diurnal variation with a negative peak around 04:00 LT and maximum during day-time hours. The higher and lower values of H T seem to be associated with positive and negative phases of the E × B drift velocities, respectively.

Description: Anomalous foreshock field-aligned beams observed by Cluster Annales Geophysicae, 29, 1967-1975, 2011 Author(s): K. Meziane, A. M. Hamza, M. Wilber, C. Mazelle, and M. A. Lee We report occasional observations of two simultaneously distinct ion foreshock components recorded by the Cluster spacecraft upstream of the Earth's bow shock. In most occurrences, the lower-energy population originates as a field-aligned beam (FAB) associated with quasi-perpendicular regions, which loses energy as the IMF rotates into oblique geometries. A second beam, with energies in excess of ~10 keV, appears sometimes in association with the onset of ultra-low frequency (ULF) waves, and sometimes ahead of the appearance of the latter. Measurements from the mass spectrometer indicate that both beams consist of protons. While the lower-speed beam is well-accounted for by a known reflection mechanism, the non-radial IMF orientations as well as other arguments seem to rule out magnetosheath or magnetospheric sources for the higher energy component. The wave characteristics are typical of the oblique foreshock and we have found that they are in cyclotron-resonance with the low speed beam (FAB). These observations constitute a theoretical challenge since conventional mechanisms described in the literature cannot account for the production of beams at two different energies.

Description: Response of the equatorial ionosphere to the total solar eclipse of 22 July 2009 and annular eclipse of 15 January 2010 as observed from a network of stations situated in the Indian longitude sector Annales Geophysicae, 29, 1955-1965, 2011 Author(s): A. Paul, T. Das, S. Ray, A. Das, D. Bhowmick, and A. DasGupta Dual-frequency GPS TEC monitors have been used to study the response of the ionosphere to the solar eclipses of 22 July 2009 and 15 January 2010. The receivers were located at three stations, Calcutta, Kharagpur and Baharampore which are situated outside the umbra zone in the Indian longitude sector with each baseline being ~200 km. Effects of obscuration of the solar disc were noted in the ambient TEC recorded at the three stations. A series of depletions in TEC along the track of a GPS satellite and associated wave-like structures were identified on some GPS links during both the eclipses.

Description: Characteristics of Arctic tides at CANDAC-PEARL (80° N, 86° W) and Svalbard (78° N, 16° E) for 2006–2009: radar observations and comparisons with the model CMAM-DAS Annales Geophysicae, 29, 1939-1954, 2011 Author(s): A. H. Manson, C. E. Meek, X. Xu, T. Aso, J. R. Drummond, C. M. Hall, W. K. Hocking, M. Tsutsumi, and W. E. Ward Operation of a Meteor Radar (MWR) at Eureka, Ellesmere Island (80° N, 86° W) began in February 2006: this is the location of the Polar Environmental and Atmospheric Research Laboratory (PEARL), operated by the "Canadian Network for the Detection of Atmospheric Change" (CANDAC). The first 36 months of tidal wind data (82–97 km) are here combined with contemporaneous tides from the Meteor Radar (MWR) at Adventdalen, Svalbard (78° N, 16° E), to provide the first significant evidence for interannual variability (IAV) of the High Arctic's diurnal and semidiurnal migrating (MT) and non-migrating tides (NMT). The three-year monthly means for both diurnal (DT) and semi-diurnal (SDT) winds demonstrate significantly different amplitudes and phases at Eureka and Svalbard. Typically the summer-maximizing DT is much larger (~24 m s −1 at 97 km) at Eureka, while the Svalbard tide (5–24 m s −1 at 97 km)) is almost linear (north-south) rather than circular. Interannual variations are smallest in the summer and autumn months. The High Arctic SDT has maxima centred on August/September, followed in size by the winter features; and is much larger at Svalbard (24 m s −1 at 97 km, versus 14–18 m s −1 in central Canada). Depending on the location, the IAV are largest in spring/winter (Eureka) and summer/autumn (Svalbard). Fitting of wave-numbers for the migrating and non-migrating tides (MT, NMT) determines dominant tides for each month and height. Existence of NMT is consistent with nonlinear interactions between migrating tides and (quasi) stationary planetary wave (SPW) S =1 (SPW1). For the diurnal oscillation, NMT s =0 for the east-west (EW) wind component dominates (largest tide) in the late autumn and winter (November–February); and s =+2 is frequently seen in the north-south (NS) wind component for the same months. The semi-diurnal oscillation's NMT s =+1 dominates from March to June/July. There are patches of s =+3 and +1, in the late fall-winter. These wave numbers are also consistent with SPW1-MT interactions. Comparisons for 2007 of the observed DT and SDT at 78–80° N, with those within the Canadian Middle Atmosphere Model Data Assimilation System CMAM-DAS, are a major feature of this paper. The diurnal tides for the two locations have important similarities as observed and modeled, with seasonal maxima in the mesosphere from April to October, and similar phases with long/evanescent wavelengths. However, differences are also significant: observed Eureka amplitudes are generally larger than the model; and at Svalbard the modeled tide is classically circular, rather than anomalous. For the semi-diurnal tide, the amplitudes and phases differ markedly between Eureka and Svalbard for both MWR-radar data and CMAM-DAS data. The seasonal variations from observed and modeled archives also differ at each location. Tidal NMT-amplitudes and wave-numbers for the model differ substantially from observations.

Description: Winds and tides in the mid-latitude Southern Hemisphere upper mesosphere recorded with the Falkland Islands SuperDARN radar Annales Geophysicae, 29, 1985-1996, 2011 Author(s): R. E. Hibbins, M. P. Freeman, S. E. Milan, and J. M. Ruohoniemi Meteor wind data from the first year of operation of the Falkland Islands SuperDARN radar (52° S, 59° W) are used to characterize the atmospheric tides and background winds in the upper mesosphere above the South Atlantic. Strong (〉40 m s −1 ) semidiurnal tides are observed in the winter time and large amplitude (〉60 m s −1 ) bursts of quasi two-day wave activity are seen in January 2011. Data are in good agreement with those presented from the SAAMER meteor radar (54° S, 68° W). Comparison with SuperDARN meteor wind data from a geographically similar Northern Hemisphere site at Goose Bay (53° N 60° W) reveal clear interhemispheric differences especially in the semidiurnal and terdiurnal components of the tides. The winter time amplitudes of the tides are much stronger in the Southern Hemisphere than in the north. Background winds are observed to be significantly more polewards and westwards throughout the year than those predicted by the empirical horizontal wind model HWM07.

Description: Role of fine mode aerosols in modulating cloud properties over industrial locations in north India Annales Geophysicae, 29, 1605-1612, 2011 Author(s): B. Abish and K. Mohanakumar The influence of aerosols on cloud properties over North India which includes the Indo-Gangetic Plain has been investigated for the years 2000 to 2010. During the years 2004, 2009 and 2010 there has been an abrupt increase in fine mode aerosol optical depth (AOD) inducing a sharp decline in cloud effective radius (CER) in the month of January. The following monsoon during these years was a failure in the region considered for the study. In the year 2010, a highest AOD value of 0.35 was recorded in the month of January. In accordance with the aerosol indirect effect, this large increase in AOD resulted in a significant reduction in CER. The monsoon season in that year was deficient in the study region even though the rest of the country received above normal rainfall. For the years when CER diminished below 12 microns in the month of May, a delay in the advancement of monsoon towards North India is noted even after a normal or early onset in southern peninsula. Meanwhile, a rapid progression took place when it was 12 microns or above. During non-monsoon months an inverse relationship existed between cloud effective radius and liquid water path and a strong positive association occurred in the monsoon months. Present analysis suggests that the excessive aerosol loading and the associated aerosol indirect effects in the months prior to the monsoon season has an effect on the propagation and onset of the south west monsoon over the region.

Description: Ducted propagation of chorus waves: Cluster observations Annales Geophysicae, 29, 1629-1634, 2011 Author(s): K. H. Yearby, M. A. Balikhin, Yu. V. Khotyaintsev, S. N. Walker, V. V. Krasnoselskikh, H. St. C. K. Alleyne, and O. Agapitov Ducted propagation of whistler waves in the terrestrial magnetosphere-ionosphere system was discussed and studied long before the first in-situ spacecraft measurements. While a number of implicit examples of the existence of ducted propagation have been found, direct observation of ducts has been hampered by the low sampling rates of measurements of the plasma density. The present paper is based on Cluster observations of chorus waves. The ability to use measurements of the spacecraft potential as a proxy for high time resolution electron density measurements is exploited to identify a number of cases when increased chorus wave power, observed within the radiation belts, is observed simultaneously with density enchantments. It is argued that the observation of ducted propagation of chorus implies modification of numerical models for plasma-wave interactions within the radiation belts.

Description: Magnetopause reconnection across wide local time Annales Geophysicae, 29, 1683-1697, 2011 Author(s): M. W. Dunlop, Q.-H. Zhang, Y. V. Bogdanova, K. J. Trattner, Z. Pu, H. Hasegawa, J. Berchem, M. G. G. T. Taylor, M. Volwerk, J. P. Eastwood, B. Lavraud, C. Shen, J.-K. Shi, J. Wang, D. Constantinescu, A. N. Fazakerley, H. Frey, D. Sibeck, P. Escoubet, J. A. Wild, Z. X. Liu, and C. Carr During April to July 2007 a combination of 10 spacecraft provided simultaneous monitoring of the dayside magnetopause across a wide range of local times. The array of four Cluster spacecraft, separated at large distances (10 000 km), were traversing the dawn-side magnetopause at high and low latitudes; the five THEMIS spacecraft were often in a 4 + 1 grouped configuration, traversing the low latitude, dusk-side magnetosphere, and the Double star, TC-1 spacecraft was in an equatorial orbit between the local times of the THEMIS and Cluster orbits. We show here a number of near simultaneous conjunctions of all 10 spacecraft at the magnetopause. One conjunction identifies an extended magnetic reconnection X-line, tilted in the low latitude, sub-solar region, which exists together with active anti-parallel reconnection sites extending to locations on the dawn-side flank. Oppositely moving FTE's are observed on all spacecraft, consistent with the initially strong IMF B y conditions and the comparative locations of the spacecraft both dusk-ward and dawn-ward of noon. Comparison with other conjunctions of magnetopause crossings, which are also distributed over wide local times, supports the result that reconnection activity may occur at many sites simultaneously across the sub-solar and flank magnetopause, but linked to the large scale (extended) configuration of the merging line; broadly depending on IMF orientation. The occurrence of MR therefore inherently follows a "component" driven scenario irrespective of the guide field conditions. Some conjunctions allow the global magnetopause response to IMF changes to be observed and the distribution of spacecraft can directly confirm its shape, motion and deformation at local noon, dawn and dusk-side, simultaneously.

Description: Characteristics of a persistent "pool of inhibited cloudiness" and its genesis over the Bay of Bengal associated with the Asian summer monsoon Annales Geophysicae, 29, 1247-1252, 2011 Author(s): Anish Kumar M. Nair, K. Rajeev, S. Sijikumar, and S. Meenu Using spatial and vertical distributions of clouds derived from multi-year spaceborne observations, this paper presents the characteristics of a significant "pool of inhibited cloudiness" covering an area of 〉10 6 km 2 between 3–13° N and 77–90° E over the Bay of Bengal (BoB), persisting throughout the Asian summer monsoon season (ASM). Seasonal mean precipitation rate over the "pool" is

Description: Coordinated Cluster/Double Star and ground-based observations of dayside reconnection signatures on 11 February 2004 Annales Geophysicae, 29, 1827-1847, 2011 Author(s): Q.-H. Zhang, M. W. Dunlop, R.-Y. Liu, H.-G. Yang, H.-Q. Hu, B.-C. Zhang, M. Lester, Y. V. Bogdanova, I. W. McCrea, Z.-J. Hu, S. R. Crothers, C. La Hoz, and C. P. Nielsen A number of flux transfer events (FTEs) were observed between 09:00 and 12:00 UT on 11 February 2004, during southward and dawnward IMF, while the Cluster spacecraft array moved outbound through the northern, high-altitude cusp and dayside high-latitude boundary layer, and the Double Star TC-1 spacecraft was crossing the dayside low-latitude magnetopause into the magnetosheath south of the ecliptic plane. The Cluster array grazed the equatorial cusp boundary, observing reconnection-like mixing of magnetosheath and magnetospheric plasma populations. In an adjacent interval, TC-1 sampled a series of sometimes none standard FTEs, but also with mixed magnetosheath and magnetospheric plasma populations, near the magnetopause crossing and later showed additional (possibly turbulent) activity not characteristic of FTEs when it was situated deeper in the magnetosheath. The motion of these FTEs are analyzed in some detail to compare to simultaneous, poleward-moving plasma concentration enhancements recorded by EISCAT Svalbard Radar (ESR) and "poleward-moving radar auroral forms" (PMRAFs) on the CUTLASS Finland and Kerguelen Super Dual Auroral Radar Network (SuperDARN) radar measurements. Conjugate SuperDARN observations show a predominantly two-cell convection pattern in the Northern and Southern Hemispheres. The results are consistent with the expected motion of reconnected magnetic flux tubes, arising from a predominantly sub-solar reconnection site. Here, we are able to track north and south in closely adjacent intervals as well as to map to the corresponding ionospheric footprints of the implied flux tubes and demonstrate these are temporally correlated with clear ionospheric velocity enhancements, having northward (southward) and eastward (westward) convected flow components in the Northern (Southern) Hemisphere. The durations of these enhancements might imply that the evolution time of the FTEs is about 18–22 min from their origin on magnetopause (at reconnection site) to their addition to the magnetotail lobe. However, the ionospheric response time in the Northern Hemisphere is about 2–4 min longer than the response time in the Southern Hemisphere.

Description: Timing mirror structures observed by Cluster with a magnetosheath flow model Annales Geophysicae, 29, 1849-1860, 2011 Author(s): V. Génot, L. Broussillou, E. Budnik, P. Hellinger, P. M. Trávníček, E. Lucek, and I. Dandouras The evolution of structures associated with mirror modes during their flow in the Earth's magnetosheath is studied. The fact that the related magnetic fluctuations can take distinct shapes, from deep holes to high peaks, has been assessed in previous works on the observational, modeling and numerical points of view. In this paper we present an analytical model for the flow lines and velocity magnitude inside the magnetosheath. This model is used to interpret almost 10 years of Cluster observations of mirror structures: by back tracking each isolated observation to the shock, the "age", or flow time, of these structures is determined together with the geometry of the shock. Using this flow time the evolutionary path of the structures may be studied with respect to different quantities: the distance to mirror threshold, the amplitude of mirror fluctuations and the skewness of the magnetic amplitude distribution as a marker of the shape of the structures. These behaviours are confronted to numerical simulations which confirm the dynamical perspective gained from the association of the statistical analysis and the analytical model: magnetic peaks are mostly formed just behind the shock and are quickly overwhelmed by magnetic holes as the plasma conditions get more mirror stable. The amplitude of the fluctuations are found to saturate before the skewness vanishes, i.e. when both structures quantitatively balance each other, which typically occurs after a flow time of 100–200 s in the Earth's magnetosheath. Comparison with other astrophysical contexts is discussed.

Description: Suprathermal electron acceleration during reconnection onset in the magnetotail Annales Geophysicae, 29, 1917-1925, 2011 Author(s): A. Vaivads, A. Retinò, Yu. V. Khotyaintsev, and M. André We study one event of reconnection onset associated to a small substorm on 27 September 2006 by using Cluster observations at inter-spacecraft separation of about 10 000 km. We focus on the acceleration of suprathermal electrons during different stages of reconnection. We show that several distinct stages of acceleration occur: (1) moderate acceleration during reconnection of pre-existing plasma sheet flux tubes, (2) stronger acceleration during reconnection of lobe flux tubes, (3) production of the most energetic electrons within dipolarization fronts (magnetic pile-up regions). The strongest acceleration is reached at the location of B z maxima inside the magnetic pile-up region where the reconnection jet stops. Very strong localized dawn-dusk electric field are observed within the magnetic pile-up regions and are associated to most of the magnetic flux transport.

Description: On the characteristics of 150-km echoes observed in the Brazilian longitude sector by the 30 MHz São Luís radar Annales Geophysicae, 29, 1905-1916, 2011 Author(s): F. S. Rodrigues, E. R. de Paula, and J. L. Chau We present long-overdue details about the intensity and spectral characteristics of 150-km echoes observed by the São Luís radar in Brazil. The São Luís observations show that the echoes usually come from multiple scattering layers that descend in altitude before local noon, and ascend during afternoon hours, similar to what has been found in observations made in other longitude sectors. The layers are usually 3–5 km thick and located, mostly, between 130 and 170 km altitude. The measurements also show variations in echo intensity that are similar to observations made at other equatorial and off-equatorial sites. Analysis of observations made during 2008 shows significant (〉37%) monthly occurrence rates for every month. Reduced occurrence rates were observed around March Equinox. We associate this reduction in occurrence rate, however, to a non-geophysical factor. An increase in the daytime sky noise in the months around March Equinox causes a decrease in the signal-to-noise ratio (SNR) of the echoes, which makes them less distinguishable in our analysis. A higher occurrence of weaker echoes around March Equinox was confirmed by an statistical analysis of the seasonal variation of echo intensities. Strong, long-lasting and, therefore, more noticeable echoing layers, however, were observed between June and early September compared to other months in 2008. Spectral analyses show that most of the echoes have negative mean Doppler shifts indicating upward velocities. The echoes also have narrow spectral widths of only a few m s −1 . Finally, we also found that the mean Doppler shift of the observed echoes can vary noticeably with altitude at times. Using spaced antenna measurements we show that this is caused by the wide field-of-view of the radar and the spatial distribution of the scatterers within the radar beam.

Description: Characteristics of Arctic winds at CANDAC-PEARL (80° N, 86° W) and Svalbard (78° N, 16° E) for 2006–2009: radar observations and comparisons with the model CMAM-DAS Annales Geophysicae, 29, 1927-1938, 2011 Author(s): A. H. Manson, C. E. Meek, X. Xu, T. Aso, J. R. Drummond, C. M. Hall, W. K. Hocking, M. Tsutsumi, and W. E. Ward Operation of a Meteor Wind Radar (MWR) at Eureka, Ellesmere Island (80° N, 86° W) began in February 2006; this is the location of the Polar Environmental and Atmospheric Research Laboratory (PEARL), operated by the "Canadian Network for the Detection of Atmospheric Change" (CANDAC). The first 36 months of wind data (82–97 km) are here combined with contemporaneous winds from the Meteor Wind Radar at Adventdalen, Svalbard (78° N, 16° E), to provide the first evidence for substantial interannual variability (IAV) of longitudinally spaced observations of mean/background winds and waves at such High Arctic latitudes. The influences of "Sudden Stratospheric Warmings" (SSW) are also apparent. Monthly meridional (north-south, NS) 3-year means for each location/radar demonstrate that winds (82–97 km) differ significantly between Canada and Norway, with winter-equinox values generally northward over Eureka and southward over Svalbard. Using January 2008 as case study, these oppositely directed meridional winds are related to mean positions of the Arctic mesospheric vortex. The vortex is from the Canadian Middle Atmosphere Model, with its Data Assimilation System (CMAM-DAS). The characteristics of "Sudden stratospheric Warmings" SSW in each of the three winters are noted, as well as their uniquely distinctive short-term mesospheric wind disturbances. Comparisons of the mean winds over 36 months at 78 and 80° N, with those within CMAM-DAS, are featured. E.g. for 2007, while both monthly mean EW and NS winds from CMAM/radar are quite similar over Eureka (82–88 km), the modeled autumn-winter NS winds over Svalbard (73–88 km) differ significantly from observations. The latter are southward, and the modeled winds over Svalbard are predominately northward. The mean positions of the winter polar vortex are related to these differences.

Description: Quasi-linear dynamics of Weibel instability Annales Geophysicae, 29, 1997-2001, 2011 Author(s): O. A. Pokhotelov and O. A. Amariutei The quasi-linear dynamics of resonant Weibel mode is discussed. It is found that nonlinear saturation of Weibel mode is accompanied by substantial modification of the distribution function in resonant region. With the growth of the wave amplitude the parabolic bell-like form of the electron distribution function in this region converts into flatter shape, such as parabola of the fourth order. This results in significant weakening of the resonant interaction of the wave with particles. The latter becomes weaker and then becomes adiabatic interaction with the bulk of the plasma. This is similar to the case of Bernstein-Greene-Kruskal (Bernstein et al., 1957) electrostatic waves. The mathematical similarity of the Weibel and magnetic mirror instabilities is discussed.

Description: Comparison of the observed dependence of large-scale Birkeland currents on solar wind parameters with that obtained from global simulations Annales Geophysicae, 29, 1809-1826, 2011 Author(s): H. Korth, L. Rastätter, B. J. Anderson, and A. J. Ridley Spatial distributions of the large-scale Birkeland currents derived from magnetic field data acquired by the constellation of Iridium Communications satellites have been compared with global-magnetosphere magneto-hydrodynamic (MHD) simulations. The Iridium data, spanning the interval from February 1999 to December 2007, were first sorted into 45°-wide bins of the interplanetary magnetic field (IMF) clock angle, and the dependencies of the Birkeland currents on solar wind electric field magnitude, E yz , ram pressure, p sw , and Alfvén Mach number, M A , were then examined within each bin. The simulations have been conducted at the publicly-accessible Community Coordinated Modeling Center using the University of Michigan Space Weather modeling Framework, which features a global magnetosphere model coupled to the Rice Convection Model. In excess of 120 simulations with steady-state conditions were executed to yield the dependencies of the Birkeland currents on the solar wind and IMF parameters of the coupled model. Averaged over all IMF orientations, the simulation reproduces the Iridium statistical Birkeland current distributions with a two-dimensional correlation coefficient of about 0.8, and the total current agrees with the climatology averages to within 10%. The total current for individual events regularly exceeds those computed from statistical distributions by factors of ≥2, resulting in larger disparities between observations and simulations. The simulation results also qualitatively reflect the observed increases in total current with increasing E yz and p sw , but the model underestimates the rate of increase by up to 50%. The equatorward expansion and shift of the large-scale currents toward noon observed for increasing E yz are also evident in the simulation current patterns. Consistent with the observations, the simulation does not show a significant dependence of the total current on M A .

Description: Global observation of 24 November 2006 Pc5 pulsations by single mid-latitude underground [SQUID] 2 system Annales Geophysicae, 29, 1977-1984, 2011 Author(s): J. Marfaing, E. Pozzo di Borgo, G. Waysand, A. Cavaillou, and M. Parrot On 24 November 2006, simultaneous observations of Pc5 pulsations, electron precipitation and whistler-mode chorus, as well as solar wind and IMF parameters have been analyzed based on data from IMAGE magnetometers, riometer array and temporal VLF station. This paper focuses on the Pc5 pulsations detected at the same time, in the 1–25 millihertz range, by the [SQUID] 2 system (SQUID magnetometer within a S hielding QU alified for I onosphere D etection) installed 518 m underground at 43.92° N, 5.48° E. As expected, the 3-D-frequency spectrum of these mid-latitude [SQUID] 2 signals exhibits frequency peaks quasi identical to those observed by polar stations of close geomagnetic longitude. The signal/noise ratio allows the observation of the wave polarization and the beatings of the frequencies. As a result, the possibility of studying, at mid-latitude, magnetic Pc5 pulsations linked with an event in the magnetosphere can improve the description of both behaviour and propagation of these waves.

Description: Comparison of accelerated ion populations observed upstream of the bow shocks at Venus and Mars Annales Geophysicae, 29, 511-528, 2011 Author(s): M. Yamauchi, Y. Futaana, A. Fedorov, R. A. Frahm, J. D. Winningham, E. Dubinin, R. Lundin, S. Barabash, M. Holmström, C. Mazelle, J.-A. Sauvaud, T. L. Zhang, W. Baumjohann, A. J. Coates, and M. Fraenz Foreshock ions are compared between Venus and Mars at energies of 0.6~20 keV using the same ion instrument, the Ion Mass Analyser, on board both Venus Express and Mars Express. Venus Express often observes accelerated protons (2~6 times the solar wind energy) that travel away from the Venus bow shock when the spacecraft location is magnetically connected to the bow shock. The observed ions have a large field-aligned velocity compared to the perpendicular velocity in the solar wind frame, and are similar to the field-aligned beams and intermediate gyrating component of the foreshock ions in the terrestrial upstream region. Mars Express does not observe similar foreshock ions as does Venus Express, indicating that the Martian foreshock does not possess the intermediate gyrating component in the upstream region on the dayside of the planet. Instead, two types of gyrating protons in the solar wind frame are observed very close to the Martian quasi-perpendicular bow shock within a proton gyroradius distance. The first type is observed only within the region which is about 400 km from the bow shock and flows tailward nearly along the bow shock with a similar velocity as the solar wind. The second type is observed up to about 700 km from the bow shock and has a bundled structure in the energy domain. A traversal on 12 July 2005, in which the energy-bunching came from bundling in the magnetic field direction, is further examined. The observed velocities of the latter population are consistent with multiple specular reflections of the solar wind at the bow shock, and the ions after the second reflection have a field-aligned velocity larger than that of the de Hoffman-Teller velocity frame, i.e., their guiding center has moved toward interplanetary space out from the bow shock. To account for the observed peculiarity of the Martian upstream region, finite gyroradius effects of the solar wind protons compared to the radius of the bow shock curvature and effects of cold ion abundance in the bow shock are discussed.

Description: On the role of oceanic entrainment temperature ( T e ) in decadal changes of El Niño/Southern Oscillation Annales Geophysicae, 29, 529-540, 2011 Author(s): J. Zhu, G. Zhou, R.-H. Zhang, and Z. Sun The role of decadal changes in ocean thermal structure in modulating El Niño/Southern Oscillation (ENSO) properties was examined using a hybrid coupled model (HCM), consisting of a statistical atmospheric model and an oceanic general circulation model (OGCM) with an explicitly embedded empirical parameterization for the temperature of subsurface water entrained into the mixed layer ( T e ), which was constructed via an EOF analysis of model-based historical data. Using the empirical T e models constructed from two subperiods, 1963–1979 ( T e 63−79 ) and 1980–1996 ( T e 80−96 ), the coupled system exhibits striking different properties of interannual variability, including oscillation periods and the propagation characteristic of sea surface temperature anomalies (SSTAs) along the equator. In the T e 63−79 run, the model features a 2–3 yr oscillation and a westward propagation of SSTAs along the equator, while in the T e 80−96 run, it is characterized by a 4–5 yr oscillation and an eastward propagation. Furthermore, a Lag Covariance Analysis (LCOA) was utilized to illustrate the leading physical processes responsible for decadal change in SST. It is shown that the change in the structure of T e acts to modulate the relative strength of the zonal advective and thermocline feedbacks in the coupled system, leading to changes in ENSO properties. Two additional sensitive experiments were conducted to further illustrate the respective roles of the changes in ocean mean states and in T e in modulating ENSO behaviors. These decadal changes in the simulated ENSO properties are consistent with the observed shift occurred in the late 1970s and a previous simulation performed with an intermediate coupled model (ICM) described in Zhang and Busalacchi (2005), indicating a dominant role T e plays in decadal ENSO changes.

Description: The dependence of Pi2 waveforms on periodic velocity enhancements within bursty bulk flows Annales Geophysicae, 29, 493-509, 2011 Author(s): K. R. Murphy, I. J. Rae, I. R. Mann, A. P. Walsh, D. K. Milling, and A. Kale Pi2s are a category of Ultra Low Frequency (ULF) waves associated with the onset of magnetic substorms. Recent work has suggested that the deceleration of bulk plasma flows in the central plasmasheet, known as bursty bulk flows (BBFs), are able to directly-drive Pi2 oscillations. Some of these studies have further shown evidence that there is a one-to-one correlation between Pi2 magnetic waveforms observed on the ground and periodic peaks in flow velocity within the BBF, known as flow bursts. Utilising a favourable conjunction between the Geotail spacecraft and the Canadian Array for Real-time Investigations of Magnetic Activity (CARISMA) magnetometer array on 31 May 1998, we examine the causality of the link between BBF flow bursts and Pi2 waveforms. Using a series of analytical tests in both the time and frequency domains, we find that while the Pi2 and BBF waveforms are very similar, the ground response for this event occurs prior to the observed flow enhancements in the magnetotail. We conclude that during this specific case study the temporal variations of the flow bursts within the BBF are not directly-driving the observed ground-based Pi2 waveforms, despite the fact that a visual inspection of both time-series might initially suggest that there is a causal relationship. We postulate that rather than there being a direct causal relation, the similar waveforms observed in both Pi2s and BBFs may result from temporal variations in a common source for both the BBFs and the Pi2s, such as magnetic reconnection in the tail, this source modulating both the Pi2 and BBF at the same frequency.

Description: Observations and modeling of forward and reflected chorus waves captured by THEMIS Annales Geophysicae, 29, 541-550, 2011 Author(s): O. Agapitov, V. Krasnoselskikh, Yu. Zaliznyak, V. Angelopoulos, O. Le Contel, and G. Rolland Discrete ELF/VLF chorus emissions are the most intense electromagnetic plasma waves observed in the radiation belts of the Earth's magnetosphere. Chorus emissions, whistler-mode wave packets propagating roughly along magnetic field lines from a well-localized source in the vicinity of the magnetic equator to polar regions, can be reflected at low altitudes. After reflection, wave packets can return to the equatorial plane region. Understanding of whistler wave propagation and reflection is critical to a correct description of wave-particle interaction in the radiation belts. We focus on properties of reflected chorus emissions observed by the THEMIS (Time History of Events and Macroscale Interactions During Substorms) spacecraft Search Coil Magnetometer (SCM) and Electric Field Instrument (EFI) at ELF/VLF frequencies up to 4 kHz at L ≥8. We determine the direction of the Poynting flux and wave vector distribution for forward and reflected chorus waves. Although both types of chorus waves were detected near the magnetic equator and have similar, discrete structure and rising tones, reflected waves are attenuated by a factor of 10–30 and have 10% higher frequency than concurrently-observed forward waves. Modeling of wave propagation and reflection using geometrical optics ray-tracing allowed us to determine the chorus source region location and explain observed propagation characteristics. We find that reflected wave attenuation at a certain spatial region is caused by divergence of the ray paths of these non-ducted emissions, and that the frequency shift is caused by generation of the reflected waves at lower L-shells where the local equatorial gyrofrequency is larger.

Description: Climatology of GPS phase scintillation and HF radar backscatter for the high-latitude ionosphere under solar minimum conditions Annales Geophysicae, 29, 377-392, 2011 Author(s): P. Prikryl, P. T. Jayachandran, S. C. Mushini, and R. Chadwick Maps of GPS phase scintillation at high latitudes have been constructed after the first two years of operation of the Canadian High Arctic Ionospheric Network (CHAIN) during the 2008–2009 solar minimum. CHAIN consists of ten dual-frequency receivers, configured to measure amplitude and phase scintillation from L1 GPS signals and ionospheric total electron content (TEC) from L1 and L2 GPS signals. Those ionospheric data have been mapped as a function of magnetic local time and geomagnetic latitude assuming ionospheric pierce points (IPPs) at 350 km. The mean TEC depletions are identified with the statistical high-latitude and mid-latitude troughs. Phase scintillation occurs predominantly in the nightside auroral oval and the ionospheric footprint of the cusp. The strongest phase scintillation is associated with auroral arc brightening and substorms or with perturbed cusp ionosphere. Auroral phase scintillation tends to be intermittent, localized and of short duration, while the dayside scintillation observed for individual satellites can stay continuously above a given threshold for several minutes and such scintillation patches persist over a large area of the cusp/cleft region sampled by different satellites for several hours. The seasonal variation of the phase scintillation occurrence also differs between the nightside auroral oval and the cusp. The auroral phase scintillation shows an expected semiannual oscillation with equinoctial maxima known to be associated with aurorae, while the cusp scintillation is dominated by an annual cycle maximizing in autumn-winter. These differences point to different irregularity production mechanisms: energetic electron precipitation into dynamic auroral arcs versus cusp ionospheric convection dynamics. Observations suggest anisotropy of scintillation-causing irregularities with stronger L-shell alignment of irregularities in the cusp while a significant component of field-aligned irregularities is found in the nightside auroral oval. Scintillation-causing irregularities can coexist with small-scale field-aligned irregularities resulting in HF radar backscatter. The statistical cusp and auroral oval are characterized by the occurrence of HF radar ionospheric backscatter and mean ground magnetic perturbations due to ionospheric currents.

Description: Statistics of lower tropospheric inversions over the continental United States Annales Geophysicae, 29, 401-410, 2011 Author(s): Y. H. Zhang, S. D. Zhang, F. Yi, and Z. Y. Chen The basic structure parameters of lower tropospheric inversions (LTIs) have been derived from 10 years (1998–2007) of high vertical resolution (~50 m) radiosonde observations over 56 United States stations. Seasonal and longitudinal variability of these parameters are presented and the formation mechanisms of LTI are also discussed. It is found that LTI seems to be a common feature over the continental United States. The LTI occurrence rates (defined as the fraction of measurements with LTI, which is calculated from the number of LTI cases divided by the number of measurements of the whole 10 years) at these 56 stations vary from 3.7% to 14.5%; the averaged base heights of LTI have a range of 3–5 km above mean sea level (a.m.s.l.); the averaged thicknesses and temperature jump ranges from 420–465 m and 1.9–2.2 K, respectively. These parameters have an obvious seasonal variation. In winter, all the occurrence rates, thicknesses and temperature jumps of LTI have much larger values than those in summer. LTI occurrence rate shows an obvious west-east increasing trend in all 4 seasons. Detailed analyses reveal that dynamical instability induced by strong zonal wind shear is responsible for LTI in winter, spring and autumn; the frontal system tends to generate LTI in summer. Since the higher occurrence rate, larger temperature jump and larger thickness of LTI occur in winter, we believe strong zonal wind shear plays a more important role in the formation of LTI.

Description: Sea surface wind perturbations over the Kashevarov Bank of the Okhotsk Sea: a satellite study Annales Geophysicae, 29, 393-399, 2011 Author(s): T. I. Tarkhova, M. S. Permyakov, E. Yu. Potalova, and V. I. Semykin Sea surface wind perturbations over sea surface temperature (SST) cold anomalies over the Kashevarov Bank (KB) of the Okhotsk Sea are analyzed using satellite (AMSR-E and QuikSCAT) data during the summer-autumn period of 2006–2009. It is shown, that frequency of cases of wind speed decreasing over a cold spot in August–September reaches up to 67%. In the cold spot center SST cold anomalies reached 10.5 °C and wind speed lowered down to ~7 m s −1 relative its value on the periphery. The wind difference between a periphery and a centre of the cold spot is proportional to SST difference with the correlations 0.5 for daily satellite passes data, 0.66 for 3-day mean data and 0.9 for monthly ones. For all types of data the coefficient of proportionality consists of ~0.3 m s −1 on 1 °C.

Description: Predicting storm-time thermospheric mass density variations at CHAMP and GRACE altitudes Annales Geophysicae, 29, 443-453, 2011 Author(s): R. Liu, S.-Y. Ma, and H. Lühr Orbit-averaged mass density measurements derived from the satellites CHAMP and GRACE are used to investigate the storm-time prediction model developed by Liu et al. (2010) at different altitudes. This model uses as input only the solar wind merging electric field. From 2002 to 2005 in total 31 major geomagnetic storms with minimum Dst

Description: Analysis of ionospheric electrodynamic parameters on mesoscales – a review of selected techniques using data from ground-based observation networks and satellites Annales Geophysicae, 29, 467-491, 2011 Author(s): H. Vanhamäki and O. Amm We present a review of selected data-analysis methods that are frequently applied in studies of ionospheric electrodynamics and magnetosphere-ionosphere coupling using ground-based and space-based data sets. Our focus is on methods that are data driven (not simulations or statistical models) and can be used in mesoscale studies, where the analysis area is typically some hundreds or thousands of km across. The selection of reviewed methods is such that most combinations of measured input data (electric field, conductances, magnetic field and currents) that occur in practical applications are covered. The techniques are used to solve the unmeasured parameters from Ohm's law and Maxwell's equations, possibly with help of some simplifying assumptions. In addition to reviewing existing data-analysis methods, we also briefly discuss possible extensions that may be used for upcoming data sets.

Description: Quasi-periodic oscillations of aerosol backscatter profiles and surface meteorological parameters during winter nights over a tropical station Annales Geophysicae, 29, 455-465, 2011 Author(s): M. G. Manoj and P. C. S. Devara Atmospheric gravity waves, which are a manifestation of the fluctuations in buoyancy of the air parcels, are well known for their direct influence on concentration of atmospheric trace gases and aerosols, and also on oscillations of meteorological variables such as temperature, wind speed, visibility and so on. The present paper reports quasi-periodic oscillations in the lidar backscatter signal strength due to aerosol fluctuations in the nocturnal boundary layer, studied with a high space-time resolution polarimetric micro pulse lidar and concurrent meteorological parameters over a tropical station in India. The results of the spectral analysis of the data, archived on some typical clear-sky conditions during winter months of 2008 and 2009, exhibit a prominent periodicity of 20–40 min in lidar-observed aerosol variability and show close association with those observed in the near-surface temperature and wind at 5% statistical significance. Moreover, the lidar aerosol backscatter signal strength variations at different altitudes, which have been generated from the height-time series of the one-minute interval profiles at 2.4 m vertical resolution, indicate vertical propagation of these waves, exchanging energy between lower and higher height levels. Such oscillations are favoured by the stable atmospheric background condition and peculiar topography of the experimental site. Accurate representation of these buoyancy waves is essential in predicting the sporadic fluctuations of weather in the tropics.

Description: Feasibility study on Generalized-Aurora Computed Tomography Annales Geophysicae, 29, 551-562, 2011 Author(s): Y.-M. Tanaka, T. Aso, B. Gustavsson, K. Tanabe, Y. Ogawa, A. Kadokura, H. Miyaoka, T. Sergienko, U. Brändström, and I. Sandahl Aurora Computed Tomography (ACT) is a method for retrieving the three-dimensional (3-D) distribution of the volume emission rate from monochromatic auroral images obtained simultaneously by a multi-point camera network. We extend this method to a Generalized-Aurora Computed Tomography (G-ACT) that reconstructs the energy and spatial distributions of precipitating electrons from multi-instrument data, such as ionospheric electron density from incoherent scatter radar, cosmic noise absorption (CNA) from imaging riometers, as well as the auroral images. The purpose of this paper is to describe the reconstruction algorithm involved in this method and to test its feasibility by numerical simulation. Based on a Bayesian model with prior information as the smoothness of the electron energy spectra, the inverse problem is formulated as a maximization of posterior probability. The relative weighting of each instrument data is determined by the cross-validation method. We apply this method to the simulated data from real instruments, the Auroral Large Imaging System (ALIS), the European Incoherent Scatter (EISCAT) radar at Tromsø, and the Imaging Riometer for Ionospheric Study (IRIS) at Kilpisjärvi. The results indicate that the differential flux of the precipitating electrons is well reconstructed from the ALIS images for the low-noise cases. Furthermore, we demonstrate in a case study that the ionospheric electron density from the EISCAT radar is useful for improving the reconstructed electron flux. On the other hand, the incorporation of CNA data into this method is difficult at this stage, because the extension of energy range to higher energy causes a difficulty in the reconstruction of the low-energy electron flux. Nevertheless, we expect that this method may be useful in analyzing multi-instrument data and, in particular, 3-D data, which will be obtained in the upcoming EISCAT_3D.

Description: Possible interaction between thermal electrons and vibrationally excited N 2 in the lower E-region Annales Geophysicae, 29, 583-590, 2011 Author(s): K.-I. Oyama, M. Shimoyama, J. Y. Liu, and C. Z. Cheng As one of the tasks to find the energy source(s) of thermal electrons, which elevate(s) electron temperature higher than neutral temperature in the lower ionosphere E-region, energy distribution function of thermal electron was measured with a sounding rocket at the heights of 93–131 km by the applying second harmonic method. The energy distribution function showed a clear hump at the energy of ~0.4 eV. In order to find the reason of the hump, we conducted laboratory experiment. We studied difference of the energy distribution functions of electrons in thermal energy range, which were measured with and without EUV radiation to plasma of N 2 /Ar and N 2 /O 2 gas mixture respectively. For N 2 /Ar gas mixture plasma, the hump is not clearly identified in the energy distribution of thermal electrons. On the other hand for N 2 /O 2 gas mixture, which contains vibrationally excited N 2 , a clear hump is found when irradiated by EUV. The laboratory experiment seems to suggest that the hump is produced as a result of interaction between vibrationally excited N 2 and thermal electrons, and this interaction is the most probable heating source for the electrons of thermal energy range in the lower E-region. It is also suggested that energy distribution of the electrons in high energy part may not be Maxwellian, and DC probe measures the electrons which are non Maxwellian, and therefore "electron temperature" is calculated higher.

Description: Climatology of the inter-hemispheric field-aligned current system in the equatorial ionosphere as observed by CHAMP Annales Geophysicae, 29, 573-582, 2011 Author(s): J. Park, H. Lühr, and K. W. Min From geomagnetic field observations of CHAMP during 2001–2009 we extracted characteristic signatures of inter-hemispheric field-aligned currents (IHFACs) in the equatorial ionosphere. The results are in general agreement with previous observations. Nighttime IHFACs are negligibly small. Solstitial IHFACs flow from the summer to winter (from winter to summer) hemisphere at dawn (around noon). Duskside IHFACs flow southbound irrespective of season. We have also found some new IHFAC properties, which may have been predicted by theories, but are not yet given observational support. IHFACs clearly exhibit a longitude dependence, which is modulated by the South Atlantic Anomaly, the offset between geographic and magnetic equators, and tidal waves. IHFACs show little dependence on the solar cycle. We provide a comprehensive assessment of the IHFAC modulation by non-migrating tides.

Description: THEMIS observations of double-onset substorms and their association with IMF variations Annales Geophysicae, 29, 591-611, 2011 Author(s): C.-C. Cheng, C. T. Russell, V. Angelopoulos, I. R. Mann, K.-H. Glassmeier, and W. Baumjohann On 16 July 2008, two pairs of consecutive bursts of Pi2 pulsations were recorded simultaneously across the THEMIS ground-based observatory system. Wavelet transformation reveals that for each high-latitude pair, the dominant frequency of the first burst is higher than that of the second. But at low latitudes, the dominant frequency does not change. It is suggested that both pairs result from fast magnetospheric cavity waves with the second burst also containing shear Alfvén waves. INTERMAGNET magnetograms at auroral latitudes showed magnetic variations affected by two recurrent electrojets for each pair. The ground-based magnetometers and those at geostationary orbit sensed magnetic perturbations consistent with the formation of the substorm current wedge. Four consecutive enhancements of energetic electron and ion fluxes detected by the THEMIS probes in the dayside magnetosphere appeared in the later afternoon and then in the early afternoon. The horizontal magnetic variation vectors had vortex patterns similar to those induced by the upward and downward field-aligned currents during substorm times. The hodogram at mid- L stations had a polarization pattern similar to the one induced by the substorm current wedge for each Pi2 burst. The mapping of ground Pi2 onset timing to the interplanetary magnetic field (IMF) observations shows that they appear under two cycles of north-to-south and then north variation. CLUSTER 4 in the south lobe observed wave-like magnetic fluctuations, probably driven by near-Earth reconnection, similar to those on the ground. These two observations are consistent with the link of double-onset substorms to magnetotail reconnection externally triggered by IMF variations.

Description: Plasma transport modelling in the inner magnetosphere: effects of magnetic field, electric field and exospheric models Annales Geophysicae, 29, 427-442, 2011 Author(s): A. Woelfflé, D. Boscher, and I. Dandouras A qualitative study is performed on plasma transport modelling in the inner magnetosphere, revealing the significance of a model use choice and its parameterization. First, we examine particle transport using comparative analysis of both magnetic and electric field models. This work reveals that the electric field plays an important role in understanding particle dynamics and the models lead to various results in terms of plasma source, energy and particle trajectory. We then concentrate particularly on proton loss assessment considering the charge exchange phenomenon. For that, models are needed to provide a neutral hydrogen density estimation. So, exospheric models were tested in light of the Dynamics Explorer 1 measurements analysed by Rairden.

Description: Time variations of the ionosphere at the northern tropical crest of ionization at Phu Thuy, Vietnam Annales Geophysicae, 29, 197-207, 2011 Author(s): H. Pham Thi Thu, C. Amory-Mazaudier, and M. Le Huy This study is the first which gives the climatology of the ionosphere at the northern tropical crest of ionization in the Asian sector. We use the data from Phu Thuy station, in Vietnam, through three solar cycles (20, 21 and 22), showing the complete morphology of ionosphere parameters by analyzing long term variation, solar cycle variation and geomagnetic activity effects, seasonal evolution and diurnal development. Ionospheric critical frequencies, fo F2, fo F1 and fo E, evolve according to the 11-year sunspot cycle. Seasonal variations show that fo F2 exhibits a semiannual pattern with maxima at equinox, and winter and equinoctial anomalies depending on the phases of the sunspot solar cycle. Δ fo F2 exhibits a semiannual variation during the minimum phase of the sunspot solar cycle 20 and the increasing and decreasing phases of solar cycle 20, 21 and 22. Δ fo F1 exhibits an annual variation during the maximum phase of solar cycles 20, 21 and 22. Δ h 'F2 shows a regular seasonal variation for the different solar cycles while Δ h 'F1 exhibits a large magnitude dispersion from one sunspot cycle to another. The long term variations consist in an increase of 1.0 MHz for fo F2 and of 0.36 MHz for fo F1. fo E increases 0.53 MHz from solar cycle 20 to solar cycle 21 and then decreases −0.23 MHz during the decreasing phase of cycle 21. The diurnal variation of the critical frequency fo F2 shows minima at 05:00 LT and maxima around 14:00 LT. fo F1 and fo E have a maximum around noon. The diurnal variation of h 'F2 exhibits a maximum around noon. The main features of h 'F1 are a minimum near noon and the maximum near midnight. Other minima and maxima occur in the morning, at about 04:00 or 05:00 LT and in the afternoon, at about 18:00 or 19:00 LT but they are markedly smaller. Only during the maximum phase of all sunspot solar cycles the maximum near 19:00 LT is more pronounced.

Description: Temperature perturbations in the troposphere-stratosphere over Thumba (8.5° N, 76.9° E) during the solar eclipse 2009/2010 Annales Geophysicae, 29, 275-282, 2011 Author(s): K. V. Subrahmanyam, G. Ramkumar, K. K. Kumar, D. Swain, S. V. Sunil Kumar, S. S. Das, R. K. Choudhary, K. V. S. Namboodiri, K. N. Uma, S. B. Veena, S. R. John, and A. Babu Measurements of atmospheric temperature profiles in the troposphere and lower stratosphere were made over Thumba Equatorial Rocket Launching Station (TERLS) (8.5° N, 76.9° E) during a partial solar eclipse (22 July 2009) and an annular solar eclipse (15 January 2010). It was observed that during the partial solar eclipse, the temperature decreased by 2–3 °C in the vicinity of the tropopause and in the lower stratosphere the temperature increased by ~2.6 °C during the maximum phase of the partial solar eclipse. During the annular solar eclipse, a temperature reduction of ~2 °C was observed around the tropopause. This study also revealed a feature of delayed effect in the form of a very intense warming of ~8 °C at 18 km after about 4 h of the annular solar eclipse. The Cold-Point Tropopause (CPT) temperature increased slowly before the beginning of the eclipse (up to 10:00 IST) and during the maximum phase of the eclipse, the difference in CPT temperature and height was −3.5 °C and ~110 m, respectively, as that of the control day. After the four hours of the eclipse, the CPT height had decreased by ~1.7 km and the CPT temperature increased by ~4.6 °C. This is for the first time that the diurnal variation of the tropopause has been reported during a solar eclipse day. The present study, thus, provided an opportunity to investigate the temperature perturbations in the troposphere and lower stratosphere during a partial and annular solar eclipse. The highlight of the present results are (1) cooling of the entire troposphere and lower stratosphere during the maximum phase of annular solar eclipse, (2) an intense heating of the lower stratosphere by 8 °C after nearly four hours from the maximum phase of the annular eclipse, and (3) drastic variations in the diurnal evolution of the tropical tropopause characteristics. The cooling effect is attributed to the radiative response of the atmosphere to the solar eclipse, where as heating is attributed to the dynamical response of the atmosphere to the solar eclipse. These results may have important implications in understanding the response of the atmosphere to the radiative, as well as dynamical, perturbations caused by any celestial or terrestrial disturbances.

Description: Pitch-angle diffusion coefficients from resonant interactions with electrostatic electron cyclotron harmonic waves in planetary magnetospheres Annales Geophysicae, 29, 321-330, 2011 Author(s): A. K. Tripathi, R. P. Singhal, and K. P. Singh Pitch-angle diffusion coefficients have been calculated for resonant interaction with electrostatic electron cyclotron harmonic (ECH) waves in the magnetospheres of Earth, Jupiter, Saturn, Uranus and Neptune. Calculations have been performed at two radial distances of each planet. It is found that observed wave electric field amplitudes in the magnetospheres of Earth and Jupiter are sufficient to put electrons on strong diffusion in the energy range of less than 100 eV. However, for Saturn, Uranus and Neptune, the observed ECH wave amplitude are insufficient to put electrons on strong diffusion at any radial distance.

Description: Multiple magnetic dipolarizations observed by THEMIS during a substorm Annales Geophysicae, 29, 331-339, 2011 Author(s): S. P. Duan, Z. X. Liu, J. Liang, Y. C. Zhang, and T. Chen The magnetic field dipolarization in the vicinity of substorm onset and during substorm expansion phase during the period of 06:00–06:40 UT on 15 February 2008 is investigated with observations from multiple probes of THEMIS. It is found that the magnetic dipolarization at the substorm onset (the onset time was about 06:14 UT) was not accompanied by obvious magnetic disturbance and ion bulk speed variation. The magnetic dipolarizations taking place during the substorm expansion phase observed by P4~(−10.97, 2.04, −3.03) R E and P3~(−11.32, 1.15, −3.10) R E were mostly accompanied by high speed earthward ion bulk flow, but the magnetic dipolarizations occurring during the substorm expansion phase observed by P5~(−9.45, 1.07, −2.85) R E were not accompanied by high speed earthward ion bulk flow. Before substorm onset THEMIS P3, P4, P5 all observed the B x component fluctuation with a period of about 300 s. After substorm onset earthward high speed ion bulk flow and significant magnetic disturbances both occurred at P3 and P4 locations. These results indicate that there is no one-to-one relationship between the near-Earth magnetic dipolarization and the earthward ion bulk flow. In particular, the magnetic dipolarization occurring on the earthward side of the inner near-Earth plasma sheet is not accompanied by high speed earthward ion bulk flow. The dipolarization at substorm onset is a local and small scale phenomenon. There are multiple magnetic dipolarizations occurring during the substorm expansion phase. The dipolarization process is very complex and is not simply an MHD process. It is accompanied by some kinds of plasma instabilities, the plasma sheet azimuthal expansion not only by earthward ion bulk flow during substorm. A sharp increase of the AE index does not always give an accurate substorm onset time for substorm analysis.

Description: A brief review on the presentation of cycle 24, the first integrated solar cycle in the new millennium Annales Geophysicae, 29, 341-348, 2011 Author(s): K. J. Li, W. Feng, H. F. Liang, L. S. Zhan, and P. X. Gao The status of the extended solar activity minimum, since the second half of 2007, has been briefly instructed to the solar-terrestrial community. Cycle 24 has the most spotless days since cycle 16, and probably even since the modern cycles, latitudes of high-latitude (〉35°) sunspots belonging to a new cycle around the minimum time of the cycle are statistically the lowest at present, compared with those of cycle 12 onwards, and there is only one or no sunspots in a month appearing at high latitudes (〉20°) for 58 months (from November 2003 to September 2008), which is observed for the first time since cycle 12 onwards. The solar wind velocity and pressure, 10.7 cm solar radio flux, the polar solar magnetic field, solar total irradiance, and so on reach their minima during the 23–24 cycle minimum time. In order to explain the present extreme low activity, we introduced here one possible mechanism using helio-seismology observations. Viewing, from the long-term running of the time scales of both the Gleissberg period and millenniums, the extended solar activity minimum becomes logical. According to the present observations, the cycle 24 should start in November 2008. Solar activity is predicted at being about 30% lower in cycle 24 than in cycle 23, synthesizing the typical predictions of solar activity, including those given by NASA and NOAA. The 24th solar cycle is sluggishly coming and it should be an opportune moment for studying solar physics and solar-terrestrial physics.

Description: Corrigendum to "The statistical studies of the inner boundary of plasma sheet" published in Ann. Geophys., 29, 289–298, 2011 Annales Geophysicae, 29, 349-349, 2011 Author(s): J. B. Cao, W. Z. Ding, H. Reme, I. Dandouras, M. Dunlop, Z. X. Liu, and J. Y. Yang No abstract available.

Description: Case study of a mesospheric wall event over Ferraz station, Antarctica (62° S) Annales Geophysicae, 29, 209-219, 2011 Author(s): J. V. Bageston, C. M. Wrasse, R. E. Hibbins, P. P. Batista, D. Gobbi, H. Takahashi, V. F. Andrioli, J. Fechine, and C. M. Denardini On 16–17 July 2007 during an observational campaign at Comandante Ferraz Antarctic Station (62° S, 58° W), a mesospheric wall was observed with an airglow all-sky imager. The wave appeared like an extensive dark region in the all-sky airglow images, with a large depletion in the OH emission. Simultaneous mesospheric winds measured with a MF radar at Rothera station and temperature profiles from SABER instrument, on board of TIMED satellite, were used to obtain the propagation condition of the wave. Wind measurements during four days, around the time of observation of the wave, are presented in order to discuss the type and consistence of the duct in which this wave was propagating. By using wavelet analysis and tidal amplitude components we found that 12 and 8 h components were the most important periodicities around the time interval of the wave observation. A collocated imaging spectrometer, for mesospheric temperature measurements, has been operated simultaneously with the all-sky imager. Direct effects of the mesospheric front have been seen in the spectrometric measurements, showing an abrupt decrease in both OH intensity and rotational temperature when the wave front passes overhead. The main contribution of the present work is the investigation of the type of duct in which the wall event was propagating. We found evidences for a thermal duct structure to support the mesospheric wall propagation. This result was obtained by two types of analysis: (a) the tidal components analysis and winds filtering (harmonic analysis), and (b) comparison between the terms of the m 2 dispersion relation.