Publications

2015

• Multipoint observations and simulation of the effects of Earth's bow shock on magnetic clouds' structure and geoeffectiveness
  
  • Fontaine Dominique
  • Turc Lucile
  • Savoini Philippe
  , 2015, 17, pp.5908. Magnetic clouds represent a sub-class of coronal mass ejection (CME) capable to trigger very strong storms in the terrestrial environment. Their magnetic structure can be described as a flux rope and statistical studies show a dependence of their geoeffectiveness on the orientation of this flux rope relative to the terrestrial magnetic field. However, this primary dependence does not fully explain by itself the storm triggering and strength. Other effects as the cloud's sheath or events on its leading or trailing edges have been discussed. We investigate here the role of the bow shock that magnetic clouds cross before interacting with the magnetosphere. From observations in the solar wind upstream of the bow shock (ACE) and downstream (CLUSTER), we show that the bow show may strongly modify the cloud's magnetic structure and even cause the reversal of the IMF Bz component: this is crucial for the development of geomagnetic activity inside the magnetosphere. We show that this modification is related to the shock configuration (quasi-parallel, quasi-perpendicular) and we derive the upstream conditions leading to strong downstream distortions. In addition, we run hybrid simulations to analyze the different plasma behaviours in both configurations. In particular we discuss the results concerning the distribution of dynamical, thermal and magnetic pressures and the associated forces.
• Production of non-gyrotropic and gyrotropic backstreaming ion distributions in the quasi-perpendicular ion foreshock region : Origin and acceleration mechanisms.
  
  • Savoini Philippe
  • Lembège Bertrand
  , 2015, pp.EGU2015-12780.
• Influence of the angular scattering on the thermal runaway acceleration mechanism
  
  • Chanrion O.
  • Bonaventura Z.
  • Bourdon Anne
  • Neubert T.
  , 2015.
• In situ spacecraft observations of suprathermal ion acceleration in the reconnection jet braking region
  
  • Retinò Alessandro
  • Vaivads A.
  • Khotyaintsev Y. V.
  • Le Contel Olivier
  • Fu H.S.
  • Zieger B.
  • Nakamura R.
  • Artemyev A. V.
  • Kronberg E. A.
  , 2015, 17, pp.11323. Reconnection jet fronts and jet braking regions are sites of strong particle energization in space and astrophysical plasmas. Jet fronts are the boundaries separating ambient from reconnection jets while braking regions is where jets are eventually stopped/diverted. Examples are jet fronts and braking regionscan be found in planetary magnetospheres, loop-top regions in the solar corona during flares and astrophysical jets. Jet braking regions have been also recently reproduced in laboratory experiments. A number of recent in situ observations in the Earth's magnetotail have allowed studying in detail electron energization mechanisms at jet fronts/braking regions. Yet, observations of suprathermal ions are scarce. Here we show Cluster spacecraft observations in the near-Earth jet braking region of suprathermal protons and oxygen up to ~ 1 MeV, that is about 10 times their thermal energy. Observations indicate that ions are trapped between large-scale oppositely-directed jets and accelerated therein by strong electric fields. We discuss possible applications of this acceleration mechanism to solar and other astrophysical plasmas.
• Magnetosheath Turbulence at MHD Scales: A Statistical Study
  
  • Huang S. Y.
  • Sahraoui Fouad
  • Hadid L. Z.
  • Yuan Z. G.
  , 2015, 17, pp.7893. Turbulence is ubiquitous in space plasmas, such as terrestrial magnetotail and magnetosheath, solar wind, or the interstellar medium. In the solar wind, it is well established that at MHD scales, the magnetic energy spectra generally follow the so-called Kolmogorov's spectrum f-5/3. In the magnetosheath, Alexandrova et al. [2006] observed a Kolmogorov-like inertial range in the frequency range f < fci. In this study, we used three years data from the Cluster mission to statistically investigate the existence of the Kolmogorov inertial range in the whole magnetosheath, including flanks and subsolar regions. Statistical results show that most spectra are shallower than the Kolmogorov one, and have a scaling ~ f-1recalling the energy containing scales of solarwind turbulence. These spectra were found to be populated by uncorrelated fluctuations. The Kolmogorov scaling is observed only away from the bock shock and in the flanks region. These results suggest that random-like fluctuations are generated behind the shock, which reach a fully developed turbulence state only after some time corresponding to their propagation (or advection) away from the shock. At kinetic scales no dependence of the turbulence scaling on the location in the magnetosheath was found.
• Magnetopause orientation and motion: comparison of constrained minimum variance and multi-spacecraft triangulation
  
  • Reddy Anekallu Chandrasekhar
  • Haaland Stein
  • Dorville Nicolas
  , 2015, 17, pp.10600. Knowledge of orientation, motion and dimensions od plasma structures in space is essential in their study. The advent of multi-spacecraft missions such as Cluster have provided new opportunities to determine these parameters using multiploint observations. Multi spacecraft methods are computationally more complicated and require more data than traditional single spacecraft methods though, and also typically imply strict assumptions about planarity and stationarity of the plasma structure studied. In this paper, we have utilized a large number of Cluster magnetopause crossings to compare triangulation methods with a constrained minimum variance method. The results suggest that the simplified method often provide boundary normal estimations very similar to the more elaborate multi-spacecraft methods.
• Total energy cascade and residual energy in MHD turbulence: homogeneous versus expanding (solar wind) turbulence
  
  • Grappin Roland
  • Müller Wolf-Christian
  • Verdini Andrea
  , 2015, 17, pp.3623. Standard phenomenologies of MHD turbulence generally neglect deviations from kinetic-magnetic energies equipartition. However, solar wind turbulence commonly shows a magnetic excess (or positive residual energy) in the inertial range, with a definite power-law. We report here direct MHD simulation results showing a magnetic excess, both in homogeneous and expanding turbulence, with the latter taking into account the radial flow (expanding box model or EBM). We show that the results on magnetic excess, both scaling laws and amplitude, can be interpreted as resulting from the competition between the nonlinear stretching of the magnetic field by the velocity field and the relaxation to equipartition by the linear propagation of Alfvén waves. We generalize in this way earlier results on homogeneous MHD turbulence.
• Multipoint measurements: a key tool for magnetic reconnection research
  
  • Hesse Michael
  • Liu Yi-Hsin
  • Chen L.-J.
  • Kuznetsova M. M.
  • Aunai N.
  • Bessho N.
  , 2015, 17, pp.2325. Despite a multitude of research efforts in recent history, theoretical and modeling predictions of the structure of the reconnection diffusion region continue to uncover a multitude of surprises. A common theme among most predictions is a complex structure, with prominent spatial gradients, which determine the variation of magnetic fields, currents, and structures of distribution functions. On the electron scales, there is growing consensus that these scales are given either by the electron bounce width or by local electron Larmor radii, but many unsolved puzzles remain, particularly in asymmetric configurations. In this presentation, we will present an overview of open scientific questions pertaining to reconnection physics, and we will put these questions into the context of NASA's Magnetospheric Multiscale mission.
• Cascade rate and 3rd order structure functions in anisotropic turbulence
  
  • Verdini Andrea
  • Grappin Roland
  • Hellinger P.
  • Landi Simone
  • Müller Wolf-Christian
  , 2015, 17, pp.11216. The measure of the third-order structure function, Y, is employed in the solar wind to compute the cascade rate of turbulence. In absence of a mean field B₀=0, Y is expected to be isotropic (radial) and independent of the direction of increments, so its measure yields directly the cascade rate. For turbulence with mean field, as in the solar wind, Y is expected to become more two dimensional (2D), that is, to have larger perpendicular components, loosing the above simple symmetry. To get the cascade one should compute the flux of Y, which is not feasible with single-spacecraft data, thus measurements rely upon assumptions about the unknown symmetry. We use direct numerical simulations (DNS) of magneto-hydrodynamic (MHD) turbulence to characterize the anisotropy of Y. We find that for strong guide field B₀=5 the degree of two-dimensionalization of Y depends on the relative importance of shear and pseudo polarizations (the two components of an Alfvén mode in incompressible MHD). The anisotropy also shows up in the inertial range. The more turbulence is 2D the more the inertial range extent differs along parallel and perpendicular directions. We finally test the two methods employed in observations and find that the so-obtained cascade rate may depend on the angle between B<SUB>0</SUB> and the direction of increments. Both methods yield a vanishing cascade rate along the parallel direction, contrary to observations, suggesting a weaker anisotropy of solar wind turbulence compared to our DNS. This could be due to a weaker mean field and/or to solar wind expansion.
• How to find magnetic null and construct field topology with MMS data?
  
  • Fu H.S.
  • Vaivads A.
  • Khotyaintsev Y. V.
  • Olshevsky V.
  • André M.
  • Cao J.B.
  • Huang S. Y.
  • Retinò Alessandro
  • Eastwood Jonathan P.
  , 2015, 17, pp.2705. In this study, we apply a new method'Taylor expansion'to find magnetic null and construct magnetic field topology, in order to use it with the data from the forth-coming MMS mission. We compare this method with the previously used Poincare index (PI), and find that they are generally consistent, except that the PI method can only find a null inside the spacecraft (SC) tetrahedron, while the Taylor expansion can find a null both inside and outside the tetrahedron and also deduce its drift velocity. Taylor expansion can also: (1) avoid the limitations of PI method such as data resolution, instrument uncertainty (Bz offset), and SC separation; (2) identify 3D null types (A, B, As, and Bs) and determine whether these types can degenerate into 2D (X and O); (3) construct the magnetic field topology. We quantitively test the accurateness of Taylor expansion in positioning magnetic null and constructing field topology, by using the data from 3D kinetic simulations. The influences of SC separation (from 0.05 to 1 di) and null-SC distance (from 0 to 1 di) on the accurateness are both considered. We find that: (1) for single null, the method is accurate when the SC separation is smaller than 1 di, and the null-SC distance is smaller than 0.5 di (weakly chaotic reconnection) or 0.25 di (strongly chaotic reconnection); (2) for null pair, the accurateness is same as the single-null situation, except at the null-null line, where the field is nonlinear. We invent a parameter xi &#8801;|(lambda1 lambda2 lambda3)|/ |lambda|max to quantify the quality of the method'the smaller this parameter the better the results. Comparing to the previously used one (eta &#8801;|downtriangle -B|/ |downtriangle × B |), this parameter is more relevant. Using the new method, we construct the magnetic field topology around a radial-type null and a spiral-type null, and find that the topologies are well consistent with those predicted in theory. This means that our method is reliable. We therefore suggest using this method to find magnetic null and construct field topology with the four-point measurements, particularly the Cluster and forth-coming MMS measurements.
• Planetary turbulence: survey of Cassini data in the Saturn's magnetosheath
  
  • Hadid L. Z.
  • Sahraoui Fouad
  • Kiyani Khurom
  • Modolo Ronan
  • Retinò Alessandro
  • Canu Patrick
  • Masters Adam
  • Dougherty Michele K.
  • Gurnett Donald A.
  , 2015, 17, pp.EGU2015-3712. Turbulence is one of the most important yet not fully understood topics of modern physics. Understanding turbulence is collisionless plasmas, where kinetic effects mediate interactions between fields and charged particles play, is crucial to apprehend many dynamical processes such as particle heating and acceleration. Among others, one key open issue of plasma turbulence is how the energy associated to magnetic and electric fields is converted, and eventually dissipated, into kinetic and internal energy of the plasma. The planets' magnetosheath present a high level of turbulence that involves both nonlinear stochastic processes and a rich variety of wave phenomena. In comparison with turbulence in the solar wind and in the terrestrial magnetosheath, turbulence around other planets is far less explored. Here, we expand our knowledge in plasma turbulence by exploring the properties of turbulence in the Kronian magnetosheath using the Cassini spacecraft data. These properties include the magnetic field energy spectra, the magnetic compressibility and intermittency at both MHD and kinetic scales. The analysis is based on in-situ data provided by the Fluxgate Magnetometer of the MAG instrument, which measures the magnetic field data with 32ms time resolution and the plasma data from the CAPS/IMS (Cassini Plasma Spectrometer) and the Electron Spectrometer (ELS), during 39 shock-crossings between 2004 and 2005. Similarities and differences with the solar wind were found, in particular about the nature of the turbulence and its scaling laws, as well as the dependence of those properties on the topology of the bow shock.
• Current Status of MPPE (Mercury Plasma Particle Experiment) on BepiColombo/MMO
  
  • Saito Y.
  • Hirahara M.
  • Barabash S.
  • Delcourt Dominique
  • André N.
  • Takashima T.
  • Asamura K.
  , 2015, 17, pp.15573. Mercury's plasma/particle environment has gradually become clear thanks to the new observations made by MESSENGER spacecraft orbiting around Mercury. However, it is also true that many questions will be left unsolved. In order to elucidate the detailed plasma structure and dynamics around Mercury, an orbiter BepiColombo MMO (Mercury Magnetospheric Orbiter) is going to be launched in 2016 as a joint mission between ESA and ISAS/JAXA. Mercury Plasma/Particle Experiment (MPPE) is a comprehensive instrument package for plasma, high-energy particle and energetic neutral atom measurements. It consists of 7 sensors: two Mercury Electron Analyzers (MEA1 and MEA2), Mercury Ion Analyzer (MIA), Mass Spectrum Analyzer (MSA), High Energy Particle instrument for electron (HEP-ele), High Energy Particle instrument for ion (HEP-ion), and Energetic Neutrals Analyzer (ENA). Currently, the MPPE sensors are on the MMO spacecraft under system integration test at ISAS/JAXA (Institute of Space and Astronautical Science / Japan Aerospace Exploration Agency). Evaluation of the sensor calibration data and the final check of the onboard processing software are being made in order to realize the flawless future plasma/particle observations around Mercury.
• Kinetic equilibrium for an asymmetric tangential layer with rotation of the magnetic field
  
  • Belmont Gérard
  • Dorville Nicolas
  • Aunai N.
  • Rezeau Laurence
  , 2015, 17, pp.5300. Finding kinetic equilibria for tangential current layers is a key issue for modeling plasma phenomena such as magnetic reconnection instabilities, for which theoretical and numerical studies have to start from steady-state current layers. Until 2012, all theoretical models -starting with the most famous "Harris" one- relied on distribution functions built as mono-valued functions of the trajectories invariants. For a coplanar anti-symmetric magnetic field and in absence of electric field, these models were only able to model symmetric variations of the plasma, so precluding any modeling of "magnetopause-like'' layers, which separate two plasmas of different densities and temperatures. Recently, the "BAS" model was presented (Belmont et al., 2012), where multi-valued functions were taken into account. This new tool is made necessary each time the magnetic field reversal occurs on scales larger than the particle Larmor radii, and therefore guaranties a logical transition with the MHD modeling of large scales. The BAS model so provides a new asymmetric equilibrium. It has been validated in a hybrid simulation by Aunai et al (2013), and more recently in a fully kinetic simulation as well. For this original equilibrium to be computed, the magnetic field had to stay coplanar inside the layer. We present here an important generalization, where the magnetic field rotates inside the layer (although restricted to a 180° rotation hitherto). The tangential layers so obtained are thus closer to those encountered at the real magnetopause. This will be necessary, in the future, for comparing directly the theoretical profiles with the experimental ones for the various physical parameters. As it was done previously, the equilibrium is presently tested with a hybrid simulation. Belmont, G.; Aunai, N.; Smets, R., Kinetic equilibrium for an asymmetric tangential layer, Physics of Plasmas, Volume 19, Issue 2, pp. 022108-022118-10, 2012 Aunai, N.; Belmont, G.; Smets, R., First demonstration of an asymmetric kinetic equilibrium for a thin current sheet, Physics of Plasmas, Volume 20, Issue 11, pp. 110702-110702-4, 2013
• Quasi-periodic modulation of equatorial noise intensity
  
  • Nemec F.
  • Santolík O.
  • Hrbackova Z.
  • Pickett J. S.
  • Cornilleau-Wehrlin Nicole
  , 2015, 17, pp.3907. Equatorial noise (EN) emissions are electromagnetic waves at frequencies between the proton cyclotron frequency and the lower hybrid frequency observed routinely in the equatorial region of the inner magnetosphere. They propagate in the extraordinary mode nearly perpendicular to the ambient magnetic field. Although their harmonic structure, which is characteristic of the proton cyclotron frequency in the source region has been known for a couple of decades, they were generally believed to be continuous in time. The analysis of more than 2000 EN events observed by the STAFF-SA and WBD instruments on board the Cluster spacecraft reveals that this is not always the case, with about 5% of events exhibiting a clear quasi-periodic (QP) modulation of the wave intensity. We perform a systematic analysis of these events, and we discuss possible mechanisms of the QP intensity modulation. It is shown that the events occur usually in the noon-to-dawn magnetic local time sector, and their occurrence seems to be related to the periods of increased geomagnetic activity. The modulation period of these events is on the order of minutes. Compressional ULF magnetic field pulsations with periods about double the modulation periods of EN were identified in about half of the events. These ULF pulsations might modulate the EN wave intensity, similarly as they modulate the intensity of formerly reported VLF whistler-mode QP events.
• Magnetic clouds' structure in the Earth's magnetosheath as observed by Cluster and Geotail
  
  • Turc Lucile
  • Fontaine Dominique
  • Savoini Philippe
  • Kilpua E. K. J.
  • Escoubet P.
  , 2015, 17, pp.5832. Magnetic clouds (MCs) are a subset of coronal mass ejections which are known to drive intense geomagnetic activity. When studying the geoeffectivity of MCs, it is generally assumed that their magnetic structure remains unchanged from the solar wind upstream of the Earth's environment, measured for example at the first Lagrangian point L1, to the magnetopause. However, before impinging on the magnetosphere, MCs first cross the Earth's bow shock which can alter the orientation of their magnetic field. In this study, we examine the effects of the bow shock crossing on the magnetic structure of MCs. To address this issue, we compare simultaneous spacecraft observations in the solar wind (ACE) and in the magnetosheath (Cluster, Geotail) and find that the variation of the magnetic field direction across the bow shock varies from one event to another, from one spacecraft to another, and even during the same MC event. We relate these differences to the shock configuration. We find that when the shock is in a quasi-perpendicular configuration, the MC's magnetic structure is similar in the solar wind and in the magnetosheath, whereas it is strongly modified if the shock is in a quasi-parallel geometry. In this case, the magnetic field reaching the magnetopause cannot be approximated by that measured in L1. We discuss the consequences for the MC's geoeffectivity.
• Ion outflow above polar cap arcs and solar illumination.
  
  • Maes Lukas
  • Maggiolo Romain
  • de Keyser J.
  • Dandouras Iannis
  • Fear R.
  • Fontaine Dominique
  • Haaland Stein
  , 2015, 17, pp.11492. We investigate the solar illumination dependence of the flux and composition of ion beams, often observed by Cluster during periods of prolonged northward interplanetary magnetic field, escaping from the polar ionosphere. They have been energized by the electric fields parallel to the magnetic field, that are also responsible for accelerating electrons downwards, which then create polar cap arcs when colliding in the polar ionosphere. To do this, we use Cluster CIS data to measure the density, composition and energy of the outflowing ions, and trace the spacecraft position back to the footpoint of the magnetic field line in the ionosphere. When characterizing the data in terms of the solar zenith angle (SZA) of this footpoint, we find a clear transition in the O&#8314; flux densities from lower SZA (<94°) to higher SZA (>107°). So a difference is found between the outflow above a sunlit polar cap and above a dark polar cap. The same is found for the H&#8314; ions, but to a lesser extent. The data shows evidence for the fact that the ions originate in the local ionosphere at the footpoint. Therefore we also explore what these results might mean for outflow from the whole polar cap, i.e. polar wind. Also the energy is analyzed in this way, and again different regimes are found above a sunlit ionosphere and a dark ionosphere. This energy corresponds to the field-aligned potential drop, and therefore this dependence is evidence for an ionosphere-magnetosphere coupling, likely through the change in ionospheric conductivity.
• The terrestrial magnetopause - an asymmetric boundary
  
  • Haaland Stein
  • Reistad J.
  • Tenfjord P.
  • Maes Lukas
  • Anekallu C.
  • Dorville Nicolas
  • de Keyser J.
  , 2015, 17, pp.8887. The terrestrial magnetopause marks the boundary between the geomagnetic field on one side and the decelerated solar wind with its embedded interplanetary magnetic field on the other side. It is thus a key region for the transfer of mass and momentum from the solar wind into the magnetosphere. In this paper, we report observations of magnetopause characteristics based on more than 15000 indvidual magnetopause crossings by the Cluster mission. The results show that both thickness, velocity and current density and current profile of the flank magnetopause can vary greatly from crossing to crossing, mainly due to internal structures such as magnetic islands and undulations. Overall, however, there is a persistent dawn-dusk asymmetry with a thinner current sheet and higher current density on dusk than on dawn. The asymmetry becomes more pronounced during active geomagnetic conditions. Possible reasons for this asymmetry and behavior is discussed.
• Three-dimensional Iroshnikov-Kraichnan Turbulence in a Mean Magnetic Field
  
  • Müller Wolf-Christian
  • Grappin Roland
  • Verdini Andrea
  • Gürcan Özgür D.
  , 2015, 17, pp.3457. We present a new cascade scenario motivated by the three-dimensional energy spectrum observed in numerical simulations of incompressible MHD turbulence in a strong mean field. It is shown that the energy distribution is not in accord with standard critical balance and the associated scale anisotropy. This is not surprising as the present setup with isotropic large-scale forcing predominantly yields fluctuations in the weak-turbulence regime.In spite of this, a measurable anisotropy of structure-function scaling exists independent of taking spatial increments with respect to the mean or local direction of the magnetic field. We, thus, propose a combination of weak Iroshnikov-Kraichnan dynamics governing energy transfer in the field-perpendicular plane and the ricochet process distributing energy quasi-resonantly along all other directions. This turbulence properties are consistent with the main numerical findings, in particular, regarding the energy spectrum: (i) an inertial-range power law exponent independent of direction, (ii) a direction-dependent power-law spectral-range extent ~ brms/B0. This spectral transfer process asymptotically approaches the 2D IK-cascade as B0 increases. The new transfer mechanism is at variance with the commonly accepted resonant weak-turbulence cascade as well as with the critically balanced strong turbulence cascade, both resulting in strictly perpendicular energy transfer. This is necessary to explain the significant field-parallel extent of the observed energy distribution, The findings also disagree with the small-scale dynamic-alignment phenomenology. It is shown that the non-universal spectral dynamics are determined by the large-scale ratio of kinetic and magnetic energy.
• The formation of the ion seed population at quasi-parallel shocks
  
  • Johlander A.
  • Vaivads A.
  • Khotyaintsev Y. V.
  • Retinò Alessandro
  • Dandouras Iannis
  • Yordanova E.
  • André M.
  , 2015, 17, pp.1765. Collisionless shocks in space plasma are known to be capable of accelerating particles to very high energies through Fermi acceleration. However, this process requires an injection of a suprathermal ion seed population. The process of how the ion seed population is formed is still not fully understood. In this work, we studied the formation of the ion seed population as a result of solar wind ions being reflected off short large amplitude magnetic structures (SLAMS) in the quasi-parallel bow shock of Earth. For our analysis, we used field and particle data from the four Cluster satellites. In order to follow the ion dynamics at fast temporal scales we used ion subspin data from times when the separation of the satellites was ~ 100 km. We have found that SLAMS nearly specularly reflect solar wind ions through magnetic mirroring. We have also performed test particle simulations of ions encountering a SLAMS using field data from the satellites. The simulations show that solar wind ions with lower energy are more likely to be reflected off the SLAMS while high energy ions pass through the SLAMS, which is consistent with observational ion data. Additionally, high energy ions are observed upstream of the shock. These are most likely ions that were reflected off a SLAMS and energy conservation in the solar wind frame causes an increase of energy in the spacecraft frame.
• MHD turbulence in the solar wind: highlights on the effects of expansion
  
  • Verdini Andrea
  • Grappin Roland
  • Müller Wolf-Christian
  • Landi Simone
  • Hellinger P.
  • Matteini Lorenzo
  • Franci Luca
  • Velli Marco
  , 2015, 17, pp.10865. Properties of solar wind fluctuations are often interpreted as those of a homogenous turbulent plasma, at MHD or ion scales. However solar wind turbulence is not homogenous, being embedded in a spherically expanding flow of approximately constant speed. We briefly review some of the recent results on MHD turbulence obtained with the Expanding Box Model (EBM), which reveal the influence of expansion on the spectral anisotropy, the component anisotropy, and the z /z- imbalance. We then focus on structure functions, computed in frames attached to the local or global mean field, and show that most of the observed features are well reproduced in our EBM simulations. We finally comment on the role of expansion in determining the injection scale of solar wind turbulence and its anisotropy.
• Impacts of CME on the TEC at middle and low latitudes during maximum of the 24th solar cycle
  
  • Migoya-Orue Y. O.
  • Amory-Mazaudier Christine
  • Radicella S.
  • Nava B.
  • Kashcheyev A.
  , 2015, 17, pp.11071. In this study we analyzed the impacts on the GNSS-derived Total Electron Content (TEC) of four selected CME hitting the Earth during the year 2013 at different stations of middle and low latitudes (Ebre, Rabat, Alexandria, San Fernando, M'barara, Matera and Dakar). In order to analyzed the seasonal behavior of TEC under these disturbed conditions in the mentioned stations we have selected four CME events occurred during the different seasons (January 19, March 17, July 9 and October 2) of year 2013, at a maximum of the sunspot cycle 24. At the beginning of each event there is an increase of TEC followed by a decrease. The first increase of TEC is a consequence of the Prompt Penetration of the Electric Field (PPEF). The depletion of the TEC is associated to the Disturbance Dynamo Electric Field (DDEF). In order to interpret the observations we analyzed the convection patterns at high latitudes given by the radar SUPERDARN. At low latitudes, we derived the ionospheric electric current disturbance Diono from ground magnetic variations. Diono is the sum of the DP2 (PPEF) and Ddyn (DDEF) electric current systems. Finally we found that the strength of the impact at middle and low latitudes depends on the time of the impact of the CME and the season.
• Alteration of a magnetic cloud's structure across the bow shock: results from 3D hybrid simulations
  
  • Turc Lucile
  • Fontaine Dominique
  • Savoini Philippe
  • Modolo Ronan
  , 2015, 17, pp.5436. Magnetic clouds (MCs) are a subset of coronal mass ejections which are characterised by an enhanced and smoothly rotating magnetic field. Recent studies have shown that their magnetic structure can be significantly modified when they cross the Earth's bow shock, and that the extent of this modification is closely related to the encountered shock configuration. In this work, we investigate the interaction of an MC with a bow shock using 3D hybrid simulations. They allow us to have access to the large-scale MC-bow shock interaction, but also to processes taking place at the ion scales, which are of critical importance in the quasi-parallel regime, in particular in the emergence of the ion foreshock region. The MC's magnetic structure is modelled by a flux rope which propagates with the solar wind flow inside the simulation domain. We examine its alteration across the different shock geometries, from quasi-perpendicular to quasi-parallel, and find a good agreement with the results from previous studies based on modelling and combined observations from different spacecraft (ACE, Cluster, Geotail). We also investigate the consequences of the MC's passage on the bow shock, the magnetosheath and the foreshock.
• Generic residue analysis and BV method comparison
  
  • Dorville Nicolas
  • Anekallu C.
  • Haaland Stein
  • Belmont Gérard
  , 2015, 17, pp.13972. Determining the orientation of the normal direction to the magnetopause layer is a key issue for studying in detail the structure of this boundary. Both conservation laws methods and the new iterative BV method, that performs a fit of the magnetic field and ion normal flow velocity with an elliptic model, have been developed for this purpose. These methods have different model assumptions and validity ranges. Unlike the conservation laws methods, the BV method also provides spatial profiles inside the layer. However, it is compatible only with a subset of magnetopause crossings with a single layer current sheet. We compare here their results on artificial magnetopause data with noise, to understand their sensibility to small departures from their physical hypothesis. Then we present a statistical study on their comparison on a list of 149 flank and dayside magnetopause crossings.
• Turbulent fluctuations at kinetic scales: from coherent structures to quasi-parallel whistler waves
  
  • Alexandrova O.
  • Lacombe C.
  • Matteini Lorenzo
  • Rossi C.
  • Maksimovic M.
  • Mangeney André
  • Santolík O.
  • Cornilleau-Wehrlin Nicole
  • de Conchy Y.
  , 2015, 17. The nature of the magnetic field fluctuations in the solar wind between the ion and electron scales is still under debate. Using the Cluster/STAFF instrument, we make a survey of the power spectral density and of the polarization of these fluctuations at frequencies f &#8712; [1,400] Hz, during five years (2001-2005) when Cluster was in the free solar wind. In most of the analyzed time intervals, the fluctuations have quasi-random polarization and they have a general spectral shape between the ion scales and a fraction of electron scales. The intensity of these spectra is well correlated to the ion thermal pressure. These fluctuations seem to have a negligible frequency in the solar wind frame, and a wavevector anisotropy k&#8869; &#8811; k||. Such time intervals are dominated by coherent structures, propagating with a finit velocity in the plasma frame, in the plane perpendicular to the mean magnetic field. In the rest ~ 10% of the selected data, we observe narrow-band, right-handed, circularly polarized fluctuations, with wave vectors quasi-parallel to the mean magnetic field, superimposed on the spectrum of the permanent background turbulence. We interpret these coherent fluctuations as whistler mode waves. The life time of such waves varies between a few seconds and several hours. We analyze in details the long-lived whistler waves, i.e. with a life time longer than five minutes. When the electron parallel beta factor betae|| is larger than 3, the whistler waves are seen along the heat flux threshold of the whistler heat flux instability.
• Properties of Jupiter's magnetospheric turbulence observed by the Galileo spacecraft
  
  • Tao Chihiro
  • Sahraoui Fouad
  • Fontaine Dominique
  • Patoul Judith De
  • Chust Thomas
  • Kasahara Satoshi
  • Retinò Alessandro
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120 (4), pp.2477--2493. In collisionless plasmas, turbulence is thought to play an important role in mass transport and energy dissipation. Magnetic fluctuations in the Jovian magnetosphere are essential in a turbulent state. Previous studies of that turbulence have focused on the large scales using low time resolution of magnetic field data. Here we extend those studies to cover a wider range of scales by combining both low and high-time-resolution data of Galileo magnetometer. We use particle data from the plasma instrument and include energetic particle contributions to estimate the local plasma parameters. We obtain 11 power spectra of magnetic field in the frequency range of 10(-4)-1Hz, which covers both magnetohydrodynamics and ion kinetic scales. The frequencies of the evidenced spectral breaks are found to be relatively well correlated with the characteristic scales of heavy ion. The spectral indices below and above the spectral breaks are found to be broad and cover the ranges of 0.6-1.9 and 1.7-2.5, respectively. An analysis of higher-order statistics shows an intermittent feature of the turbulence, found to be more prominent in the plasma sheet than in the lobe. Furthermore, a statistical survey of the power of the fluctuations using low-time-resolution data suggests a radially varying dawn-dusk asymmetry: the total power is larger in the duskside (dawnside) at \textless50 R-J (\textgreater80 R-J), which would reflect flow shear and global magnetospheric activity. (10.1002/2014JA020749)
  DOI : 10.1002/2014JA020749