Publications

2017

• The Alfvén Mission for the ESA M5 Call
  
  • Fazakerley A.
  • Berthomier Matthieu
  • Pottelette Raymond
  • Forsyth C.
  , 2017, 19, pp.16809. The Alfvén mission will explore particle acceleration processes and their consequences for electromagnetic radiation and energy transport in strongly magnetised plasmas. In particular it will address the following three key questions. Alfvén will discover where and how particle acceleration occurs in strongly magnetized plasmas. Charged particle acceleration in strongly magnetized plasmas requires the conversion of electromagnetic energy into magnetic-field-aligned particle kinetic energy. Several pathways of energy conversion have been proposed; to understand which are important, Alfvén will measure for the first time in a strongly magnetized plasma the occurrence and distribution of small scale parallel electric fields in space and time. In order to determine the relative efficiency of the different conversion mechanisms, Alfvén will also measure the corresponding particle energy fluxes locally and into the aurora. Alfvén discoveries will inform efforts to understand similar processes in other strongly magnetized plasmas, such as recent work to resolve paradoxes in models of solar flares. Alfvén will discover how electromagnetic radiation is generated in the acceleration region and how it escapes. One of the most important consequences of particle acceleration in strong magnetic fields is the generation of non-thermal electromagnetic radiation. Some of the brightest astrophysical radio signals are from coherent generation in plasmas, which also occurs on every magnetized planet. Alfvén will make key measurements of Earth's powerful Auroral Kilometric Radiation (AKR) needed to test competing models of wave generation, mode conversion and escape from their source region. These will reveal the mode conversion processes and which information is ultimately carried by the polarization of radio waves reaching free space. The resulting discoveries will make a strong contribution to a better understanding of astrophysical radio sources. Alfvén will discover the global impact of particle acceleration on the dynamic coupling between a magnetized object and its plasma environment. Energy can be transported over vast distances in several forms regulated by the magnetic field, including Poynting flux of plasma waves, accelerated particle fluxes, and bulk plasma flows. A key to understanding the coupling between a magnetized object and the surrounding plasma is how the energy converts from one type to another. Dual spacecraft measurements offer the unique opportunity to unambiguously determine which part of the energy flowing into the ionosphere is eventually dissipated in this collisional plasma and which part is transmitted to outflowing ions of ionospheric origin. Alfvén will discover what combination of plasma and magnetic conditions controls the conversion of Poynting flux into particle energy at Earth. These conditions will be compared to those at the outer planets, illuminating the theoretical descriptions of energy deposition in these remote environments. The Alfvén mission design involves use of two simple identical spacecraft, a comprehensive suite of inter-calibrated particles and fields instruments, cutting edge auroral imaging, easily accessible orbits that frequently visit the region of scientific interest and straightforward operations. This has not previously been possible, but is now compelling and timely. It is a low risk mission that is compatible with the M5 cost cap.
• New insights into sub-ion scale turbulence in Earth's magnetosheath using MMS data
  
  • Breuillard Hugo
  • Andriopoulou M.
  • Graham Daniel
  • Le Contel Olivier
  • Huang S. Y.
  • Hadid Lina
  • Sahraoui Fouad
  • Alexandrova O.
  • Berthomier Matthieu
  • Retinò Alessandro
  • Nakamura R.
  • Baumjohann W.
  , 2017, 19, pp.16664. On January 22nd 2016, MMS was located in Earth's magnetosheath and detected intense lion roars showing a secondary bandwidth. Detailed polarization analysis, using burst data from SCM and EDP instruments, and numerical simulation, using WHAMP, are performed in this study. They show that these mainly perpendicular fluctuations are highly nonlinear whistler wave packets, and that a high sampling rate is needed to pick up the peaks of the signal. As a result, their amplitude might have been underestimated in previous missions such as Cluster, which can have a significant impact on electron dynamics. Using FPI burst data, we show that electron velocity distribution functions exhibit a gyrophase-bunched signature in the presence of these lion roars. The analysis of magnetic and density fluctuations, inferred from spacecraft potential, also show the highly-compressible nature of turbulence up to electron scales.
• Reproducing the Solar Wind proton temperature profile via DNS of MHD turbulence
  
  • Montagud-Camps Victor
  • Grappin Roland
  • Verdini Andrea
  , 2017, 19, pp.8273. Context: The Solar Wind proton temperature Tp shows a radial profile R-0.9 significantly shallower than the adiabatic R-4/3 profile [Totten et al 1996]. This temperature profile has been attributed to turbulent heating, which requires a dissipation rate equal to Q = 3.610-5TpU/R[J/(kg s)] (1) [Vasquez et al 2007]. The possibility of a turbulent heating large enough to modify the radial profile of the temperature has not been verified yet via direct numerical simulations. Aim: We want to test if MHD turbulence developing in the range [0.2,1] AU is able to reproduce the observed R-0.9 temperature profile. Method: We use the expanding box model (EBM) [Grappin & Velli 1996] which incorporates the effects of expansion into the compressible MHD equations, and so allows to follow the evolution of the plasma advected by the solar wind between 0.2 and 1 AU. In the absence of turbulence, the R-4/3 temperature profile is obtained. We start at 0.2 AU with mean field almost aligned with the radial and k⊥-1 spectrum perpendicular to the mean field [Verdini, Grappin 2016]. Simple phenomenology (Kolmogorov) suggests that the ratio between turbulent heating and the required heating (1) is close to M2/ε, where M is the Mach number of the large eddies and ε is the nonlinear time normalized by the transport time of the plasma by the wind. We thus explore the (M,ε) parameter space and examine whether a large enough value of M2/ε indeed allows to recover the temperature profile observed by Totten et al (1996). Results: We have obtained significant slowing down of the adiabatic cooling by considering increasing Mach numbers and/or decreasing ε and approach in some cases the R-0.9 temperature profile. The role of the compressibility in the cascade is examined.
• On statistics of electric field amplitudes in Langmuir turbulence
  
  • Voshchepynets A.
  • Volokitin A.
  • Krasnoselskikh V.
  • Krafft C.
  , 2017, 19, pp.4519. A systematic study of the properties of Langmuir wave turbulence generated by electron beams via bump-on-tail instabilities in strongly non-homogeneous plasmas is presented. A statistical analysis of the Langmuir waves' fields' amplitudes using numerical simulations based on two different theoretical models is performed : a probabilistic one and a dynamical one. The former describes the self-consistent dynamics of wave-particle and wave-wave interactions in inhomogeneous plasmas. The latter is a modified version of the standard quasi-linear theory which requires much less computational resources. To analyze the simulation data provided by the probabilistic model, a Pearson technique is used to classify the calculated probability distribution functions (PDFs) of the logarithm of the wave fields' intensities. It is demonstrated that the core parts of the PDFs belong to the Pearson types I, IV and VI distributions, depending on the spatial profiles of the density fluctuations, rather than to the normal distribution. Moreover it is shown that the high-amplitude parts of the PDFs follow power-law or exponential decay distributions, depending on the type of the corresponding cores' distributions. The PDFs of the fields' amplitudes calculated using the numerical simulations based on the dynamical model are in the whole consistent with those provided by the probabilistic model. Moreover, these simulations lead to a series of additional results. First, in the small fields' amplitudes' parts of the PDFs (i.e. in the linear stage of the system's evolution), an universal scaling parameter is found, with a value not depending on the average levels of the density fluctuations and of the Langmuir turbulence. Second, the PDFs are obtained in the presence of wave 28 decay processes, which are not taken into account in the probabilistic model. When those are weak, the PDFs show at large fields' amplitudes an exponential asymptotic behavior; during the time evolution, the corresponding scaling parameter decreases until a universal probability distribution is reached, what is realized when the wave decay processes are sufficiently strong. This distribution is analogous to that obtained for a quasi-homogeneous plasma. Such exponential type of distribution is a specific signature of transition states in the Langmuir turbulence. Third, the square of the statistical field amplitude maximum is found to be proportional to the average energy of the Langmuir waves.
• Evaluation of the generalized Ohm's law at the subsolar magnetopause diffusion region with MMS data
  
  • Cozzani Giulia
  • Retinò Alessandro
  • Le Contel Olivier
  • Califano F.
  • Chasapis A.
  • Khotyaintsev Y. V.
  • Mirioni Laurent
  • Vaivads A.
  • Lavraud Benoit
  • Breuillard Hugo
  , 2017, 19, pp.1330. Magnetic reconnection is a fundamental process occurring in thin current sheets where a change in the magnetic field topology leads to fast magnetic energy conversion into energy of charged particles. A key yet poorly understood aspect is how the reconnection electric field is sustained in the diffusion region by the different terms in the generalized Ohm's law. In particular, the role of the pressure and inertia terms is not yet fully understood as well as the importance of the anomalous resistivity term and its source. Simulations have provided some estimations of the different terms; however direct observations have been scarce so far. The four-spacecraft Magnetospheric Multiscale Mission (NASA/MMS) allows, for the first time, the full evaluation of the generalized Ohm's law in the diffusion region. Here we present MMS observations at a few subsolar diffusion region crossings on October,3 rd 2015 where MMS spacecraft were separated by 25 km. We compare the measured electric field with the electric field due to both kinetic effects (electron pressure tensor, electron inertia terms) and to anomalous resistivity associated to different wave modes. The electric field is balanced by the Hall term at ion scales as expected. At smaller scales, preliminary results indicate that the electric field is mainly balanced by the divergence of the electron pressure tensor, although the contribution of anomalous resistivity is not negligible.
• Multi-spacecraft Observation of Electrostatic Solitary Waves in the Reconnection Separatrix Region
  
  • Khotyaintsev Y. V.
  • Graham Daniel B.
  • Norgren Cecilia
  • Li Wenya
  • Vaivads A.
  • Divin A. V.
  • Andre M.
  • Lindqvist Per-Arne
  • Wilder Frederick
  • Ergun Robert
  • Le Contel Olivier
  • Russell Christopher T.
  • Magnes Werner
  • Torbert Roy B.
  • Giles B. L.
  • Burch Jim
  , 2017, 19, pp.13278. Electrostatic solitary waves (ESWs) are often observed in a vicinity of reconnection regions in association with streaming electron distribution. Such ESWs can be generated by the bump-on-tail, electron two-stream or Buneman instabilities, and lead to transfer of the energy initially contained in electron streaming to heating and acceleration of electrons and ions. Accurate knowledge of ESW potentials and scales is needed to quantitatively address the interaction between ESWs and particles. We present Magnetospheric Multiscale (MMS) observations of ESWs at small inter-spacecraft separation, which allows the same ESW to be observed by all four spacecraft. This provides a substantially longer baseline for interferometry compared to typical 100 m and shorter when using double-probe measurements on a single spacecraft, which provides a much more accurate estimate of the phase speed, potential and spatial scales of ESWs.
• Ray tracing study of rising tone EMIC-triggered emissions
  
  • Hanzelka Miroslav
  • Santolik O.
  • Grison B.
  • Cornilleau-Wehrlin Nicole
  , 2017, 19, pp.12065. ElectroMagnetic Ion Cyclotron (EMIC) triggered emissions have been subject of extensive theoretical and experimental research in last years. These emissions are characterized by high coherence values and a frequency range of 0.5 - 2.0 Hz, close to local helium gyrofrequency. We perform ray tracing case studies of rising tone EMIC-triggered emissions observed by the Cluster spacecraft in both nightside and dayside regions off the equatorial plane. By comparison of simulated and measured wave properties, namely wave vector orientation, group velocity, dispersion and ellipticity of polarization, we determine possible source locations. Diffusive equilibrium density model and other, semi-empirical models are used with ion composition inferred from cross-over frequencies. Ray tracing simulations are done in cold plasma approximation with inclusion of Landau and cyclotron damping. Various widths, locations and profiles of plasmapause are tested.
• Intermittent energy dissipation by turbulent reconnection
  
  • Fu H.S.
  • Vaivads A.
  • Khotyaintsev Y. V.
  • Andre M.
  • Cao J.B.
  • Olshevsky V.
  • Eastwood Jonathan P.
  • Retinò Alessandro
  , 2017, 19, pp.9103. Magnetic reconnection−-the process responsible for many explosive phenomena in both nature and the laboratory−-is efficient at dissipating magnetic energy into particle energy. To date, exactly how this dissipation happens remains unclear, owing to the scarcity of multi-point measurements of the 'diffusion region' at the sub-ion scale. Here we report such a measurement by Cluster−-four spacecraft with separation of 1/5 ion scale. We discover numerous current filaments and magnetic nulls inside the diffusion region of magnetic reconnection, with the strongest currents appearing at spiral nulls (O-lines) and the separatrices. Inside each current filament, kinetic-scale turbulence is significantly increased, and the energy dissipation, E'*J, is 100 times larger than the typical value. At the jet reversal point, where radial nulls (X-lines) are detected, the current, turbulence, and energy dissipations are surprisingly small. All these features clearly demonstrate that energy dissipation in magnetic reconnection occurs at O-lines but not X-lines.
• Protons and heavy ions acceleration by electromagnetic fluctuations in the Earth's magnetotail
  
  • Catapano F.
  • Zimbardo G.
  • Perri S.
  • Greco A.
  • Delcourt Dominique
  • Retinò Alessandro
  , 2017, 19, pp.1260. Energetic protons and heavy ions are very often observed in the Earth's magnetotail, as shown by numerous spacecraft observations. Yet the acceleration mechanism causing such energization is still under debate. One important candidate is the acceleration by electromagnetic fields fluctuations, which are also very often observed in the magnetotail. Here we perform test particle simulations in which protons and heavier ions are injected in three-dimensional time-dependent stochastic electromagnetic perturbations superposed to an unperturbed magnetic field configuration. We study the energization process for H , He and O ions by performing a detailed analysis of particle dynamics. We find that light ions are preferentially energized and that the level of fluctuations affects the energization rate. We also compare the results from the model with MMS spacecraft observations.
• MMS observations of coherent structures in the turbulent magnetosheath plasma
  
  • Huang S. Y.
  • Sahraoui Fouad
  • Yuan Z. G.
  • Retinò Alessandro
  • He Jiansen
  • Le Contel Olivier
  • Zhao J. S.
  • Chasapis A.
  • Deng X. H.
  • Zhou M.
  • Fu H.S.
  • Aunai N.
  • Breuillard Hugo
  • Pang Y.
  • Wang D. D.
  • Shi Q. Q.
  • Yang J.
  , 2017, 19, pp.6946. Thanks to the unprecedented high time resolution data of the MMS mission, we identified two types of coherent structures in the turbulent magnetosheath plasma. The first one is ion-scale magnetic island/flux rope. The magnetic island is characterized by bipolar variation of magnetic fields with magnetic field compression, strong core field, density depletion and strong currents dominated by the parallel component to the local magnetic field. Distinct particle behaviors and wave activities inside and at the edges of the magnetic island are observed: parallel electron beam accompanied with electrostatic solitary waves and strong electromagnetic lower hybrid drift waves inside the magnetic island; bidirectional electron beams, whistler waves, weak electromagnetic lower hybrid drift waves and strong broadband electrostatic noise at the edges of the magnetic island. Our observations demonstrate that highly dynamical, strong wave activities and electron-scale physics occur within ion-scale magnetic islands in the magnetosheath turbulent plasma. The second one is electron vortex magnetic hole. The magnetic hole is characterized by a magnetic depression, a density peak, a total electron temperature increase (with a parallel temperature decrease but a perpendicular temperature increase), and strong currents carried by the electrons. The current has a dip in the core region of the magnetic hole and a peak in the outer region of the magnetic hole. There is an enhancement in the perpendicular electron fluxes at 90° pitch angles inside the magnetic hole, implying that the electrons are trapped within it. The variations of the electron velocity components Vem and Ven suggest that an electron vortex is formed by trapping electrons inside the magnetic hole in the circular cross-section. These observations demonstrate the existence of a new type of coherent structures behaving as an electron vortex magnetic hole in turbulent space plasmas as predicted by recent kinetic simulations.
• THOR - Turbulence Heating ObserveR
  
  • Vaivads A.
  • Retinò Alessandro
  • Escoubet C. Philippe
  • Khotyaintsev Y. V.
  • Soucek J.
  • Valentini F.
  • Chen C. H. K.
  • Fazakerley A.
  • Lavraud Benoit
  • Marcucci M. F.
  • Narita Y.
  • Vainio Rami O.
  • Voirin Thomas
  • Wielders A.
  • Boudin N.
  • Osuna Pedro
  • Gehler M.
  , 2017, 19, pp.1959. THOR (Turbulence Heating ObserveR) is one of the three candidates for selection as the next ESA M-class mission (M4). THOR will be the first mission ever flown in space that is fully dedicated to study plasma turbulent fluctuations and associated energization mechanisms. Turbulent fluctuations are ubiquitous in astrophysical plasmas and reach up scales as large as stars, bubbles and clouds blown out by stellar winds as well as entire galaxies. However, most of the irreversible energy dissipation associated to turbulent fluctuations occurs at very small scales, the so-called kinetic scales, where the plasma no longer behaves as a fluid and the properties of individual plasma species (electrons, protons, and other ions) become important. THOR will explore the kinetic plasma processes that determine the fundamental behavior of plasma in the universe. THOR will lead to an understanding of the basic plasma heating and particle acceleration mechanisms, of their effect on different plasma species and of their relative importance in different turbulent regimes. THOR will achieve this by making detailed in situ measurements of the closest available dilute and turbulent magnetized plasmas - the Near-Earth's space - at unprecedented temporal and spatial resolution. THOR focuses on particular regions in space: the pristine solar wind, the Earth's bow shock and interplanetary shocks, and the compressed solar wind regions downstream of shocks. These regions are selected because of their different turbulence properties and they reflect the properties of a number of distant astrophysical environments. Here we present THOR's science and summarize the results of the mission and payload studies that finished earlier this year.
• Whistler waves at the Earth bow shock
  
  • Wei Hanying
  • Russell Christopher T.
  • Strangeway Robert J.
  • Schwartz Steve J.
  • An Xin
  • Fischer David
  • Le Contel Olivier
  • Argall Matthew
  • Paterson William R.
  • Torbert Roy B.
  , 2017, 19, pp.3392. The Magnetospheric Multiscale (MMS) spacecraft, with their state-of-the-art plasma and field instruments onboard, allow us to investigate electromagnetic waves at the bow shock and their association with small-scale disturbances in the shocked plasmas. Understanding these waves could improve our knowledge on the heating of electrons and ions across the shock ramp and the energy dissipation of supercritical shocks. We have found broad-band and narrow band waves across the shock ramp and slightly downstream. The broad-band waves propagate obliquely to the magnetic field direction and have frequencies up to the electron cyclotron frequency. Simultaneously, the electrons have quite disturbed velocities and are anisotropic in velocity space, leading to multiple possible instabilities, such as kinetic cross-field streaming instability, low-hybrid drift instability, etc. In the same region with the broad-band wave, there are narrow-band waves at a few hundred Hertz with durations under a second. These waves are right-handed circularly polarized and propagate along the magnetic field lines. The broad-band waves are only observed at the shock ramp, but the narrow-band waves are observed more frequently further downstream in the magnetosheath. Both wave types are likely to be whistler mode with different generation mechanisms. In this paper, we examine the electric and magnetic fields of these waves, as well as the plasma observations to understand the wave generation and their effects on the shock and magnetosheath plasmas.
• Field-aligned currents observed by MMS in the near-Earth plasma sheet during large-scale substorm dipolarizations.
  
  • Nakamura R.
  • Nagai Tsugunobu
  • Giles B. L.
  • Le Contel Olivier
  • Stawarz J. E.
  • Khotyaintsev Y. V.
  • Artemyev A. V.
  , 2017, 19, pp.8362. During substorms significant energy conversion has been reported to take place at the sharp dipolarization front in the flow braking region where the probability of observing bursty bulk flows (BBFs) significantly drops. On 10 August 2016, MMS traversed the pre-midnight near-Earth plasma sheet when dipolarization disturbances were detected in an extended nightside local time region by Cluster, Geotail, GOES 13, 14 and 15, and the Van Allen Probes. In an expanding plasma sheet during the dipolarization, MMS detected sub-ion scale field-aligned current layers that are propagating both Earthward (equatorward) as well as tailward (outward). These multi-scale multi-point observations enable a unique investigation of both the meso-scale evolution of the disturbances and the detailed kinetic structures of the fronts and boundaries relevant to the dipolarizations.
• Evidence for quasi-adiabatic motion of plasma particles in strong current sheets in the solar wind
  
  • Malova H. V.
  • Popov V. Y.
  • Grigorenko E. E.
  • Petrukovich A. A.
  • Delcourt Dominique
  • Sharma A. S.
  • Khabarova O. V.
  • Zelenyi L. M.
  , 2017, 19, pp.3852. We investigate the quasi-adiabatic dynamics of charged particles in strong current sheets (SCSs) in the solar wind, including the heliospheric current sheet (HCS). A self-consistent hybrid model of a SCS is developed in which the dynamics of ions is described using the quasi-adiabatic approach, while the electron motion is assumed to be magnetized and described by the guiding center approximation. The model shows that the SCS profile is determined by the relative contributions of two currents: (i) the current supported by demagnetized protons which follow open quasi-adiabatic orbits, and (ii) the electron drift current. The simplest SCS is found to be a multi-layered structure that consists of a thin current sheet embedded into a much thicker analogue of plasma sheet. This result is in good agreement with observations of SCSs at 1 AU. The fine structure of different SCSs, including the HCS, is shown, independently of the SCS origin, to consist of a narrow current layer (with thickness of 10 000 km) embedded within a wider region of about 105 km at 1 AU. Also, multi-scale structure is shown to be an intrinsic feature of SCSs in the solar wind.
• Kinetic features revealed by top-hat electrostatic analysers: numerical simulations and instrument response results
  
  • de Marco R.
  • Marcucci M. F.
  • Brienza D.
  • Bruno Roberto
  • Consolini G.
  • Perrone Denise
  • Valentini F.
  • Servidio S.
  • Stabile S.
  • Pezzi O.
  • Sorriso-Valvo L.
  • Lavraud Benoit
  • de Keyser J.
  • Retinò Alessandro
  • Fazakerley A.
  • Wicks R. T.
  • Vaivads A.
  • Salatti M.
  • Veltri P.
  , 2017, 19, pp.15779. Turbulence Heating ObserveR (THOR) is the first mission devoted to study energization, acceleration and heating of turbulent space plasmas, and designed to perform field and particle measurements at kinetic scales in different near-Earth regions and in the solar wind. Solar Orbiter (SolO), together with Solar Probe Plus, will provide the first comprehensive remote and in situ measurements which are critical to establish the fundamental physical links between the Sun's dynamic atmosphere and the turbulent solar wind. The fundamental process of turbulent dissipation is mediated by physical mechanism that occur at a variety of temporal and spatial scales, and most efficiently at the kinetics scales. Hybrid Vlasov-Maxwell simulations of solar-wind turbulence show that kinetic effects manifest as particle beams, production of temperature anisotropies and ring-like modulations, preferential heating of heavy ions. We use a numerical code able to reproduce the response of a typical electrostatic analyzer of top-hat type starting from velocity distribution functions (VDFs) generated by Hybrid Vlasov-Maxwell (HVM) numerical simulations. Here, we show how optimized particle measurements by top-hat analysers can capture the kinetic features injected by turbulence in the VDFs.
• Quasi-adiabatic transport in Mercury's magnetotail
  
  • Delcourt Dominique
  • Malova H. V.
  • Zelenyi L. M.
  , 2017, 19, pp.2658. MESSENGER observations have revealed that the magnetotail of Mercury is fairly dynamical, possibly subjected to series of magnetic field line dipolarization on time scales of a few seconds. Because of the sharp reversal of the magnetic field, ions may not travel adiabatically in this region of space, and their behavior can be organized according to different categories. Among these categories, quasi-adiabatic (Speiser) ions are such that they experience negligible net change of magnetic moment upon crossing of the field reversal and can thus travel back to low altitudes. We examine the robustness of this quasi-adiabatic behavior during magnetic field line dipolarization where ions are subjected to a large induced electric field. We demonstrate that, although this surging electric field possibly yields substantial nonadiabatic heating, quasi-adiabaticity is robust for ions with velocities larger than the peak ExB drift speed, a behavior that we refer to as "strong" quasi-adiabaticity (as opposed to "weak" quasi-adiabaticity that is violated during dipolarization). We show that the impulsive energization of such quasi-adiabatic ions during dipolarization events can lead to prominent energy-time dispersion structures at low altitudes.
• Electromagnetic waves and electron phase-space hole like signatures detected by MMS during a substorm
  
  • Le Contel Olivier
  • Nakamura R.
  • Berthomier Matthieu
  • Pottelette Raymond
  • Breuillard Hugo
  • Retinò Alessandro
  • Chust Thomas
  • Mirioni Laurent
  • Argall Matthew
  • Fischer David
  , 2017, 19, pp.9295. In August 2016, the MMS constellation was in the magnetotail with an orbit apogee of 12 Earth radii and an average inter-satellite distance of 60 km (i.e. between electron and ion scales). On August 10, 2016 although MMS was located quite far from the magnetic equator, it detected multiple dipolarization signatures associated with substorm events. In this study, we focus on the wave activity detected during one of the dipolarization event and in particular we analyze in details the electromagnetic electron phase-space hole like signatures observed by three of the four MMS spacecraft. Such signatures have been already detected by one of the THEMIS probes under similar magnetospheric conditions. However, the MMS tetrahedral configuration with its small inter-satellite separation allows us to better analyze the characteristics of these structures such as their velocity, their direction of propagation, their internal structure and/or their time evolution. The consistency of these observations with existing models will be discussed.
• 2d axisymmetric beambulk modelling of the generation of runaway electrons by streamers
  
  • Chanrion Olivier
  • Bonaventura Z.
  • Bourdon Anne
  • Neubert Torsten
  , 2017, 19 (EGU 2017-15611).
• The spatial evolution of the mixing layer in the Kelvin-Helmholtz instability at the Martian ionopause
  
  • Aizawa Sae
  • Terada N.
  • Kasaba Y.
  • Yagi M.
  • Matsumoto Y.
  • Delcourt Dominique
  , 2017, 19, pp.14629. We investigate the growth of the mixing layer thickness in the Kelvin-Helmholtz instability (KHI) using an extended-local MHD model to estimate the ion loss rate from the Martian ionopause. The KHI is expected to play a major role in transporting mass, momentum and energy across the ionopause between the sheath flow and ionospheric plasmas. Since the mixing layer has a finite thickness between them, this layer has a potential for the removal of a huge amount of ions from Mars through its history. The recent MAVEN observation reported that the density ratio across the ionopause reaches as high as 100 5000. With such a large density ratio, compressible effects are expected to modify the structure of the KH vortices and the evolution of the mixing layer by generating high-amplitude nonlinear fast-mode plane waves from ridges of the KH waves. In order to reproduce Martian ionopause, we developed an extended-local MHD model with aperiodic boundary condition for the evaluation of traveling waves along the dayside Martian ionopause ( 6,000km). Spatial resolution is set with 3km to resolve the thin mixing layer. We find two factors that accelerate the growth of the mixing layer. Firstly, the KH wave with the fastest growing mode behaves like a wall to the leading vortex in the aperiodic condition. The sheath flow is stagnated by this wall-like structure and induces an enhanced vortex return flow, resulting in a deeper excavation of the ionospheric plasma. Secondly, fast-mode rarefaction waves generated by compressible effects make wall-like structures more effective by lowering pressure around antinodes of the KH waves. Such a pressure profile further accelerates the stagnation and the excavation. In addition, KH vortices are merging not one by one but also some vortices are merged together at time. Thus, the large wave like structure can be seen when the effect of compressibility is not so large. The mixing layer spread with the lapse of time and it depends on the density ratio, its relation can be described the function of the logarithm. Furthermore, we estimate the ion loss rate from our results and it agrees with the MAVEN observational results.
• Turbulence Heating ObserveR - THOR: mission overview and payload summary
  
  • Escoubet C. Philippe
  • Voirin Thomas
  • Wielders A.
  • Vaivads A.
  • Retinò Alessandro
  • Khotyaintsev Y. V.
  • Soucek J.
  • Valentini F.
  • Chen C. H. K.
  • Fazakerley A.
  • Lavraud Benoit
  • Marcucci M. F.
  • Narita Y.
  • Vainio R.
  • Romstedt Jens
  • Boudin N.
  • Junge A.
  • Osuna Pedro
  • Walsh Andrew P.
  , 2017, 19, pp.6286. The Turbulence Heating ObserveR (THOR) mission was selected as one of the three candidates, following the Call for Medium Class Missions M4 by the European Space Agency, with a launch planned in 2026. THOR is the first mission ever flown in space dedicated to plasma turbulence. THOR will lead to an understanding of the basic plasma heating and particle energization processes, of their effect on different plasma species and of their relative importance in different turbulent regimes. The THOR mission features one single spinning spacecraft, with the spin axis pointing toward the Sun, and 10 state-of-the-art scientific instruments, measuring electromagnetic fields and waves and electrons and ions at the highest spatial and temporal resolution ever achieved. THOR focuses on particular regions: pristine solar wind, Earth's bow shock and interplanetary shocks, and compressed solar wind regions downstream of shocks, that will be observed with three different orbits of 6 x 15 RE, 6 x 25 RE and 6 x 45 RE. These regions are selected because of their differing turbulent fluctuation characteristics, and reflect similar astrophysical environments. The THOR mission, the conceptual design of the spacecraft and a summary of the payload will be presented. Furthermore, driving requirements and their implications for the spacecraft like Electromagnetic Compatibility and cleanliness will be discussed.
• Localized Energy Conversion within a Reconnection Diffusion Region
  
  • Burch J. L.
  • Torbert Roy
  • Ergun Robert
  • Rager A.
  • Giles B. L.
  • Webster J. M.
  • Genestreti Kevin
  • Allen Robert
  • Phan T. D.
  • Dorelli J. C.
  • Gershman D. J.
  • Chen L.-J.
  • Le Contel Olivier
  • Russell C. T.
  • Strangeway R. J.
  • Wang S.
  • Wilder Frederick
  • Graham Daniel
  • Cassak P. A.
  • Hesse Michael
  , 2017, 19, pp.5880. The four MMS spacecraft encountered an electron diffusion region near 13:07:02.2 UT on 16 Oct. 2015. Electron distribution functions with 30-ms cadence show non-gyrotropic distributions with predicted crescent-shaped peaks near the stagnation point on the magnetosphere side of the reconnection X-line. Breaking and reconnection of field lines is indicated by the transition of the crescent feature from perpendicular to parallel to the local magnetic field line with downward magnetosheath electrons and upward magnetospheric electrons populating open field lines. Multiple bipolar electric field pulses (possibly solitary waves) with magnitudes from 20 - 100 mV/m were observed in the L and M boundary normal coordinates by MMS2 and MMS3 along with a quasistatic positive normal electric field component. The strongest of these events, which resulted in significant J dot E dissipation and quenching of widespread magnetosonic waves at 30 - 40 Hz, occurred at the precise location of field-line breaking and reconnection. Weaker J dot E signatures were observed at some of the other events, suggesting the occurrence of multiple, or patchy energy conversion within the diffusion region.
• The Search-coil Magnetometer for the THOR mission
  
  • Sahraoui Fouad
  • Jannet Guillaume
  • Pinçon Jean-Louis
  • Mansour Malik
  • Henri Pierre
  • Chalumeau Gilles
  • Hachemi Tedjani
  • Jeandet Alexis
  • Briand Nicolas
  • Le Contel Olivier
  • Rezeau Laurence
  , 2017, 19 (EGU2017-14590), pp.Poster. Turbulence Heating ObserveR (THOR) is the first mission ever flown in space fully dedicated to plasma turbu- lence. The search-coil magnetometer (SCM) of THOR is a triaxial dual-band antenna dedicated to measuring the magnetic field fluctuations in the frequency range [1Hz,4kHz] and [1,200]kHz. THOR/SCM has a long heritage from earlier space missions such as Cluster, Themis, MMS, BepiColombo, Taranis, Solar orbiter and Solar Probe. In comparison to those missions, the SCM of THOR has a higher sensitivity level, which makes it capable of mea- suring very low amplitude magnetic fluctuations, in particular in the solar wind. Those measurements are crucial to address the problem of turbulence and energy dissipation at electron scales, a central goal of the THOR mission.
• How is the quasi perpendicular ion foreshock filled in ? Self-consistent 2D Full-Particle and Test-particles simulations
  
  • Savoini Philippe
  • Lembège Bertrand
  , 2017, 19, pp.7979. Backstreaming ion populations propagating along the interplanetary magnetic field are evidenced upstream of the Terrestrial curved bow shock and form the ion foreshock. Two distinct backstreaming populations have been firmly identified by spacecrafts within the quasi-perpendicular shock region (i.e. for 45° <= Theta<SUB>Bn</SUB> <= 90°, where Theta<SUB>Bn</SUB> is the angle between the shock normal and the upstream magnetostatic field): so called (i) field-aligned ion beams (
• The firehose instability during multiple reconnection in the Earth's magnetotail
  
  • Alexandrova Alexandra
  • Divin A. V.
  • Retinò Alessandro
  • Deca J.
  • Catapano F.
  • Cozzani Giulia
  , 2017, 19, pp.18183. We found unique events in the Cluster spacecraft observations of the Earth's magnetotail which correspond to the case of multiple reconnection sites. The ion temperature anisotropy of more energized ions in the direction parallel to the magnetic field, rather than in the perpendicular direction, is observed in the region of dynamical interaction between two active X-lines. The magnetic field and plasma parameters associated with the anisotropy correspond to the firehose instability conditions. We discuss possible scenarios of development of the firehose instability in multiple reconnection by comparing the observations with numerical simulations. Conventional Particle-in-Cell simulations of 2D magnetic reconnection starting from Harris equilibria are performed using implicit PIC code iPIC3D [Markidis, 2010]. At earlier stages the evolution creates fronts which push the weakly magnetized current sheet plasma away from the X-line. Fronts accelerate and reflect particles, producing parallel ion beams and increasing parallel ion temperature ahead of the front. If multiple X-lines are present, then the counterstreaming ion beams appear inside the original current sheet between colliding reconnection jet fronts. For large enough parallel ion pressure anisotropy, the firehose-like mode is excited inside the original current sheet with a flapping-like appearance along the X GSM direction but not Y GSM (current) direction. One should note that our simulations do not include the Bz magnetic field component (normal to the current sheet), hence ion beams cannot escape into the lobes and the whole region between two colliding fronts is unstable to firehose-like instability. In the Earth's magnetotail such configuration likely occurs when two active X-lines are close enough to each other, similar to a few cases we found in the Cluster observations.
• Non-Maxwellianity of electron distributions and their source regions
  
  • Graham Daniel
  • Vaivads A.
  • Khotyaintsev Y. V.
  • André M.
  • Chasapis A.
  • Retinò Alessandro
  • Valentini F.
  • Matthaeus W. H.
  , 2017, 19, pp.13910. Identifying regions where electron distributions strongly deviate from Maxwellian distributions and understanding the basic physical processes driving this deviation is of high importance in space plasma physics. These are the regions where kinetic plasma process can strongly affect the large-scale plasma properties and dynamics. Examples include reconnection diffusion regions, shocks, strong turbulence, and current sheets. Using the Magnetospheric Multiscale (MMS) spacecraft, we evaluate and quantify the deviations of electron distributions from bi-Maxwellian distribution functions; for example, using agyrotropy measures and comparisons of observed distributions with the Maxwellian distributions predicted from electron moments. We investigate where these distributions develop, focusing on turbulent regions in the solar wind, foreshock and bowshock, magnetosheath, and the magnetopause and determine under what conditions these deviations from bi-Maxwellian distributions occur. We discuss these how these measures relate to turbulence and applications to the potential THOR mission.