Publications

2016

• Characterization of Bismuth Silicon Oxide Crystals Using Imaging Mueller Ellipsometry
  
  • Slikboer Elmar
  • Yoo Sang Hyuk
  • Guaitella Olivier
  • Sobota Ana
  • Garcia-Caurel Enric
  , 2016.
• High sensitivity ultra-broad-band absorption spectroscopy of inductively coupled chlorine plasma
  
  • Marinov Daniil
  • Foucher Mickaël
  • Campbell Ewen
  • Brouard Mark
  • Chabert Pascal
  • Booth Jean-Paul
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (3), pp.035019. We propose a method to measure the densities of vibrationally excited Cl 2 (v) molecules in levels up to v = 3 in pure chlorine inductively coupled plasmas. The absorption continuum of Cl 2 in the 250 – 450 nm spectral range is deconvoluted into the individual components originating from the different vibrational levels of the ground state, using a set of ab-initio absorption cross sections. It is shown that gas heating at constant pressure is the major depletion mechanism of the Cl 2 feedstock in the plasma. In these line-integrated absorption measurements, the absorption by the hot (and therefore rarefied) Cl 2 gas in the reactor centre is masked by the cooler (and therefore denser) Cl 2 near the walls. These radial gradients in temperature and density make it difficult to assess the degree of vibrational excitation in the centre of the reactor. The observed line-averaged vibrational distributions, when analyzed taking into account the radial temperature gradient, suggest that vibrational and translational degrees of freedom in the plasma are close to local equilibrium. This can be explained by efficient VT relaxation between Cl 2 and Cl atoms. Besides the Cl 2 (v) absorption band, a weak continuum absorption is observed at shorter wavelengths, and is attributed to photodetachment of Cl ‒ negative ions. Thus, line-integrated densities of negative ions in chlorine plasmas can be directly measured using broad-band absorption spectroscopy. (10.1088/0963-0252/25/3/035019)
  DOI : 10.1088/0963-0252/25/3/035019
• Predator Prey Dynamics in Magnetized Fusion Plasmas
  
  • Morel Pierre
  • Kobayashi Sumire
  • Donnel Peter
  • Berionni Vincent
  • Honoré Cyrille
  • Pisarev V.
  • Hennequin Pascale
  • Gürcan Özgür D.
  , 2016.
• GNSS, Space Weather and Capacity building
  
  • Amory-Mazaudier Christine
  , 2016. During the last decade, the deployment of GPS over Africa and the Southern hemisphere allowed a great advance in Research in the countries where GPS receivers were installed. These research advances concern the Geodesy, ionospheric and atmospheric physics especially at low and equatorial latitudes, and Space weather effects on GPS system. The GNSS tools are cheap compared to satellite tools, and as a consequence permit the participation of all the countries to the research in various fields. The GNSS data are complementary of the satellite data. The GNSS tools became very useful for capacity building in research, especially in Space Weather. Another important point is that GNSS data are used for many applications useful for the development of a country (aviation, cadastre, agriculture, infrastructures of the country etc). It is important that all the users of GNSS data be aware of the progress in research on GNSS data, in order to take into account the scientific results in the development of their applications. Now with GNSS (GPS GLONASS GALLILEO ....), we can multiply the number of GNSS receivers and develop more research and application studies. This presentation will be devoted to the success story of the use of GNSS for capacity building in Space Weather, in Africa and Southern hemisphere.
• Exploring turbulent energy dissipation and particle energization in space plasmas: the science of THOR mission
  
  • Retinò Alessandro
  , 2016.
• Synthetic diagnostic of Doppler Back-Scattering measurements on the Tore Supra Plasma #TS45511 with a dedicated GYSELA non-linear gyrokinetic simulation
  
  • Morel Pierre
  • Gassama Banda
  • Leybros Robin
  • Dif-Pradalier Guilhem
  • Hennequin Pascale
  • Vermare Laure
  • Gürcan Özgür D.
  • Grandgirard V.
  • Latu G.
  • Sarazin Y.
  • Ghendrih Ph.
  • Garbet X.
  , 2016.
• EMC ASPECTS OF TURBULENCE HEATING OBSERVER (THOR) SPACECRAFT
  
  • Soucek Jan
  • Ahlen L.
  • Bale Stuart
  • Bonnell J W
  • Boudin N.
  • Brienza D.
  • Carr C.
  • Cipriani Fabrice
  • Escoubet C. P.
  • Fazakerley Andrew F
  • Gehler M.
  • Génot Vincent
  • Hilgers A.
  • Hanock B.
  • Jannet G.
  • Junge A.
  • Khotyaintsev Yuri
  • de Keyser Johan
  • Kucharek H.
  • Lavraud B
  • Lau R.
  • Leblanc François
  • Magnes W.
  • Mansour M.
  • Marcucci M.F.
  • Nakamura Rumi
  • Němeček Z.
  • Owen C.
  • Phal Y.
  • Retinò Alessandro
  • Rodgers David
  • Šafránková J.
  • Sahraoui Fouad
  • Vainio R.
  • Wimmer-Schweingruber R.
  • Steinhagen J.
  • Vaivads A.
  • Wielders A.
  • Zaslavsky A.
  , 2016, 738, pp.1-4. Turbulence Heating ObserveR (THOR) is a spacecraft mission dedicated to the study of plasma turbulence in near-Earth space. The mission is currently under study for implementation as a part of ESA Cosmic Vision program. THOR will involve a single spinning spacecraft equipped with state of the art instruments capable of sensitive measurements of electromagnetic fields and plasma particles. The sensitive electric and magnetic field measurements require that the spacecraft-generated emissions are restricted and strictly controlled; therefore a comprehensive EMC program has been put in place already during the study phase. The THOR study team and a dedicated EMC working group are formulating the mission EMC requirements already in the earliest phase of the project to avoid later delays and cost increases related to EMC. This article introduces the THOR mission and reviews the current state of its EMC requirements.
• Unveiling coherent structures through entropy-complexity analysis
  
  • Chrisment Antoine M.
  • Firpo Marie-Christine
  , 2016.
• Counter-Propagating Laser Produced Radiative Shocks at the Orion Laser Facility
  
  • Clayson Thomas
  • Suzuki-Vidal Francisco
  • Lebedev S.
  • Swadling G.
  • Burdiak G.
  • Patankar S.
  • Smith R.
  • Foster J.
  • Skidmore J.
  • Gumbrell E.
  • Graham P.
  • Charles R.
  • Treadwell P.
  • Hopps N.
  • Danson C.
  • Stehlé Chantal
  • Chaulagain Uddhab
  • Spindloe C.
  • Kozlová Michaela
  • Larour Jean
  • Awe Team The
  , 2016. The Orion high-power laser facility at AWE Aldermaston, UK, was used to produce hyper-sonic (M>>1) radiative shocks in a variety of noble gases. These experiments aimed to study the radiative precursor, a heat and ionization wave preceding the shock front and, for the first time, the dynamics of colliding, counter propagating radiative shocks. Laser ablation of a piston, at intensities of ~6x1014 W/cm2, drove counterpropagating shocks, with velocities between 60 km/s and 120 km/s, into a gas cell, filled to pressures between 0.1 bar and 1.0 bar. Targets were 4mm long octagonal gas cells, produced by SciTech Precision, with a diameter of 8mm to remove the effect of wall shocks. X-ray backlighting and optical self-emission streak Imaging were used to image the shock front and collision dynamics. Multi-frame and streaked interferometry were used to image the radiative precursor and determine its electron density. These experiments compared the shock and collision dynamics in different gases (e.g. Ne, Ar, Kr, Xe), maintaining a constant mass density in order to keep the hydrodynamics in the shock consistent while varying the strength of the radiative precursor losses. In some cases the shocks exhibited features suggesting the formation of hydrodynamic or radiative instabilities. The experimental data is in good agreement with 2-D numerical radiative-hydrodynamic simulations and provides a new benchmark for codes to be tested against. Project supported by Orions Academic Access Programme (through AWE and CLF), and in part by the Royal Society, and by EPSRC through a DTA Scholarship and by Labex PLAS@PAR (ANR-11-IDEX-0004-02). The authors would like to acknowledge the help and support from the staff at AWE Aldermaston.
• Experimental study of the collision of two laser-driven radiative shocks at the PALS laser
  
  • Singh Raj Laxmi
  • Stehlé Chantal
  • Suzuki-Vidal Francisco
  • Larour Jean
  • Chaulagain Uddhab
  • Clayson Thomas
  • Nejdl Jaroslav
  • Krus Miroslav
  • Kozlová Michaela
  • Dostál Jan
  • Acef Ouali
  • Barroso Patrice
  • Cotelo M.
  • Velarde P.
  • Rodriguez Perez R.
  • Gil J. M.
  , 2016. Radiative shocks are present in various astrophysical contexts and can be proxy of fundamental accretion processes, for instance X-ray signatures from accretion shocks can be related to the rate of mass accretion onto forming stars. Thus the study of hypersonic shocks (Mach number >> 1) in the laboratory Under controlled conditions is of primary interest in order to study the influence of radiation, and to compare with numerical simulations. In the past decade, several experiments on radiative shocks have been performed on various large-scale laser facilities, demonstrating the formation of shocks in Xenon with velocities ~50 - 150 km/s in background gas pressures ~0.1 - 1 bar (Bouquet et al. 2004, Gonzalez et. al. 2006, Reighard et. al. 2007, Stehlé et al. 2010, Doss et al. 2011, Drake et al. 2011, Dizière et al. 2012, Stehlé et al. 2012, Chaulagain et al. 2015). Many advances have been achieved in understanding the effect of radiation on the different shock components (radiative precursor, shock collapse, wall heating etc.), however, these studies were focused solely on the case of a single radiative shock. We have recently conducted experiments at the PALS laser facility, looking at the collision between two counter streaming radiative shocks. Besides providing a new experimental platform, these experiments aimed at studying how one radiative precursor is influenced by the presence of another. The experiments launched shocks with different shock speeds (~30-55 km/s and 10-25 km/s), at a range of different pressures (~0.1-0.3 bar), and with different gases (Ar, Xe). Optical interferometry allowed us to estimate several physical parameters such as shock speed and electron density in the precursor, whereas the use of time and spatially resolved optical spectroscopy led to a number of spectral signatures. We will present preliminary results together with numerical simulations.
• The importance of surface interaction probabilities in low pressure plasma simulations for process design
  
  • Gibson Andrew
  • Foucher Mickaël
  • Marinov Daniil
  • Chabert Pascal
  • Gans T.
  • Guerra V.
  • Kushner M.J.
  • Booth Jean-Paul
  , 2016.
• Studying the plume neutralization process of the PEGASES thruster
  
  • Cichocki F.
  • Rafalskyi D.V.
  • Aanesland Ane
  , 2016.
• Origin of energetic ions observed in the terrestrial ion foreshock : 2D full-particle simulations
  
  • Savoini Philippe
  • Lembège Bertrand
  , 2016, 18, pp.EGU2016-6218. Collisionless shocks are well-known structures in astrophysical environments which dissipate bulk flow kinetic energy and accelerate large fraction of particle. Spacecrafts have firmly established the existence of the so-called terrestrial foreshock region magnetically connected to the shock and filled by two distinct populations in the quasi- perpendicular shock region (i.e. for 45 ̊ ≤ ΘBn ≤ 90 ̊, where ΘBn is the angle between the shock normal and the upstream magnetic field) : (i) the field-aligned ion beams or “ FAB ” characterized by a gyrotropic distribution, and (ii) the gyro-phase bunched ions or “ GPB ” characterized by a NON gyrotropic distribution. The present work is based on the use of two dimensional PIC simulation of a curved shock and associated foreshock region where full curvature effects, time of flight effects and both electrons and ions dynamics are fully described by a self consistent approach. Our previous analysis (Savoini et Lembège, 2015) has evidenced that these two types of backstreaming populations can originate from the shock front itself without invoking any local diffusion by ion beam instabilities. Present re- sults are focussed on individual ion trajectories and evidence that "FAB" population is injected into the foreshock mainly along the shock front whereas the "GPB" population penetrates more deeply the shock front. Such differ- ences explain why the "FAB" population loses their gyro-phase coherency and become gyrotropic which is not the case for the "GPB". The impact of these different injection features on the energy gain for each ion population will be presented in détails.
• Exploring turbulent energy dissipation and particle energization in space plasmas: the science of THOR mission
  
  • Retinò Alessandro
  , 2016, 18, pp.EPSC2016-15240. The Universe is permeated by hot, turbulent magnetized plasmas. They are found in active galactic nuclei, supernova remnants, the intergalactic and interstellar medium, as well as in the solar corona, the solar wind and the Earth's magnetosphere. Turbulent plasmas are also found in laboratory devices such as e.g. tokamaks. Our comprehension of the plasma Universe is largely based on measurements of electromagnetic radiation such as light or X-rays which originate from particles that are heated and accelerated as a result of energy dissipation in turbulent environments. Therefore it is of key importance to study and understand how plasma is energized by turbulence. Most of the energy dissipation occurs at kinetic scales, where plasma no longer behaves as a fluid and the properties of individual plasma species (electrons, protons and other ions) become important. THOR (Turbulent Heating ObserveR - http://thor.irfu.se/) is a space mission currently in Study Phase as candidate for M-class mission within the Cosmic Vision program of the European Space Agency. The scientific theme of the THOR mission is turbulent energy dissipation and particle energization in space plasmas, which ties in with ESA's Cosmic Vision science. The main focus is on turbulence and shock processes, however areas where the different fundamental processes interact, such as reconnection in turbulence or shock generated turbulence, are also of high importance. The THOR mission aims to address fundamental questions such as how plasma is heated and particles are accelerated by turbulent fluctuations at kinetic scales, how energy is partitioned among different plasma components and how dissipation operates in different regimes of turbulence. To reach the goal, a careful design of the THOR spacecraft and its payload is ongoing, together with a strong interaction with numerical simulations. Here we present the science of THOR mission and we discuss implications of THOR observations for space, astrophysical and laboratory turbulent plasmas.
• Importance of the magnetic field cone angle in magnetic clouds
  
  • Turc Lucile
  • Escoubet P.
  • Fontaine Dominique
  • Kilpua E. K. J.
  , 2016, 18, pp.EPSC2016-6101. The properties of magnetic clouds (MCs) observed by either ACE or Wind in L1 from 2000 to 2014 are investigated. The results obtained for their basic properties, such as the average density, velocity and magnetic field intensity, are consistent with previous studies. We then focus on the orientation of the magnetic field inside MCs. We find that in most cases the direction of the MCs' magnetic field departs from that of the Parker-spiral interplanetary magnetic field usually encountered during quiet times. As a consequence, the position of the quasi-parallel and quasi-perpendicular regions of the Earth's bow shock will differ from the textbook picture (quasi-parallel on the dawnside, quasi-perpendicular on the duskside). The distribution of the different shock geometries along the shock's surface can have a strong impact on the solar wind-magnetospheric coupling (e.g. asymmetries in the magnetosheath, modification of the MCs' magnetic structure...). Using events during which observations inside the magnetosheath are available and with the help of a magnetosheath model, we obtain an estimate of the ThetaBn angle, between the shock normal and the upstream magnetic field, encountered upon entering the magnetosheath. We show that for a given cone angle, the magnetosheath observations are associated with only a limited range of ThetaBn values. Therefore, it appears that the cone angle gives a good approximation of the encountered shock configuration. In the context of space weather forecasting, we discuss the importance of predicting not only the Bz component of the magnetic field inside MCs, but also the other components and in particular the Bx component, as they affect the position of the quasi-parallel and quasi-perpendicular regions of the terrestrial bow shock, which in turn influences the driving of the magnetosphere.
• THOR Ion Mass Spectrometer instrument - IMS
  
  • Retinò Alessandro
  • Kucharek H.
  • Saito Y.
  • Fraenz Markus
  • Verdeil Christophe
  • Leblanc F.
  • Techer Jean-Denis
  • Jeandet A.
  • Macri J.
  • Gaidos John
  • Granoff M.
  • Yokota S.
  • Fontaine Dominique
  • Berthomier Matthieu
  • Delcourt Dominique
  • Kistler L. M.
  • Galvin A. B.
  • Kasahara S.
  • Kronberg E. A.
  , 2016, 18, pp.EPSC2016-15367. Turbulence Heating ObserveR (THOR) is the first mission ever flown in space dedicated to plasma turbulence. Specifically, THOR will study how turbulent fluctuations at kinetic scales heat and accelerate particles in different turbulent environments within the near-Earth space. To achieve this goal, THOR payload is being designed to measure electromagnetic fields and particle distribution functions with unprecedented resolution and accuracy. Here we present the Ion Mass Spectrometer (IMS) instrument that will measure the full three-dimensional distribution functions of near-Earth main ion species (H , He , He and O ) at high time resolution (~ 150 ms for H , ~ 300 ms for He ) with energy resolution down to ~ 10% in the range 10 eV/q to 30 keV/q and angular resolution ~ 10°. Such high time resolution is achieved by mounting multiple sensors around the spacecraft body, in similar fashion to the MMS/FPI instrument. Each sensor combines a top-hat electrostatic analyzer with deflectors at the entrance together with a time-of-flight section to perform mass selection. IMS electronics includes a fast sweeping high voltage board that is required to make measurements at high cadence. Ion detection includes Micro Channel Plates (MCP) combined with Application-Specific Integrated Circuits (ASICs) for charge amplification, discrimination and time-to-digital conversion (TDC). IMS is being designed to address many of THOR science requirements, in particular ion heating and acceleration by turbulent fluctuations in foreshock, shock and magnetosheath regions. The IMS instrument is being designed and will be built by an international consortium of scientific institutes with main hardware contributions from France, USA, Japan and Germany.
• A new technique for the investigation of the energy cascade associated with coherent structures in Kelvin-Helmholtz turbulence
  
  • Rossi C.
  • Camporeale E.
  • Califano F.
  • Cerri Silvio Sergio
  • Retinò Alessandro
  • Sorriso-Valvo L.
  • Carbone V.
  , 2016, 18, pp.EPSC2016-13318. The dissipation of turbulent energy at small scales in space plasmas is an open and challenging problem. Coherent structures at the kinetic scales could play a fundamental role in redistributing the plasma energy thus replacing in some sense the role of collisions. Coherent structures in the form of current sheets (CS) are associated with localized particle heating, and are generally responsible for the observed intermittent nature of plasma turbulence. Still, the contribution of such structures to the local energy spectrum shaping is not well understood. Here, for the first time, we apply a 'space-filter' technique to two-fluid plasma simulations of Kelvin-Helmholtz turbulence to obtain a local measure of the inter-scale transfer and to characterize the contribution of coherent structures to the energy spectrum. This technique, used in hydrodynamics and in Large-Eddy-Simulation communities, is applied here for the first time to space plasma turbulence. Specifically, we study in detail the current sheets forming in turbulent Kelvin-Helmholtz vortices by the Partial Variance of Increments (PVI) technique, and we discuss the correlation between the inter-scale transfer and high values of the PVI index.
• Turbulence Heating ObserveR - THOR: Mission overview and payload summary
  
  • Wielders A.
  • Boudin N.
  • Vaivads A.
  • Khotyaintsev Y. V.
  • Lavraud Benoit
  • Sahraoui Fouad
  • Nakamura R.
  • Owen C.
  • Fazakerley A.
  • Nemecek Z.
  • Soucek J.
  • Marcucci M. F.
  • Wimmer-Schweingruber R. F.
  • Retinò Alessandro
  • Gehler M.
  • Escoubet P.
  , 2016, 18, pp.EPSC2016-15629. The Turbulence Heating ObserveR (THOR) mission was selected as one of the three candidates of the Call for Medium Class Missions M4 in the European Space Agency's Science Programme 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 10 scientific instruments focusing on particular regions in three different elliptical orbits around the Earth; pristine solar wind, Earth's bow shock and interplanetary shocks, and compressed solar wind regions downstream of shocks. 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.
• New Exact Relations for Helicities in Hall Magnetohydrodynamic Turbulence
  
  • Banerjee Supratik
  • Galtier Sébastien
  , 2016, 18, pp.EPSC2016-7291. Hall magnetohydrodynamics is a mono-fluid plasma model appropriate for probing Finalsome of the physical processes (other than pure kinetic effects) at length scales smaller than the scales of standard MHD. In sub-ionic space plasma turbulence (e.g. the solar wind) this fluid model has been proved to be useful. Three-dimensional incompressible Hall magnetohydrodynamics (MHD) possesses three inviscid invariants which are the total energy, the magnetic helicity and the generalized helicity. In this presentation, we would like to discuss new exact relations for helicities (magnetic helicities and generalized helicities) which are derived for homogeneous stationary (not necessarily isotropic) Hall MHD turbulence (and also for its inertialess electron MHD limit) in the asymptotic limit of large Reynolds numbers. The universal laws are written only in terms of mixed second-order structure functions, i.e. the scalar product of two different increments and are written simply as eta<SUB>M</SUB> = d<SUB>i</SUB> < delta ( b × j) · delta b >, with eta<SUB>M</SUB> the average magnetic helicity flux rate, b the magnetic field, j the current and ± eta<SUB>G</SUB> = < delta ( v × Omega ) · delta Omega > , with eta<SUB>M</SUB> the average generalized helicity flux rate, v the fluid velocity and Omega = b d<SUB>I</SUB> omega being the generalized helicity where omega is simply the fluid vorticity ( = nabla × v). It provides, therefore, a direct measurement of the dissipation rates for the corresponding helicities even in case of an anisotropic plasma turbulence. This study shows that the generalized helicity cascade is strongly linked to the left polarized fluctuations while the magnetic helicity cascade is linked to the right polarized fluctuations. The newly derived relations also show that like energy, a non-zero helicity flux can only be associated to a departure of Beltrami flow state. Reference S. Banerjee & S. Galtier, Chiral Exact Relations for Helicities in Hall Magnetohydrodynamic Turbulence (submitted).
• The role of waves and DC electric fields for electron heating and acceleration in the diffusion region
  
  • Graham Daniel
  • Khotyaintsev Y. V.
  • Vaivads A.
  • Norgren Cecilia
  • Andre M.
  • Lindqvist Per-Arne
  • Le Contel Olivier
  • Ergun Robert
  • Goodrich Katherine
  • Torbert Roy
  • Burch J. L.
  • Russell C. T.
  • Magnes Werner
  • Giles B. L.
  • Pollock Craig
  • Mauk Barry
  • Fuselier Stephen
  , 2016, 18, pp.EPSC2016-10458. Magnetic reconnection is a fundamental process in solar and astrophysical plasmas. The processes operating at electron spatial-scales, which enable magnetic field lines to reconnect, are generally difficult to resolve and identify. However, the recently launched Magnetospheric Multiscale (MMS) mission is specifically designed to resolve electron spatial scales. We use the MMS spacecraft to investigate the process operating within the diffusion region to determine the causes of electron heating and acceleration. In particular, we investigate the type of electrostatic and electromagnetic waves that develop and how they affect the electron distributions. We also compare the roles of wave-particle interactions with DC electric fields to determine which is responsible for the electron heating observed in diffusion regions.
• Multi-Spacecraft Analysis of Plasma Jet Events and Associated Whistler-Wave Emissions using MMS Data
  
  • Breuillard Hugo
  • Le Contel Olivier
  • Retinò Alessandro
  • Chasapis A.
  • Chust Thomas
  • Cohen Ian
  • Wilder Frederick
  • Graham Daniel
  • Khotyaintsev Y. V.
  , 2016, 18, pp.13754. Plasma jets aka bursty bulk flows play a crucial role in Earth's plasmasheet dynamics, in particular during substorms where they can sometimes even penetrate down to the geosynchronous orbit. The energy input from the solar wind is partly dissipated in jet fronts(also called dipolarization fronts) in the form of strong whistler waves that can heat and accelerate energetic electrons. The ratio of the energy transported during jets to the substorm energy consumption is still under debate due to instrumental limitations. In May 2015 the Magnetospheric Multiscale (MMS) mission evolves in a string-of-pearls configuration with an average inter-satellite distance of 300 km which allows us to study in detail the microphysics of these phenomena. Thus in this study we employ MMS data to investigate the properties of jet fronts propagating earthward and their associated whistler-mode wave emissions. We show that the spatial dynamics of jet fronts are of the order of the ion gyroradius and whistler-wave dynamics have a temporal scale of a few seconds. We also investigate the energy dissipation associated with such waves and their interaction with energetic electrons in the vicinity of the flow/jet braking region. In addition, we make use of ray tracing simulations to evaluate their propagation properties, as well as their impact on particles in the off-equatorial magnetosphere.
• The Search-coil Magnetometer for the THOR mission
  
  • Sahraoui Fouad
  • Pinçon Jean-Louis
  • Jannet Guillaume
  • Mansour Malik
  • Henri Pierre
  • Chalumeau Gilles
  • Hachemi Tedjani
  • Jeandet Alexis
  • Briand Nicolas
  • Le Contel Olivier
  • Rezeau Laurence
  , 2016. Turbulence Heating ObserveR (THOR) is the first mission ever flown in space fully dedicated to plasma turbulence. 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 measuring 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.
• Open questions on particle acceleration in strongly magnetized plasmas and how to answer them
  
  • Berthomier Matthieu
  • Fazakerley A.
  , 2016, 18, pp.EPSC2016-16842. Particle acceleration mechanisms in solar system plasmas usually imply the conversion of electromagnetic energy into particle kinetic energy. These processes may take different forms depending on plasma magnetization but in most cases they involve multi-scale phenomena that cannot be described by ideal MHD. Little evidence has been gathered on how particle acceleration works in strongly magnetized plasmas. We will show how Earth's auroral regions provide the unique opportunity to address the open questions on particle acceleration in low beta plasmas. Single point observations in the auroral regions have suggested that acceleration by Alfvén waves would be responsible for filamentary acceleration along magnetic field lines. In the auroral regions, this mechanism would be associated with the generation of the sub-km scale auroral arcs. However single spacecraft measurements cannot evaluate the energy exchanged over a large volume of space between waves and particles. They cannot assess the efficiency of this mechanism, nor can they tell us where and when it is effective and how it relates to the evolving boundary conditions of the system. Numerical simulations alone cannot fully describe this multi-scale and non-local process in the inhomogeneous auroral plasma. Alternatively, it has been proposed from high-time resolution particle measurements in the auroral regions that localized parallel electric fields would explain the larger scale arcs that can be observed by onboard imagers. Single spacecraft measurements cannot follow the formation and evolution of these transient structures or the complex transport phenomena associated with the strong plasma turbulence that develop along magnetic field lines around these structures. Multi-point CLUSTER observations have shown how these potential acceleration structures were distributed in space and time. However we still miss the dynamic picture of how these structures are created on how they can be maintained in space and time. We will show that the only way to distinguish between these models describing acceleration processes in strongly magnetized plasmas is to combine advanced numerical simulations with high-time resolution in situ measurements, multi-point measurements, and auroral arc imaging. We will describe how the Alfvén mission concept, that will be proposed to the ESA M5 AO, will allow a major breakthrough in our understanding of particle acceleration mechanisms in solar system plasmas.
• Testing THEMIS wave measurements against the cold plasma theory
  
  • Taubenschuss U.
  • Santolik O.
  • Le Contel Olivier
  • Bonnell John
  , 2016, 18, pp.7903. The THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission records a multitude of electromagnetic waves inside Earth's magnetosphere and provides data in the form of high-resolution electric and magnetic waveforms. We use multi-component measurements of whistler mode waves and test them against the theory of wave propagation in a cold plasma. The measured ratio cB/E (c is speed of light in vacuum, B is magnetic wave amplitude, E is electric wave amplitude) is compared to the same quantity calculated from cold plasma theory over linearized Faraday's law. The aim of this study is to get estimates for measurement uncertainties, especially with regard to the electric field and the cold plasma density, as well as evaluating the validity of cold plasma theory inside Earth's radiation belts.
• Topology of kinetic range turbulence in the solar wind: observations and simulations
  
  • Kiyani K. H.
  • Chapman S. C.
  • Meyrand Romain
  • Sahraoui Fouad
  • Hadid Lina
  • Osman Kareem
  , 2016, 18, pp.EPSC2016-18250. There is now considerable evidence that below ion gyroscales there is a kinetic range of turbulence that shows non-trivial scaling both in the power spectral density and in the higher order moments of fluctuations. We present an investigation of magnetic field fluctuations in sub-ion scale plasma turbulence in the solar wind, using high-cadence measurements from the STAFF search coil instrument on Cluster. We will compare observational results with sub-ion scale fluid model simulations such as Electron MHD and Electron Reduced MHD to shed light on the type of topological coherent structures that we might expect to see on these scales. Our results suggest current sheet domination at the MHD scales transitioning to filament domination at the sub-ion scales, which we attribute to the force-free structures (Beltrami fields) forming from the dominant Hall physics. Comparison of magnetic compressibility ratios (magnetic field component polarization ratios) seen as a function of plasma beta, with those exhibited by the different linear plasma modes from solutions of the linearised compressible Hall-MHD model. These suggest that the fluctuations seen in our observations share polarizations akin to highly oblique (near perpendicular) Alfven and Kinetic Alfven wave modes.