Electron acceleration by an Alfvénic pulse propagating in an auroral plasma cavity

International audienceWith the help of a 2.5-D particle-in-cell simulation code, we investigate the physics of the acceleration of auroral electrons, through the interaction of an isolated Alfvén wave packet with a plasma density cavity. The cavity is edged by density gradients perpendicular to the magnetic field. We show that a single passing of an isolated wave packet over a (infinite) cavity creates an electron beam. It triggers local current and beam-plasma instabilities and small-scale coherent electric structures. The energy flux of downgoing electrons is significantly increased, whereas upgoing electrons are also accelerated, even if no beam is formed. Accelerated electrons remain after the passage of the Alfvénic pulse, allowing the observation of energetic particles without any significant electromagnetic perturbation. The dependence of this process on the electron to ion mass ratio is consistent with its control by inertial effects

Spacecraft potential effects on electron moments derived from a perfect plasma detector

International audienceA complete computation of the effect of the spacecraft potential on electron moments is presented. We adopt the perfect detector concept to estimate how measured density, velocity and temperature are affected by the constraints imposed by the detector, such as the finite lower energy cutoff and the spacecraft potential. We investigate the role of the potential in different plasma regimes usually crossed by satellites. It appears that the solar wind is the region where the moments are most compromised, as the particle temperature is low. To a lesser extent the moments calculated in the magnetosheath may also deviate from the real moments, displaying up to 40% overestimation for the density under typical detector operation. The analysis allows us to identify a range of spacecraft potential values which minimizes the variation in the estimation; it is found that it corresponds to the common value adopted by potential controlling experiments

Secondary electron emission causing potential barriers around negatively charged spacecraft

Low-energy secondary electrons have been observed to be reflected back to the spacecraft during eclipse conditions. It has been argued that the presence of negative potential barriers can be caused by the secondary electron emission space charge and may play a role in the spacecraft charging process. The barriers turn back the lowenergy spacecraft-emitted electrons and prevent the low-energy ambient electrons from reaching the detector. Two numerical methods previously presented by Whipple and by Parrot et al. in the literature have been used to study the effect of secondary electrons on potential barriers negatively charged spacecrafts. The former method provides an upper bound for the potential barriers when the sheath is large compared to spacecraft dimension. The latter one provides in principle the exact sheath profile subject to accurate integration of the density distribution over the energy. The application of the methods to data provided by the ATS6 and Freja spacecraft suggests that the high level negative charging is not due to barriers induced by secondary electron emission space charge

Validating coronal magnetic field reconstruction methods using solar wind simulations and synthetic imagery

We present an ongoing effort within the ESA Modeling and Data Analysis Working Group (MADAWG) to determine automatically the magnetic connectivity between the solar surface and any point in interplanetary space. The goal is to produce predictions of the paths and propagation delays of plasma and energetic particle propagation. This is a key point for the data exploitation of the Solar Orbiter and Solar Probe Plus missions, and for establishing connections between remote and in-situ data. The background coronal magnetic field is currently determined via existing surface magnetograms and PFSS extrapolations, but the interface is ready to include different combinations of coronal field reconstruction methods (NLFFF, Solar Models), wind models (WSA, MULTI-VP), heliospheric models (Parker spiral, ENLIL, EUHFORIA). Some model realisations are also based on advanced magnetograms based on data assimilation techniques (ADAPT) and the HELCATS catalogue of simulations. The results from the different models will be combined in order to better assess the modelling uncertainties. The wind models provide synthetic white-light and EUV images which are compared to coronographic imagery, and the heliospheric models provide estimations of synthetic in-situ data wich are compared to spacecraft data. A part of this is work (wind modelling) is supported by the FP7 project #606692 (HELCATS)

The Comet Interceptor Mission

Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA’s F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum ΔV capability of 600 ms−1. Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes – B1, provided by the Japanese space agency, JAXA, and B2 – that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission’s science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule

Comment on 'A class of exact two-dimensional kinetic current sheet equilibria' by Peter H. Yoon and Anthony T. Y. Lui

The analytical derivation of equilibrium solutions described in Yoon and Lui (2005) is discussed

Étude des phénomènes d'accélération de particules dans les régions aurorales des magnétosphères

Particle acceleration is a topic of general astrophysical interest which can be studied in a convenient natural laboratory: the Earth's magnetosphere and, more widely, the magnetospheres of magnetised planets. In particular, numerous physical processes occur in the auroral regions, for example the northern and southern auroras, which are very spectacular but rather imperfectly understood phenomena. Numerous satellite measurements have shown the existence of highly energetic populations of particles flowing towards the Earth, and this implies the existence of a strong electric potential difference which most of the proposed models fail to predict. Generally speaking, an understanding of this acceleration is necessary to explain the dynamic coupling between the magnetosphere, where the energy is released during substorms, and ionosphere where the energy is dissipated. The study of the dissipation, which takes place on short spatial and temporal scales, constitutes the main theme of the present work. We consider as electromagnetic perturbation an Alfvén wave propagating along the geomagnetic field lines. Its interaction, in the auroral zones, with the highly inhomogeneous structures known as plasma cavities, leads to the formation of parallel electric fields able to accelerate particles, as well as to a significant energy transfer from the waves to the electrons. Finally, this study enables us to suggest a new scenario for the formation of auroral arcs. This work was conducted both analytically and numerically using a "particles in cell" code.L'accélération de particules est une thématique d'astrophysique générale qui peut être étudiée dans un laboratoire naturel : les régions aurorales de la Terre, et plus globalement, celles des planètes magnétisées. Ces régions sont en effet le siège de nombreux processus qui donnent, entre autres, naissance aux aurores boréales et australes, phénomènes spectaculaires mais dont de nombreux aspects restent incompris. En particulier, de multiples mesures de satellites ont montré l'existence de populations de particules énergétiques précipitant vers la Terre, nécessitant le maintien d'une différence de potentiel électrique élevée que la plupart des modèles proposés sont incapables de reproduire. D'une manière générale, comprendre l'accélération, c'est comprendre une partie du couplage dynamique entre la magnétosphère, où l'énergie est libérée lors des sous-orages, et l'ionosphère où l'énergie est dissipée. L'étude de cette dissipation, qui opère sur de courtes échelles spatiales et temporelles, constitue le thème principal de ce travail. Dans ce but, nous considérons une perturbation électromagnétique, sous forme d'onde d'Alfvén, se propageant le long des lignes de champ magnétique. Son interaction, en région aurorale, avec les cavités de plasma, structures fortement inhomogènes, conduit à l'apparition de champs électriques parallèles susceptibles d'accélérer les particules, ainsi qu'à un transfert d'énergie significatif des ondes vers les électrons. Finalement, cette étude permet de dégager un nouveau scénario de formation des arcs auroraux. Ce travail a été mené de façon analytique avant d'être traité numériquement grâce à un code particulaire

Mirror and Firehose Instabilities in the Heliosheath

International audienceWe investigate the nature of the heliosheath plasma behind the termination shock across which jump relations in anisotropic MHD are formulated. Along side analytical results for downstream parameters in the strictly parallel and perpendicular cases we numerically solve the Rankine-Hugoniot relations for arbitrary shock angle and strength. We then focus on two temperature anisotropy driven instabilities which have attracted attention in many other astrophysical situations, namely the mirror and firehose instabilities. It is revealed that the firehose instability is mainly controlled by the shock strength with little influence of the shock angle contrary to the mirror instability for which both parameters intervene. We confirm results showing that the heliosheath plasma observed by Voyager 1 immediately behind the termination shock is mirror unstable. Similar conditions are probable in the heliosheath recently encountered by Voyager 2. Finally, by comparison with studies in the Earth's magnetosheath context, we formulate predictions on the shapes of mirror-associated magnetic fluctuations in the heliosheath. Both hole and peak magnetic structures were indeed observed by Voyager 1 and these shapes correspond to different stages of the mirror instability

Analytical solutions for anisotropic MHD shocks

International audienceA new method to analytically solve the anisotropic MHD system of equations describing shock transitions is presented. As this system is known to be under-determined (there is more unknown parameters than available equations) free parameters must be chosen. From observational contraints it appears that the magnetic amplitude jump is a good candidate as it is generally available more frequently and more precisely than other jump variables. With this approach we obtain an explicit expression for the density compression ratio for arbitrary upstream parameters and shock geometry. Downstream anisotropy and pressure are also calculated. The results are tested against an other approach and compared with observations from the Earth's bow shock and the solar wind termination shock