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Accueil > Séminaires & Soutenances > Séminaires, soutenances de thèses et HDR précédents > 2019 > Soutenances de thèses et HDR 2019 > Le lundi 30 septembre 2019 à 10h
Le lundi 30 septembre 2019 à 10h
Soutenance de thèse de Giulia Cozzani
Lieu : Amphi Sauvy, Ecole polytechnique
Title : Microphysics of magnetic reconnection in near-Earth space : spacecraft observations and numerical simulations
Abstract :
Magnetic reconnection is a space and laboratory plasmas. Reconnection takes place in thin current sheets leading to the reconfiguration of magnetic field topology and to conversion of magnetic energy into acceleration and heating of particles. Today reconnection is recognized to play a key role in the Earth-solar environment, from the so- lar corona to the solar wind, to magnetosheath, at the Earth’s magnetopause, and in the magnetotail. Reconnection is initiated in the Electron Diffusion Region (EDR), where electrons decouple from the magnetic field and are en- ergized by electric fields. Despite the very significant advances that have been made in the understanding of the magnetic reconnection process by means of in-situ measurements (notably provided by the Cluster mission) and by nu- merical simulations, the small electron scale physics of the dissipation region remains basically unsolved. It is only the last years, with the launch of the Magnetospheric MultiScale mission (MMS) together with the recent impressive increasing of computational capabilities of supercomputers, that the dynamics of the Electron Diffusion Region has started to be enlightened. One of the key, yet still open questions, is whether the EDR has a preferred homogeneous or inhomogeneous structure at electron scales and below. The purpose of this The- sis is to advance in the understanding of the structure of the Electron Diffusion Region using two different approaches, notably MMS spacecraft observations and kinetic full Vlasov simulations. The first part presents MMS observations of an EDR encounter at the subsolar magnetopause when the four MMS probes were located at the smallest interspacecraft separation of ∼ 6 km, which is com- parable to a few electron inertial length (de ∼ 2 km). We find that the EDR is rather inhomogeneous and that the pattern of the energy conversion is patchy, showing that the structure of the EDR at the magnetopause can be much more complex than it has been found in other MMS events and than it is usually depicted by kinetic PIC simulations. Our MMS data analysis has pointed out the need of simulations with better spatial resolution and low noise on the elec- tron scales, in particular on the electric field, in order to better understand the kinetic physics at play. Following this motivation, the second part of the Thesis aims at studying the EDR by using a novel fully-kinetic Eulerian Vlasov- Darwin model which we have implemented in the numerical ViDA code. The ViDA code is specifically designed to improve our understanding of the kinetic dynamics of collisionless plasmas at electron scales by giving access to the fine phase space details of the electron distribution function. A first part is devoted to the testing of the code by performing 2D symmetric magnetic reconnection simulations. Then, low-noise simulation data have been used to investigate the contribution of the different terms in the Ohm’s law in the EDR, focusing on the contribution of the electron inertia term which is responsible for the decoupling of the electron dynamics from the magnetic field.
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