Soutenance de thèse Giulio Ballerini le 13 septembre 2024 à 15:00

Annonce transmise par Laurence Rezeau (LPP)

Bonjour à toutes et tous,

J'ai le plaisir de vous annoncer que ma soutenance de thèse aura lieu vendredi le 13 septembre à 15:00 dans la salle RC27 du Bâtiment Atrium à Jussieu (la salle se trouve au rez-de-chaussée de l'Atrium). Le sujet ainsi qu'un résumé sont fournis ci-après. N'hésitez pas à y assister, vous êtes tous les bienvenus! N'hésitez pas non plus à venir au pot prévu après la délibération du jury qui sera dans la salle de réunion du LPP à Jussieu au 5eme étage!

Pour ceux qui souhaitent assister à distance, vous trouverez le lien Zoom en bas de l'email.

Au plaisir de vous y retrouver,

Giulio

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https://zoom.us/j/98131124212?pwd=SC9oaTR0ekV3YlJQZ252SGtsek1rZz09
ID de réunion: 981 3112 4212
Code secret: 654321
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Titre

The Magnetopause: an almost tangential interface between the magnetosphere and the magnetosheath

Resumé

This thesis aims to study the Earth's magnetopause, defined as the boundary between the Earth's magnetosphere and the solar wind. Although considered in first approximation as a clear barrier between the two plasmas, the reality is more complex, as the solar wind plasma and the magnetosphere plasma mix with each other in the magnetopause in ways not yet fully understood. One example of this interaction is magnetic reconnection, which creates a flow of mass and magnetic field between the two media.
In this thesis, we focus on regions of the magnetopause away from areas of magnetic reconnection. In these regions, which make up most of the structure, the magnetopause often takes on a one-dimensional, stationary structure and is generally modeled as a discontinuity through the Classic Theory of Discontinuities (CTD). However, in situ data from recent space missions show how this theory does not adequately describe the magnetopause. In fact, at the magnetopause, both a rotation of the magnetic field in the plane tangent to the structure and compressive characteristics are observed. In order to describe these properties simultaneously, the magnetopause is described in CTD as a tangential discontinuity. However, this classification is a singularity in the theory that requires the normal component of the magnetic field to the structure to be zero. Instead, we observe from the data that this component is small but not zero, emphasizing the need to introduce a "quasi-tangential" description in order to describe the magnetopause.
In this thesis, therefore, the CTD is used as a starting point, exploiting its limitations in describing the magnetopause, in order to determine which terms are relevant in its equilibrium. To this end, we use in situ measurements from the Magnetospheric Multiscale Mission (MMS, NASA). The first part of the work aimed to develop an instrument, called GF2, that estimates the direction of the normal to the magnetopause more accurately than current instruments. Indeed, accurate estimation of the normal is of fundamental importance in order to determine which experimentally relevant terms are not included in the classical theory. This instrument was tested both on the MMS mission data, analyzing in detail a December 28, 2015 magnetopause crossing, and through a numerical simulation obtained through the hybrid-PIC code Menura, demonstrating good skill in determining the normal.
The same magnetopause crossing of MMS was also used to study the magnetopause equilibrium in detail. In particular, taking advantage of the normal obtained through the previously developed instrument, we show that the divergence of the pressure tensor plays a key role in this equilibrium, unlike the assumption in CTD. Specifically, we show that the effects of finite Larmor radius (FLR) play an important role in the quasi-tangential discontinuity when the Larmor radius of the ions is not completely negligible with respect to the thickness of the magnetopause. To generalize the result, a similar statistical study was also conducted on a database of MMS magnetopause crossings, which confirmed that these results are common in the magnetopause.
Finally, one part of the project focused on Mercury's magnetosphere, deviating slightly from the main objective of this thesis. In this analysis, full-kinetic simulations were used in order to analyze the generation of whistler waves in the reconnection region in the magnetotail. In this study, the small size of Mercury's magnetosphere compared with that of Earth is exploited in order to learn new insights about Earth's magnetosphere.