J’ai le plaisir de vous inviter à ma soutenance de thèse, prévue le lundi 1er décembre à 14:00 dans la salle du château sur le site de l’Observatoire à Meudon, et intitulée “Solar wind radial evolution and dynamics of the heliospheric current sheet : Observations during a Parker Solar Probe – Solar Orbiter radial alignment and hybrid simulations of magnetic reconnection”. Il y aura également une retransmission en direct sur Youtube.
Directeurs de thèse: Olga Alexandrova et Pascal Démoulin (LIRA – Observatoire de Paris)
Membres du Jury:
- Fabrice Mottez (LUX – Observatoire de Paris) – Président
- Marco Velli (Department of Earth, Planetary and Space Sciences – UCLA) – Rapporteur
- Benoit Lavraud (Laboratoire d’Astrophysique de Bordeaux – Université de Bordeaux, CNRS) – Rapporteur
- Miho Janvier (European Space Research and Technology Centre – ESA) – Examinatrice
- Lorenzo Matteini (The Blackett Laboratory, Department of Physics – Imperial College London) – Examinateur
- Petr Hellinger (Institute of Atmospheric Physics – Czech Academy of Sciences) – Invité
- Alexandra Alexandrova – Invitée
Abstract:
In the solar wind, large-scale magnetic sectors of opposite polarities are separated by a thin layer called the heliospheric current sheet (HCS). The HCS, as many other current sheets, can undergo magnetic reconnection, changing the magnetic topoloy and abruptly converting magnetic energy towards heating, particle acceleration, and outflowing jets. Thus, the magnetic reconnection process strongly affects the dynamic and structuration of the HCS.
The goal of this thesis is the investigation of the solar wind radial evolution, as well as magnetic reconnection dynamics in the context of a weakly collisional plasma.
The original approach is to study the same plasma passing through two radially aligned spacecraft, here Parker Solar Probe (PSP) and Solar Orbiter (SolO). We developed a ballistic propagation method, allowing the identification of the same density structure of ~10^6 km radial extension passing through both spacecraft. This density structure was remarkably conserved during its ∼5.75 days of propagation between PSP at 0.1 au and SolO at 0.9 au. It is closely associated with the HCS, where reconnection jets are also detected at both radial distances.
The second main focus of this thesis is the study of a reconnecting current sheet using hybrid-PIC numerical simulations with novel Möbius periodic boundary conditions. Global dynamics of the tearing instability, where several reconnection events are triggered along the same current sheet, and its transition from the linear to non-linear stage are described. During the non-linear stage, a strong proton parallel temperature anisotropy, with T_parallel > T_perp, triggers the kinetic proton firehose instability. The energy conversion rates evolution in the whole domain and around different regions of the unstable current sheet is quantified. In particular, these reveal that more heating is happening within the magnetic islands rather than around the X-points as is usually thought.
(Transmis par Étienne Berriot)