Soutenance de thèse Souhail Dahani - 02 octobre 2024 à 09h30 à l'IRAP

Dear everyone,

I am pleased to invite you to attend my PhD defense, which will take place on Wednesday, October 2nd at 09h30 in the conference room at IRAP-Roche (9 avenue du Colonel Roche, 31400 Toulouse). The defense will be in English.

Title: Magnetic Reconnection in Near-Earth Space: Energy Conversion Mechanisms and the Formation of Flux Transfer Events

Abstract:

The solar wind interaction with the Earth is governed by plasma physics. The solar wind is continuously expelled from the Sun, carrying with it the Interplanetary Magnetic Field (IMF) to Earth. This interaction shapes the near-Earth plasma environment, comprising regions like the bow shock, magnetosheath, and magnetosphere. Magnetic reconnection at the magnetopause is a key consequence of this interaction, converting electromagnetic energy into kinetic and thermal energies. Flux Transfer Events (FTEs), transient phenomena at the dayside magnetopause, are one result of reconnection. This work is divided into two parts. The first one focuses on FTE formation and orientation, while the second focuses on energy conversion in magnetic reconnection (association with Electron Diffusion Regions (EDRs) and FTEs), with a particular focus on kinetic energy and force balance.

Concerning FTEs, while it is known that they are generated by magnetic reconnection, it remains unclear how the details of magnetic reconnection control their properties. A recent study showed that the helicity sign of FTEs positively correlates with the east-west (By) component of the IMF. With data from Cluster and MMS, I performed a statistical study of FTEs. I focused on their helicity sign and possible association with upstream solar wind conditions and local magnetic reconnection properties. Using both in situ data and magnetic shear modeling, it was found that FTEs whose helicity sign corresponds to the IMF By are associated with moderate magnetic shears, while those that do not correspond to the IMF By are associated with higher magnetic shears. While uncertainty in IMF propagation to the magnetopause may lead to randomness in the determination of the flux rope core field and helicity, this work rather proposes that for small IMF By, which corresponds to high shear and low guide field, the Hall pattern of magnetic reconnection determines the FTE core field and helicity sign. In that context, I explain how the temporal sequence of multiple X- line formation and the reconnection rate are important in determining the flux rope helicity sign.

In the second part of this thesis, I focus on the main physical terms affecting the changes in kinetic energy. These are pressure-gradient-related terms and electromagnetic terms. The former account for plasma acceleration/deceleration from a pressure gradient, and the latter from an electric field. Although limited spatial and temporal deviations are expected, a statistical balance between these terms is fundamental to ensure the overall conservation of energy and momentum. I use in-situ observations from the MMS mission to study the relationship between these terms. I perform a statistical analysis of those parameters in the context of magnetic reconnection by focusing on small-scale EDRs and large-scale FTEs. The analysis reveals a correlation between the two terms in the monofluid force balance, and in the ion force and energy balance. However, the expected relationship cannot be verified from electron measurements. Generally, the pressure-gradient-related terms are smaller than their electromagnetic counterparts. I perform an error analysis to quantify the expected underestimation of gradient values as a function of the spacecraft separation compared to the gradient scale. These findings highlight that MMS is capable of capturing energy and force balance for the ion fluid, but that care should be taken for energy conversion terms based on electron pressure gradients, owing to measurement limitations.

Jury members:

Matteo Faganello, Reviewer, Aix-Marseille Université

Olivier Le Contel, Reviewer, LPP-CNRS-Sorbonne Université

Sergio Toledo-Redondo, Examiner, University of Murcia

Aurélie Marchaudon, Examiner, IRAP-CNRS-Université Toulouse III Paul Sabatier

Vincent Génot, Supervisor, IRAP-CNRS-Université Toulouse III Paul Sabatier

Benoît Lavraud, Supervisor, LAB-CNRS-Univ. Bordeaux

Zoom link:

https://univ-tlse3-fr.zoom.us/j/97159835034