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Home > About us > Media > Archived news > 2022 > Soboh Alqeeq defended his PhD "Energy conversion processes related to dipolarization fronts in the Earth magnetotail"
Soboh Alqeeq defended his PhD "Energy conversion processes related to dipolarization fronts in the Earth magnetotail"
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On December 21, 2022, Soboh Alqeeq defended his PhD "Energy conversion processes related to dipolarization fronts in the Earth magnetotail".
Abstract:
Fast plasma flows have been detected in the nightside of the Earth’s magnetosphere, the magnetotail, for a long time. Different processes such as magnetic reconnection or kinetic ballooning-interchange instability are still investigated. While propagating Earthward, these flows generate a sharp increase of the northward component of the magnetic field named dipolarisation front (DF). These kinetic scale structures contribute significantly to the energy dissipation in the magnetosphere. This study aims at better understanding the energy conversion processes described by J.E (J being the current density and E the electric field) which occur in the vicinity of these fronts and at determining their role in the energy global cycle of the magnetosphere. Using in situ measurements from the Magnetospheric Multiscale mission, which consists of four identical satellites evolving in a tetrahedral configuration and separated at electron scales, I have first investigated the current density structures, the different terms of the Ohm’s law, and the energy conversion processes for 6 DFs. I found that for all DFs, ions are mainly decoupled from the magnetic field by the Hall electric field but the electron pressure gradient also contributes. It implies that electrons are decoupled by their own pressure gradient at DF. Regarding the energy conversion processes in the spacecraft frame, the energy is transferred from the electromagnetic fields to the plasma ahead of the DF (dissipation or loading region) whereas it is transferred from the plasma to the fields (dynamo or generator region) behind the front. This energy conversion reversal is caused by the reversal of the ion diamagnetic current at DF. In the fluid frame, the energy is always transferred to the fields, due to the electric field generated by the electron pressure gradient, which could contribute to the slowdown of the flow. Furthermore, it is shown that the energy conversion processes are not homogeneous at the electron scale mostly due to the variations of the electric fields. These case study results have been extended thanks to a statistical study carried out over the full 2017 magnetotail season. From this study, it turns out that two DF classes can be distinguished: class I (74.4%) corresponds to the standard DF properties and energy dissipation whereas a new class II (25.6%), which includes the 6 DF previously discussed, corresponds to a bump of the magnetic field associated with a minimum of the ion and electron pressures and a reversal of the energy conversion process. The possible origin of this second class is discussed. For both DF classes, it is shown that the energy conversion process in the spacecraft frame is driven by the diamagnetic current dominated by the ion pressure gradient. In the fluid frame, it is driven by the electron pressure gradient.
Jury:
Karine Issautier, Directrice de recherche, CNRS-Observatoire de Paris Présidente
Claire Foullon, Senior Lecturer, University of Exeter, UK Rapporteure
Thierry Passot, Directeur de recherche, CNRS-Observatoire de la Côte d’Azur Rapporteur
Cecilia Norgren, Researcher, University of Bergen, Norway Examinatrice
Christian Jacquey, Astronome, Observatoire Midi-Pyrénées et Université Toulouse III Examinateur
Suleiman Baraka, Researcher, University of Alberta, Canada Examinateur
Patrick Canu, Directeur de recherche, CNRS-LPP Directeur de thèse
Olivier Le Contel, Chargé de recherche (HDR), CNRS-LPP Co-directeur de thèse
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