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Accueil > A propos du LPP > Communication > Actualités archivées > 2023 > Gaetan Gauthier defended his PhD "Study of non-linear electronic structures in the magnetosphere and so- lar wind : theory and simulations"
Gaetan Gauthier defended his PhD "Study of non-linear electronic structures in the magnetosphere and so- lar wind : theory and simulations"
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On December, 21, 2023, Gaëtan Gauthier defended his PhD "Study of non-linear electronic structures in the magnetosphere and so- lar wind : theory and simulations".
Summary :
In the course of this thesis, we carried out a study in two distinct parts, with one thing in common : beam-plasma (bump-on-tail) instability. Firstly, we studied electromagnetic wave emissions at the plasma frequency and its first harmonic in the heliospheric context. Our study was essentially numerical, based on massively-parallel 2D3V Particle-In-Cell (PIC) simulations (Smilei) generating electrostatic and then electromagnetic waves by re- laxation of an electron beam at the origin of type III radio bursts propagating in the solar wind plasma. By generalizing previous studies, the physical and numerical characteristics of our simulations have enabled us to study the principal modes of the waves associated with these emissions generated by non-linear coupling. Through a choice of parameters, we showed that the numerical noise (inherent in PIC codes) could be reduced sufficiently to allow us to model the density fluctuations observed in the solar wind. This is a prerequisite for showing that these fluctuations, although very small, can modify the emission characteristics.
We then turned our attention to the non-linear kinetic structures known as electron holes in phase space (or EH for short) observed in many regions of the magnetosphere. Our study has been carried out using two approaches : (i) A theoretical study based on the BGK (Bernstein-Greene-Kruskal) integral method to determine the distributions of particles (electrons and ions) associated with these EHs, as well as their conditions of existence. We have thus developed a 3D model of revolution symmetry around the ambient magnetic field, which takes into account both electron polarization drift and a more realistic description of plasma boundary conditions with the introduction of the EH’s velocity relative to the ambient plasma. This model has enabled us to characterize the relationship between parallel and perpendicular scales of EHs in different regions of the magnetosphere, as well as some restrictions on their conditions of existence. (ii) The second approach is a numerical PIC study, which allows us to generate these
EHs with realistic initial conditions and to compare them with in situ spatial observations. Thanks to a parametric study, we have shown that environmental conditions (ambient magnetic field, beam density) have an impact on their generation and nature (quasi-electrostatic or with an internal magnetic field component). This qualitative and quantitative numerical study has made it possible to specify certain parameters, such as beam density, which is still not easily accessible to space mission measurements, as well as other fundamental characteristics of EHs, such as their propagation speed or even the conservation and conversion of energy within them.
Jury :
• Olga Alexandrova (Lesia), examinatrice ;
• Andréa Ciardi (Lerma), examinateur ;
• Vincent Génot (Irap), rapporteur ;
• Pierre Henri (Lagrange), rapporteur ;
• Chadi Salem (Ssl Berkeley), examinateur ;
• Philippe Savoini (Lpp), directeur de thèse ;
• Olivier Le Contel (Lpp), invité (co-directeur) ;
• Thomas Chust (Lpp), invité (co-directeur)
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