Etude des couches frontières dans les plasmas : Structure et stabilité de la magnétopause terrestre

Abstract : The terrestrial magnetopause is the boundary between the solar wind (compressed by a shock) and the terrestrial magnetosphere. This kind of thin and nearly impenetrable boundary naturally forms each time two magnetized plasmas are pushed one toward another. It happens here, like for several astrophysical situations, in a collisionless medium. For these reasons, the terrestrial magnetopause, accessible experimentally with a lot of satellite missions, is representative of a very general type of interfaces. Key phenomena like plasma transport across the boundary, heating and acceleration of charged particles or magnetic reconnection, take place at these interfaces. Therefore, studying and deeply understanding such kind of boundary is critical to understand the fundamental plasma physics. The terrestrial magnetopause is the boundary between two plasmas of different densities and temperatures. The magnetic fields of the magnetosphere and the solar wind have also different directions and intensities. The transition observed at the magnetopause therefore concerns matter, with two interpenetrating plasmas, and fields. How do these different kinds of variations combine and what structure does it give to the boundary? These are the questions we study in this work. The simplest case, when the boundary can be considered locally as a plane and is stationary, will be the basis of the study, but we will also show how a boundary shaken by instabilities and magnetic reconnection can deviate from these simple models. In the first part of the thesis, we show an experimental study of the magnetopause using the data from the European Cluster mission. We show how to combine magnetic and ion data to obtain a characterization of the normal direction to the boundary and a coordinate along this normal, and validate this new tool. Then, we show that when the normal magnetic field is nonzero, the boundary can be a succession of small layers bearing separately the rotational and compressional variations. We give clues on the good way to study these in detail. In the second part of the thesis, we develop a theoretical model of the structure as a 1D-stationay equilibrium of a current layer like the magnetopause. This equilibrium is a kinetical one, that means it is valid for the distribution function, and not only its first moments like density, fluid velocity, and pressure. This is necessary in a collisionless medium as soon as the characteristic scale of the particle motion, particularly the Larmor radius, is not negligible with respect to the thickness of the layer. Such kinds of equilibria are necessary to initialize the numerical simulations that are used to study the magnetopause and the instabilities that can happen at the boundary like the tearing instability (which implies reconnection). Finally, we present a new tool for building Fourier spectra and phases for space plasmas turbulence studies.
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Nicolas Dorville. Etude des couches frontières dans les plasmas : Structure et stabilité de la magnétopause terrestre. Physique [physics]. Ecole Polytechnique, 2015. Français. ⟨tel-01181616⟩

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