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Accueil > A propos du LPP > Communication > Actualités archivées > 2016 > Ecole Polytechnique Thesis Award for Nicolas Dorville, a former LPP PhD student

Ecole Polytechnique Thesis Award for Nicolas Dorville, a former LPP PhD student

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 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 that have been studied in this work. The simplest case, when the boundary can be considered locally as a plane and stationary, will be the basis of the study, but the thesis also shows how a boundary shaken by instabilities and magnetic reconnection can deviate from these simple models.

The first part of Nicolas Dorville’s thesis consists in an experimental study of the magnetopause using data from the European Cluster mission. It shows how to combine magnetic and ion data to obtain a characterization of the normal direction to the boundary and a coordinate along this normal. Then, he shows that, when the normal magnetic field is nonzero, the boundary can be a succession of small layers bearing separately the rotational and compressional variations.

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Quand on trace le vecteur champ magnétique dans le plan tangentiel à la magnétopause, l’extrémité de ce vecteur décrit un "hodogramme" qui peut être de forme complexe. Cela indique que le satellite traverse une frontière constituée successivement des couches rotationnelles (arcs de cercle) et compressionnelles (variations radiales).

In the second part of the thesis, a theoretical model of the structure is developed, as a 1D-stationay equilibrium of a current layer, like the magnetopause. This equilibrium is a kinetic one, which means that 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 particularly necessary for initializing 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).

References :

- Thèse : http://www.theses.fr/2015EPXX0015

- Dorville N., Belmont G., Rezeau L., Grappin R., Retinò A., Rotational/compressional nature of the magnetopause : Application of the BV technique on a magnetopause case study, Journal of Geophysical Research Space Physics 119 1898-1908 (2014)

- Dorville, N., G. Belmont, L. Rezeau, N. Aunai, and A. Retinò (2014), BV technique for investigating 1-D interfaces, J. Geophys. Res. Space Physics, 119, 1709–1720, doi:10.1002/2013JA018926.

- Dorville, N., Belmont, G., Aunai, N., Dargent, J., Rezeau, L., Asymmetric kinetic equilibria : Generalization of the BAS model for rotating magnetic profile and non-zero electric field, Physics of Plasmas, 22, 092904 (2015) ; doi : 10.1063/1.4930210

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Tutelles : CNRS Ecole Polytechnique Sorbonne Université Université Paris Sud Observatoire de Paris Convention : CEA
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Directeur de la publication : Pascal Chabert (Directeur)