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Etude de la dynamique des electrons en presence de fortes densites de courant

Abstract : The study of the dynamics of collisionnal plasma under the influence of a parallel electric field in the edges of auroral arcs is a young and interesting science. The existence of large field-aligned current densities has been inferred over the last years by using satellites and numerical models. Different authors and kinds of studies (experimental and modeling) agree that current densities can reach up to hundreds of μA.m−2 in the edges of auroral arcs. These large current densities can be the cause of many phenomena such as tall red rays or triggering unstable ion acoustic waves. These current densities imply the presence of a parallel electric field which can yield kinetic effects such as the creation of runaway electrons. The runaway effect is not a new topic and is considered in different fields : nuclear fusion, heating of the solar corona or transient luminous events (Sprites). In this thesis, we are interested in runaway electrons in the ionosphere which is an original issue.
Thus, we decide to study the dynamics of the electrons which carry these large current densities. We consider the issue of electrons moving through an ionospheric gas of positive ions and neutrals under the influence of a parallel electric field. We develop a kinetic model of collisions including electrons/electrons, electrons/ions and electrons/neutrals collisions. We use a Fokker-Planck approach to describe binary collisions between charged particles with a long-range interaction. We present the essential elements of this collision operator: the Langevin equation for electrons/ions and electrons/electrons collisions and the Monte-Carlo and null collision methods for electrons/neutrals collisions. A computational example is given illustrating the approach to equilibrium and the impact of the different terms (electrons/electrons and electrons/ions collisions on the one hand and electrons/neutrals collisions on the other hand).
Then, a static electric field is applied in a new sample run. In this run, the electrons move in the z direction, parallel to the electric field. The first results show that all the electron distribution functions are non-Maxwellian. Furthermore, runaway electrons can carry a significant part of the total current density up to 20% of the total current density. Nevertheless, we note that the divergence free of the current density is not conserved.
We introduce major changes such as a feedback on the electric field or the resolve of the fluid equations in order to take into account the variation of the different moments of the ion distribution functions. We observe that the electron distribution functions are still non-Maxwellian. Runaway electrons are created and carry the current density. The core distribution stay at rest. As these electrons undergo less collisions,they increase the plasma conductivity.
Last but not least, we realize a parametric study in order to underline the influence of various parameters such as current density, electron density or time to reach the maximal current density, on the electron distribution functions. We fit the electron distribution function by two Maxwellians : one corresponds to the core distribution and the other one to the suprathermal distribution. We show that the time to reach the maximal current density is a key point. Thus, when we increase this time, we modify the temperatures : not only the mean electron temperature but also the electron temperature of the core and the suprathermal distribution. The current density plays a primary role. When the current density increases, all the moments of the distributions increase : electron density and mean velocity of the suprathermal distribution and the electron temperature of the core and suprathermal distributions. We also point out that the increase of the total electron density decreases the temperature and the mean velocity of the suprathermal distribution.
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Contributor : Geraldine Garcia <>
Submitted on : Sunday, February 10, 2008 - 2:38:57 PM
Last modification on : Wednesday, December 9, 2020 - 3:05:58 PM
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  • HAL Id : tel-00250116, version 1

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Geraldine Garcia. Etude de la dynamique des electrons en presence de fortes densites de courant. Astrophysique [astro-ph]. Université Pierre et Marie Curie - Paris VI, 2007. Français. ⟨tel-00250116⟩

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