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From Vlasov to STEREO: nonlinear coupling in the solar wind

Abstract : Many astrophysical plasmas, among them the solar wind, are collisionless. The typical frequencies of the system dynamics are indeed much higher than the collision frequency, so that the solar wind cannot be considered at local thermodynamic equilibrium. In this context, kinetic and/or nonlinear processes (wave-particle and wave-wave interactions) are of primary importance for the redistribution of the energy in the plasma. Kinetic processes and nonlinear plasma theory have been extensively studied since the 1960s, but observational evidence for such processes is still lacking. In the particular case of electron and ion electrostatic coupling and in the context of space plasma, the main goals of this thesis are (i) to understand the physics of kinetic and/or nonlinear processes and (ii) to provide observational evidence of such processes. The study is based on electric field observations and kinetic plasma simulations. Insitu waveform measurements are provided by the TDS observing mode of the WAVES instrument onboard the STEREO mission. The kinetic simulations were performed using a 1D-1V Vlasov-Poisson code in the electrostatic approximation. The results of this thesis are the following. (1) A method has been developed to measure and to calibrate in-situ small scale density fluctuations, through the quasistatic fluctuations of the spacecraft floating potential. (2) The first observational evidence for ponderomotive effects (which nonlinearly couples solar wind ion dynamics to high-amplitude Langmuir waves) is obtained by analyzing a data set of density and electric field measurements over more than three years. (3) This work also provides the first direct, complete set of evidence for Langmuir electrostatic decay (an archetype of wave-wave interaction) associated to a Type III burst. The resonant character of the interaction is shown in the observations, by verifying the conservation of momentum and energy during the interaction (relation of resonance), as well as the phase coherence through a bicoherence analysis. A new expression for the Langmuir decay threshold is computed from the simulations, taking into account the dynamics of the process, and is shown to be in agreement with the observations. (4) Finally, the long time evolution of weak Langmuir turbulence (in the Langmuir electrostatic decay regime) is shown to asymptotically end up with strong turbulence and the formation of Langmuir cavitons, through the Vlasov-Poisson system. This thesis shows the importance of a complementary approach between observations and simulations.
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Submitted on : Thursday, November 4, 2010 - 11:38:14 AM
Last modification on : Monday, December 14, 2020 - 9:48:52 AM
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  • HAL Id : tel-00532627, version 1

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Pierre Henri. From Vlasov to STEREO: nonlinear coupling in the solar wind. Astrophysics [astro-ph]. Université Pierre et Marie Curie - Paris VI, 2010. English. ⟨tel-00532627⟩

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