Simulations of Magnetic Reconnection - Kinetic Mechanisms Underlying the Fluid Description of Ions

Because of its ability to transfer the energy stored in magnetic field together with the breaking of the flux freezing constraint, magnetic reconnection is considered as one of the most important phenomena in plasma physics. When it happens in a collision less environment such as the terrestrial magnetosphere, it should a priori be modelled with in the framework of kinetic physics. The evidence of kinetic features has incidentally for a long time, been shown by researchers with the help of both numerical simulations and satellite observations. However, most of our understanding of the process comes from the more intuitive fluid interpretation with simple closure hypothesis which do not include kinetic effects. To what extent are these two separate descriptions of the same phenomenon related? What is the role of kinetic effects in the averaged/fluid dynamics of reconnection? This thesis addresses these questions for the proton population in the particular case of anti parallel merging with the help of 2D Hybrid simulations. We show that one can not assume, as is usually done, that the acceleration of the proton flow is only due to the Laplace force. Our results show, for symmetric and asymmetric connection, the importance of the pressure force, opposed to the electric one on the separatrices, in the decoupling region. In the symmetric case, we emphasize the kinetic origin of this force by analyzing the proton distribution functions and explain their structure by studying the underlying particle dynamics. Protons, as individual particles, are shown to bounce in the electric potential well created by the Hall effect. The spatial divergence of this well results in a mixing in phase space responsible for the observed structure of the pressure tensor. A detailed energy budget analysis confirms the role of the pressure force for the acceleration; but, contrary to what is sometimes assumed, it also reveals that the major part of the incoming Poynting flux is transferred to the thermal energy flux rather than to the convective kinetic energy flux, although the latter is generally supposed dominant. In the symmetric case, we propose the pressure tensor to be an additional proxy of the ion decoupling region in satellite data and verify this suggestion by studying a reconnection event encountered by the Cluster spacecrafts. Finally, the last part of this thesis is devoted to the study of the kinetic structure of asymmetric tangential current sheets where connection can develop. This theoretical part consists in finding a steady state solution to the Vlasov-Maxwell system for the protons in such a configuration. We present the theory and its first confrontation to numerical tests

Comparison between hybrid and fully kinetic models of asymmetric magnetic reconnection: coplanar and guide field configurations

Magnetic reconnection occurring in collisionless environments is a multi-scale process involving both ion and electron kinetic processes. Because of their small mass, the electron scales are difficult to resolve in numerical and satellite data, it is therefore critical to know whether the overall evolution of the reconnection process is influenced by the kinetic nature of the electrons, or is unchanged when assuming a simpler, fluid, electron model. This paper investigate this issue in the general context of an asymmetric current sheet, where both the magnetic field amplitude and the density vary through the discontinuity. A comparison is made between fully kinetic and hybrid kinetic simulations of magnetic reconnection in coplanar and guide field systems. The models share the initial condition but differ in their electron modeling. It is found that the overall evolution of the system, including the reconnection rate, is very similar between both models. The best agreement is found in the guide field system, which confines particle better than the coplanar one, where the locality of the moments is violated by the electron bounce motion. It is also shown that, contrary to the common understanding, reconnection is much faster in the guide field system than in the coplanar one. Both models show this tendency, indicating that the phenomenon is driven by ion kinetic effects and not electron ones.Comment: 11 pages, 8 figures, accepted in Physics of Plasma

Processus de transports à la magnétopause terrestre

The thin interface between the solar wind flowing around the Earth and the inner magnetosphere is a key region for mass, momentum and energy transfer. Yet, it is very controversial to identify the procress of such transfer as well as any quantitative measure of their efficiency. This transition region, called t he magnetopause, is the loci of importants gradient in the particle density, bulk flow velocity, magnetic field direction and magnitude. We review in this t hesis the nonlinear particle dynamics in such thin current sheet and the anomalous transport resulting from the Kelvin-Helmholtz instability (chapter 1), th e diffusion process resulting from electromagnetic fluctuations (chapter 2), the collisionless magnetic reconnection process in asymmetric current sheets (c hapter 3), and some new perspectives on magnetic reconnection (chapter 4).La fine interface entre le vent solaire en écoulement autour de la Terre et sa magnétosphère interne est une région clé pour le transfert de masse, d'impulsion et d'énergie. L'identification de processus de transport ainsi qu'une mesure quantitative de leur efficacité est encore un sujet controversé. Cette région de transition que l'on appelle la magnétopause est le lieu d'importants gradients de la densité de particules, de la vitesse du flot moyen, et de la direction et/ou du module du champ magnétique. Dans ce manuscrit, nous revenons sur la dynamique non lin\'eaire des particules dans une telle couche de courant ainsi que sur le transport anormal qui résulte de l'instabilité Kelvin-Helmholtz (chapitre 1), sur les processus de diffusion associ\'es aux fluctuations électromagnétiques (chapitre 2), sur les processus de reconnection magnétique non-collisionnelle dans les couches de courant asymetriques (chapitre 3), ainsi que sur de nouvelles perspectives sur la reconnection magnétique (chapitre 4)

Processus de transports à la magnétopause terrestre

The thin interface between the solar wind flowing around the Earth and the inner magnetosphere is a key region for mass, momentum and energy transfer. Yet, it is very controversial to identify the procress of such transfer as well as any quantitative measure of their efficiency. This transition region, called t he magnetopause, is the loci of importants gradient in the particle density, bulk flow velocity, magnetic field direction and magnitude. We review in this t hesis the nonlinear particle dynamics in such thin current sheet and the anomalous transport resulting from the Kelvin-Helmholtz instability (chapter 1), th e diffusion process resulting from electromagnetic fluctuations (chapter 2), the collisionless magnetic reconnection process in asymmetric current sheets (c hapter 3), and some new perspectives on magnetic reconnection (chapter 4).La fine interface entre le vent solaire en écoulement autour de la Terre et sa magnétosphère interne est une région clé pour le transfert de masse, d'impulsion et d'énergie. L'identification de processus de transport ainsi qu'une mesure quantitative de leur efficacité est encore un sujet controversé. Cette région de transition que l'on appelle la magnétopause est le lieu d'importants gradients de la densité de particules, de la vitesse du flot moyen, et de la direction et/ou du module du champ magnétique. Dans ce manuscrit, nous revenons sur la dynamique non lin\'eaire des particules dans une telle couche de courant ainsi que sur le transport anormal qui résulte de l'instabilité Kelvin-Helmholtz (chapitre 1), sur les processus de diffusion associ\'es aux fluctuations électromagnétiques (chapitre 2), sur les processus de reconnection magnétique non-collisionnelle dans les couches de courant asymetriques (chapitre 3), ainsi que sur de nouvelles perspectives sur la reconnection magnétique (chapitre 4)

Etat de l'art des simulations HPC de processus eruptifs

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Magnetic Reconnection in HEDP : Hybrid simulations

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First demonstration of an asymmetric kinetic equilibrium for a thin current sheet

International audienceThe modeling of steady state collisionless asymmetric tangential current layers is a challenging and poorly understood problem. For decades now, this difficulty has been limiting numerical models to approximate equilibria built with locally Maxwellian current layers and theoretical analyses to the very restricted Harris equilibrium. We show how the use of any distribution functions depending only on local macroscopic quantities results in a strong alteration of the current layer internal structure, which converges toward an unpredictable quasi-steady state with emission of ion scale perturbations. This transient can be explained in terms of ion kinetic and electron fluid physics. We demonstrate, for the first time, the validity of an asymmetric kinetic equilibrium model as well as its usability as an initial condition of hybrid kinetic simulations. This offers broad perspectives for the current sheet modeling, for which the early phase of instabilities can be studied within the kinetic formalism

Ion acceleration in antiparallel collisionless magnetic reconnection: Kinetic and fluid aspects

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Proton acceleration in antiparallel collisionless magnetic reconnection: Kinetic mechanisms behind the fluid dynamics

International audienceThis paper investigates the proton kinetic mechanisms leading to the formation of plasma jets in antiparallel magnetic reconnection. In particular, the interaction of the protons with the Hall electric field in the proton non-ideal region is discussed. The study, based on a two-dimensional hybrid simulation, details the important role of the proton pressure force in the acceleration process and its role in maintaining open and steady the proton outflow channel. When no fluid closure is assumed, it is found that this force arises from a strong anisotropy in velocity space which comes from kinetic effect. By analyzing the distribution functions and the individual particle dynamics, it is shown that the mixing of protons bouncing in a divergent electrostatic potential well associated to the Hall effect statistically couples the two in-plane velocity components of the particles. This coupling results, from the macroscopic point of view, in off-diagonal components of the pressure tensor

Energy budgets in collisionless magnetic reconnection: Ion heating and bulk acceleration

International audienceThis paper investigates the energy transfer in the process of collisionless antiparallel magnetic reconnection. Using two-dimensional hybrid simulations, we measure the increase of the bulk and thermal kinetic energies and compare it to the loss of magnetic energy through a contour surrounding the ion decoupling region. It is shown, for both symmetric and asymmetric configurations, that the loss of magnetic energy is not equally partitioned between heating and acceleration. The heating is found to be dominant and the partition ratio depends on the asymptotic parameters, and future investigations will be needed to understand this dependence