13 research outputs found

    Magnetopause study by means of a multi-fluid approach

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    New methods are presented which are able to obtain a "realistic" multi-fluid analysis of the Earth’s magnetopause. The goal is reached in three steps. The analysis of spacecraft data acquired across the magnetopause is done by means of new techniques which relax most of the hypotheses usually assumed about the observed plasmas. These techniques help in disentangling the principal causes of misunderstanding in data interpretations by discerning whether the observed variations are due to the magnetopause motion in the spacecraft frame or due to the purely temporal variations of the magnetopause structure. Optimization techniques help in determining automatically the parameters the methods are dependent by. The spatial profiles feed a new 3fluid analytical model (two ion and one electron populations) able to confine the magnetospheric and magnetosheath plasmas in their own regions and letting them to partially overlap close to the contact boundary. This model determines the two ions contributions to the total ion population ratio in case it is not provided by the distribution functions. A 3fluid equilibrium is then perturbed and evolved in time by means of a new 3fluid numerical code, coded to take the 3fluid model outputs as inputs. The numerical model of the magnetopause develops a magnetic reconnection instability, in agreement to what is observed close to the analyzed magnetopause crossing and leads to conclusions about the spatial distribution of the mixing processes

    Étude magnétopause avec une simulation numérique multi-fluide

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    In this thesis, new methods are presented which are able to obtain a "realistic" multi-fluid analysis of the Earth’s magnetopause. This goal is reached in three main steps. The analysis of spacecraft data acquired across the magnetopause is done by means of new techniques which relax most of the hypotheses usually assumed about the observed plasma structures. These techniques help in disentangling the principal causes of misunderstanding in data interpretations by discerning whether the observed variations are due to the magnetopause motion in the spacecraft frame or due to the purely temporal variations of the magnetopause structure. Optimization techniques help in determining automatically the parameters (thresholds) the methods are dependent by.The spatial profiles feed a new 3fluid analytical model (two ion and one electron populations) able to spatially confine the magnetospheric and magnetosheath plasmas in their own regions and letting them to partially overlap close to the contact boundary. This model helps also in determining the two ions contributions to the total ion population where this information is not accessible analyzing the distribution functions. The 3fluid equilibrium computed by the analytical model is then perturbed and evolved in time by means of a new 3fluid numerical code, explicitly coded to take the 3fluid model outputs as inputs. The numerical model of the magnetopause develops a magnetic reconnection instability, in agreement to what is observed close to the analyzed magnetopause crossing and leads to conclusions about the spatial distribution of the mixing processes.Dans cette thèse, je présente des nouvelles méthodes qui permettent d'obtenir une analyse multi-fluide “réaliste” de la magnétopause de la Terre. Ce but est atteint en trois étapes. L'analyse des données est réalisée grâce à de nouvelles techniques qui relâchent la plupart des hypothèses le plus souvent faites pour ces analyses. Ces techniques aident à distinguer si les variations observées sont causées par les mouvements de la magnétopause ou par la modification de sa structure au cours du temps. Techniques d'optimisation aident à choisir automatiquement les paramètres (seuils) dont les méthodes sont dépendent. Les profils spatiaux fournissent l'information primaire pour un nouveau modèle analytique 3fluides (deux populations ioniques et une population d'électrons), qui permet de faire en sorte que la population de chaque région voit sa densité s'annuler dans la région opposée avec une région de superposition au milieu. Le modèle aide aussi a déterminer dans quelle proportion les deux populations ioniques contribuent à la population ionique globale, même lorsque cette information n'est pas directement accessible dans les données, permettant en particulier d'expliquer la forme de la fonction de distribution ionique. L'équilibre décrit par ces profils analytiques est ensuite perturbé et pris comme condition initiale d'un code 3fluides, qui a été développé dans ce but. Le modèle de la magnétopause montre une instabilité de reconnexion magnétique, en accord avec ce qui est observé dans les données proches du cas analysé et mènent à conclusions en ce qui concerne la distribution spatiale du mélange des deux populations

    A multi-fluid model of the magnetopause

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    International audienceObservation of the solar wind-magnetosphere boundary provides a unique opportunity to investigate the physics underlying the interaction between two collisionless magnetized plasmas with different temperature, density and magnetic field topology. Their mixing across the interface as well as the boundary dynamics are affected by the development of fluid (and kinetic) instabilities driven by large-scale inhomogeneities in particle and electromagnetic fields. Building up a realistic initial equilibrium state of the magne-topause according to observations is still a challenge nowadays. In this paper, we address the modeling of the particles and electromagnetic field configuration across the Earth's magnetopause by means of a three-fluid analytic model. The model relies on one hot and one cold ion population as well as a neutralizing electron population. The goal is to create an analytic model that is able to reproduce the observations as closely as possible. Some parameters of the model are set using a fitting procedure that aims to minimize their difference with respect to experimental data provided by the Magne-tospheric MultiScale (MMS) mission. All of the other profiles , concerning the electron pressure and the relative densities of the cold and hot ion populations, are calculated in order to satisfy the fluid equilibrium equations. Finally, using a new tri-fluid code, we check the stability of the large-scale equilibrium model for a given experimental case and provide proof that the system is unstable to reconnection. This model could be of interest for the interpretation of satellite results and for the study of the dynamics at the magnetosphere-solar wind boundary

    A new multi-fluid model for space plasma simulations

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    International audienceWe propose a new numerical code based on a new multi-species theoretical model to study the mass, momentum and energy exchanges (MMEE) that happen across the magnetospheric boundaries. We use two distinct populations for ions, one cold and one hot (plus one neutralising electron population), to take into account the differences between the properties of the plasmas coming from the magnetosphere and from the solar wind. This approach represents a step forward in the context of the study of coupled large-scale plasma systems being a new and efficient compromise between fluid and kinetic codes in tracing the different plasma contributions during MMEE. Due to the very important role that magnetic reconnection plays in connecting the shocked Solar Wind to the Earth's magnetosphere, we show and discuss the results we obtained about the simulations of the tearing mode instability occurring across an Earth's magnetopause that we modelled thanks to our most recents MMS observations [Rezeau 2018].

    Analyzing the Magnetopause Internal Structure: New Possibilities Offered by MMS Tested in a Case Study

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    International audienceWe explore the structure of the magnetopause using a crossing observed by the Magnetospheric Multiscale (MMS) spacecraft on 16 October 2015. Several methods (minimum variance analysis, BV method, and constant velocity analysis) are first applied to compute the normal to the magnetopause considered as a whole. The different results obtained are not identical, and we show that the whole boundary is not stationary and not planar, so that basic assumptions of these methods are not well satisfied. We then analyze more finely the internal structure for investigating the departures from planarity. Using the basic mathematical definition of what is a one‐dimensional physical problem, we introduce a new single spacecraft method, called LNA (local normal analysis) for determining the varying normal, and we compare the results so obtained with those coming from the multispacecraft minimum directional derivative (MDD) tool developed by Shi et al. (2005). This last method gives the dimensionality of the magnetic variations from multipoint measurements and also allows estimating the direction of the local normal when the variations are locally 1‐D. This study shows that the magnetopause does include approximate one‐dimensional substructures but also two‐ and three‐dimensional structures. It also shows that the dimensionality of the magnetic variations can differ from the variations of other fields so that, at some places, the magnetic field can have a 1‐D structure although all the plasma variations do not verify the properties of a global one‐dimensional problem. A generalization of the MDD tool is proposed
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