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Accueil > Séminaires & Soutenances > Séminaires, soutenances de thèses et HDR précédents > 2009 > Séminaires 2009 > le jeudi 2 juillet à 11h
le jeudi 2 juillet à 11h
A. Bhattacharjee -
Center for Integrated Computation and Analysis of Reconnection and Turbulence
Institute for the Study of Earth, Oceans, and Space
University of New Hampshire
Fast Reconnection and Secondary Instabilities of Thin Current Sheets : Applications to Sawtooth Crashes in a Tokamak and Substorms in the Magnetotail
le jeudi 2 juillet à 11 heures
Palaiseau, Salle de conférence du CPHT
Résumé :
Sawtooth crashes in a tokamak and substorms in the Earth’s magnetotail are among the most interesting and as yet unresolved problems in plasma physics. Both are examples of explosive plasma dynamics. Despite the great differences in plasma parameters and boundary conditions of the tokamak and the magnetotail, we will demonstrate that certain features of both types of phenomena can be understood in the framework of Hall MHD (or two-fluid) equations.
The Hall MHD model incorporates a generalized Ohm’s law which includes the Hall current and electron pressure gradient (assumed to be a scalar). In this model, the mechanism that breaks field lines is a small but finite resistivity and electron inertia. It is shown that under certain conditions, the kink-tearing instability grows near-explosively. While the qualitative features of this near explosive growth are similar to those obtained earlier by using reduced equations, the present work using the full equations at finite aspect ratio gives a fuller picture of the conditions under which one obtains near-explosive growth, and when near-explosive growth is thwarted by nonlinear diamagnetic stabilization. Thus the theory delineates conditions under which the development of thin current sheets produces a sawtooth crash, and when it does not.
The role of magnetic reconnection and ballooning instabilities in magnetotail substorms onset is also studied within the framework of the Hall MHD model. It is shown that collisionless reconnection facilitates the growth of an extended and dynamic, thin current sheet that exhibits an impulsive pre-onset enhancement at near-Earth distances. Subsequently, the thin current sheet appears to be unstable to Hall MHD (or drift-ballooning) instabilities. The Hall MHD ballooning instability is characterized by a real frequency that corresponds to a westward propagating wave. Recent numerical simulations of nonlinear ballooning modes show that the mode continues to grow nonlinearly at the linear growth rate, producing large sheared flows, and a reduction of the pressure gradient nearly everywhere except at the local stagnation point of the flow where a shock-like coherent structure forms. We compare the predictions of theory and simulation with several substorm events observed by Wind, Polar and Cluster.
Several open questions remain. We will discuss these questions, and possible strategies for further work.
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