[hal-00779719] Dynamics of collisionless shocks: Introduction to basic kinetic processes and stationary shocks

Wherever a supersonic flow exists, the generation of a shock wave is an inevitable consequence when the flow encounters a surrounding plasma medium as an obstacle or when a large quantity of dense and hot ionized matter is suddenly released in a background plasma. Collisionless shocks are quite common throughout the heliosphere, as evidenced by planetary and cometary shocks, by travelling interplanetary shocks, by shocks associated with coronal mass ejection (CME) in solar physics, or by the heliospheric termination shock. Such shocks represent excellent energy converters from bulk energy to particles kinetic energy, and are quite often invoked as attractive candidates for being responsible for particles acceleration. A large part of this conversion takes place at and/or around the shock front itself. Accelerated particles may reach energies large enough to represent noticeable sources of radiation. Such shocks are named collisionless since their characteristic spatial width is much smaller than the particle mean free path (distance covered by particles between two successive collisions). Then, all energy exchanges between the shock itself and the medium cannot take place by collisions but by intricated waves-particles interactions. In this first approach, collisionless shocks will be described as the result of a compromise between three efffects : (i) nonlinear effects which are balanced by (ii) dispersive and (ii) dissipative effects. This balance can be analyzed with the help of different types of numerical simulations which will be summarized. One main difficulty is to describe and to identify the dissipative effects which contribute to this balance, since many different source mechanisms can participate. The basic mechanisms of particles interactions with the macroscopic fields at the shock front will be presented. Moreover, in situ measurements reveal a large variety of quite different shocks signatures. This variety has stimulated the necessity for two main classes of shocks: (a) supercritical/subcritical shocks according to the Mach number regime, and (b) quasiperpendicular /quasiparallel shock according to the direction of the shock normal. The main goal of this first presentation is to reach a view of the collisionless shocks dynamics as synthetic as possible, taking into account this classification and the basic mechanisms involved both in planar and curved shocks. This first presentation will be based on stationary shocks mainly.



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