Ion distributions and thermalization at perpendicular and quasi‐perpendicular supercritical collisionless shocks

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Abstract
Computer simulations and observations in laboratory and space plasmas have revealed that some incident ions are reflected at perpendicular and quasi‐perpendicular high Mach number (i.e., supercritical) shocks. Moreover, these studies have established that the gyration and subsequent thermalization of these ions play a dominant role in the the shock dissipation process. We use a hybrid kinetic simulation in order to study the selection of an incoming ion as either reflected‐gyrating or transmitted. We find that the reflected ions come from a limited region in the upstream velocity space distribution. None of the reflected ions come from the core of the distribution. Whether a particle becomes reflected depends on its energy in the upstream frame and its gyrophase as it encounters the shock. In the simulations we have carefully identified and separated the two subpopulations: transmitted and reflected. The transmitted ions do not heat appreciably in passing through the shock, although there may be wave‐particle effects neglected in the hybrid simulation. There is a contribution to the total downstream pressure due to the gyration of the two subpopulations about their common center of mass, in addition to the pressure associated with the reflected component relative to its own center of mass.

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