Production of nongyrotropic and gyrotropic backstreaming ion distributions in the quasi-perpendicular ion foreshock region
Résumé
A curved shock is analyzed in the whole quasi-perpendicular propagation region (90° ≥ θBn≥45°) in a supercritical regime with the help of a 2-D particle-in-cell code including self-consistent effects such as the shock front curvature and the time-of-flight effects. Two distinct ion populations are observed within the foreshock: a (gyrotropic) field-aligned beam population, hereafter named “FAB,” and a (nongyrotropic) gyrophase bunched population, hereafter named “GPB.” The origin of these high-energy particles and their corresponding acceleration mechanisms are analyzed in details in the present paper. Both FAB and GPB populations are shown to be produced by the shock front itself and more important, do have exactly the same origin. At the shock front, the two populations gain a nongyrotropic distribution, but FAB population loses its initial phase coherency after suffering several bounces along the curved front. This result has one main consequence: the time evolution of the two populations does not involve some distinct reflection processes as often claimed in the literature, but results only from the particle time history at the shock front. This important result was not expected and greatly simplifies the question of their origin. More precisely, a new parameter, the injection angle θinj has been defined between the shock normal direction and the ion gyrating velocity vector. We found that the FAB population is formed by ions injected almost along the shock front, while GPB population is formed by ions injected almost along the shock normal.
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