Origin of the Ion Foreshock in a Quasi-perpendicular Curved Collisionless Shock: Particles Trajectory Analysis in 2D PIC Simulations

Philippe Savoini (1) , Bertrand Lembège (2)

1 LPP - Laboratoire de Physique des Plasmas
2 HELIOS - LATMOS

The ion foreshock located upstream of the Earth's shock wave is populated with ions having interacted with the shock, and then, reflected back with an high energy gain. Spacecrafts have clearly established the existence of two distinct populations in the quasi-perpendicular shock region (i.e. for 45° <= ThetaBn <= 90°, where ThetaBn is the angle between the shock normal and the upstream magnetic field) : (i) field-aligned ion beams or « FAB » characterized by a gyrotropic distribution, and (ii) gyro-phase bunched ions or « GPB » characterized by a NON gyrotropic distribution. One of the important unresolved problem is the exact origin of the particles contributing to these two populations. To our knowledge, it was the first time that full-particle simulations have been performed including self-consistently the shock front curvature and nonstationarity, and the time-of-flight effects. Our analysis evidences that these two backstreaming populations may be reflected by the front itself and can be differentiated both in terms of interaction time and trajectory within the shock front. In particular, simulations evidence that "GPB" population is characterized by a short interaction time (DeltaTinter = 1 to 2 tauci) while the "FAB" population corresponds to a much larger time range (from 1 tauci to 10 tauci), where tci is the upstream ion gyroperiod. Present individual ion trajectories evidence that "FAB" population shows a strong perpendicular drift at the shock front (i.e. strong dependence of the pitch angle to the perpendicular velocity) whereas the "GPB" population shows no perpendicular drift (i.e. its pitch angle is mainly driven by the parallel velocity). Such differences explain why the "FAB" population loses their gyro-phase coherency and become gyrotropic which is not the case for the "GPB". This important result was not expected and greatly simplifies the question of their origin.

Domaines: Physique des plasmas [physics.plasm-ph]; Astrophysique [astro-ph]

Philippe Savoini, Bertrand Lembège. Origin of the Ion Foreshock in a Quasi-perpendicular Curved Collisionless Shock: Particles Trajectory Analysis in 2D PIC Simulations. American Geophysical Union, Fall Meeting, Dec 2015, San Francisco, California, United States. pp.SM21C-07. ⟨hal-02800203⟩