J. Stienlet

Origin of backstreaming electrons within the quasi‐perpendicular foreshock region: Two‐dimensional self‐consistent PIC simulation

The foreshock region is populated by energetic backstreaming particles (electrons and ions) issued from the shock after having interacted with it. Several aspects concerning the origin of these high-energy particles and their corresponding acceleration mechanisms are still unresolved. The present study is focused on a quasi-perpendicular curved shock and associated electron foreshock region (i.e., for 90° ≥ θBn ≥ 45°, where θBn is the angle between the shock normal and the upstream magnetostatic field). Two-dimensional full-particle simulation is used in order to include self-consistently the electron and ion dynamics, the full dynamics of the shock, the curvature effects and the time-of-flight effects. All expected salient features of the bow shock are recovered both for particles and for electromagnetic fields. Present simulations evidence that the fast-Fermi acceleration (magnetic mirror) mechanism, which is commonly accepted, is certainly not the unique process responsible for the formation of energetic backstreaming electrons. Other mechanisms also contribute. More precisely, three different classes of backstreaming electrons are identified according to their individual penetration depth within the shock front: (i) “magnetic mirrored” electrons which only suffer a specular reflection at the front, (ii) “trapped” electrons which succeed to penetrate the overshoot region and suffer a local trapping within the parallel electrostatic potential at the overshoot, and (iii) “leaked” electrons which penetrate even much deeper into the downstream region. “Trapped” and “leaked” electrons succeed to find appropriate conditions to escape from the shock and to be reinjected back upstream. All these different types of electrons contribute together to the formation of energetic field-aligned beam. The acceleration mechanisms associated to each electron class and/or escape conditions are analyzed and discussed.

Evidence of ion foreshock in 2-D PIC simulations of a curved collisionless shock: Statistical and individual trajectory approach

2-D Full particle simulations are used to investigate the so-called foreshock region which is filled with energized backstreaming particles. Two populations are observed for 90° ≥ ΘBn ≥ 45°: (i) field-aligned ion beams collimated along the IMF and having a gyrotropic distribution and (ii) gyro-phase bunch ions having a global gyration around the magnetic field. Our analysis evidences that these two populations are reflected by the shock itself and can have different origins both in term of interaction time, drift along the shock front and distance of penetration (leaked ions are observed).