Full-particle 2-D Simulations of the Ion Foreshock associated to a Supercritical Quasi-perpendicular Curved Collisionless Shock : Origin of Backstreaming Energetic Particles

Abstract:

The ion foreshock located upstream of the Earth's bow shock is populated with ions reflected back by the shock front. In-situ spacecraft measurements have clearly established the existence of two distinct populations in the upstream of the quasi-perpendicular shock region (i.e. for 45^o ≤ Θ_Bn≤ 90^o, where Θ_Bn is the angle between the shock normal and the upstream magnetostatic 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, which exhibits a non-vanishing perpendicular bulk velocity.

The use of 2D PIC simulations where full curvature effects, time of flight effects and both electrons and ions dynamics are fully described, has evidenced that the shock front itself can be the possible source of these two characteristic populations.

A recent analysis has evidenced that both populations can be discriminated in terms of interaction time (Δt_inter) with the shock front. 'GPB' and 'FAB' populations are characterized by a short (Δt_inter ~ 1 τ_ci) and much larger (Δt_inter ≥ 2 τ_ci) interaction time respectively, where τ_ci is the ion upstream gyroperiod.

In addition, present statistical results evidence that:

(i) backstreaming ions are splitted into 'FAB' and 'GPB' populations depending on their injection angle when hitting the shock front (defined between the local normal to the shock front and the gyration velocity vector).

(ii) As a consequence, ion trajectories strongly differ between the 'FAB' and 'GPB' populations at the shock front. In particular, 'FAB' ions suffer multi-bounces along the curved front whereas 'GPB' ions make only one bounce. Such differences may explain why the 'FAB' population loses their gyro-phase coherency and become gyrotropic which is not the case for the 'GPB'.

Then, the differences observed between 'FAB' and 'GPB' populations do not involve some distinct reflection processes as often claimed in the literature but follow from different particle time histories at the shock front. Both 'FAB' and 'GPB' ions suffer the same reflection process but only 'FAB' population loose their initial phase coherency by suffering several bounces. This important result was not expected and greatly simplifies the question of their origin.