A deep insight into the Ion Foreshock with the help of Test-particles Two-dimensional simulations

Two dimensional test-particles simulations based on shock profiles issued from 2D full PIC simulations are used in order to analyze the formation processes of ions backstreaming within the upstream region after these interact with a quasi-perpendicular curved shock front. Two different types of simulations have been performed based on (i) a FCE (Full Consistent Expansion) model which includes all self-consistent shock profiles at different times, and (ii) a HE (Homothetic Expansion) model where shock profiles are fixed at certain times and artificially expanded in space. The comparison of both configurations allows to analyze the impact of the front non stationarity on the backstreaming population. Moreover, the role of the space charge electric field is analyzed by switching it in/off in the simulations. A detailled comparison of these two last different configurations allows to show that the electric field component plays a key role in the ion reflection process within the whole quasi-perpendicular propagation range. Simulations evidence that the different populations observed in-situ namely the FAB (Field-Aligned Beam) and GBP (Gyro-Phase Bunch) populations are essentially formed by a E→t × B→ drift involving the convective electric field E→t. Simultaneously, the study emphasizes the leading role of the electrostatic (longitudinal) field E→l built up within the shock front in the acceleration process in addition to the magnetic mirror reflection (Fast Fermi). This electrostatic field component appears as essential to form backstreaming ions at high θBn angles and in particular at the edge of the ion foreshock around 70°. Moreover, the HE model shows that the rate BI% of reflected ions is strongly dependent on the shock front profile which varies because of the shock front non stationarity. In particular, reflected ions appear to escape periodically from the shock front as bursts with an occurrence time period associated to the self-reformation of the shock front.