On the origin of the quasi-perpendicular ion foreshock: Full-particle simulations

Many space missions have already evidenced the existence of the ion foreshock region located upstream of the Earth's bow shock and populated by energetic backstreaming ions reflected by the shock front. In order to analyze this region, a curved shock is simulated with a 2-D particle-in-cell (PIC) code. The analysis is presently restricted to the quasi-perpendicular angular range defined by 45 degrees <= theta(Bn) <= 90 degrees. In agreement with experimental data, present results evidence two distinct ion populations backstreaming from the shock front along the interplanetary magnetic field: (i) the field-aligned beam population (hereafter "FAB") and (ii) the gyrophase bunched population (hereafter "GPB") which differ from each other by their gyrotropic or non-gyrotropic behavior, respectively. Excluded by a simulation time which is too short, ion instabilities pitch-angle scattering cannot be the source of "GPB." Two new criteria are proposed to identify more precisely each population: their interaction time Delta t(int) with the shock front and their downstream penetration depth. These criteria show that (i) the "FAB" population moves back and forth between the upstream edge of the shock front and the overshoot, and is characterized by a Delta t(int) covering several upstream gyroperiods. (ii) In contrast, the "GPB" ions suffer a short interaction time (i.e., 1 < tau(ci)). We observe that the "FAB" ions may have different origins although all "GPB" ions seem to be produced by the electrostatic field built up at the shock and are emitted in a burst-like mode rather than in continuous way.