Evidence of different backstreaming populations in the quasi-perpendicular ion foreshock : 2D full particle simulation results

Many space missions have already evidenced the existence of the foreshock region located upstream of the terrestrial bow shock and populated by energetic backstreaming particles (electrons and ions) issued from the shock. In order to analyse the foreshock region, a curved shock is simulated with the help of a 2-D full particle (PIC) code, where full curvature and time of flight effects are fully described by a self consistent approach. The analysis is restricted to the quasi-perpendicular angular range defined by 45° ≤ QBn ≤ 90°. Herein, we focus on the ion foreshock. In a good agreement with experimental data, present preliminary results evidence two distinct ion populations collimated along the interplanetary magnetic field (IMF): (i) the Field-Aligned Beam population (hereafter named "FAB'') and (ii) the gyro-phase bunch population (hereafter named "GPB'') which differ from each other by their gyrotropic or non-gyrotropic behavior, respectively. The analysis shows that no pitch angle scattering mechanism needs to be invoked to account for the generation of the "GPB''. Present results show that additional criteria are necessary namely: the interaction time Dint of backstreaming ions with the shock front and their downstream penetration depth. These criteria allow to evidence that the "FAB'' population correspond to particles which move back and forth between the upstream edge of the front and the overshoot, and are characterized by a quite large Dint (covering several local gyro-periods). In contrast, the "GPB'' ions have suffered a very short interaction time (i.e. less than one gyro-period) and escaped into the upstream region with a parallel velocity lightly higher than the "FAB'' population. "GPB'' particles seem to be produced by the electrostatic field built up at the shock front and are emitted in a burst-like mode rather than in continuous way