Origin of gyrotropic/non gyrotropic ion populations in the earth´s quasi-perpendicular ion foreshock: Full-particle 2D simulation results

The ion foreshock located upstream of the Earth´s bow shock is populated with ions which are reflected back by the shock front with an high energy gain and are backstreaming into the incoming solar wind. In-situ spacecraft measurements [1-8] have clearly established the existence of two distinct populations in the foreshock upstream of the quasi-perpendicular shock region (i.e. for 45° ≤ Θ_Bn ≤ 90°, where 0B 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.