The Link Between Individual Velocities in the Incident Plasma Flow and Ion Populations Upstream of Collisionless Shocks

Abstract

AbstractThe interaction of the solar wind with Earth’s bow shock gives rise to several distinct ion populations upstream and downstream of the shock. Using numerical computation of trajectories in the de Hoffmann‐Teller frame of reference and previous analytical results, we investigate whether membership of these populations is determined by a particle’s incident velocity in the solar wind rest frame. The shock is assumed to be a planar surface acting as an electrostatic potential jump. Ions with insufficient energy to overcome this potential are assumed to reflect specularly. We find that ions escaping upstream come from the wings of the incident distribution, assumed to be Maxwellian, and have incoming velocities in the plasma rest frame that point upstream close to the magnetic field direction. Ions escaping after several reflections have more field‐aligned incident velocities in the upstream frame on average than those escaping after one reflection. Different ion populations occupy separate regions of the incident distribution.