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

The 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. Plain Language Summary The Earth’s bow shock decelerates the approaching supersonic solar wind. Unlike at ordinary hydrodynamic shock waves, the conversion of kinetic into thermal energy is accomplished in a collisionless way. An important contribution to this heating process is made by solar wind ions that reflect at the shock. After gyration in the upstream magnetic field, most of these ions return to the shock and pass downstream but some reflect again or even escape without further shock interactions. We use a mixture of previous analytical results and computation of ion trajectories in a model planar shock to try to establish whether these various groups of ions originate from different parts of the incident solar wind velocity distribution. We find that a particle’s individual upstream velocity in the solar wind rest frame does determine its belonging to one or other of the above ion populations. In particular, ions that escape the shock after reflection have high speeds in the incident distribution and velocities that are oriented along the upstream magnetic field. Key Points Different ion populations near low Mach number collisionless shocks arise from separate portions of the incident velocity distribution Incoming plasma frame velocities for escaping ions point upstream close to the magnetic field and are from the wings of the distribution Most escaping ions have further reflections and incident velocities more field aligned t