Ion reflection and downstream thermalization at the quasi‐parallel bow shock

Abstract

It is well known that for sufficiently high Mach numbers (typically Mf>2) ion reflection is the primary process by which ion energy dissipation is initiated at quasi‐perpendicular, collisionless shocks in space. In this paper we present evidence from ISEE 2 measurements that coherent ion reflection is also often an important element of the ion energy dissipation process at high Mach number, quasi‐parallel shocks. Such coherent reflection is not evident in a number of published hybrid simulations of quasi‐parallel shocks wherein the dissipation process appears to be associated primarily with incoherent scattering of solar wind ions in the upstream and downstream wave fields. However, such reflection is present in other simulation results. The main evidence for coherent ion reflection as an important element in achieving dissipation at quasi‐parallel shocks is the following: (1) A cold secondary (to the solar wind) beam of ions is often observed within the ramp of the shock where the magnetic field intensity rises precipitously and where the major electron heating occurs. Typically this secondary beam of ions has a velocity space position roughly consistent with specular reflection of a portion of the solar wind ions incident on the shock. (2) Downstream from the shock, a relatively dense and cool core ion population is typically observed, accompanied by either multiple ion bunches or, more commonly, a suprathermal ion shell. (These populations are in addition to the extremely hot, tenuous diffuse ions which appear nearly ubiquitously both upstream and downstream from the shock.) The downstream core and shell (or bunches) are believed to represent directly transmitted solar wind ions and reflected and later scattered ions, respectively, with the motions of the latter population accounting for a large fraction of the ion energy dissipated at the shock.