Near‐specular reflection of ions at quasi‐parallel shocks

Abstract

Previous numerical simulations have shown that ions are reflected at quasi‐parallel shocks in a bursty manner. At times, relatively cold and dense beams of ions that appear to be specularly reflected are seen, consistent with observations at the quasi‐parallel portions of the bow shock. The intermittent reflection of ions has been shown to be connected to the periodic disruption and re‐formation of the shock front. Though the connection of the reflected ions to the re‐formation cycle has been studied extensively, the reflection process itself has received little attention. Using one‐dimensional hybrid simulations and a semianalytic model of the shock front, the reflection process is investigated to determine the source regions in incident ion phase space of reflected and transmitted ions, the relative importance of the electric and magnetic forces in the reflection of incident ions, and how these characteristics change in time. We find that the phase space origin of reflected particles and the fraction of incident ions that reflect depend upon the electromagnetic field structure of the shock front at the time the ions encounter it. The reflection fraction is maximized when the electric field along the shock normal (Ex) and the noncoplanar magnetic field (By) are at their maximum values. This happens because the phase space source region of reflected particles lies closer to the center of the incident ion distribution function when those two field components are large than it does when they are small. Also, when Ex and By are large, the reflection process produces a beam that is cooler, more dense, and closer to specular than when they are small. We also find that, in general, the electric field provides most of the force involved in the reflection of incident ions, although the magnetic field is also needed to reflect all but a few ions. When Ex is near zero, almost no ions reflect in the shock ramp.