Particle acceleration and generation of diffuse superthermal ions at a quasi‐parallel collisionless shock: Hybrid simulations

Abstract

A large scale one‐dimensional hybrid simulation is performed to investigate the generation mechanism of diffuse superthermal ions at a high Mach number quasi‐parallel collisionless shock. The shock exhibits a cyclic behavior and reforms periodically. The generation of the diffuse ions is associated with the reformation of the shock. At the beginning of the reformation cycle, a part of ions are reflected by the shock due to the existence of the cross shock potential. At the same time, an upstream wave is brought back by the upstream plasma and interacts with the shock. The upstream wave begins to steepen as it approaches the shock, and then a new shock front is formed. The reflected ions are trapped between the new and old shock fronts. They are accelerated every time they are reflected by the new shock front until the reformation cycle of the shock is finished and the particles escape from the shock. In this way, diffuse superthermal ions are generated in the quasi‐parallel shock, which may be further accelerated to higher energy due to shock diffusive acceleration.