Dissipation, Ion Injection, and Acceleration in Collisionless Quasi-Parallel Shocks

Abstract

The structure of and dissipation at magnetic collisionless shocks depend considerably on upstream parameters, in particular, the upstream flow speed along the shock normal (i.e., Mach number) and the angle between the upstream magnetic field and the shock normal direction (shock normal angle Θ Bn ). Whereas the physical nature of quasi-perpendicular shocks (Θ Bn > 45°) is fairly well understood, this is not so in the case of quasi-parallel shocks. This is due to the complex nature of quasi-parallel shocks in space, in particular at the quasi-parallel bow shock: unlike quasi-perpendicular shocks which have a well-defined and smooth magnetic profile through the shock ramp, the quasi-parallel shock exhibits large-amplitude waves in the upstream and downstream region. Equally complex are the ion populations observed at the quasi-parallel shock: in the upstream region there exists a more energetic, (in velocity space) diffuse ion component, while beams of relative cold ions are often observed at the shock ramp or downstream of the main transition. A number of theoretical concepts have been developed in the past. Parker (1961) suggested that the shocks forms by means of the firehose instability, or more properly termed by the electromagnetic ion-ion nonresonant instability as two ion streams interact. Kennel and Sagdeev (1967) also considered the firehose instability driven by a temperature anisotropy in the downstream region, which is due to steepening ion acoustic waves at the shock front. Golden et al. (1973) proposed the excitation of the resonant electromagnetic ion beam instability to be responsible for shock formation. The instability is driven by the interaction of the upstream and the downstream ion distribution. A third mechanism involves the excitation of short-wavelength whistler waves. Jackson and