Ion injection and acceleration at quasi‐perpendicular shocks

Abstract

We present and discuss results of a new model for ion injection and acceleration at quasi‐perpendicular collisionless shocks. We use the one‐dimensional hybrid simulation (kinetic ions/fluid electrons) and impose an assumption on the ion motion so that diffusion across the magnetic field (normal to the shock front) is possible. These motions are otherwise suppressed by both one‐ and two‐dimensional simulations. We find that, even in strictly perpendicular shocks, when scattering normal to the field is included, a fraction of the incident ions are accelerated to suprathermal energies. When reasonable scattering times are considered, only pickup ions are injected, whereas thermal solar wind ions are not. The acceleration of these ions is very rapid. We have found that a few of the initially low‐energy pickup ions can reach many tens to a few hundred times the plasma ramming energy in less than 100 gyroperiods. Furthermore, highly field aligned energetic ions are found to exist upstream of the slightly oblique shock. The most direct application of this study is toward the interpretation of observations of both solar wind and interstellar pickup ion distributions in the vicinity of interplanetary shocks which are most often quasi‐perpendicular. This work also directly addresses a fundamental issue with regard to our current understanding of the anomalous component of cosmic rays.