THE INJECTION AND ACCELERATION OF PARTICLES IN OBLIQUE SHOCKS - A UNIFIED MONTE-CARLO DESCRIPTION

Standard Fermi particle acceleration may, for certain shock parameters, be enhanced in oblique shocks, in which the upstream magnetic field makes a significant angle THETA(Bn1) to the shock normal, compared to parallel ones (THETA(Bn1) approximately 0). However, the complexity of oblique shocks has prevented, until now, any determination of the efficiency of the injection and acceleration process (apart from extremely limited hybrid plasma simulation results; e.g., Burgess 1989). As a first step in producing a self-consistent model capable of simultaneously describing shock structure and particle acceleration in shocks of arbitrary obliquity, we have generalized a Monte Carlo simulation, previously used for parallel shocks, to describe oblique geometries. Here we present initial results showing how the injection and acceleration efficiency varies with Mach number and obliquity. In this report we consider only test particles (the shock remains discontinuous on the scale of a particle's mean free path) drawn from the upstream thermal distribution and neglect cross-field diffusion. We show that, for high Mach number parallel shocks, significant numbers of these thermal test particles are drawn (''injected'') into the acceleration process, but that the injection efficiency drops rapidly as THETA(Bn1) increases. For sonic Mach numbers above approximately 20, the fraction of energy density in superthermal particles falls from approximately 97% when THETA(Bn1) = 0-degrees to approximately 30% when THETA(Bn1) = 25-degrees. At low Mach numbers (below M(s) approximately 3) the energy density in superthermal particles drops from approximately 87% to approximately 72% as THETA(Bn1) increases from 0-degrees to 25-degrees. These test-particle results suggest that the acceleration efficiency may be too low in oblique shocks to explain cosmic-ray production in most sources. However, the inclusion of cross-field diffusion and shock smoothing from the backpressure of accelerated particles, effects likely to occur in astrophysical shocks, may significantly modify these predictions.