ACCELERATION RATES AND INJECTION EFFICIENCIES IN OBLIQUE SHOCKS

The rate at which particles are accelerated by the first-order Fermi mechanism in shocks depends on the angle, Theta(Bn1), that the upstream magnetic field makes with the shock normal. The greater the obliquity, the greater the rate, and in quasi-perpendicular shocks (i.e., Theta(Bn1) --> 90 degrees), rates can be hundreds of times higher than those seen in parallel shocks. In many circumstances pertaining to evolving shocks (e.g., supernova blast waves and interplanetary traveling shocks) or where acceleration competes with losses (e.g., through synchrotron cooling), high acceleration rates imply high maximum particle energies, and obliquity effects may have important astrophysical consequences. However, as is demonstrated in this paper, the efficiency for injecting thermal particles into the acceleration mechanism also depends strongly on obliquity and, in general, varies inversely with Theta(Bn1); shocks which accelerate particles most rapidly are least capable of injecting thermal particles into the acceleration process. In addition, the degree of turbulence and the resulting cross-field diffusion strongly influences both injection efficiency and acceleration rates. The test particle Monte Carlo simulation of shock acceleration used here assumes large-angle scattering, computes particle orbits exactly in shocked, laminar, nonrelativistic flows (in contrast to our previous simulations of oblique shocks, which assumed magnetic moment conservation), and calculates the injection efficiency as a function of obliquity, Mach number, and degree of turbulence. We find that turbulence must be quite strong for high Mach number, highly oblique shocks to inject significant numbers of thermal particles and that only modest gains in acceleration rates can be expected for strong oblique shocks over parallel ones if the only source of seed particles is the thermal background.