Nonlinear Particle Acceleration in Oblique Shocks

We have developed a Monte Carlo technique for self-consistently calculatingthe hydrodynamic structure of oblique, steady-state shocks, together with thefirst-order Fermi acceleration process and associated non-thermal particledistributions. This is the first internally consistent treatment of modifiedshocks that includes cross-field diffusion of particles. Our method overcomesthe injection problem faced by analytic descriptions of shock acceleration, andthe lack of adequate dynamic range and artificial suppression of cross-fielddiffusion faced by plasma simulations; it currently provides the most broad andversatile description of collisionless shocks undergoing efficient particleacceleration. We present solutions for plasma quantities and particledistributions upstream and downstream of shocks, illustrating the strongdifferences observed between non-linear and test-particle cases. It is foundthat there are only marginal differences in the injection efficiency andresultant spectra for two extreme scattering modes, namely large-anglescattering and pitch-angle diffusion, for a wide range of shock parameters,i.e., for subluminal shocks with field obliquities less than or equal to 75degrees and de Hoffmann-Teller frame speeds much less than the speed of light.Comment: 38 pages, 15 figures, AASTeX format, to appear in the Astrophysical Journal, December 20, 199