MONTE-CARLO SIMULATIONS OF PARTICLE-ACCELERATION AT OBLIQUE SHOCKS

The fermi shock acceleration mechanism may be responsible for the prod uction of high-energy cosmic rays in a wide variety of environments. M odeling of this phenomenon has largely focused on plane-parallel shock s, and one of the most promising techniques for its study is the Monte Carlo simulation of particle transport in shocked fluid flows. One of the principal problems in shock acceleration theory is the mechanism and efficiency of injection of particles from the thermal gas into the accelerated population. The Monte Carlo technique is ideally suited t o addressing the injection problem directly, and previous applications of it to the quasi-parallel Earth bow shock led to very successful mo deling of proton and heavy ion spectra, as well as other observed quan tities. Recently this technique has been extended to oblique shock geo metries, in which the upstream magnetic field makes a significant angl e Theta(B1) to the shock normal. In this paper, spectral results 0from test particle Monte Carlo simulations of cosmic-ray acceleration at o blique, nonrelativistic shocks are presented. The results show that lo w Mach number shocks have injection efficiencies that are relatively i nsensitive to (though not independent of) the shock obliquity, but tha t there is a dramatic drop in efficiency for shocks of Mach number 30 or more as the obliquity increases above 15 degrees. Cosmic-ray distri butions just upstream of the shock reveal prominent bumps at energies below the thermal peak; these disappear far upstream but might be obse rvable features close to astrophysical shocks.