##
Monte Carlo simulations of particle acceleration
at oblique shocks

Baring, Matthew G.; Ellison, Donald C.; Jones, Frank C.
*Published in:*
ApJS, 90, 547

### Abstract

The Fermi shock acceleration mechanism may be responsible for the production
of high-energy cosmic
rays in a wide variety of environments. Modeling of this phenomenon has
largely focused on
plane-parallel shocks, 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 to
addressing the injection problem
directly, and previous applications of it to the quasi-parallel Earth bow
shock led to very successful
modeling of proton and heavy ion spectra, as well as other observed
quantities. Recently this technique has
been extended to oblique shock geometries, in which the upstream magnetic
field makes a significant
angle Theta(sub B1) to the shock normal. Spectral resutls from test particle
Monte Carlo simulations of
cosmic-ray acceleration at oblique, nonrelativistic shocks are presented. The
results show that low Mach
number shocks have injection efficiencies that are relatively insensitive to
(though not independent of) the
shock obliquity, but that there is a dramatic drop in efficiency for shocks of
Mach number 30 or more as
the obliquity increases above 15 deg. Cosmic-ray distributions just upstream
of the shock reveal
prominent bumps at energies below the thermal peak; these disappear far
upstream but might be
observable features close to astrophysical shocks.