##
Acceleration of Solar Wind Ions by Nearby Interplanetary
Shocks: Comparison
of Monte Carlo Simulations with Ulysses Observations

Baring, Matthew G.; Ogilvie, Keith W.; Ellison, Donald C.;
Forsyth, Robert J.
*Published in:*
ApJ, 476, 889

### Abstract

Various theoretical techniques have been devised to determine distribution
functions of particles
accelerated by the first-order Fermi mechanism at collisionless astrophysical
shocks. The most stringent
test of these models as descriptors of the phenomenon of diffusive acceleration
is a comparison of the
theoretical predictions with observational data on particle populations. Such
comparisons have yielded
good agreement between observations at the quasi-parallel portion of the Earth's
bow shock and three
theoretical approaches, namely, Monte Carlo kinetic simulations, hybrid plasma
simulations, and
numerical solution of the diffusion-convection equation. Testing of the Monte
Carlo method is extended
in this paper to the realm of oblique interplanetary shocks: here observations
of proton and He2+
distributions made by the SWICS ion mass spectrometer on Ulysses at nearby
interplanetary shocks (less
than about 3 AU distant from the Sun) are compared with test-particle Monte
Carlo simulation
predictions of accelerated populations. The plasma parameters used in the
simulation are obtained from
measurements of solar wind particles and the magnetic field upstream of
individual shocks; pickup ions
are omitted from the simulations, since they appear, for the most part, at
greater heliospheric distances.
Good agreement between downstream spectral measurements and the simulation
predictions are obtained
for two shocks by allowing the parameter lambda /rg, the ratio of the mean-free
scattering length to the
ionic gyroradius, to vary in an optimization of the fit to the data; generally
lambda /rg <~ 5, corresponding
to the case of strong scattering. Simultaneous H+ and He2+ data, presented only
for the 1991 April 7 shock
event, indicate that the acceleration process is roughly independent of the mass
or charge of the species.
This naturally arises if all particles interact elastically with a massive
background, as occurs in
collisionless "scattering" off a background magnetic field, and is a patent
property of the Monte Carlo
technique, since it assumes elastic and quasi-isotropic scattering of particles
in the local plasma frame.