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Direct Acceleration of Pickup Ions at the Solar Wind Termination Shock: The
Production of Anomalous Cosmic Rays

Ellison, D. C., Jones, F. C.,
Baring, M. G.
*Published in:*
ApJ, 512, 403

### Abstract

We have modeled the injection and acceleration of pickup ions at the solar wind
termination shock and investigated the parameters needed to
produce the observed anomalous cosmic-ray (ACR) fluxes. A nonlinear Monte Carlo
technique was employed that in effect solves the
Boltzmann equation and is not restricted to near-isotropic particle distribution
functions. This technique models the injection of thermal and
pickup ions, the acceleration of these ions, and the determination of the shock
structure under the influence of the accelerated ions. The
essential effects of injection are treated in a mostly self-consistent manner,
including effects from shock obliquity, cross-field diffusion, and
pitch-angle scattering. Using recent determinations of pickup ion densities, we
are able to match the absolute flux of hydrogen in the ACRs
by assuming that pickup ion scattering mean free paths, at the termination
shock, are much less than an AU and that modestly strong
cross-field diffusion occurs. Simultaneously, we match the flux ratios He^+/H^+
or O^+/H^+ to within a factor ~5. If the conditions of
strong scattering apply, no pre-termination-shock injection phase is required
and the injection and acceleration of pickup ions at the
termination shock are totally analogous to the injection and acceleration of
ions at highly oblique interplanetary shocks recently observed by
the Ulysses spacecraft. The fact that ACR fluxes can be modeled with standard
shock assumptions suggests that the much discussed ``injection
problem'' for highly oblique shocks stems from incomplete (either mathematical
or computer) modeling of these shocks rather than from any
actual difficulty shocks may have in injecting and accelerating thermal or
quasi-thermal particles.