Particle Acceleration at Interplanetary Shocks

The acceleration of interstellar pick-up ions as well as solar wind species has been observed at a multitude of interplanetary (IP) shocks by different spacecraft. The efficiency of injection of the pick-up ion component differs from that of the solar wind, and is strongly enhanced at highly oblique and quasi-perpendicular shock events. This paper expands upon previous work modeling the phase space distributions of accelerated ions associated with the shock event encountered on day 292 of 1991 by the Ulysses mission at 4.5 AU. As in the prior work, a kinetic Monte Carlo simulation is employed here to model the diffusive acceleration process. This exposition presents recent developments pertaining to the incorporation into the simulation of the diffusive characteristics incurred by field line wandering (FLW), according to the work of Giacalone and Jokipii. Resulting ion distributions and upstream diffusion scales are presented and compared with Ulysses data. For a pure field-line wandering construct, it is determined that the upstream spatial ramp scales are too short to accommodate the HI-SCALE flux increases for 200 keV protons, and that the distribution function for H+ somewhat underpopulates the combined SWICS/HI-SCALE spectra at the shock. This contrasts our earlier theory/data comparison where it was demonstrated that diffusive transport in highly turbulent fields according to kinetic theory can successfully account for both the proton distribution data near the shock, and the observation of energetic protons upstream of this interplanetary shock, using a single turbulence parameter. The principal conclusion here is that, in a FLW scenario, the transport of ions across the mean magnetic field is slightly less efficient than is required to effectively trap energetic ions within a few Larmor radii of the shock layer, and thereby sustain acceleration at levels that match the observed distributions. This highlights the contrast between ion transport in highly turbulent shock environs and remote, less-disturbed interplanetary regions.