A Focused Transport Approach to Pickup Ion Shock Acceleration: Implications for the Termination Shock

Abstract

The discovery of low-energy suprathermal ions coming directly from the solar wind termination shock by the Voyager 1 spacecraft revealed strong beaming of these particles along the direction of the large-scale magnetic field. Since such large anisotropies exclude the use of standard cosmic-ray transport theory for nearly isotropic particle distributions in modeling the acceleration of these particles at the termination shock, we developed a shock acceleration model based on the numerical solution of the standard focused kinetic transport equation. After investigating the appropriateness of the physical content of this equation for shock acceleration, and applying the model to shocks of varying shock obliquity, we find that (1) this approach is a viable alternative to more sophisticated particle codes such as hybrid codes for simulating the shock acceleration of pickup ions and (2) that the model has the potential to reproduce many of the interesting features of energetic ions observed by the Voyager spacecraft at the termination shock.