The Injection Problem for Anomalous Cosmic Rays

The precise mechanism by which some interstellar pickup ions are selected to be accelerated up to anomalous cosmic-ray (ACR) energies at the heliospheric termination shock is a central puzzle in the physics of the outer heliosphere. Observations by Gloeckler et al. of the mass dependence of the efficiency with which low-energy pickup ions are accelerated at an interplanetary shock is in the opposite sense of related ACR observations made by Cummings and Stone. Furthermore, the rapidity with which ions must be accelerated at the termination shock requires that the scattering of these ions be weak, an effect which requires that relatively energetic ions exist initially. It is shown here that these three sets of observations can be reconciled by using a multiply reflected ion acceleration mechanism to provide a preaccelerated population of pickup ions at the termination shock. The precise fraction of preenergized pickup ions that can subsequently be diffusively shock accelerated depends on the ion scattering strength at the termination shock. It is shown that the assumption of weak scattering yields an anomalously low injection efficiency for pickup H+ compared to those of He+, O+, and Ne+, and those inferred for C+ and N+. The predicted injection efficiencies of C+, N+, O+, and Ne+ are clustered together closely whereas He+ is a little less efficiently injected. Computed ACR termination shock and modulated fluxes compare well to those observed and inferred by Cummings and Stone. Finally, strong scattering of ions at the termination shock is found to eliminate the mass dependence of the ACR injection efficiency.