"Pre-Acceleration of Anomalous Cosmic Ray Ions at Recurrent Solar Wind Shocks"

Interstellar pickup ions and solar wind ions are two main sources of anomalous cosmic ray (ACR) ions. An important unresolved theoretical issue is how such low-energy seed ions are pre-accelerated to energies sufficiently high that they are readily injected and accelerated to ACR energies at the termination shock. Specifically, is injection accomplished by local processes at the termination shock itself, or by distributed processes that take place in the solar wind? The former scenario, although of great interest from the perspective of shock physics, remains speculative, whereas the latter is supported directly by LECP (Low Energy Charged Particle Instrument) measurements made in the outer heliosphere. The LECP data show that ion injection at the termination shock is not likely to be a problem, at least for protons and helium nuclei, since heliospheric shocks can pre-accelerate such ions, of both solar wind and interstellar origin, to highly superthermal energies. Shocks observed in the outer heliosphere fall in two main classes: (1) traveling shocks associated with solar transients prevalent during active solar periods, and (2) recurrent or corotating shocks that bound corotating interaction regions (CIRs) prevalent during quiet solar periods. In this note we focus on ACR pre-acceleration at CIR shocks that have been the dominant large-scale structures in the outer heliosphere from 1992 to recent (declining phase of solar Cycle 23). The intensities of accelerated ions at these relatively weak recurrent shocks tend to be lower than those at stronger, yet less frequent, transient shocks. However, trains of recurrent shocks can last for large fractions of a year, thereby providing the termination shock with a stead