Energetic particle abundances at CIR shocks

Abstract

The forward and reverse shocks that bound corotating interaction regions (CIRs) travel into different solar wind regimes. In order to test whether the energetic ions associated with the CIR shocks reflect the ambient solar wind, we investigate the elemental composition of 0.1–4.0 MeV/N ions in CIRs observed from 1 to 5 AU with instrumentation on board the Wind and Ulysses spacecraft. This is the first comparison of the abundances of energetic minor ions in CIR forward and reverse shocks. We find that apart from Helium (and Neon at 1 AU) the energetic particle abundances are roughly in agreement with the respective solar wind thermal abundances. He is overabundant by a factor of 2.5 at the CIR reverse shock at distances of 4 to 5 AU, while the forward shock abundance shows no He enhancement. There is no energy cutoff for the He acceleration observed below 2.0 MeV/N at 2 to 5 AU. This result is in qualitative agreement with models for pickup abundances in the inner heliosphere and suggests that only pickup He dominates the suprathermal solar wind He spectrum in coronal high speed streams.