The Transition from Singly to Multiply-Charged Anomalous Cosmic Rays: Simulation and Interpretation of SAMPEX Observations

Abstract

Multiply-charged anomalous cosmic rays (ACRs) can arise when singly-charged ACR ions are stripped of one or more of their electrons during their acceleration via, e.g., the process of diffusive shock-drift acceleration at the solar-wind termination shock. Recent measurements of the charge states of ACR neon, oxygen, and nitrogen by SAMPEX at 1 AU have shown that above ≈ 25 MeV/nucleon these ions are multiply charged. In addition, SAMPEX observations have also established that the transition from mostly singly-charged to mostly multiply-charged ACRs (defined as the 50% point) occurs at a total kinetic energy of ≈ 350 MeV. Preliminary simulations for ACR oxygen based on a theory of multiply-charged ACRs were able to show a transition energy at ≈ 300 MeV. However, the simulated intensity distribution among the various charge states was inconsistent with observations. This paper reexamines the predictions of the theory in light of new SAMPEX ACR observations and recently developed and refined estimates of hydrogen-impact ionization cross sections. Based on simulations for multi-species ACR ions, we find that the transition energy is only weakly dependent on characteristic transport parameters, and that the new ionization rates distribute the intensity among the charge states in a manner consistent with observations. The calculated transition energy is in excellent agreement with the measured value.