CME-driven Shock Simulations and Observations: Variability of SEP Abundances, Mechanisms, and Validation

In the past decade, much progress has been made by way of satellite observations regarding the origin and acceleration mechanisms of solar energetic particles (SEPs). In comparison, relatively little work has been done on the side of event-based simulations. In particular in the context of developing quantitative models of SEP fluxes and spectra, it is of great concern to understand their intrinsic possible variability, and to address the question whether the prevalence and efficiency of different contributing mechanisms can be estimated or predicted. Using ACE data, we have selected a number of characteristic "energetic storm particle" (ESP) events, i.e., SEP events in which the CME-driven shock passes the spacecraft, to compare observed local proton flux profiles with those obtained from large-scale hybrid simulations (kinetic ions, electron fluid). The events were selected for relatively undisturbed solar wind, isolation from other events, and flux profiles that clearly indicate local shock acceleration. Interestingly, in the sub-MeV range, we find very little variation of peak proton fluxes with shock normal angle. In our simulations we have investigated the role of seed particles, the acceleration processes at oblique shocks, and other effective mechanisms such as mirroring of energetic ions in downstream converging fields. In addition, shock curvature on various scales can play a role. Via direct comparison with the observed events, we discuss the pertinent acceleration mechanisms and the feasibility of predicting their respective, relative importance and occurrence.