Diffusive Ion Acceleration by CME‐Driven Shocks

Abstract

The acceleration of solar energetic ions in events at coronal/ interplanetary shocks is reviewed including: a brief observational motivation for a shock origin of gradual events, the historical development of the theory of shock acceleration as applied to solar energetic particles, the basic features of the process of diffusive shock acceleration (first-order Fermi acceleration at the shock, wave excitation by the energetic protons, magnetic focusing and upstream escape of the ions), challenges to developing a successful theory, the basic equations of ion transport and acceleration, and current theoretical research. Finally, outstanding issues which must be addressed by future work are listed including: ion injection into the process of diffusive shock acceleration, careful evaluation of the wave intensity upstream of the shock, acceleration and injection at quasi-perpendicular shocks, long-time behavior of gradual events, the inclusion of wave propagation velocities in the effective shock compression ratio, and a rigorous treatment of the time-dependence.