Solar Energetic Particles as a Laboratory for the Study of Shock Physics

Shocks driven by fast coronal mass ejections (CMEs) are generally believed to be the dominant accelerators in large, gradual solar energetic particle (SEP) events. A key challenge for this notion has been the highly variable spectral and compositional characteristics above a few tens of MeV per nucleon. Although this variability is a daunting problem, there are also high levels of correlation among the variable factors, which provide compelling clues to their origin. I will review observations from ACE, Wind and other spacecraft that have yielded these clues. I will also review recent efforts to understand this high-energy variability in terms of the interplay of two factors: evolution in the shock-normal angle as the shock moves outward from the Sun; and a compound seed population, typically comprising at least suprathermals from the corona (or solar wind) and suprathermals from flares. A simple analytical implementation of these ideas has been shown to semi-quantitatively account for a wide range of SEP phenomenology, including facets that have been known but unexplained for more than 20 years. These simple calculations also highlight a number of questions where we should be able to use SEP observations to deepen our understanding of shock acceleration.