The role of quasi-perpendicular shocks in solar energetic particle events

Solar energetic particles (SEPs) are an important venue for testing and refining our understanding of acceleration processes that are ubiquitous in astrophysical plasmas. Large SEP events occur at a rate of about 10 per year during solar maximum. The dominant accelerators in these events are believed to be shocks driven by fast coronal mass ejections (CMEs). A particular challenge has been the dramatic event-to-event variability in composition and spectral characteristics at energies above a few tens of MeV per nucleon. I discuss recent efforts to understand this variability in terms of the interplay of two factors: seed populations, typically comprising at least suprathermals from flares and suprathermals from the corona or solar wind; and shock geometry, which generally begins as quasiperpendicular near the Sun but evolves toward quasi-parallel as the shock moves outward.