Electron and Proton Acceleration during the First Ground Level Enhancement Event of Solar Cycle 24

Where do the highest-energy particles in a major SEP event originate? This cartoon vividly describes the quandary posed by a GLE ("Ground Level Event"), an event energetic enough for primary particles actually to reach ground level at the Earth (normally they are detected by secondary neutrons in a neutron monitor). Notably the earlier McCracken cartoon called attention to the two alternatives, still much debated: do these highest-energy particles come from some part of the flare itself, or from the interplanetary shock wave? There has been progress in this field, as can be noted from the very early Meyer cartoon which so-innocently described a "field free region" in the solar corona. We've learned better since!