Magnetic Topology of Impulsive and Gradual Solar Energetic Particle Events

We examine the magnetic topology at the Sun that leads to the two classes of impulsive and gradual solar energetic particle (SEP) events so as to address new observations that seem to blur this classification, for example, that coronal mass ejections (CMEs) can accompany events of both classes. In our model, the unusual element abundances in impulsive SEP events result from resonant stochastic acceleration in magnetic reconnection regions that incorporate open magnetic field lines, allowing both accelerated ions and ejected plasma to escape. In the large gradual events that produce classic CMEs, reconnection occurs on closed field lines beneath the CME where the accelerated particles are trapped so they plunge into the solar atmosphere to produce a flare; they cannot escape. The SEPs seen at 1 AU in these large gradual events are accelerated by the shock wave driven outward by the CME. The shock-accelerated particles are derived from the local plasma and from reaccelerated suprathermal ions from previous impulsive or gradual SEP events.