University of Tasmania
Classes of SEP Events
It is well established that solar energetic particles (SEPs) are accelerated in two separate ways, in flaring regions and at coronal mass ejection (CME) shocks in the corona and interplanetary medium. It is often assumed that this leads to two well separated classes of SEP events, so-called `impulsive' and `gradual', and that flares make no contribution in the larger (gradual) events. This talk examines the characteristics of various groups of particle events and of the associated flares and coronal mass ejections (CMEs). Since all gradual events have flares and the more energetic impulsive events have CMEs it is clear that the presence/absence of either phenomenon does not separate classes of events. The data appear consistent with the idea that gradual events are ones in which the CME drives a shock that produces additional acceleration of the flare particles; these are likely to be produced in a lower density environment than is the case for impulsive events. In addition to a dependence on flare intensity, flare duration and shock speed (when present), abundance variations depend on how well the observer is connected to the flare site. Connectivity is not only important for determining the relative contribution of particles accelerated close to the Sun versus those accelerated in the interplanetary medium but whether immediate flare particles are detected or ones that might have traveled across the low corona. These, and other, variations are reasons why lists of SEP events based on different species and different energy ranges show limited overlap.