ENERGETIC PARTICLE ABUNDANCES IN SOLAR ELECTRON EVENTS

We report on the results of a comprehensive search of the ISEE 3 energetic particle data for solar electron events with associated increases in elements with atomic number Z ≥ 6. A sample of 90 such events was obtained. The events support earlier evidence of a bimodal distribution in Fe/O or, more clearly, in Fe/C. Most of the electron events belong to the group that is Fe-rich in comparison with the coronal abundance. The Fe-rich events are frequently also 3He-rich and are associated with type IH and type V radio bursts and impulsive solar flares. Fe-poor events are associated with type IV bursts and with interplanetary shocks. Some large well-connected events show Fe-rich intensity profiles early in the event, followed almost immediately by Fe-poor material associated with the rapidly approaching shock. These large two-component events usually show radio signatures of multiple phases of acceleration. With some exceptions, event-to-event enhancements in the heavier elements vary smoothly with Z (or, probably, with the charge-to-mass ratio, Q/A) and with Fe/C. In fact these variations extend across the full range of events despite inferred differences in acceleration mechanism. The origin of source material in all events appears to be coronal and not photospheric. To the extent that the abundance variations are smooth with Z and with Fe/C, they argue against any first ionization potential-dependent fluctuations in the coronal abundances from one flare site to the next. The Ne/C ratio does show a discontinuity between the Fe-poor and the Fe-rich regimes that is consistent with an increase in the source abundance of Ne in impulsive flares or with a particularly large slope in Ne/C as a function of Fe/C.