New Properties of [TSUP]3[/TSUP]H[CLC]e[/CLC]-rich Solar Flares Deduced from Low-Energy Particle Spectra

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Abstract
Using advanced instrumentation on the Advanced Composition Explorer spacecraft, we have surveyed a largely unexplored spectral region of impulsive solar particle events, namely the energy range from 50 keV nucleon-1 to ~2 MeV nucleon-1, over the period of 1997 November through 2000 May. Particular attention was paid to select only events that were observed with minimal interruptions or dropouts due to a varying magnetic connection to the flare site. Twelve impulsive events met the selection criterion, and all showed large enrichments of the rare isotope 3He and more modest enrichments of heavy ions such as Fe. Spectral forms for the events fell into two classes. Class 1 showed power-law or steeper spectra that were similar for all elements. Class 2 showed 3He and Fe spectra that were distinctly different from the spectra of other species such as 4He and O: the 3He spectra had peaks in the range of ~100-400 keV nucleon-1. In class 2 events, the Fe spectrum also showed a peak in the range of 100 keV nucleon-1 or below, and in general for this class, the spectra for different species appeared to be better organized by magnetic rigidity than by kinetic energy per nucleon. Class 2 events also had larger 3He enrichments, including what may be the largest 3He/4He ratio ever observed: 33.4 ± 5.2 near 385 keV nucleon-1 in the event of 2000 January 6. We conclude that the class 2 spectra and heavy ion abundances show that the 3He enrichment in these flares is unique to that species and that the impulsive flare 3He acceleration mechanism peaks out in the range of a few hundred keV per nucleon.