A search for ³He enhancements in SEP and CIR-associated events with Ulysses/HI-scale.

Abstract

We identify 17 solar energetic particle (SEP) events with enhanced abundances of ³He relative to ⁴He detected by the Heliosphere Instrument for Spectra, Composition, and Anisotropy at Low Energies (HI-SCALE) on the Ulysses spacecraft. These events were all detected while Ulysses was within 5deg of the ecliptic plane, but at distances from the Sun of 1-5 AU. We compare these results to those obtained with the ONR-604 instrument on the Combined Release and Radiation Effects Satellite (CRRES) mission (Chen et al. 1995). Chen et al. found 13 events with enhancements of ³He above normal solar abundances at energies >50MeV/nucleon. The HI-SCALE instrument measures He ions with energies between 0.3 and 3.1MeV/nucleon. We identify flux increases in the HI-SCALE data which correspond to 6 of the CRRES events, but we find an enhancement in ³He in only one of these events. We believe this single coincident detection to be a chance occurrence. We use these results, together with the multitude of previous results at different energies, to discuss the overall picture of ³He-rich events as a function of energy. We also analyze the data for ³He enhancements in high latitude co-rotating interaction region (CIR)-associated events observed while the spacecraft was in the southern solar hemisphere above the streamer belt. We find, within the sensitivity of the HI-SCALE instrument, that there is no ³He enhancement in these CIR-associated events. We then briefly discuss how this relates to particle acceleration mechanisms in solar flares.