Enhanced Abundances of ^3He in Large Solar Energetic Particle Events

Abstract

Observations of a number of relatively large solar energetic particle (SEP) events that have occurred since the launch of ACE in August 1997 have shown that the ratio of ^3He/^4He can be enhanced over the solar wind value (~4×10^(–4)) by more than an order of magnitude in such events. Since particle acceleration in these "gradual" SEP events is thought to be caused by CME-driven shocks traveling through the solar corona and interplanetary medium, a source of ^3He in addition to the solar wind appears required to provide the seed material. Using data from the Solar Isotope Spectrometer on ACE, we have carried out a more detailed investigation of the characteristics of the ^3He enhancements at energies >5 MeV/nucleon in three large SEP events (4 Nov 1997, 6 May 1998, and 14 Nov 1998). We find that the ^3He/^4He ratios are essentially time-independent during the events, that the ^3He energy spectra are markedly harder than those commonly observed in impulsive events, and that the spectra of ^3He may be harder than those of ^4He.