The 3He/4He ratios for solar energetic particle events during the Combined Release and Radiation Effects Satellite mission
Helium data measured by the University of Chicago instrument, ONR-604, are employed to determine the ratio of 3He to 4He for solar energetic particle (SEP) events over an energy range 50-110 MeV nucleon-1 during the 1990/1991 Combined Release and Radiation Effects Satellite mission. The Sun in this period is extremely active. A total of 29 separate SEP events have been identified; among them 16 major events have been analyzed to obtain 3He/4He ratios, with a mass resolution of 0.10 amu. Thirteen events have a 3He/4He ratio larger than 0.005, one order of magnitude greater than the solar coronal value. The 3He/4He ratio at energies of 50-110 MeV nucleon-1 is independent of the size of the SEP event, for the moderately large flares analyzed here. The helium energy spectra are represented by power laws. During the 1991 June flare period, different large-particle injections associated with different solar flares, but occurring from the same active region, have a similar average spectral index and a similar 3He/4He ratio. The spectral index of 4He varies from event to event, i.e., from as small as 1.5 to as large as 7.5. A correlation is found between the inferred spectral index from gamma-ray measurements and our measured spectral indices for the 1991 June 11 and June 15 events, suggesting that the high-energy SEPs may come from the same acceleration event as the particles that interact at the Sun and produce the gamma rays. The implications of these results for particle acceleration and propagation at the flare site and in the solar corona are discussed.