THE UPPER LIMIT ON ~3He FLUENCE IN SOLAR ENERGETIC PARTICLE EVENTS

摘要

We investigated 201 ~3He-rich (~3He/~4He > 0.004 at 0.2-2.0 MeV nucleon~(-1)) solar energetic particle (SEP) events from 1997 September through 2003 December using the Ultra Low Energy Isotope Spectrometer on the A dvanced Composition E xplorer. Both "impulsive" (flare-related) and "gradual" (CME-related) events are included. The ~3He fluences varied only by a factor of 100 above our instrument sensitivity threshold, while the ~4He fluences varied by factor of 10,000 above the same threshold. Moreover, there appears to be no significant correlation between the ~3He and ~4He fluences. We find it striking that with more than 6 years of continuous SEP data, we could not find any SEP event that has ~3He fluence higher than 2.0 x 10~5 particles (cm~2 sr MeV nucleon~(-1))~(-1), while the largest ~4He fluence observed was 7.0 x 10~7 particles (cm~2 sr MeV nucleon~(-1))~(-1). To the approximation that the event fluence is to first order proportional the number of particles released from the Sun, the observed upper limit for the ~3He fluence seems to indicate that only a limited number of 0.2-2 MeV nucleon~(-1) ~3He can be released from the Sun in an SEP event.