The Upper Limit on 3He Fluence in Solar Energetic Particle Events

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 2.0×10⁵ particles (cm² sr MeV nucleon^-1)^-1, while the largest ⁴He fluence observed was 7.0×10⁷ particles (cm² 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 3}He can be released from the Sun in an SEP event.