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

We investigated 201 3 He-rich ( 3 He/ 4 He > 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 3 He fluences varied only by a factor of 100 above our instrument sensitivity threshold, while the 4 He fluences varied by factor of 10,000 above the same threshold. Moreover, there appears to be no significant correlation between the 3 He and 4 He fluences. We find it striking that with more than 6 years of continuous SEP data, we could not find any SEP event that has 3 He fluence higher than 2.0 × 10 5 particles (cm 2 sr MeV nucleon -1 ) -1 , while the largest 4 He fluence observed was 7.0 × 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 3 He 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.