Upper limit on 3He fluence in solar energetic particle events

We investigated 195 ^3He -rich (^3He/^4He > 0.004 at 0.2-2.0 MeV/nucleon) solar energetic particle (SEP) events from September 1997 through December 2003 using the ULEIS instrument on ACE. Both impulsive (flare-related) and gradual (CME-related) events are included. The ^3He fluences varied only by a factor of 100 above our sensitivity threshold while the ^4He fluences varied by factor of 100,000 above the same threshold. Moreover, we find 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 found any SEP event that has a ^3He fluence higher than 10^5/cm^2-sr-MeV/nucleon, while the largest ^4He fluence observed was 3.0×10^7/cm^2-sr-MeV/nucleon (two orders of magnitude larger than the ^3He upper limit). Since the event fluence should be same fraction of the particle net flux from the Sun at 1 AU, the observed upper limit for ^3He fluence indicates that there is a limit to the number of energetic ^3He ions that can be released from the Sun in a SEP event.