M. E. Wiedenbeck1, E. R. Christian2, C. M. S. Cohen3, A. C. Cummings3, R. A. Leske3, 
R. A. Mewaldt3, P. L. Slocum1, E. C. Stone3, and T. T. von Rosenvinge2

1 Jet Propulsion Laboratory
2 NASA / Goddard Space Flight Center
3 California Institute of Technology

Observations of a set of relatively large solar energetic particle (SEP) events over the past two years have shown that previous schemes for classifying SEP events are deficient. ACE is providing measurements of ³He content, heavy element abundances, charge state distributions, time-intensity profiles, and energy spectra over a broad range of energies in SEP events. From this wealth of detail it is evident that events are not confined to two distinct, mutually-exclusive categories: "gradual" events in which particles are thought to be accelerated by CME-driven interplanetary shocks and "impulsive" events in which acceleration is believed to occur at the site of a solar flare. In particular, a number of the larger events with gradual-event characteristics are found to have ³He/⁴He ratios well in excess of the solar wind value, which would suggest flare-associated acceleration. A recent proposal by Mason et al. (1999) attempts to explain these characteristics in terms of shock acceleration of a population of interplanetary ions that has been seeded with ³He by previous small impulsive events.

We report additional observations of the isotopic composition of helium above ~5 MeV/nuc made using the ACE Solar Isotope Spectrometer (SIS) in the larger SEP events the have occurred since the ACE launch. We use these data to further characterize the properties of "mixed"-type SEP events.
This research was supported by NASA at Caltech (under grant NAG5-6912), JPL, and GSFC.