Measurements of $^{3}$He in Large Solar Energetic
Particle Events
M. E. Wiedenbeck$^{1}$, E. R. Christian$^{2}$, C. M. S. Cohen$^{3}$,
A. C. Cummings$^{3}$, R. A. Leske$^{3}$, R. A. Mewaldt$^{3}$,
P. L. Slocum$^{1}$, E. C. Stone$^{3}$, and T. T. von Rosenvinge$^{2}$
$^{1}$ 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 $^{3}He$ 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 $^{3}$He/$^{4}$He
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 $^{3}$He by previous small impulsive events.
We report additional observations of the isotopic composition of
helium above $\sim$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.