The Isotopic Composition of Solar Energetic Particles
C.M.S. Cohen, A.C. Cummings, R.A. Leske, R.A. Mewaldt, E.C. Stone
California Institute of Technology
P.L. Slocum, M.E. Wiedenbeck
Jet Propulsion Laboratory
E.R. Christian, T.T. von Rosenvinge
NASA/Goddard Space Flight Center
Since the launch of ACE in August 1997, the Solar Isotope Spectrometer
(SIS) has observed 11 large solar events in which elemental and
isotopic composition was determined over a large energy range. The
composition of these events has raised many issues and challenged
generally accepted characterizations of solar energetic particle (SEP)
events. In particular 3He/4He enhancements have been observed in
several large events as well as enhancements of heavy ions typically
associated with smaller impulsive events. The isotopic composition
varies substantially (a factor of 3 for 22Ne/20Ne) from event to event
with enhancements that are generally well correlated with elemental
enhancements. This correlation suggests the isotopic enhancements may
be related to the Q/M fractionation typically evident in elemental
composition of SEP events. However, significant deviations may imply
other processes, such as wave-particle resonance or mass
fractionation, may be involved. We will review the recent isotopic
observations made with ACE and discuss possible explanations and their
implications for particle acceleration and transport.