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.