Time Variations of Solar Energetic Particle Abundances Observed by the ACE Spacecraft
T.T. von Rosenvinge1, C.M.S. Cohen2, E.R. Christian1, A.C. Cummings2, R.A. Leske2, 
R.A. Mewaldt2, P.L. Slocum3, E.C. Stone2, and M.E. Wiedenbeck3

1 NASA/Goddard Space Flight Center, Greenbelt, MD  20771
2 California Institute of Technology, Pasadena, CA  91125 
3 Jet Propulsion Laboratory, Pasadena, CA  91109

The abundances of elements from Helium to Iron have now been observed in 18 different solar energetic particle (SEP) events using the Solar Isotope Spectrometer (SIS) on-board the Advanced Composition Explorer (ACE). SIS has a large geometry factor (~ 40 cm2-sr), enabling us to observe abundances on a time scale of hours. In this paper we report on substantial temporal variations of the observed abundances within events and from event to event. We wish to understand these results in terms of acceleration and transport processes, taking into account factors such as first ionization potential, particle charge states, spectral shape, event type (gradual and impulsive), and the event's initial source location at the sun. It is essential that these variations be understood if we are to reliably estimate the composition of the sun from direct observations of SEP abundances.