Using SEP Elemental Abundance Measurements to Infer Coronal Composition

The composition of solar energetic particle (SEP) events has been observed to be quite variable. At energies above 25 MeV/nucleon the event-averaged Fe/O ratio may differ by 2 orders of magnitude from one event to the next. It is necessary to understand such variability to extract the composition of initial seed population which presumably originates in the solar corona. With the high statistical accuracy of today's SEP composition measurements, the elemental composition can be studied both as a function of time and energy within a given event for 13 or more elements from carbon to nickel. We present the observed elemental composition (above 12 MeV/nucleon) of more than 50 SEP events as measured by the Solar Isotope Spectrometer on the ACE spacecraft. Accounting for different fractionation mechanisms believed to occur during the acceleration and/or transport of the ions, we obtain the seed populations and compare them to solar wind and coronal abundances. This work was supported by NASA at Caltech (under grants NAG5-6912 and NAG5-12929), JPL, and GSFC.