R.A. Mewaldt, C.M.S. Cohen, R.A. Leske, A.C. Cummings and E.C. Stone Space Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125 P.L. Slocum and M.E. Wiedenbeck Jet Propulsion Laboratory, Pasadena, CA 91109 E.R. Christian and T.T. von Rosenvinge NASA/Goddard Space Flight Center, Greenbelt, MD 20771
Variations in the elemental composition of solar energetic particles (SEPs) from event to event are usually ascribed to acceleration and transport processes that depend on the charge-to-mass (Q/M) ratio of the particles. In addition, it is well known that the abundance of elements with first ionization potential (FIP) > 10 eV is depleted in SEPs by a factor of ~4 with respect to photospheric abundances. Indeed, it was this observation that led to the realization that the composition of the solar corona is depleted in high-FIP elements when compared to the photosphere.
In a 1994 paper, Garrard and Stone presented evidence that the degree of FIP-fractionation in SEPs varies by a factor of 50% from event to event, based mainly on a study of events observed by Voyager and IMP-8 in the 1970s. It has also been observed that there is a variable degree of FIP-fractionation in the solar wind and in spectroscopic studies of coronal abundances.
In this paper we re-examine the evidence for event-to-event variations in the FIP-fractionation of solar particles by combining data from the 1970s with new observations from the Solar Isotope Spectrometer on ACE. We fit the observed abundances in a given event with a combination of FIP and Q/M dependent fractionation, and compare the results with evidence for FIP fractionation effects in other samples of solar system material.
This work was supported by NASA under grant NAG5-6912.