Dependence of Solar Energetic Particle Abundances on First Ionization Potential

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More than two decades ago it was discovered that the abundances of elements with first ionization potential (FIP) > 10 eV are depleted in solar energetic paricles (SEPs) by a factor of ~4 with respect to photospheric abundances. This led to the discovery that the solar corona is depleted in high-FIP elements when compared to the photosphere. It is not generally known that this FIP-depletion factor varies from one SEP event to another. Variations in the elemental composition of solar energetic particles (SEPs) are usually ascribed to acceleration and transport processes that depend on the charge-to-mass (Q/M) ratio of the particles. However, in 1994, Garrard and Stone presented evidence that the degree of FIP-fractionation in SEPs varies by a factor of ~2 from event to event, based mainly on a study of events observed by Voyager and IMP-8 in the 1970s and 1980's. A variable degree of FIP-fractionation is also observed in the solar wind and in spectroscopic studies of coronal abundances. We re-examine the evidence for event-to-event variations in the FIP-fractionation of solar particles by combining data from the 1970's and 1980's 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 the solar wind and other samples of solar system material. This work was supported by NASA under grant NAG5-6912.

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