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Published September 1, 2003 | Published
Journal Article Open

Elemental Fractionation in Small Solar Energetic Particle Events


Using the Solar Isotope Spectrometer on the Advanced Composition Explorer (ACE), we have measured the 11.0-21.8 MeV nucleon^(-1) heavy (Z ≥ 6) element abundances of 39 small solar energetic particle (SEP) events that occurred between 1998 April 3 and 2002 February 26. Using He isotope data from the Ultra-Low-Energy Isotope Spectrometer on ACE, we have classified the events according to their 0.5-2.0 MeV nucleon^(-1) ^3He/^4He ratios. We find that their average heavy-element composition is similar to that of either large gradual events or ^3He-rich events, depending on their ^3He/^4He ratio. As seen in recent studies of small SEP events, we find that the heavy-element intensities relative to C increase with the ^3He/^4He ratios. The dependence of the heavy-element abundances on first ionization potential (FIP) has been derived, using a model consisting of a power law in Q/M (and Z) times a step function in FIP. We report the magnitude of the FIP factor in each event and find that it varies from ~2 to 7 with no clear dependence on the ^3He/^4He ratio.

Additional Information

© 2003 American Astronomical Society. Received 2003 January 13; accepted 2003 May 5. This research was supported by NASA at Caltech (under grant NAG5-6912), JPL, GSFC, The Aerospace Corporation (under NASA Cooperative Agreement 26979B), and the University of Maryland (under grant PC-251429). The authors thank R. E. Gold, D. K. Haggerty, and S. E. Hawkins for providing the electron and ion intensities, which were useful in this analysis. We thank C. W. Smith, H. Qiang, and R. M. Skoug for providing information about the interplanetary conditions relevant to this study. We are also grateful to H. V. Cane for helpful discussions related to this work.

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