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Published August 20, 2010 | Published
Journal Article Open

Heavy-ion Fractionation in the Impulsive Solar Energetic Particle Event of 2002 August 20: Elements, Isotopes, and Inferred Charge States


Measurements of heavy-ion elemental and isotopic composition in the energy range ~12-60 MeV nucleon^(–1) are reported from the Advanced Composition Explorer/Solar Isotope Spectrometer (ACE/SIS) instrument for the solar energetic particle (SEP) event of 2002 August 20. We investigate fractionation in this particularly intense impulsive event by examining the enhancements of elemental and isotopic abundance ratios relative to corresponding values in the solar wind. The elemental enhancement pattern is similar to those in other impulsive events detected by ACE/SIS and in compilations of average impulsive-event composition. For individual elements, the abundance of a heavy isotope (mass M_2) is enhanced relative to that of a lighter isotope (M_1) by a factor ~(M_(1)/M_2)^α with α ≃ 15. Previous studies have reported elemental abundance enhancements organized as a power law in Q/M, the ratio of estimated ionic charge to mass in the material being fractionated. We consider the possibility that a fractionation law of this form could be responsible for the isotopic fractionation as a power law in the mass ratio and then explore the implications it would have for the ionic charge states in the source material. Assuming that carbon is fully stripped (Q_C = 6), we infer mean values of the ionic charge during the fractionation process, Q_Z , for a variety of elements with atomic numbers 7 ≤ Z ≤ 28. We find that Q_(Fe) ≃ 21-22, comparable to the highest observed values that have been reported at lower energies in impulsive SEP events from direct measurements near 1 AU. The inferred charge states as a function of Z are characterized by several step increases in the number of attached electrons, Z – Q_Z . We discuss how this step structure, together with the known masses of the elements, might account for a variety of features in the observed pattern of elemental abundance enhancements. We also briefly consider alternative fractionation laws and the relationship between the charge states we infer in the source material and those derived from in situ observations.

Additional Information

© 2010 American Astronomical Society. Received 2009 November 16; accepted 2010 June 28; published 2010 July 27. We are grateful to J. Mazur for providing charge state estimates from SAMPEX/LICA, S. Krucker for providing information about the RHESSI X-ray imaging measurements in the 2002 August 20 event, A. Labrador for providing some unpublished charge-state data included in Figure 8, Y.-K. Ko for advice about the comparison with solar wind charge states, and G. Mason for providing the ULEIS data shown in Figure 12. In addition, we acknowledge useful discussions with E. Möbius, V. Petrosian, and J. Drake. This study also benefited from discussions at international team meetings sponsored by the International Space Science Institute (ISSI) in 2006 and 2007 and at the NSF-sponsored SHINE workshop in 2008. We thank E. Chollet and the anonymous referee for helpful comments on the manuscript. We gratefully acknowledge the ACE/SWICS team for providing solar wind charge state data used in this study through the ACE Science Center Web site. This research was supported by NASA at Caltech (under grants NAG5-12929 and NNX08AI11G), JPL, and GSFC.

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