HEAVY-ION FRACTIONATION IN THE IMPULSIVE SOLAR ENERGETIC PARTICLE EVENT OF 2002 AUGUST 20: ELEMENTS, ISOTOPES, AND INFERRED CHARGE STATES
- Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States)
- California Institute of Technology, Pasadena, CA 91125 (United States)
Measurements of heavy-ion elemental and isotopic composition in the energy range {approx}12-60 MeV nucleon{sup -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 {sub 2}) is enhanced relative to that of a lighter isotope (M{sub 1}) by a factor {approx}(M {sub 1}/M {sub 2}){sup {alpha}} with {alpha} {approx_equal} -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{sub C} = 6), we infer mean values of the ionic charge during the fractionation process, Q{sub Z} , for a variety of elements with atomic numbers 7 {<=} Z {<=} 28. We find that Q{sub Fe} {approx_equal} 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{sub 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.
- OSTI ID:
- 21457103
- Journal Information:
- Astrophysical Journal, Vol. 719, Issue 2; Other Information: DOI: 10.1088/0004-637X/719/2/1212; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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