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The Astrophysical Journal


Since its 1997 August launch, the Ultra Low Energy Isotope Spectrometer (ULEIS) on board the Advanced Composition Explorer (ACE) has observed a large number of solar energetic particle (SEP) events in the 0.4-3 MeV nucleonˉ¹ range. These events show remarkable variability of both the elemental and isotopic compositions, with ²²Ne/²⁰Ne varying, from event to event, by up to a factor of 10, Fe/O by up to a factor of 100, and ³He/⁴He by up to a factor of 104. In this paper we present new measurements of the ²²Ne/²⁰Ne, ²⁶Mg/²⁴Mg, and ³He/⁴He ratios at ∼ 1 MeV nucleonˉ¹ and compare these ratios to the iron abundances measured during the same time periods. We find that the elemental abundances, Fe/O and Fe/Mg, and the isotopic abundances, ²²Ne/²⁰Ne and ²⁶Mg/²⁴Mg, are all well ordered by both the ³He/⁴He ratio and the average Fe charge state as measured by ACE/SEPICA. These variations in ²²Ne/²⁰Ne and ²⁶Mg/²⁴Mg are much larger than expected from the charge to mass dependence derived for the elemental abundances, and we suggest that the large fluctuations seen for the neon and magnesium isotopes are related to the amount of impulsive flare material present.



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