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The Astrophysical Journal


We have surveyed the ∼0.1-10 MeV nucleonˉ¹ abundances of heavy ions from ³He through Fe in 64 large solar energetic particle (LSEP) events observed on board the Advanced Composition Explorer from 1997 November through 2005 January. Our main results are (1) the 0.5-2.0 MeV nucleonˉ¹ ³He/⁴He ratio is enhanced between factors of ∼2-150 over the solar wind value in 29 (∼46%) events. (2) The Fe/O ratio in most LSEP events decreases with increasing energy up to ∼60 MeV nucleonˉ¹. (3) The Fe/O ratio is independent of CME speed, flare longitude, event size, the ³He/⁴He ratio, the pre-event Fe/O ratio, and solar activity. (4) The LSEP abundances exhibit unsystematic behavior as a function of M/Q ratio when compared with average solar wind values. (5) The survey-averaged abundances are enhanced with increasing M/Q ratio when compared with quiet coronal values and with average gradual SEP abundances obtained at 5-12 MeV nucleonˉ¹. (6) The event-to-event variations in LSEP events are remarkably similar to those seen in CME-driven IP shocks and in ³He-rich SEP events. The above results cannot be explained by simply invoking the current paradigm for large gradual SEP events, i.e., that CME-driven shocks accelerate a seed population dominated by ambient coronal or solar wind ions. Instead, we suggest that the systematic M/Q-dependent enhancements in LSEP events are an inherent property of a highly variable suprathermal seed population, most of which is accelerated by mechanisms that produce heavy-ion abundances similar to those observed in impulsive SEP events. This heavy-ion-enriched material is subsequently accelerated at CME-driven shocks near the Sun by processes in which ions with higher M/Q ratios are accelerated less efficiently, thus causing the Fe/O ratios to decrease with increasing energy



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