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Observations of systematic temporal evolution in elemental composition during gradual solar energetic particle events

The WIND/EPACT experiment offers a ∼100 fold increase in collecting power over instruments flown in previous solar cycles, thus allowing unprecedented detailed studies of temporal evolution in gradual solar energetic particle (SEP) events. We present hourly WIND/EPACT observations at ∼2–10 MeV/nuc from the 20 April 1998 and 26 August 1998 SEP events. These observations show striking patterns in elemental composition which evolve in a systematic fashion throughout the events' several‐day durations. These data, combined with theoretical modeling in a companion Letter [Ng et al. 1999], suggest that a dynamic Alfvén wave field, generated primarily by streaming energetic protons, is responsible for the complex behavior which is observed.