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

The WIND/EPACT experiment offers a similar to 100 fold increase in collecti ng power over instruments flown in previous solar cycles, thus allowing unp recedented detailed studies of temporal evolution in gradual solar energeti c particle (SEP) events. We present hourly WIND/EPACT observations at simil ar to 2-10 MeV/nuc from the 20 April 1998 and 26 August 1998 SEP events. Th ese observations show striking patterns in elemental composition which evol ve in a systematic fashion throughout the events' several-day durations. Th ese data, combined with theoretical modeling in a companion Letter [Ng et a l. 1999], suggest that a dynamic Alfven. wave field, generated primarily by streaming energetic protons, is responsible for the complex behavior which is observed.