Ion Abundances in Large Solar Energetic Particle Events

Large solar energetic particle (SEP) events are believed to occur as a result of solar wind ions being accelerated by coronal or interplanetary shocks driven by fast coronal mass ejections or CMEs. Although such events have been observed routinely over the last four decades, the origin of the seed population has remained highly controversial. Composition measurements in SEP events can provide vital clues regarding the seed population as well as provide insights into the physical processes that govern their acceleration and transport through the interplanetary medium. We will present results of a survey of the ~0.1-5.0 MeV/n He and heavy ion abundances measured during 64 large SEP events by the Ultra-Low Energy Isotope Spectrometer (ULEIS) on board the Advanced Composition Explorer (ACE) spacecraft from November 1997 through December 2004. In particular we investigate the event-to-event variability observed in the SEP ion abundances in terms of properties of the associated flare and CME. We also survey the ~1 MeV/n 3He/4He abundance ratio during the events and compare the heavy ion abundances with those measured in various particle populations such as the solar wind, and gradual and impulsive SEP events. We discuss our new results in terms of the origin of the seed population and current models of SEP acceleration and transport.