Temporal Variations in Solar Energetic Particle Events

Abstract

The Solar Isotope Spectrometer (SIS) on-board the Advanced Composition Explorer (ACE) spacecraft has measured elemental abundances in large solar energetic particle (SEP) events as a function of time for more than 25 such events. There are large temporal variations in the observed abundances within events, from event to event, and as a function of particle energy. Such variations have been previously attributed to the combination of an initial impulsive phase having enhanced heavy element abundances with a longer gradual phase with coronal abundances. More recently they have been attributed to rigidity dependent escape from CME-driven shocks through plasma waves generated by wave-particle interactions. Both these models can be expected to depend upon solar longitude since impulsive events are associated with longitudes well-connected magnetically to the observer, and shock properties and connection of the observer to the shock are also longitude dependent. We present evidence of longitude related variations. In addition, we show that there are events with little time variation and heavy element enhancements similar to those of impulsive events. These events appear to be difficult to explain in current models in which particles are accelerated with coronal abundances. Rather, these events seem to require abundance enhancements either in the source material or in the acceleration process.