Don Reames
IPST, University of Maryland, College Park
Taking the Temperature of Solar Energetic Particles
It has been known for over 30 years that element abundances in large solar energetic particle (SEP) events consist of two factors: 1) average abundances that differ from solar photospheric abundance by a factor that depends upon the first ionization potential (FIP) of the element, and 2) event-to-event variations and time dependences that vary as a power of the mass-to-charge ratio, A/Q, of the elements. The FIP effect is a property of the common coronal origin of the particles, but the A/Q dependence can be a measure of source plasma temperature T since the pattern of ionization values, Q, depends upon T. In the large gradual SEP events, particles are accelerated by shock waves driven out from the Sun by coronal mass ejections (CMEs). As they propagate away from the shock, scattering by magnetic fluctuations depends upon a power of A/Q so that Fe scatters less than C or O, producing Fe-rich abundances early and Fe-depleted regions behind. In the small impulsive SEP events, A/Q-dependent enhancements occur in the regions of magnetic reconnection where they are accelerated. We find that ions in impulsive SEP events come from solar active regions with T= 2 - 4 MK. In the large gradual events, 24% of events also come from T= 2 - 4 MK, but 69% come from T= 0.8 - 1.6 MK typical of the ambient corona outside active regions.