Gradual sep events

Abstract

Gradual solar energetic-particle (SEP) events are the “big proton events” of the past and are usually much more “gradual” in their decay than in their onset. As their intensities increase, particles streaming away from the shock can amplify Alfvén waves that scatter subsequent particles, eventually limiting their flow at the “streaming limit.” Waves generated by higher-speed protons running ahead can also throttle the flow of lower-energy ions, flattening spectra and altering abundances in the biggest SEP events. Yet, we find that the A/Q-dependent scattering causes abundance patterns, varying in space and time, which determine source-plasma temperatures, since the pattern of Q values of the ions depends upon temperature. Different source-plasma temperatures explain much of the variation in element abundances in gradual SEP events. In nearly 70% of gradual events, SEPs are shock-accelerated from ambient coronal plasma of ~0.8–1.6MK, while 24%of the events involve accelerated material from active-region temperatures of 2–4 MK and include residual impulsive-suprathermal ions with pre-enhanced abundances. Non-thermal variations of the ions in gradual SEP events from 2–4 MK source plasma are greatly reduced, relative to those in impulsive SEPs, from similar plasma, probably because the accelerating shock waves average over impulsive-suprathermal ions from multiple jet sources. Later, SEPs become trapped in a reservoir behind the CME in gradual events, where spectra are uniform in space and decrease adiabatically in time as the magnetic bottle containing them slowly expands.