Seeds and sequences of element abundances in solar energetic particle events

Abstract

AbstractSolar energetic particles (SEPs) in the small “impulsive” events, primarily accelerated during magnetic reconnection in solar jets, have strong enhancements of the abundances of increasingly heavy elements. In contrast, the shock acceleration of ambient coronal plasma in most large “gradual” SEP events produces flat or decreasing abundances vs. element mass-to-charge ratios A/Q. However, heavy-ion enhancements in the largest gradual SEP events can occur in two ways: (1) strong streaming of protons away from the shock amplifies Alfvén waves that preferentially scatter and retard protons near the shock while increasingly heavy ions can leak out, and (2) strong shock waves reaccelerate SEPs fed from persistent impulsive SEP events streaming from some active regions, with their pre-enhanced heavy ions becoming dominant. Power-law fits of abundance enhancements versus A/Q can distinguish the latter events by the presence of both impulsive and coronal seed components and the best-fit charges Q define characteristic source temperatures. Ironically, ions with high observed charges, e.g., QFe ≈ 20, are yet another signature of impulsive seed ions that are routinely stripped after initial acceleration. Intense impulsively seeded events can occur in sequences fed from a single persistent active region as it rotates across the disk of the Sun. Three-week-long event sequences, each producing two or three very large events, occur early in the strong solar-cycle 23 (1997 – 2008). The weak solar cycle 24 produces only one impulsively seeded event sequence—perhaps a dearth of both impulsive seeds and sufficiently strong shocks. Solar cycle 25 has produced an unusual active period of short strong impulsive events. In contrast, there are other active regions where large events alternate SEPs with and without impulsively seeded sources. We also find that events with moderate Alfvén-wave trapping near the shock can release ions slowly or rapidly as a function of A/Q. This A/Q-dependent trapping acts almost as a magnetic spectrometer that separates elements in space and time.