Donald V Reames

Solar energetic particles (SEPs) are produced in two fundamental ways: at magnetic reconnection sites in solar jets and at collisionless shock waves driven by fast coronal mass ejections (CMEs). "Impulsive" SEP events, on open field lines from jets, have signature abundance enhancements of 3He and of increasingly heavy elements, and their outward streaming electrons drive type-III radio bursts. Similar acceleration for particles trapped on closed loops energizes solar flares. In contrast, fast, wide, CME-driven shocks accelerate seed ions from the ambient corona that grow resonant Alfven waves as they stream outward. These waves can scatter and trap lower-rigidity ions near the shock, limiting outflow, and flattening low-energy spectra upstream at the "streaming limit." Downstream, a spatially-uniform "reservoir" of SEPs is shed by the expanding shock between it and the Sun. These trapped invariant SEP spectra decrease in intensity adiabatically as the volume of the reservoir expands. Ions from the reservoir can seed further acceleration in multi-shock event and energetic proton precipitation can prolong solar gamma-ray emission. Shocks are often additionally seeded by residual impulsive ions which dominate the SEP heavy-ion abundances with their signature enhancements. As samples of the corona, SEP abundances also probe differences with the photosphere that depend upon the first ionization potential (FIP) of the elements.

Solar 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 Alfven waves that preferentially scatter and retard protons near the shock while increasingly-heavy ions can leak out, and (2) strong shock waves re-accelerate 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 vs. 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. 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. 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 Alfven-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.

Acceleration and transport of solar energetic particles (SEPs) causes their abundances, measured at constant velocity, to be enhanced or suppressed as a function of each ion's magnetic rigidity, and hence its atomic mass-to-charge ratio A/Q. Ion charges, in turn, depend upon source electron temperature. In small "impulsive" SEP events, arising from solar jets, acceleration during magnetic reconnection causes steep power-law abundance enhancements. These impulsive SEP events can have 1000-fold enhancements of heavy elements from sources at ~2.5 MK, and similar enhancements of 3He/4He and of streaming electrons that drive type-III radio bursts. Gamma-ray lines show that solar flares also accelerate 3He-rich ions, but their electrons and ions remain trapped on magnetic loops so they dissipate their energy as X-rays, gamma-rays, heat, and light. "Gradual" SEPs accelerated at shock waves, driven by fast coronal mass ejections (CMEs), can show power-law abundance enhancements or depressions, even with see ions from the ambient solar corona. In addition, shocks can reaccelerate seed particles from residual impulsive SEPs with their pre-existing signature heavy-ion enhancements. Different patterns of abundance often show that heavy elements are dominated by a different source from that of H and He. Nevertheless, the SEP abundances averaged over many large events define the abundances of the corona itself, which is found to differ from the solar photosphere as a function of the first ionization potential (FIP) since ions, with FIP < 10 eV, are driven upward by forces of electromagnetic waves which neutral atoms, with FIP > 10 eV, cannot feel. Thus, SEPs provide a measurement of element abundances in the solar corona, distinct from the solar wind, and may even better define the photosphere for some elements.