Donald V. Reames
IPST, University of Maryland
Element Abundances in Solar Energetic Particles:Two Physical Processes, Two Abundance Patterns
Abundances of elements in solar energetic particles (SEPs) come in two flavors, historically called “impulsive” and “gradual” events, now observed for nearly 20 years by the Wind spacecraft. Gradual SEP events are produced when shock waves, driven by coronal mass ejections (CMEs), accelerate the ambient coronal plasma yielding ions of 1–10 MeV/amu where measured average abundances of 21 elements differ from the solar photospheric abundances by a well-known dependence on the first ionization potential (FIP) of the element. The abundances of heavier elements, near Fe, are fractionated by differential scattering during transport, through intense proton-generated Alfvén waves, that varies with the mass-to-charge ratio A/Q of the ion. This process makes Fe/O, for example, vary in space and time, but averaging over the spatial distribution using many SEP events can recover the FIP dependence of the coronal abundances.
The smaller impulsive (or “3He-rich”) SEP events are associated with magnetic reconnection involving open field lines in solar flares or jets that also eject plasma to produce CMEs recently found associated with at least ~70% of these SEP events. These events produce striking heavy-element abundance enhancements relative to coronal abundances, with a strong, power-law dependence on A/Q with a ~3.6 power, on average, with Q values determined at coronal temperatures of 2.5–3.2 MK. This produces average enhancements by a factor of 3 at Ne, 9 at Fe, and 900 for elements with 76 ≤ Z ≤ 82. Small SEP events with the steepest enhancements (~6 power) from ~2.5 MK plasma, are associated with B- and C-class X-ray flares, and with narrow CMEs. In contrast, larger events have flatter spectra, more modest enhancements (~3 power), tend to come from ~3.2 MK plasma and to associate with M- and even X-class flares. The 1000-fold enhancements of heavy elements in some events are uncorrelated with the 1000-fold enhancements in 3He / 4He in others, suggesting that two different acceleration mechanisms contribute to each event.