Four Distinct Pathways to the Element Abundances in Solar Energetic Particles

Donald V. Reames

Published 2019-12-13Version 1

Recent evidence from abundance patterns of chemical elements in solar energetic particles (SEPs), and, ironically, the belated inclusion of H and He, has provided evidence for four distinct physical pathways of SEP acceleration. Abundance measurements divide each of the previous categories of impulsive and gradual SEP events, based upon the presence or absence of shock acceleration, or upon the dominance of either preaccelerated ions or ambient coronal plasma. SEPs are drawn from plasma that originally entered the solar corona on magnetic loops in active regions where, relative to the photosphere, abundances of elements depends upon the first ionization potential (FIP) so that SEPs differ from the solar wind which rises on open magnetic fields. After FIP fractionation, acceleration of impulsive-source SEP1 ions, at islands of magnetic reconnection in solar jets, generates an additional steep power-law dependence of abundance enhancements vs. the ion mass-to-charge ratio A/Q, with Q-values determined by a source temperature of T~3 MK. For these SEP1 ions, proton abundances at A/Q=1 fall on the power-law-fit line from elements Z=6-56. More acceleration of SEP1 ions by shock waves can add protons from the ambient plasma to supply "excess protons" above the fit line, a new signature of shock acceleration, either locally in the same jet to define SEP2 ions or at an extensive shock averaging residual ions from a multi-jet active region to defining SEP3. Larger gradual SEP events, with stronger shocks, sample more deeply the ambient coronal material with T =0.8-1.6 MK, and, for these SEP4 events, proton abundances can again fit the power-law extrapolated from the fit of Z>2 ions. Particle acceleration and transport with simple power-law dependences on velocity and A/Q has provided an unexpected but powerful new tool with signatures of the dominant physical processes involved.