  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.
