  From a turbulent history, the study of abundances of elements in solar
energetic particles (SEPs) has grown into an extensive field that probes the
solar corona and the physical processes of SEP acceleration and transport.
Underlying SEPs are the abundances of the solar corona, which differ from
photospheric abundances as a function of the first ionization potentials (FIPs)
of the elements. The FIP-dependence of SEPs also differs from that of the solar
wind; each has a different magnetic environment where low-FIP ions and high-FIP
neutral atoms rise toward the corona. Two major sources generate SEPs: The
small "impulsive" SEP events are associated with magnetic reconnection in solar
jets that produce 1000-fold enhancements from H to Pb as a function of
mass-to-charge ratio A/Q, and also 1000-fold enhancements in 3He/4He produced
by resonant wave-particle interactions. In large "gradual" events, SEPs are
accelerated at shock waves driven out from the Sun by wide, fast coronal mass
ejections (CMEs). A/Q dependence of ion transport allows us to estimate Q and
hence the source plasma temperature T. Weaker shock waves favor reacceleration
of suprathermal ions accumulated from earlier impulsive SEP events, along with
protons from the ambient plasma. In strong shocks the ambient plasma dominates.
Ions from impulsive sources have T ~ 3 MK; those from ambient coronal plasma
have T = 1 - 2 MK. These FIP- and A/Q-dependences explore complex new
interactions in the corona and in SEP sources.
