  Recent studies of the abundances of H and He relative to those of heavier
ions in solar energetic particle (SEP) events suggest new features in the
underlying physics. Impulsive SEP events, defined by uniquely large
enhancements of Fe/O, emerge from magnetic reconnection in solar jets. In
small, "pure," shock-free, impulsive SEP events, protons with mass-to-charge
ratio A/Q = 1 fit the power-law dependence of element abundance enhancements
versus A/Q extrapolated from the heavier elements 2 < Z < 57. Sometimes these
events have order-of-magnitude suppressions of He, even though H fits with
heavier elements, perhaps because of the slower ionization of He during a rapid
rise of plasma from the chromosphere. In larger impulsive SEP events, He fits,
but there are large proton excesses relative to the power-law fit of Z > 2
ions, probably because associated coronal mass ejections (CMEs) drive shock
waves fast enough to reaccelerate the impulsive SEPs but also to sample protons
from the ambient solar plasma. In contrast, gradual SEP events are accelerated
by wide, fast CME-driven shock waves, but those with smaller, weaker shocks,
perhaps quasi-perpendicular, favor impulsive suprathermal residue left by many
previous jets, again supplemented with excess protons from ambient coronal
plasma. In the larger, more common gradual SEP events, faster, stronger shock
waves sample the ambient coronal plasma more deeply, overwhelming any
impulsive-ion component, so that proton abundances again fit the same power-law
distribution as all other elements. Thus, studies of the power-law behavior in
A/Q of SEP element abundances give compelling new information on the varying
physics of SEP acceleration and properties of the underlying corona.
