Hydrogen and the Abundances of Elements in Gradual Solar Energetic-Particle Events

Despite its dominance, hydrogen has been largely ignored in studies of the abundance patterns of the chemical elements in gradual solar energetic particle (SEP) events; those neglected abundances show a surprising new pattern of behavior. Abundance enhancements of elements with 2Z56, relative to coronal abundances, show a power-law dependence versus their average mass-to-charge ratio A/Q, which varies from event to event and with time during events. The ion charge states Q depend upon the source plasma temperature T. For most gradual SEP events, shock waves have accelerated ambient coronal material with T<2MK with decreasing power laws in A/Q. In this case, the proton abundances agree rather well with the power-law fits extrapolated from elements with Z6 at A/Q>2 down to hydrogen at A/Q=1. Thus the abundances of the elements with Z6 fairly accurately predict the observed abundance of H, at a similar velocity, in most SEP events. However, for those gradual SEP events where ion enhancements follow positive powers of A/Q, especially those with T>2MK where shock waves have reaccelerated residual suprathermal ions from previous impulsive SEP events, proton abundances commonly exceed the extrapolated expectation, usually by a factor of order ten. This is a new and unexpected pattern of behavior that is unique to the abundances of protons and may be related to the need for more streaming protons to produce sufficient waves for scattering and acceleration of more heavy ions at the shock.