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

TL;DR

This study reveals a new pattern in hydrogen abundance in gradual solar energetic-particle events, showing how it correlates with element charge states and shock wave reacceleration, differing from prior assumptions.

Contribution

It uncovers a novel behavior of hydrogen abundances in SEP events, especially in cases involving shock reacceleration and higher plasma temperatures.

Findings

Hydrogen abundance patterns vary with event type and plasma temperature.

Proton abundances often exceed predictions in certain SEP events with T > 2 MK.

Element abundances with Z >= 6 can predict hydrogen abundance in most cases.

Abstract

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 2 <= Z <= 56, relative to coronal abundances, show a power-law dependence, versus their average mass-to-charge ratio A/Q, that 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 < 2 MK 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 Z >= 6 at A/Q > 2 down to hydrogen at A/Q = 1. Thus the abundances of the elements with Z >= 6 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 > 2 MK 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.