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

TL;DR

This study investigates hydrogen's role in impulsive solar energetic particle events, revealing its variable abundance and suggesting shock acceleration as a key process, especially in larger events with high intensities.

Contribution

It is the first comprehensive analysis of hydrogen abundance in impulsive SEPs, highlighting its significance and the influence of shock waves and coronal mass ejections.

Findings

Hydrogen fits abundance patterns in 24% of smaller impulsive SEP events.

In large impulsive events, hydrogen can be 10 to 100 times expected levels.

Shock waves and coronal mass ejections significantly influence hydrogen abundance in impulsive SEPs.

Abstract

Hydrogen has been almost completely ignored in studies of the abundance patterns of the chemical elements in solar energetic particles (SEPs). We seek to find impulsive events where H fits these abundance patterns and document the events that do not, suggesting possible reasons for the disparity. For 24 % of the smaller impulsive SEP events, the relative abundance of H fits within one standard deviation of the power-law fit of the abundances of elements 6 <= Z <= 56, relative to coronal abundances. In impulsive events with high intensities, H can be 10 to 100 times its expected value. In a few of these larger events, increased scattering at high wavenumber may preferentially detain H, perhaps with self-amplified waves; in some events pre-event proton background may con-tribute. In most large impulsive SEP events, however, associated shock waves must play a much greater role than previously thought; fast (>500km/s) coronal mass ejections con-tribute to 62 % of impulsive events. Shocks may sample protons from the ambient coronal plasma or residual background as well as reaccelerating heavier impulsive SEP ions injected from the region of magnetic reconnection in solar jets. Excess H may be a signature of shock acceleration.