Solar Energetic Particles: Spatial Extent and Implications of the H and He Abundances

TL;DR

This paper investigates the spatial extent and elemental abundance variations of solar energetic particles, emphasizing how shock acceleration, seed populations, and different solar events influence SEP composition and distribution.

Contribution

It provides new insights into how shock waves and seed populations affect SEP abundances, especially for helium isotopes, across different solar event types.

Findings

Large shock waves can reaccelerate impulsive ions, affecting SEP composition.

Differences in 4He/O ratios depend on seed populations and acceleration modes.

SEP events sampling multiple seed populations show higher 4He/O ratios.

Abstract

One of the earliest indicators of the importance of shock acceleration of solar energetic particles (SEPs) was the broad spatial extent of the "gradual" SEP events produced as the shock waves, driven by wide, fast coronal mass ejections (CMEs), expand across the Sun with cross-field transport mediated by the shocks. Contrasting "impulsive" SEP events, with characteristic enhancements of 3He and of heavy elements, are now associated with magnetic reconnection on open field lines in solar jets. However, large shock waves can also traverse pools of residual impulsive suprathermal ions and jets can produce fast CMEs that drive shock waves; in both cases shocks reaccelerate ions with the "impulsive" abundance signatures as well as coronal plasma. These more-complex events produce "excess protons" that identify this process, and recently, differences in the distribution of 4He abundances have also been found to depend upon the combination of seed population and acceleration mode. Extreme differences in the 4He abundances may reflect underlying differences in the abundances of the coronal regions being sampled by solar jets and, surprisingly, SEP events where shock waves sample two seed-particle populations seem to have about twice the 4He/O ratio of those with a single source.