The Origin of Element Abundance Variations in Solar Energetic Particles

TL;DR

This paper investigates how element abundance variations in solar energetic particles relate to source plasma temperature and transport processes, revealing differences between impulsive and gradual events and proposing a revised He/O reference ratio.

Contribution

It introduces a method to determine source plasma temperature from abundance enhancement patterns and compares SEP event characteristics based on temperature and acceleration mechanisms.

Findings

Abundance enhancements follow power laws in A/Q, enabling temperature estimation.

Impulsive events show broader abundance variation than gradual events at similar temperatures.

A revised He/O ratio of 91 is proposed for hotter plasma, improving abundance normalization.

Abstract

Abundance enhancements, during acceleration and transport in both gradual and impulsive solar energetic particle (SEP) events, vary approximately as power laws in the mass-to-charge ratio [A/Q] of the ions. Since the Q-values depend upon the electron temperature of the source plasma, this has allowed a determination of this temperature from the pattern of element-abundance enhancements and a verification of the expected inverse-time dependence of the power of A/Q for diffusive transport of ions from the SEP events, with scattering mean free paths found to be between 0.2 and 1 AU. SEP events derived from plasma of different temperatures map into different regions in typical cross-plots of abundances, spreading the distributions. In comparisons of SEP events with temperatures above 2 MK, impulsive events show much broader non-thermal variation of abundances than do gradual events. The extensive shock waves accelerating ions in gradual events may average over much of an active region where numerous but smaller magnetic reconnections, "nanojets", produce suprathermal seed ions, thus averaging over varying abundances, while an impulsive SEP event only samples one local region of abundance variations. Evidence for a reference He/O abundance ratio of 91, rather than 57, is also found for the hotter plasma. However, while this is similar to the solar-wind abundance of He/O, the solar-wind abundances otherwise provide an unacceptably poor reference for the SEP abundance enhancements, generating extremely large errors.