Element Abundances and Source Plasma Temperatures of Solar Energetic Particles

TL;DR

This paper investigates how element abundance patterns in solar energetic particles reveal source plasma temperatures, distinguishing between impulsive and gradual events and explaining their variations through A/Q-dependent fractionation and seed populations.

Contribution

It provides a detailed analysis of element abundance enhancements in SEP events to determine source plasma temperatures and explains the differences between impulsive and gradual events.

Findings

Impulsive SEP events show A/Q-dependent enhancements up to 1000 times for Z=76-82.

Gradual SEP events have a broad temperature range, with most below 1.6 MK.

Variations in element abundances are largely due to source plasma temperature and transport effects.

Abstract

Thirty years ago Breneman and Stone observed that the enhancement or suppression of element abundances in large solar energetic-particle (SEP) events varies as a power of the mass-to-charge ratio, A/Q, of the elements. Since Q during acceleration or transport may depend upon the source plasma temperature T, the pattern of element enhancements can provide a best-fit measure of T. The small SEP events we call 3He-rich or "impulsive" show average enhancements, relative to coronal abundances, rising as the 3.6 power of A/Q to a factor of ~1000 for (76<=Z<=82)/O and temperature in the range 2-4 MK. This acceleration is believed to occur in islands of magnetic reconnection on open field lines in solar flares and jets. It has been recently found that the large shock-accelerated "gradual" SEP events have a broad range of source plasma temperatures; 69% have coronal temperatures of T < 1.6 MK, while 24% have T ~ 3 MK, the latter suggesting a seed population containing residual impulsive suprathermal ions. Most of the large event-to-event abundance variations and their time variation are largely explained by variations in T magnified by A/Q-dependent fractionation during transport. However, the non-thermal variance of impulsive SEP events (~30%) exceeds that of the ~3 MK gradual events (~10%) so that several small impulsive events must be averaged together with the ambient plasma to form the seed population for shock acceleration in these events.