Element Abundances in Solar Energetic Particles and the Solar Corona

TL;DR

This study analyzes element abundances in solar energetic particles during 54 events over 18 years, revealing how transport effects influence observed variations and providing insights into coronal composition.

Contribution

It demonstrates that transport effects cause abundance variations in SEPs and offers a new method to estimate solar coronal element abundances from SEP data.

Findings

Fe abundance correlates with spectral index.

Clustering of Fe during reservoir periods.

SEP abundances are energy independent.

Abstract

This is a study of abundances of the elements He, C, N, O, Ne, Mg, Si, S, Ar, Ca, and Fe in solar energetic particles (SEPs) in the 2 - 15 MeV amu-1 region measured on the Wind spacecraft during 54 large SEP events occurring between November 1994 and June 2012. The origin of most of the temporal and spatial variations in abundances of the heavier elements lies in rigidity-dependent scattering during transport of the particles away from the site of acceleration at shock waves driven out from the Sun by coronal mass ejections (CMEs). Variation in the abundance of Fe is correlated with the Fe spectral index, as expected from scattering theory but not previously noted. Clustering of Fe abundances during the "reservoir" period, late in SEP events, is also newly reported. Transport-induced enhancements in one region are balanced by depletions in another, thus, averaging over these variations produces SEP abundances that are energy independent, confirms previous SEP abundances in this energy region, and provides a credible measure of element abundances in the solar corona. These SEP-determined coronal abundances differ from those in the solar photosphere by a well-known function that depends upon the first ionization potential (FIP) or ionization time of the element.