Abundances, Ionization States, Temperatures, and FIP in Solar Energetic Particles

TL;DR

This paper reviews how element abundances and ionization states in solar energetic particles reveal information about their origins, acceleration mechanisms, and the solar environment, highlighting differences between impulsive and gradual events.

Contribution

It provides a comprehensive analysis of SEP abundances, ionization states, and FIP effects, connecting observations to solar plasma conditions and wave-particle interactions.

Findings

Impulsive SEPs show large enhancements in 3He/4He and heavy elements due to wave interactions.

Gradual SEPs reflect shock acceleration and transport effects, with abundances affected by scattering.

Differences in element abundances relate to source plasma temperatures and magnetic field configurations.

Abstract

The relative abundances of chemical elements and isotopes have been our most effective tool in identifying and understanding energetic particles. The early surprise in solar energetic particles (SEPs) was 1000-fold enhancements in 3He/4He from resonant wave-particle interactions in the small "impulsive" SEP events that emit electron beams that produce type III radio bursts. Further studies found enhancements in Fe/O, then extreme enhancements in abundances that increase with mass-to-charge ratio A/Q by a factor of 1000 from He to Pb arising in magnetic reconnection on open field lines in solar jets. In contrast, in the largest SEP events, the "gradual" events, acceleration occurs at shock waves driven out from the Sun by fast, wide coronal mass ejections (CMEs). Averaging many events provides a measure of solar coronal abundances, but A/Q-dependent scattering during transport causes variations with time; thus if Fe scatters less than O, Fe/O is enhanced early and depleted later. To complicate matters, shock waves often reaccelerate impulsive suprathermal ions above active regions that have spawned many impulsive events. Since both impulsive and gradual SEP events have abundance enhancements that vary as powers of A/Q, we can use abundances to deduce Q-values and 1 - 3 MK source plasma temperatures during acceleration. Comparing coronal abundances from SEPs and from the slow solar wind as a function of the first ionization potential (FIP) of the elements, remaining differences are for the elements C, P, and S. The theory of the fractionation of ions by Alfv\'en waves shows that C, P, and S are suppressed because of wave resonances during chromospheric transport on closed magnetic loops but not on open magnetic fields that supply the solar wind. Shock waves can accelerate ions from closed coronal loops that easily escape as SEPs, while the solar wind must emerge on open fields.