On the Correlation between Energy Spectra and Element Abundances in Solar Energetic Particles

TL;DR

This paper investigates the relationship between energy spectra and element abundances in solar energetic particle events, revealing correlations in gradual events and complex behaviors in large events that inform shock acceleration and transport physics.

Contribution

It provides the first detailed analysis of how energy spectra and element abundances are correlated in different SEP event types, highlighting new behaviors and implications for acceleration mechanisms.

Findings

Correlations exist in gradual SEP events but not in impulsive events.

Large SEP events show significant temporal variation in spectra and abundances.

Streaming-limited transport influences spectral shapes and element enhancements.

Abstract

In solar energetic particle (SEP) events, the physical processes of both shock acceleration and scattering during transport can cause energy-spectral indices to be correlated with enhancement or suppression of element abundances versus mass-to-charge ratios A/Q. We observe correlations for those "gradual" SEP events where shock waves accelerate ions from the ambient coronal plasma, but there are no such correlations for "impulsive" SEP events produced by magnetic reconnection in solar jets, where abundance enhancement in different events vary from (A/Q)^+2 to (A/Q)^+8, nor are there correlations when shock waves reaccelerate these residual impulsive ions. In these latter events the abundances are determined separately, prior to the accelerated spectra. Events with correlated spectra and abundances show a wide variety of interesting behavior that has not been described previously. Small and moderate gradual SEP events, with relative abundances typically depending approximately upon (A/Q)^-1 and the spectra upon energy E^-2.5, vary little with time. Large SEP events show huge temporal variations skirting the correlation line; in one case O spectra vary with time from E^-1 to E^-5 while abundances vary from (A/Q)^+1 to (A/Q)^-2 during the event. In very large events, streaming-limited transport through proton-generated resonant Alfve'n waves flattens the spectra and enhances heavy ion abundances prior to local shock passage, then steepens the spectra and reduces enhancements afterward, recapturing the typical correlation. Systematic correlation of spectra and element abundances provide a new perspective on the "injection problem" of ion selection by shocks and on the physics of SEP acceleration and transport.