Particle Acceleration in Solar Flares and Associated CME Shocks

TL;DR

This paper investigates the origins and acceleration mechanisms of solar energetic particles during flares and CMEs, showing how different transport and re-acceleration processes explain observed spectral differences and event types.

Contribution

It introduces a refined electron transport model and demonstrates how re-acceleration in CME shocks accounts for spectral hardening and event diversity.

Findings

Transport modeling explains prompt and delayed SEP spectral differences.

Re-acceleration in CME shocks leads to harder SEP spectra.

The model distinguishes between impulsive and gradual SEP events.

Abstract

Observations relating the characteristics of electrons seen near Earth (SEPs) and those producing flare radiation show that in certain (prompt) events the origin of both population appears to be the flare site, which show strong correlation between the number and spectral index of SEP and hard X-ray radiating electrons, but in others(delayed), which are associated with fast CMEs, this relation is complex and SEPs tend to be harder. Prompt event spectral relation disagrees with that expected in thick or thin target models. We show that using a a more accurate treatment of the transport of the accelerated electrons to the footpoints and to the Earth can account for this discrepancy. Our results are consistent with those found by Chen and Petrosian (2013) for two flares using non-parametric inversion methods, according to which we have weak diffusion conditions, and trapping mediated by magnetic field convergence. The weaker correlations and harder spectra of delayed events can come about by re-acceleration of electrons in the CME shock environment. We describe under what conditions such a hardening can be achieved. Using this (acceleration at the flare and re-acceleration in the CME) scenario we show that we can describe the similar dichotomy that exists between the so called impulsive, highly enriched ($^3$He and heavy ions) and softer SEP events, and stronger more gradual SEP events with near normal ionic abundances and harder spectra. These methods can be used to distinguish the acceleration mechanisms and to constrain their characteristics.