GeV Particle Acceleration in Solar Flares and Ground Level Enhancement (GLE) Events

TL;DR

This paper investigates the timing and location of particle acceleration in solar flares and CMEs that lead to ground level enhancements, finding evidence for both flare-related and shock-related acceleration processes.

Contribution

It provides a detailed analysis of GLE event timing relative to solar flare activity, highlighting the roles of impulsive flare phases and CME-associated shocks in particle acceleration.

Findings

50% of GLEs are accelerated during the impulsive flare phase

Prompt GLE component linked to flare-accelerated particles in the lower corona

Delayed GLE component may involve extended acceleration, trapping, or CME shock acceleration

Abstract

{\sl Ground Level Enhancement (GLE)} events represent the largest class of {\sl solar energetic particle (SEP)} events that require acceleration processes to produce $\gapprox 1$ GeV ions in order to produce showers of secondary particles in the Earth's atmosphere with sufficient intensity to be detected by ground-level neutron monitors, above the background of cosmics rays. Although the association of GLE events with both solar flares and coronal mass ejections (CMEs) is undisputed, the question arises about the location of the responsible acceleration site: coronal flare sites or heliospheric CME-associated shocks? To investigate the first possibility we explore the timing of GLE events with respect to hard X-ray production in solar flares, considering the height and magnetic topology of flares, the role of extended acceleration, and particle trapping. We find that 50% (6 out of 12) of recent (non-occulted) GLE events are accelerated during the impulsive flare phase, while the remaining half are accelerated significantly later. It appears that the prompt GLE component, which is observed in virtually all GLE events, is caused by flare-accelerated particles in the lower corona, while the delayed gradual GLE component can be produced by both, either by extended acceleration and/or trapping in flare sites, or by particles accelerated in CME-associated shocks during their propagation through the heliosphere.