Sigmoid eruption associated with X9.3 flare from AR 12673 drives gradual SEP event on 2017 September 6

TL;DR

This study investigates the origin and release mechanisms of solar energetic particles during large SEP events from active region 12673, revealing that plasma from the AR's core and sigmoidal loops contributes to SEP acceleration and transport.

Contribution

It identifies the source of SEP seed particles as plasma from the AR's core and sigmoidal loops, linking eruptions to particle escape pathways.

Findings

Enhanced Si/S abundance ratio observed post-eruption

Plasma originates from AR core and moss regions

Sigmoid eruption provides a direct escape channel for particles

Abstract

Large gradual solar energetic particle (SEP) events can pose a radiation risk to crewed spaceflight and a significant threat to near-Earth satellites however, the origin of the SEP seed particle population, how these particles are released, accelerated and transported into the heliosphere are not well understood. We analyse NOAA active region (AR) 12673, that was the source responsible for multiple large gradual SEP events during September 2017, and found that almost immediately after each significant eruptive event associated with SEPs an enhanced Si/S abundance ratio was measured by Wind, consistent with the previous work by Brooks et al. Hinode/EIS took data roughly 8~hours before the second SEP event on 2017 September 6 that allowed the regions of enhanced Si/S abundance ratio in the AR to be determined. We have shown that the AR contains plasma with elemental abundance values detected in situ by Wind. In particular, the plasma originates from the core of the AR, similar to Brooks et al., but in the moss (footpoints) associated with hot sigmoidal AR loops. The sigmoid, that contains highly fractionated plasma, erupts and propagates towards an Earth-connected magnetic null point, providing a direct channel for the highly fractionated plasma to escape and be detected in the near-Earth environment.