Sigmoid Eruption Associated with the X9.3 Flare from AR 12673 Drives the Gradual Solar Energetic Particle Event on 2017 September 6

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, and how these particles are released, accelerated and transported into the heliosphere are not well understood. We analyze NOAA active region (AR) 12673, which was the source responsible for multiple large gradual SEP events during 2017 September, 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. The EUV Imaging Spectrometer (EIS) onboard Hinode took data roughly 8 hr before the second SEP event on 2017 September 6, which 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, which contains highly fractionated plasma, erupts and propagates toward 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.