The Large Energetic Storm Particle Event of September 18, 2017 Observed by STEREO-A

Abstract: Solar Cycle 24 solar activity ended during September 2017 with a series of ”bangs” that included four X-Class flares, a record-breaking 3000 km/s CME, and a large ground-level event, all recorded by Earth-based observers. Less well known is the eruption of a far-side CME from the same active region on September 17, which resulted in a spectacular energetic storm particle (ESP) event observed on September 18-19 at STEREO- We report the time history, energy spectra, and composition of ~0.1 to 100 MeV/nucleon ions and 0.1 to 4 MeV electrons measured during this period by the SEPT, LET and HET instruments on STEREO-A. We also compare this event with the intense July 23, 2012 ESP event also observed by STEREO-A. The composition of ESP ions will be compared with possible seed-particle sources. Finally, these observations will serve as a basis for a modeling effort to be reported by Hu et al. at this conference.