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

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 similar to 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. There have been several studies of solar-maximum-ending activity during September 2017, including a special issue of Space Weather. With a few exceptions (e.g., Luhmann et al. (2018, Bruno et al. (2018) most of these have focused on activity observable from near-Earth spacecraft and ground-based facilities. This paper will focus on a far-side (eastern) 1380 km/s CME on September 18 that resulted in a spectacular energetic-storm-particle (ESP) event observed by STEREO-A. We report on the magnitude of the ESP enhancements of different species observed in this event. We have also measured the composition and energy spectra observed by instruments from the STEREO-IMPACT collaboration as well as solar-wind data from STEREO IMPACT & PLASTIC instruments. The observed properties of the 9/18/17 event will be compared with the large SEP/ESP event of January 23, 2012