The Ground-Level Enhancement Event of September 2017 and Other Large Solar Energetic Particle

The 10 September 2017 solar energetic particle (SEP) event was the largest since June 2015 and one of only two ground-level enhancement (GLE) events of solar cycle 24. GLE events are subset of large SEP events (similar to 15% of events identified by Space Weather Prediction Center) with particularly hard spectra, making them a substantial space weather hazard to space-based instrumentation and exposed astronauts. We present analysis of the 10 September 2017 event and compare it to the other cycle 24 GLE events, to those of cycle 23, and also to two extreme SEP events observed by Solar Terrestrial Relations Observatory (STEREO). We find the 10 September 2017 event had a broken power law spectrum typical of GLE events but was softer than average at high energies. However, it was hard at low energies with a relatively high break energy, which led to 100-MeV proton fluences within a factor of 4.5 of the largest cycle 23 GLE events. The composition was nominal, except for a low Fe/O ratio, which has also been seen in large SEP events this cycle, but is somewhat atypical of the cycle 23 GLE events. The extreme events seen by STEREO exhibited very hard high-energy spectra, with one event producing -80-MeV proton fluences larger than the largest cycle 23 GLE event. However, even including STEREO events, the top 10 largest cycle 24 events are, on average, 2.4 times smaller than the top 10 of cycle 23 based on >10-MeV proton fluences.