SOLAR CYCLE 24 EXTREME EVENTS--

From the ACE newsletter-

On 28 August 2017, active region (AR) 12673 emerged on the solar disk; before rotating over the western limb it generated 27 M-class and 4 X-class flares, including the largest of solar cycle 24 (X9.3 on 6 September). Four coronal mass ejections (CMEs) with velocities >1000 km/s and widths >120° were launched from AR 12673, not surprisingly resulting in multiple solar energetic particle (SEP) events observed by near-Earth spacecraft. Of these events, that of 10 September was the largest seen since June 2015 (in >10 MeV proton peak intensities) and one of only two ground-level enhancement (GLE) events of the current cycle. GLE events are extreme SEP events which typically exhibit significantly harder spectra (on average E-3.18) at energies above ~40 MeV; this, combined with their large intensities of >50 MeV/nuc ions, makes GLE events extremely hazardous to exposed astronauts and space-based instrumentation.

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