A GLE on 10 September 2017 – the first since 2012

After a series of major flares, the solar active region that had produced an X9.3 flare on September 6 is about to rotate across the western solar limb. Activity seemed to be decreasing on September 9 and 10, and no major flare was expected to occur. The Sun surprised us with a new X8 flare on September 10, starting 15:35 UT (information from NOAA via https://www.solarmonitor.org). Unlike its predecessor on September 6, this flare was accompanied by solar energetic particles at very high energies, producing nuclear cascades in the Earth's atmosphere and detected by neutron monitors on the Earth. The figures below show the time histories of solar energetic particles at energies above 10, 50 and 100 MeV as observed by the GOES satellites in geostationary orbit (top panel; NOAA). Particles at higher energies are observed from ground. Data collected by the neutron monitor database NMDB (www.nmdb.eu) are displayed in the central and bottom panels. The central panel shows the count rate increases of neutron monitors with nominal cutoff rigidity up to 3.9 GV, corresponding to a proton energy of 3.2 GeV, relative to the count rate between 15h and 16h UT. The bottom panel shows the count rate increases of stations with higher cutoff rigidity. There is no clear signature of this event at these energies, although a small monotonous rise seems to appear upon closer inspection. This particle event was seen by more neutron monitors than the previous event on 2012 May 17. The neutron monitor observations show that the Sun accelerated protons up to at least 3 GeV. (click to see the full size image)

GLE 72 / protons from GOES
GLE 72 / low cutoffGLE 72 / high cutoff
Same plot as the second one, but relative to a mean count rate between 14h and 15h UT:
GLE 72 / low cutoff
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GLE72_sup3.9GV.png57.2 KB
GLE72_protonsGOES.png23.97 KB
GLE72_inf3.9GV.png66.89 KB
GLE72_inf3.9GV_base14.png65.64 KB