Investigation of exceptional solar activity in September 2017: GLE 72 and unusual Forbush decrease in GCR

Abstract

The exceptional solar activity in early September 2017 at minimum of solar cycle 24 is analyzed. Intensive solar-terrestrial disturbances was caused by Active Region AR2673, which produced four powerful eruptions class X, including the strongest flare X9.3 of Solar Cycle 24 on September 6, 2017, after which began G4 – Severe geomagnetic storm on 07-08.09.2017 with Ap = 96, and also the second strongest flare X8.2 of Solar Cycle 24 on September 10, 2017, which generated Ground Level Enhancement (GLE) of cosmic rays. This was GLE72 with increase of solar cosmic ray flux 6% in Oulu Station (Finland) (effective vertical geomagnetic cutoff rigidity: 0.8 GV), and increase 9% in DOMC Antartica and 14% in DOMB Antartica (in the latter case - lead free neutron monitors with effective vertical cutoff rigidity <0.01 GV). The GLE72 develops under the conditions of a deep Forbush decrease (around 15%) in South Pole cusp caused by September 7th Coronal Mass Ejection. The Forbush effect ends on September 11th (http://cosmicrays.oulu.fi). But cosmic ray measurements by flying balloons to the stratosphere over California show that after solar eruptions in September 2017 the radiation levels in stratosphere took more than two months to fully rebound to the conditions of minimal solar activity. This is interesting fact which deserves to be explored in detail. It is precisely the study and interpretation of this process that is concerned with this work.