Analysis of extreme solar activity in early September 2017: G4 - severe geomagnetic storm (07-08.09) and GLE72 (10.09) in solar minimum

The extreme solar activity in early September 2017 at minimum of solar cycle 24 is analysed. The beginning of the intensive solar-terrestrial disturbances was the 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 = 106, and also the second strongest flare of Solar Cycle 24 on September 10, 2017, which generated instantly the ground level enhancement of cosmic rays or Ground Level Event No 72 (GLE72). The three space regions of impact are investigated and analysed in the present paper: 1) Sun – photosphere and solar corona; 2) Interplanetary space with solar wind and its basic parameters – velocity V , density or concentration N , temperature T p and intensity of the magnetic field B ; 3) Earth and its magnetosphere. Calculations of the solar wind parameters from measurements by SOHO and DSCOVR space probes in the point of Lagrange are made: the kinetic (dynamic) energy density Ek , thermal energy density E t and magnetic energy density Em during the investigated period September 2–15, 2017. We found two interesting phenomena in the investigated period: 1) tunnel effect in the Earth's environment, and 2) specific distribution of the solar wind energy density during and after the CME. It is likely that both kinetic and magnetic energies can be used as predictors of strong geomagnetic storms. Key words: solar activity, flares, coronal mass ejection (CME), G4 – Severe geomagnetic storm, solar energetic particles (SEPs), ground-level enhancement (GLE), space weather