Long term decrease of stratospheric ionization near the 24-th solar cycle minimum after G4 – severe geomagnetic storm and GLE72 on September 8–10, 2017

The mechanism of galactic cosmic ray (GCR) intensity decrease – during and several days after geomagnetic storms (known as Forbush decrease) – is well understood. Recent measurements of atmospheric ionization reveal, however, a prolonged depletion of atmospheric ionization after the severe geomagnetic storm (class G4) on 07–08.09.2017 (with Ap = 106), and following ground level enhancement of cosmic rays No 72 (GLE72) on September 10, 2017. This paper reports a similar long-lasting decrease found in neutron measurements. We also offer an explanation of this post-storm effect (PSE) found in atmospheric ionization and neutron component of GCR. Except that the effect is stronger at mid-latitudes, we suggest that it is related to the compression of the plasmasphere and reduced population of trapped particles in radiation belts. Due to the heterogeneous geomagnetic field, which focuses energetic particles in some regions, the long-lasting decrease of GCR is better pronounced over the American continent. The effect of highly energetic particles on the chemistry and, respectively, thermo-dynamics of the middle stratosphere is already fairly well recognized. In most of the recently developed models, the induced by GCRs atmospheric ionization plays a significant role. This means that clarification of factors and mechanism influencing the amount of atmospheric ionization are still very important. Key words: galactic cosmic rays (GCRs), geomagnetic storms, Forbush effect, stratosphere physics and chemistry, radiation belts (RBs)