Observed and Calculated Ionograms of Oblique Ionospheric Sounding on HF Radio Paths During a Magnetic Storm of September 7–8, 2017

Ionograms of the oblique ionospheric sounding radio paths Cyprus—Lovozero and Lovozero—Gor’kovskaya (near St. Petersburg) are analyzed using magnetometers and riometers. The behavior of the maximum observed frequencies (MOF) of the F 2 and E s layers during a magnetic storm of September 7–8, 2017 is considered. The storm has a specific, not classical form and can be divided into two parts, namely, the first and the second storms. The main results are as follows. 1) The manifestation of the first storm is accompanied by a significant increase in absorption and frequent absence of signals on the paths. The second storm is characterized by increased values of MOF-E s and the presence of signals. 2) The signals on the midlatitude Cyprus—Lovozero path propagate both as 1F 2 , 2F 2 , and 3F 2 modes and as 2E s and 3E s modes. 3) On the subpolar Lovozero—Gor’kovskaya path, during the disturbance, the signals mainly propagate via reflection from the sporadic E s layer of very high intensity. 4) Modeling of the propagation of HF radio waves using the IRI-2016 model adapted to the vertical sounding data made it possible to match the model ionograms with the real oblique sounding ionograms, including the cases of presence of the M and N modes, as well as triplets, in both quiet and disturbed conditions, which is a new result.