Effect of Weak Magnetic Storms on the Propagation of HF Radio Waves

Vertical and oblique sounding data for northeastern Russia have been used to analyze the conditions for the propagation of radio waves during weak geomagnetic storms observed in fall seasons of 2018–2020 at minimal solar activity. Even during weak storms, the maximum observed frequencies have been found to decrease by 25–35\% in daytime and by 40–50\% at night. Variations in the parameters of the distribution of high frequency radio waves during disturbances depend on the spatio-temporal dynamics of large scale structures of the high-latitude ionosphere, which, in turn, depends on the processes of magnetosphere–ionosphere interaction. Here, the depth and duration of the negative disturbance are larger if the geomagnetic storm occurs on a disturbed background.