Effect of the Ionosphere on the Controlled-Source Field in the Frequency Range Between 0.4 and 95 Hz

This letter addresses the effect of the ionosphere on the extremely low-frequency and lower frequency waves excited in the Earth-ionosphere waveguide by a controlled source. The experiment carried out on the Kola Peninsula is described and the results of measurements in the frequency range 0.4–95 Hz are presented. Nonmonotonic behavior of the magnetic field with time is revealed. It is shown that variations in the magnetic field are related to the state of the ionosphere and depend on the geomagnetic activity. The importance of the effect of the topside ionosphere on the structure of the studied field is discussed.