About the Propagation of RSDN‐20 “Alpha” Signals in the Earth‐Ionosphere Waveguide During Geomagnetic Disturbances

The paper presents the results of the modeling of VLF radio waves propagation in horizontally inhomogeneous region of the high‐latitude Earth‐ionosphere waveguide during different types of the geomagnetic disturbances. The frequencies of the model source of signal correspond to the frequencies of the long‐distance navigation radio system RSDN‐20 “Alpha” constantly broadcasting in the VLF range in the Russian Federation. It is considered the geomagnetic storm on 24 January 2012 and geomagnetic substorm on 11 December 2015. Electron density profiles and collision rates of electrons with neutrals are taken from the measurements of EISCAT radar in Tromso and EISCAT 42m radar on Svalbard. The numerical experiments were conducted with a highly horizontally inhomogeneous electron density in the ionosphere. It is shown that the amplitudes and phases of the VLF signals at these frequencies show a noticeable reaction to changes in the ionospheric parameters during the considered geomagnetic disturbances. Calculations show that changes in amplitude of RSDN‐20 “Alpha” signals caused by the action of the Earth's magnetic field and horizontal inhomogeneities depend on the direction of signal propagation relative to the Earth's magnetic field vector and cannot be reversed if the direction of propagation changes to the opposite. The results of the numerical experiment were compared with the corresponding characteristics of the RSDN‐20 signals calculated from the observations of electromagnetic signals at the Lovozero observatory for the case on 11 December 2015. Plain Language Summary The main task of the work is to provide researchers involved in the processing of ground‐based observations of VLF radio signals by additional information about the typical behavior of the signal characteristics during geomagnetic disturbances, such as substorm ans SSC events. Information about the behavior of signal characteristics associated with both magnetic and electrical components can push researchers to modernize observation stations for joint synchronized recording of these components. The responses to unusual waveguide states presented in the work were obtained by synthetic methods of a computational experiment and are free from the influence of any unaccounted factors, which makes them more informative than the measurement results. The work uses ionospheric plasma density models, EISCAT incoherent scatter radar data, interpolation methods, and results from other research teams to