Wave Atmospheric Disturbances from the Solar Terminator in the Morning and Evening Hours Based on Measurements of Amplitudes of VLF Radio Signals

Wave disturbances from the solar terminator in the morning and evening hours were investigated using a ground-based network of very low frequency (VLF) radio stations. The data of measurements of the amplitudes of VLF radio signals on the GQD–A118 radio path with a transmitter in Great Britain (GQD, f = 22.1 kHz) and a receiving point in France (A118) were used. Amplitudes of radio signals change as a result of the propagation of atmospheric waves at the altitudes of localization of the upper wall of the Earth-ionosphere VLF waveguide. This makes it possible to use a network of VLF radio stations to monitor wave activity in the mesosphere (lower ionosphere). Based on the analysis of experimental data, it was established that pronounced periodic fluctuations in the amplitudes of radio signals are observed in the evening and in the morning for several hours after the passage of the solar terminator. Histograms of the distribution of these fluctuation periods for several months were constructed. The predominance of periods of radio signal fluctuations of 20–25 min was revealed both in the evening and in the morning hours. For the evening terminator, this result is consistent with our previous studies. The predominance of approximately the same wave periods in the morning was established for the first time. It is assumed that the observed fluctuations are caused by the propagation of acoustic-gravity waves (AGWs) from the solar terminator. The existence of a dominant period probably indicates that these perturbations represent a fundamental wave mode moving synchronously with the solar terminator.