The investigation of short wave signal structure in the vicinity of the caustic by means of a high-directed antenna

Variations in short-wave radio signals near the boundary of the dead zone were investigated at sunrise and sunset by means of a high-directed antenna of the radio telescope UTR-2. The angles of arrival of lower and upper rays and Doppler frequencies and amplitudes were measured. Within the framework of the parabolic model of the F region of the ionosphere, the inverse problem was solved by reconstructing the time dependence of the critical frequency. The field structure in the vicinity of the caustic was determined, and its velocity was established. The findings demonstrate that at periods of sunrise or sunset, the nonhomogeneous structure of the field in the vicinity of the caustic is displaced with a constant speed of several tens to several hundreds of meters per second.