Experimental studies of medium radio wave propagation in the magnetosphere

Abstract

The paper presents experimental studies of anomalous propagation of medium radio waves. Radio echo signals with unusually long delay times of 310–324 ms were recorded at the transmitter point of the sounding signal. The experimental results can be explained by the guiding effect, when a radio wave penetrates into a waveguide channel oriented along the Earth's magnetic field line. In this case, the radio wave propagates to the magnetic conjugated point in the southern hemisphere and returns to the transmission point traveling a distance of 93,000 km. The echo signals were recorded with the use of AARI-developed transmitting and receiving measuring equipment that has a minimum radiation power of 1 kW, while in the previous LDE observation experiments the radiation power was 5 and 17 kW. For the first time, echo signals that radiated from the Earth's surface were recorded not at a single fixed frequency, but in a frequency band of 400 kHz, from 2.100 to 2.400 kHz. The noise environment at frequencies below 2.100 kHz did not allow us to determine the lower boundary of the channel. Analysis of the background geophysical conditions was performed. It was shown that the long delayed echo (LDE) signals were observed under disturbed magnetic conditions (the planetary magnetic index Kp = 4+) in evening hours. The echo signal frequencies exceeded the critical frequencies of the ionosphere at the transmitter point and were less than the critical frequencies at the magnetically conjugate point. A distinctive feature seen from the CADI ionograms was the presence of the F3S layer, which is the main signature of the development of a subauroral polarization stream (SPS) near the station's zenith. Swarm satellite observations revealed that the Gorkovskaya observatory was located at the bottom of the main ionospheric trough (MIT), near its equatorial boundary. The projection of the plasmapause was also located at the MIT bottom, between its polar boundary and Gorkovskaya. Plausible mechanisms for the creation of a waveguide along the magnetic field line were considered. The guiding effect may find practical significance in the development of means and methods for ground-based monitoring of space weather parameters, as well as radar sounding of the near-Earth space.