Source Bearing of Extremely Low Frequency (ELF) Waves in the Earth-Ionosphere Cavity With Day-Night Nonuniformity

Impact is modeled of the day-night nonuniformity of the Earth-ionosphere cavity on the source bearing. The smooth day-night interface is considered. The source is located at the 22.5 degrees N; 0 degrees E point, and the observer is positioned at 22.5 degrees S; 0 degrees E. Propagation path position corresponds to 4 hr UT (the night side) and to 8 hr UT (the dayside of morning terminator). We apply the full wave solution to determine the propagation parameters of extremely low frequency radio waves and solve the problem by using the 2-D telegraph equations. The following results were obtained: Terminator impact is absent when the propagation path is at the center of night or day hemisphere. Deviation of source bearing varies with frequency similarly to Schumann resonance pattern and may reach 3-4 degrees at certain frequencies. A weak elliptic polarization is observed for the monochromatic radiation; its sign changes when propagation path shifts from one side of the day-night interface to the other. Temporal variations of the pulsed orthogonal components of horizontal magnetic field form a hodograph outlining complex loops, which obstructs the source bearing. Waveforms change noticeably due to frequency response of a receiver, but the maximum instantaneous deviations do not exceed 1-2 degrees. Effect of the day-night nonuniformity on the Schumann resonance fields does not exceed the level of natural fluctuations caused by the background lightning activity. Therefore, deviations in the source bearing caused by the day-night nonuniformity might be detected exclusively for exceptionally great extremely low frequency transients.