Radio Physics and Radio Astronomy (Sep 2020)
SEISMIC-IONOSPHERIC EFFECTS: RESULTS OF RADIO SOUNDINGS AT OBLIQUE INCIDENCE
Abstract
Purpose: The object of the radio study is to investigate dynamic processes, which occurred over the People’s Republic of China following three moderate (magnitudes 5.9-6.6) earthquakes in Japan in 2018–2019. The distances between the earthquake epicenters and the radio paths midpoints varied from approximately 1300 to 2000 km The aim of the study is to present observations of the dynamic processes in the ionosphere, which accompanied the earthquakes in Japan, and the analysis of intercomparison between the events. Design/methodology/approach: To continuously observe the ionosphere state over the ~ 100-300 -km altitude range, the multi-frequency multiple path radio system for oblique incidence soundings of the ionosphere has been designed by the specialists at the V. N. Karazin National University (Ukraine) and the Harbin Engineering University, PRC (45.78 N, 126.68 E). The basic premise upon which the system operation is based are the measurements of the Doppler shift of frequency, fD, and of the amplitude of radio waves reflected from the ionosphere. The Doppler spectra are calculated over the 20-s intervals, with the Doppler resolution of 0.02 Hz and the time resolution of 7.5 s. Findings: The seismic activity in Japan on July 7, 2018 was accompanied by an increase in a number of rays, by a significant broadening of the Doppler spectra, and by aperiodic processes in the ionosphere at distances no less than 1000–2000 km from the earthquake epicenters. Also, wave disturbances, generated by the seismic waves (speeds of ≈3 km/s), have been revealed in the 4–5-min infrasonic period range; the amplitude, δN, of the quasi-periodic variations in the electron density, N, was observed to be 4.5–9 %, and the duration of the oscillation trains to vary in the 24–55-min range. The relative amplitude δNa, of the electron concentration variations with the period ≈15-30 min caused by the propagation of atmospheric gravity wave (AGW) was estimated to be 30–55 %, the oscillation train duration was observed to be approximately 100 min, and the speed 0.3 km/s. The character of the Doppler spectrum variations, the Doppler shift of frequency over the main ray, and of the signal amplitude were found to be notably different during the September 5, 2018 earthquake and on the reference days. Two characteristic apparent speeds of 3.3 km/s and of ≈500 m/s were revealed. The former is close to that of seismic waves, and the latter to the speed of AGWs in the terrestrial ionosphere. The relative amplitudes in the infrasonic and AGW wave fields were estimated to be δN≈1.5-3 % and δNa≈6-7.5 %, respectively. The April 11, 2019 earthquake was accompanied by the Doppler spectrum broadening by 1-1.5 Hz in the 5–9.8 MHz frequency range, the generation of AGWs with 0.5-1 -km/s speeds and 8-20 -min periods, and by the generation of infrasonic waves with 2-5 -min periods and 0.3-0.4 -km/s speeds. Conclusions: Moderate earthquakes of Richter magnitudes ≈6 have been determined to give rise to dynamic processes in the ionosphere at distances no less than 1000-2000 km. The disturbances are transported by seismic waves with ≈3 km/s speeds and by acoustic and atmospheric gravity waves with 0.3–1 km/s speeds and periods varying from units to tens of minutes.
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