LAUSR

Abstract The response of the ionosphere to the earthquake of a moderate magnitude (M ≈ 5.9) that occurred in Japan at 11:23:50 UT on 7 July 2018 has been studied using a newly developed coherent multi-frequency radio diagnostic system for remotely probing the ionosphere at oblique incidence. The seismic activity in Japan on 7 July 2018 was accompanied by aperiodic processes in the ionosphere at distances of no less than (1–2) × 103 km from the epicentre, with an enhancement in multiple-mode propagation, and a significant Doppler spectrum broadening. Three ways of transporting disturbances from the earthquake to the changes in the character of Doppler spectra variations have been identified by examination. First, the disturbances are generated by a surface Raleigh wave launched at the earthquake epicentre. They have been ascertained in the infrasonic range (a 3- to 4-min period) of oscillations. The relative amplitude of these quasi-periodic oscillations in the electron density is equal to 1.7–9%. The duration of the oscillation trains is found to be in the range of 24–55 min. The wave disturbance speed of propagation is approximately 3 km/s. Second, wave disturbances have also been ascertained in a 15- to 30-min period range. They could be generated in the vicinity of the epicentre and then propagated as atmospheric gravity waves modulating the electron density in the ionosphere. The relative amplitude of the quasi-periodic disturbances in the electron density is equal to 14 – 34%. The wave train attains a temporal duration of about 100 min and a speed of approximately 0.3 km/s. Third, the broadening of the Doppler spectra toward negative Doppler shifts with the time delay estimated to be 49–124 min, depending on the orientation of the propagation path, is the most pronounced Doppler signature of the disturbances caused by the earthquake. This time delay corresponds to a speed of about 0.3 km/s, and consequently, it suggests that this effect most likely is caused by the atmospheric gravity waves launched at the earthquake epicentre. Apparently, the rearrangement of the ionosphere acts to reverse the sign of the Doppler spectrum shift when the atmospheric gravity waves arrive at the reflection level.