Periodic Oscillations in the D Region Ionosphere After the 2011 Tohoku Earthquake Using LF Standard Radio Waves

We report the first observations of ~100-s periodic oscillations of intensity in low-frequency (LF) standard radio waves over Japan at 05:51–05:56 UT, which was about 4 min and 42 s after mainshock onset of “the 2011 off the Pacific coast of Tohoku earthquake” on 11 March 2011 (Mw 9.0). The LF radio wave propagation paths used in this study were JJY (Japan, 60 kHz)-Rikubetsu (RKB, Japan), and BPC (China, 68.5 kHz)-RKB, where the minimum distances between the epicenter and the LF propagation paths were 413.6 and 561.5 km, respectively. The observed modulations in intensity and phase were about 0.1 dB and 0.5°, respectively. Based on a numerical simulation of the neutral atmosphere and the wave-hop method, the occurrence time of the ~100-s periodic oscillations was in good agreement with the total propagation time of the Rayleigh wave that spread concentrically from the epicenter to the LF propagation paths and acoustic waves propagating vertically from the Earth's surface to the bottom of the ionosphere. Variations in synthesized electric field intensity of ground waves and sky waves, calculated by the wave hop method up to 10 hops, reproduced similar ~100-s periodic oscillations that were caused by the difference in arrival time of the acoustic waves excited by the Rayleigh waves at each hop point. The amplitude of the D region electron density variations at 70 km altitude during the ~100-s periodic oscillation was estimated to be about 1% compared to the background electron density, based on the numerical simulation.