Variation in the D-region ionosphere after the 2015 Nepal earthquake using LF transmitter signals

. We report variation in the D-region ionosphere after the 2015 Nepal earthquake using low-frequency (LF) transmitter signals. The Nepal earthquake (Mw 7.8) occurred at 6:11:26 UT on April 25, 2015. In this study, we used the BPC (China, 68.5 kHz)-Takine (TKN, Japan) LF radio wave propagation path, which has a great circle distance between the epicenter and the LF propagation path of 3,025 km. The observed periods of variation in the LF amplitude and phase were 100-300 s. The observed variation in the LF amplitude and phase was approximately +/-0.1 dB and +/-1°, respectively. The vertical velocity of the ground oscillation near the midpoint of the LF propagation path had a similar period to the LF waves. The Rayleigh wave spread concentrically from the epicenter to the LF path, and then the acoustic waves propagated vertically at the midpoint of the LF path from the Earth’s surface to the D-region ionosphere height. The maximum coherences between the LF amplitude and vertical seismic velocity, and between the LF phase and vertical seismic velocity were 0.90 (period: 146 s) and 0.77 (256 s), respectively, which were both significant at the 95% confidence level. The variation in the LF amplitude and phase was caused by acoustic waves excited by the Rayleigh wave.