Coseismic Ionospheric Disturbances at Multiple Altitudes Associated with the Foreshock of Tohoku Earthquake Observed by HF Doppler Sounding

Many studies have reported that ionospheric disturbances occur after big earthquakes. One of the causes of these coseismic ionospheric disturbances (CIDs) is the infrasound wave excited by Rayleigh wave propagated on the ground from the epicenter. The infrasound wave propagates upward and produces CIDs. In this study, using HF Doppler sounding system (HFD), a CID associated with the forshock of Tohoku Earthquake was examined. The HFD sounding system used in this study observes radio waves at 4 different frequencies. In association with the foreshock, the disturbances were observed at all these frequencies. The waveform of the lower ionosphere is very similar to that observed by a seismometer located the reflection point between the transmitter and the receiver. On the other hand, the shorter-period disturbance decays in higher altitude of the ionosphere. We determined the vertical profiles of amplitude ratios of infrasound wave in 3 bands (10.0-25.6, 25.6-45.5, and 45.5- 76.9 mHz) and compared the theoretical estimates of the speed of the infrasound wave as described by Chum et al.(2012). As a result of the comparison, the variations of the vertical profiles of the amplitude ratio observed by HFD is consistent with those estimated theoretically as higher frequency components are more greatly damped in at high altitude. However, absolute ratio observed by HFD is smaller than theoretically estimated values. The most possible cause of this differnece is the reflection of the infrasound wave at the lower atmosphere at about 40 km, where the refractive index of the infrasound wave is significantly varies in altitude due to that of the temparature.