Ionospheric disturbances (infrasound waves) over the Czech Republic excited by the 2011 Tohoku earthquake

Abstract

A train of large amplitude infrasound wave packets was observed by multipoint Continuous Doppler sounding system in the ionosphere over the Czech Republic on 11 March 2011. It is shown that these infrasound wave packets originated from vertical motion of the ground surface that was caused by arrival of seismic waves generated by the strong Tohoku earthquake. The infrasound wave packets were observed in the ionosphere at heights of ∼210–220 km about 9 min after the detection of corresponding wave packets on the ground, which is consistent with the calculated time for vertically propagating infrasound waves. Absolute values of cross‐correlation coefficients between ionospheric and ground measurements are typically higher than 0.9 (for two wave packets ∼0.98). The individual wave packets recorded on the ground have different observed horizontal velocities and correspond to different types of seismic waves. A comparison of vertical velocities of ground motion with oscillation velocities of air particles in the ionosphere indicates that almost 1/10 of the infrasound energy flux excited at the ground reached the altitudes of ∼210–220 km for wave periods longer than ∼30 s. Estimates of sound attenuation are performed. It is also shown that it is necessary to consider the value of electron density gradient at the reflection height of the sounding radio wave, and air (plasma) compression owing to the infrasound wave to get reasonable estimates of oscillation velocities of air particles from Doppler shift frequencies.