Vertical Propagation of Coseismic Ionospheric Disturbances Associated With the Foreshock of the Tohoku Earthquake Observed Using HF Doppler Sounding

The vertical propagation of coseismic ionospheric disturbances (CIDs) associated with the foreshock of the Tohoku earthquake was examined using an HF Doppler sounding system. The sounding system used in this study received radio waves at four different frequencies (5.006, 6.055, 8.006, and 9.595 MHz), indicating that ionospheric disturbances can be observed at up to four altitudes. CIDs were observed at these four frequencies in association with the foreshock of the Tohoku earthquake. From the propagation speed of the coseismic disturbances, it is assumed that the ground motion caused by the Rayleigh wave propagating from the epicenter generates the acoustic mode wave propagating upward, causing the neutral atmospheric particles to move vertically. To examine the characteristics of the acoustic wave, we determined the ratio of the vertical speed of the neutral particles to that of the ground motion observed by a seismometer. The vertical profiles of the ratio are roughly explained in terms of the characteristics of the acoustic mode wave resulting from the ground motion. However, the absolute values of the ratio are smaller than the theoretical ratio estimated by the atmospheric mass density and the attenuation model considering the viscosity, thermal conductivity, and relaxation losses. This result implies that the CIDs in this event may be affected by the nonlinear behavior of the acoustic mode wave. Plain Language Summary In association with an earthquake, the ground oscillates, and atmospheric waves are generated by the motion of the ground. The law of conservation of energy states that the amplitudes of these waves increases with altitude owing to the decrease in density of air, indicating that waves with small amplitudes just above the ground can be detected in the ionosphere. Ionospheric observations using radio waves can capture the motion of plasma in the ionosphere. The behavior of the atmospheric waves can therefore be captured by ionospheric sounding systems because neutral atmospheric particles drag the particles of plasma along when they move. To capture the vertical motions of the plasma, in this study, we utilized the HF Doppler sounding system which enables to receive radio waves at four frequencies. Thus, we were able to distinguish ionospheric disturbances at four altitudes. In the event of the foreshock of the Tohoku earthquake, it was confirmed that the vertical profile of the disturbance roughly coincides with that of the acoustic mod