Onset Altitudes of Co-Seismic Ionospheric Disturbances Determined by Multiple Distributions of GNSS TEC After the Foreshock of the 2011 Tohoku Earthquake on March 9, 2011

We investigate ionospheric disturbances using the total electron content (TEC) data obtained from the radio signals emitted from three satellites after the foreshock of the 2011 Tohoku Earthquake on March 9, 2011. Co-seismic ionospheric disturbances (CIDs) appeared to extend from an onset point concentrically in all of the satellite data. We have found, however, that the geographic coordinates of the onset points did not coincide if the observed CIDs were assumed to occur at one altitude. Admitting that the altitudes of the onset points are different, we searched for coinciding geographic coordinates of the onset points by changing the altitudes and identified the altitude of the three onset points to be 107.8, 131.8, and 133.3 km and the onset time at these altitudes. As a result, the vertical velocity of an acoustic-gravity wave is estimated to be 448 m/s from the travel time between the altitudes of 107.8 and 131.8 km and 370 m/s between the altitude of 107.8 and 133.3 km. This is comparable to the speed of sound calculated using the empirical model NRLMSISE-00. The present study provides a method of determining the altitude and geographic coordinates of the onset point from the ionospheric TEC analysis independently of the seismic data. Furthermore, the source location of the acoustic-gravity wave is simultaneously determined because its wavefront propagating vertically arrives earliest at an altitude that TEC disturbance is detectable.