The ionospheric anomalies prior to the M9.0 Tohoku-Oki earthquake

The GPS total electron content (TEC) data were used to investigate the ionospheric abnormal behaviors prior to the 2011 Tohoku-Oki earthquake. The observations show a significant enhancement in TEC on March 8, 2011, and during the same period the solar activity also has a large increase. Both an empirical model and a theoretical model were used to check whether the TEC anomalies were entirely contributed by the increase in solar radiation. The comparison between the observations and the simulation results shows that only the solar radiation enhancement was not enough to produce the observed significant TEC enhancement. It means that some additional mechanisms, such as the per-earthquake ionospheric disturbance or the geomagnetic activities, may play a significant role in the significant TEC enhancement on March 8. The temporal-spatial distribution of the extreme TEC enhancement within 30. days before the earthquake was particularly pursued. It is found that the extreme enhancement was persistently located in the region adjacent to the epicenter and the magnetic conjugate point for a long time of 16. h. In addition, a geomagnetic disturbance with Kp. =4 occurred on March 7. Therefore, the significant TEC enhancement on March 8 might be related to the M9.0 Tohoku-Oki earthquake and the geomagnetic disturbance.