Long time series analysis of ionospheric TEC disturbance over seismically active region in southwest China during low solar activity

Using data of Crustal Movement Observation Network of China (CMONC) we analyzed variations of total electron content (TEC) over southwest China during a period of low solar and geomagnetic activity in April-October 2008. During that time, six large earthquakes with magnitude M≥6.0 occurred around the southwest region of China. To determine the abnormal TEC signals, a quartile-based process is performed. At each time point we calculated the median M using the TEC values at the same local time for the preceding 15 days. In addition, we calculated the maps of differential TEC from global ionosphere maps (GIM) in the above period. Known that the decisive role in the ionosphere state is performed by space weather effects, we compared the TEC variations with time series of EUV solar radiation, Bz component of the interplanetary magnetic field (IMF), index of geomagnetic activity D_st and planetary index K_p, respectively. It is found that the observed anomalies in the regional TEC are in good correlation with the variations of above solar and geomagnetic activities. We have not detected obvious TEC disturbance before five earthquakes expect for Wenchuan earthquake. In order to additionally check the spatial occurrence of the observed anomalies, we chose a "check-region" with the same geomagnetic latitudes as the epicentral area of the six considered earthquakes, and analyzed the TEC changes. The result shows that the time series of anomalous TEC in "check-region" is similar to the epicentral area, which is dominated by space weather. Therefore, it indicates the difficulties in identifying earthquake precursors in the ionosphere TEC. The seismo-ionospheric coupling process is complex, and it is necessary to strengthen the observation of multi-parameters on the ground, in the atmosphere and ionosphere synchronously.