Seismoionospheric anomalies observed before Wenchuan earthquake using GPS and DEMETER data

Based on GPS TEC data and ISL detector data recorded by the French DEMETER satellite, ionospheric perturbations before the Wenchuan earthquake were analyzed. It is found that GPS TEC above the epicenter continuously decreased in the afternoon period during 6 May to 10 May but increased in the afternoon of 9 May. The spatial distribution of the TEC anomaly mainly located in the south of the epicenter, and the size of perturbation area was 1100~1670km and 1600~3700km from the epicenter in the latitudinal and longitudinal directions, respectively. It is further found that negative anomaly area located southeast on 6 May and moved to southwest on 7 May from epicenter. On 9 May, the anomaly turned to be positive and occurred in the southeast of the epicenter. The intensity of negative anomaly in the southwest or south area from epicenter was stronger than that in the southeast area, compared with the southwest area, the intensity of positive anomaly in southeast or south area was stronger. Moreover, it could be seen that the perturbation was also observed in the geomagnetic conjugate points of epicenter at the southern hemisphere. In addition, contour analysis on ionopheric plasma shows that electron density (N _e) and ion density (N _i) also reduced during 6 to 10 May. The anomalous distribution was in good conformity with TEC, which was concentrated mainly in the south of epicenter. There were two negative anomaly peaks along the earthquake longitude, which were symmetrical about the magnetic equator. Base on the above observation, we finally discuss the physical mechanism of the seismoionospheric anomaly according to Lithosphere-Atmosphere-Ionosphere Couling(LAIC) model.