The analysis of ionospheric variations before Wenchuan earthquake with DEMETER data

The French micro-satellite DEMETER was launched on June 29, 2004 in a polar and circular orbit with an altitude of about 665 km. The main objective of the satellite is to research ionospheric disturbance related to seismic activities. This paper reports the ionospheric anomalies observed by DEMETER within one week before the M-s 8.0 Wenchuan earthquake (31.02 degrees N, 103.37 degrees E) which occurred at 0628UT on May 12, 2008. The observation results include: (1) Electron density, electron temperature and oxygen ion density changed sharply (greater than 20%) near the epicenter four and five days prior to the shock. (2) Increased electromagnetic emissions were registered when the satellite passed the epicenter three and seven days before the shock. A further analysis (calculations of electric and magnetic power spectra, the wave vector direction and polarization sense) of the increased electromagnetic emissions is carried out based on I the three-component electric and magnetic ELF (extremely low frequency, 0 similar to 1250 Hz) data. In addition, the geophysical conditions are also investigated and it can be inferred that the observed anomalies are not likely due to geomagnetic activity. Finally, a possible mechanism which could account for these pre-earthquake ionospheric perturbations is discussed.