中文摘要：

DEMETER卫星由法国于2004年6月发射,高度约为665km,圆轨道且经过极区,其主要目的是研究与地震有关的电离层异常.2008年5月12日6∶28（世界时）在四川省汶川县（31.02°N,103.37°E）发生了Ms8.0级的特大地震.本文分析了在震前一周内DEMETER卫星观测到的电离层异常.这些异常主要包括：（1）卫星在震前4天和5天经过震中附近时,记录到电子浓度、电子温度和氧离子浓度出现剧烈变化（变化率均超过20%）;（2）在震前3天和7天,卫星探测到增强的电磁辐射.同时利用ELF频段（0～1250Hz）的三分量电场和磁场数据,对异常电磁辐射作了进一步的分析（计算了电场功率谱、磁场功率谱、波矢方向和极化特性等）.另外,考查了观测期间的地磁指数,由此得出观测到的异常现象不大可能由地磁活动引起.最后,对震前出现的这些电离层异常的可能的物理机理进行了讨论.

英文摘要：

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 Ms 8.0 Wenchuan earthquake（31.02°N,103.37°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 the three-component electric and magnetic ELF（extremely low frequency,0-1250 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.