Abstract: 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.

Key words: TEC(total electron content), strong earthquakes, solar and geomagnetic activities, long time series analysis

摘要： 通过中国地壳运动网络提供的GPS观测数据，获取了高精度电离层TEC分布，采用滑动四分位法分析了中国西南区域2008年4—10月（太阳和地磁活动平静时段）6次连续的M_W6.0以上地震期间孕震区电离层TEC长时间变化及其异常分布；并在此基础上利用GIM数据对比分析了全球TEC变化特征。鉴于电离层主要受到太阳和地磁等空间天气的影响，将TEC变化与太阳EUV辐射、行星际磁场南向分量IMF Bz以及地磁活动指数D_st和K_p进行了比较。结果发现，该时段内电离层TEC异常扰动与太阳和地磁活动有很好的相关性；除汶川地震外，其他地震前没有发现明显的跟地震相关的TEC异常扰动现象。同时，对比分析了与上述研究区位于同一地磁纬度的“检验区”（30°～50°E，15°～35°N）的GPS TEC随时间变化和异常分布情况，结果显示TEC异常分布的时空特征与研究区域较为一致。由于电离层是一个复杂的系统，其扰动具有多源性，而且地震电离层扰动现象是复杂多变的，因此需要联合地基和天基手段共同观测，并加强其机理研究。

关键词: 电离层TEC, 强震, 太阳和地磁活动, 长时间序列, 中国西南地区