关键词: Langmiur探针, DEMETER, 电子浓度, 电子温度, 汤加地震, 台湾地震

Abstract: Using Ne(electron density) and Te(electron temperature) data of Langmuir Probe onboard DEMETER(Detection of Electro-magnetic Emissions Transmitted from Earthquake Regions) satellite,the paper studies the variations of electron density and electron temperature associated with strong earthquakes.The specific scientific objectives of Langmuir Probe are to map the bulk plasma parameters and to study their variations associated with seismic activity,volcanism and human activity and other sources of perturbations.Langmuir probe sweeps in voltage allow to obtain a current-voltage(Ⅰ-Ⅴ) characteristic every second.Bulk plasma parameters are obtained from the analysis of the Ⅰ-Ⅴ characteristics.The work principle,data format of Langmuir Probe onboard DEMETER is briefly introduced.We mainly studied the 2006 M_W 7.9 Tonga earthquake and the M_W 7.1 Taiwan earthquake in this paper.For each earthquake,we choose the orbits which passed over the area about 2000km around epicenter,including four months data before and two months data after the earthquakes.There are mainly three steps to reprocess the raw data,in order to avoid disturbances resulting from solar activities and geomagnetic activities.The result on the M_W 7.9 Tonga earthquake shows that at the equatorial region the electron density was relatively low before 28 April,while it began to increase from 28 April,and got the highest at 2 May one day before the earthquake.Until 4 May after the main shock,Ne began to decline.Based on comparison between revisited orbits,the orbit 097681(before shock) and 098721(after shock) observed certain disturbance.To further investigate the features of the preearthquake ionospheric anomalies,using 5 days as time window and 1 day as step size,we examined the temporal and spatial evolution of electron density within the area of 2000km around epicenter.Results show that the normal distribution of Ne kept at a relatively high level,except a relatively sharp reduction of Ne from 29 April to 2 May.The result on the M_W 7.1Taiwan earthquake shows that Ne changed randomly,while Te kept at a relatively high level,but dropped gradually before the quake.Currently we have no evidence to conclude that these changes are the results from the seismic activity,but based on data reprocessing,we can exclude the disturbances induced by solar activity and magnetic field at certain degree.If these changes of Ne/Te really resulted from earthquake,we may prudently conclude that ionospheric disturbance associated with seismic activity may possibly occur in a short time impending an earthquake with a short duration.

Key words: Langmiur, DEMETER, electron density(Ne), electron temperature(Te), Tonga earthquake, Taiwan earthquake