基于GPS和DEMETER卫星数据的地震电离层电子浓度异常变化研究
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摘要
地震电磁现象的观测与研究是认识地球内部物质特性、地震孕育发生过程以及空间信号传播和介质特性变化的重要途径。空间对地观测技术的发展，推动了地震电磁信号在大气层、电离层的传播与耦合机制的研究。从上世纪60年代发现美国阿拉斯加大地震震中区上空出现电离层异常扰动现象以来，有关地震空间电磁、电离层扰动的现象引起了广泛的关注，成为了地震电磁学领域一个新的研究热点。研究表明，在地震孕育发生过程中，地下介质由弹性变形进入非弹性形变阶段，原有微裂隙扩展、新生微裂隙发育、并定向排列，体积膨胀，水溶液进入和重新分布等物理过程，产生压磁、压电、动电、感应电磁、热磁效应等物理电磁效应的同步和定向激发，从而引起在距离震源一定范围可观测的明显地震电磁前兆现象。大量的震例和统计分析显示，大地震和火山喷发前几天到几小时内，对应孕震区、火山喷发区上空大气层、电离层存在电磁异常扰动现象。主要包括：①L F/VLF/ELF/ULF信号相位、振幅异常变化；②电离层电子浓度总含量（TotalElectron Content，TEC）和电离层F2层临界频率foF2异常；③电离层等离子体参数变化；④高能粒子扰动；⑤热红外辐射异常等。
全球定位系统（Global Positioning System，GPS）作为当前探测电离层的有效手段已经得到广泛应用，通过其获取的电离层TEC则是揭示电离层时空变化规律和特征的重要物理参量。2000年以来，俄罗斯、美国、法国、乌克兰等国先后发射了地震电磁卫星，尤其是法国DEMETER卫星，其科学数据被广泛的应用于地震电离层扰动研究，取得了极大进展。中国的地震电磁监测试验卫星（ChinaSeismo-Electromagnetic Satellite，CSES）计划已经列入“十二五”立项。
本论文依托地震电磁立体观测体系发展思路，配合中国地震电磁卫星计划的推进，开展了基于DEMETER卫星和GPS数据的地震空间电磁电离层扰动现象的研究。本文研究内容主要可分为三大部分。第一部分，研究GPS测量电离层TEC的原理，掌握电磁卫星数据处理方法和异常提取技术；开展DEMETER卫星各载荷数据处理及异常提取方法研究。第二部分，分别针对汶川地震、日本地震和玉树地震开展了震前电离层扰动的多参数分析，研究和对比了3次强震前电离层异常的特征；分别利用DEMETER卫星数据和GPS数据统计分析了全球7.0级以上强震和中国境内6.0级以上强震期间的电子浓度异常变化规律；针对我国西南地震多发区，开展伴随着多次地震的电离层TEC长时间序列分析，深化对地震电离层异常扰动的研究。第三部分，调研和总结目前地震电离层耦合模拟的最新成果，详细阐述孕震区扰动电场SEF的计算过程，分析其对电离层的效应。
本文研究工作获得以下主要认识：
1．汶川地震的电离层扰动多参数分析结果表明震前多种参量都出现了一定程度的扰动现象，不同参数在时间和空间上的扰动特征并不一致，但这些参数之间存在相应的联系。
利用GPS TEC、DEMETER卫星和NOAA/AVHRR卫星的观测数据，系统梳理了汶川地震前多种参数（包括电离层TEC、Ne、Ni、Tb、Ti以及高能粒子通量等）的变化特征，探索总结了汶川地震前大气层–电离层响应。结果表明，电离层电子浓度在5月6～10日有连续的负异常现象，5月9日则明显增强；异常的范围主要位于震中南部区域，纬向约1100～1670km，经向约1600～3700km，扰动的峰值距震中约700～900km。NOAA/AVHRR卫星数据得到汶川地震前5月7日和8日出现了显著的热红外亮温异常，而DEMETER卫星的原位探测则发现5月9日离子温度有明显增高，幅度约12.5%；以上异常区域都位于震中西北，范围约100～300km。此外，DEMETER卫星IDP探测器高能粒子数据结果显示5月6日夜间100～600KeV能谱段出现明显增强，增幅达6σ；这与前人得到的异常电场增强时间相一致。研究结果表明，不同参数在时间和空间上的扰动特征并不一致，但这些参数之间存在相应的联系，因此有必要联合地基和天基手段共同观测。
2．日本地震和玉树地震的电离层电子浓度变化研究结果表明，震前TEC都出现了不同程度的增强现象，而异常同时可能受到太阳或地磁活动的共同作用。
利用IGS提供的TEC数据和日本NICT提供的电离层测高仪数据分析了2011年3月11日日本Mw9.0级特大地震（Tohoku-oki Earthquake）前上空的电离层变化。研究发现，震前3天即3月8日04～14UT期间赤道异常区电离层TEC出现了明显的增强现象，异常最大幅度达40TECU，并伴随有南半球磁共轭区增强现象。日本境内的4个电离层测高仪数据分析显示，3月8日4个台站同时出现了foF2增强现象，其中Okinawa和Yamagawa两个站增幅明显。需要指出，3月6日～12日太阳活动水平有所增强，可见太阳和地磁活动对3月8日的大幅度的TEC扰动有所贡献。然而，分析表明仅仅是太阳活动增强无法造成如此强度的电离层扰动，即无论从持续时间、空间分布特征以及异常强度等方面都表明，以上TEC变化跟此次大地震有很大的关系。
同时，综合GPS和DEMETER卫星两种观测手段研究了玉树地震前的电离层等离子体参量变化情况。结果表明，GPS观测的电离层TEC在震前20小时左右有明显的增强现象，TEC增强的区域位于震中南部15°N～30°N纬度范围内，扰动的峰值距震中约1000km。TEC增强区域显示为典型的EIA增强现象，与汶川和日本地震前观测到的TEC空间扰动特征较为一致。通过对DEMETER卫星IAP和ISL载荷记录的电离层等离子体参数时序变化和空间纬度变化分析，没有发现明显的跟地震相关的扰动现象。以上也表明了两种观测手段由于测量电子浓度的方式不同，其观测结果存在一定的差别。
3．DEMETER卫星数据统计全球强震期间电子浓度Ne变化结果表明，在赤道和中低纬地区更容易观测到明显的Ne扰动，震级大小则与扰动幅度基本呈正相关。基于GPS数据全国强震期间TEC变化统计分析结果则表明，TEC异常扰动多集中在震前2～6天，同样以增强为主，且集中在地方时12～20LT。
基于法国DEMETER卫星ISL升轨数据，统计分析了2005～2009年全球37个M≧7.0级地震前后电离层电子浓度变化。结果发现，共有19个地震（51%）前观测到了较为明显的电子浓度异常扰动现象，其中大部分表现为异常增强；观测到的电子浓度异常现象一般出现在震前1～5天内，也有部分地震前出现两次及以上扰动现象。分析表明，在赤道和中低纬地区更容易观测到明显的电子浓度扰动，即异常震例基本发生在纬度±40°以内（18次），更是以±20°内居多（13次）。此外，震源深度对电子浓度扰动的影响并不明显，而震级大小则与扰动幅度基本上呈正相关；震前出现的电子浓度异常有时会受到地磁活跃的共同影响，此时的扰动幅度一般较大。
基于GPS数据统计分析了2000～2010年期间中国境内（包括台湾地区）30次Mw6.0级以上地震的TEC异常扰动情况，结果表明，20次地震前（66.7%）观测了较为明显的TEC异常扰动，扰动以增强为主；扰动多集中在震前2～6天，而且具有明显的地方时特点，扰动一般集中在地方时12～20LT，持续一般平均4～6小时，有的地震可以持续几天。震前电离层TEC异常的空间变化主要表现为对电离层赤道异常双峰（EIA）的影响上，不但会增强或降低峰值位置的电子浓度值，而且会造成驼峰最大值位置向磁赤道方向偏移。
4．通过对中国西南地震多发区以及“检验区”电离层TEC变化长时间序列分析结果表明，电离层是一个复杂的系统，受到太阳、地磁以及地震活动等因素影响，其扰动具有多源性。
通过中国地壳运动网络提供的GPS观测数据，获取高精度电离层TEC分布，采用滑动四分位法分析研究了中国西南区域2008年4月～10月（太阳和地磁活动平静时段）6次连续的Mw6.0级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（Global Ionosphere Maps）数据分析了全球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，我们将TEC变化与太阳EUV（Extreme ultraviolet）辐射、行星际磁场（Interplanetary magnetic field，IMF）南向分量Bz以及地磁活动指数Dst和Kp进行了比较。研究发现，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；而除汶川地震外，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。结果表明，电离层是一个复杂的系统，其扰动具有多源性。
5．通过对地震电离层耦合机理研究初探，认识到电离层TEC在SEF的作用下出现明显变化，在赤道向较为明显。
介绍了电离层电流和电导率张量以及电离层O+光化学作用的数学物理方程，总结了孕震区扰动电场（Seisomogenic electrostatic field，SEF）的数学模型及公式推导，结果表明夜间扰动电场更容易传播到电离层，这与观测现象相一致。扰动电场向上传播至电离层F层高度，导致电离层电子浓度变化。结果显示电离层TEC在SEF的作用下出现明显变化，而在赤道向（longitude equatorward）上更为明显，这同样也与观测现象一致。
Recently, there are growing interests in studying the atmosphere-ionosphereresponse prior to earthquakes, including case studies and seismo-ionospheric couplingmechanism analysis. The existing studies have shown that many physical effects suchas piezomagnetism, piezoelectrics and pryromagnetic are associated with anearthquake, due to the extending of original microcrack and the development of theregenerate microcrack in the crust occurring in the earthquake preparation process.Consequently, some seismo-electromagnetic phenomena could then be observed byground based facilities and satellites within a certain range from the seismic source. Abrief list of such phenomena includes:①the phase or amplitude changes of theVLF/ELF/ULF signals,②total electron content (TEC) and critical frequency ofF2-layer (foF2) perturbations in ionosphere,③the anomalies in the electric field,magnetic field as well as plasma parameters,④thermal-infrared emission anomaly,⑤energetic particles bursts.
Global Positioning System (GPS) measurement technique can be used fordetecting ionospheric response to earthquakes, in addition to investigating crustaldeformation characteristics and studying the motion of tectonic. Since the new century,several countries including Russia, USA, France and Ukraine have launched theirseismic electromagnetic satellites. Particularly, the scientific data of FranceDEMETER satellite are widely used to search for and characterize ionosphericperturbations associated with earthequakes. Now, the China Seismo-ElectromagneticSatellite (CSES) has been officially included in the national plans of satellite launch.
This work focuses on the seismo-electromagnetic phenomena based on GPS andDEMETER satellite data. This thesis includes three parts. Firstly, it gives anintroduction about the principle of TEC measurement by GPS technique, theprocessing methods of satellite data as well as the ways to identify the abnormalsignals. Secondly, this thesis analyzes the observations of the multi-parameterseismo-ionospheric anomalies before the Wenchuan, Tohoku-oki and Yushuearthquake, respectively, and compares the characteristics of electron density variations before these three earthquakes. In addition, it presents the statisticalanalysis results of electron density perturbation before major earthquakes observed onDEMETER satellite and GPS measurements. Moreover, aiming to deepen theunderstanding of seismo-ionospheric phenomenon, it analyzes variations of totalelectron content (TEC) over southwest China during a period of low solar andgeomagnetic activity in April-October2008. Thirdly, it presents the research progressof modeling the ionospheric effect of the seismogenic electrostatic field (SEF) seen atthe earth's surface as a perturbation of the vertical atmospheric electrostatic field inthe earthquake preparation zone.
The main results of this work are as follows:
1. Multi-parameter Seismo-ionospheric Anomalies Observation before the2008Wenchuan, China Mw7.9Earthquake
This thesis analyzes the atmosphere-ionosphere response before2008Mw7.9Wenchuan earthquake based on GPS, DEMETER and NOAA/AVHRR satellite data.It is found that GPS TEC above the epicenter continuously decreased in the afternoonfrom6to10May but increased in the afternoon of9May. Contour analysis onionopheric plasma using ISL data onboard DEMETER satellite shows that electrondensity (Ne) and Ion density (Ni) also reduced from6to10May. The anomalousregions mainly lie in the south of epicenter, attesting to the anomalous TEC area. Thebrightness temperature (Tb) from NOAA/AVHRR data, calculated by the K indexmethod, shows a noticeable enhancement on the northwest side of the epicenter on7May, while Ion temperature (Ti) from DEMETER data increased on9May, with theamplitude of~12.5%. Besides, the energetic particle flux exhibited an obviouslyenhancement for the spectrum at100-600KeV energy range on6May. Preliminarystudy suggests the perturbations of these parameters (TEC, Ne, Ni, Tband Ti) beforethe Wenchuan earthquake may be related to the changes of vertical electric field in theatmosphere and ionosphere, induced by the enhancement of stress in tectonic regionsand the electromagnetic signal propagation from the earthquake preparation area.
2. Study of the seismo-ionospheric perturbation before the Tohoku-okiearthquake and Yushu earthquake
The relationship between the behavior in the ionosphere and the2011Mw9.0Tohoku-Oki earthquake was analyzed using the GPS TEC data and ionosonde data. Itcan be found from the GIM TEC maps that during04-14UT on8March2011,3daysbefore the earthquake, there was an enhancement of TEC with an amplitude peak of40TECU appearing in the EIA region, southwest of the epicenter. In addition, thefoF2also recorded an unusual enhancement on8March by the four ionosondestations in Japan, of which the increase was larger in Okinawa and Yamagawa. Thetemporal-spatial distribution of the extreme TEC enhancement within30days beforethe earthquake was particularly pursued. It is found that the extreme enhancement waspersistently located in the region adjacent to the epicenter and the magnetic conjugatepoint for a long time of16h. It is noted that during the same period the solar activityalso had a large increase. The study indicates that only the solar radiationenhancement was not enough to produce the observed significant TEC enhancement.It means that the significant TEC enhancement on March8might be related to boththe M9.0Tohoku-Oki earthquake and the solar activity.
Meanwhile, this thesis presents the ionospheric plasma variations before theYushu earthquake using both GPS and DEMETER data. The result from GPS showsthat TEC began to enhance20hours before the earthquake, appearing in the region15°N～30°N. The spatial characteristics of TEC perturbation were similar to thatbefore the Wenchuan earthquake. On the other hand, based on the DEMETERsatellite IAP and ISL data, this work studies the time series and spatial variations ofthe ionospheric plasma including Ne, Ni, O+, H+and He+before the earthquake, andfinds no disturbance correlated with the shock. It demonstrates that there are certaindifferences over the ionospheric disturbance with the elevation changes before theearthquake.
3. Statistical analysis of electron density perturbation before major earthquakesobserved by DEMETER satellite and GPS measurements
Based on the data of DEMETER satellite, this work attempts to make a statisticalanalysis about the electron density perturbation before37major earthquakes ofmagnitude above7.0during January2005to December2009. The orbits, recorded in local nighttime by satellite, are chosen by a range of longitude20degrees and latitude15degrees during the9days around these earthquakes, with7days before and1dayafter. Among the37earthquakes, electron density perturbations were detected before19earthquakes, nearly accounting for51%. The anomalies were positive before18events and occurred mostly within1-6days before the shocks, part of which appearedmore than one time. Results indicate that it is more easily to observe the electrondensity disturbance around the equatorial and low-latitude regions than thehigh-latitude areas. The disturbance amplitude presents a positive correlation with themagnitude of earthquakes basically, while the depth of earthquakes does not showsignificant influence on the perturbation. It is worth noting that before several events,the anomalies are in the combined effect of earthquake and geomagnetic activity andpresent a higher disturbance level.
This thesis studied the ionospheric anomalies associated with30earthquakesoccurred in China (including Taiwan) during2000to2010by examining theGPS-inferred TEC variations. It is shown that TEC perturbations were detected before20earthquakes among the30events, nearly67%of the total. The TEC anomaliesrepresent increase more often and occurred mostly within2-6days before the shocks,part of them appeared more than one time. And the pre-earthquake ionosphericanomalies appeared during12:00–20:00LT significantly. The analysis indicates thatthe anomalies were not so relevant to the magnitude of the forthcoming earthquakes.Both these upper and lower anomalies were also found in the magnetically conjugatedarea of the seismogenic zone to the geomagnetic equator. The spatial distributions ofthese anomalies are not of an east-west asymmetry and have an equatorward shift ofboth crests of the EIA.
4. Long time series analysis of ionospheric TEC disturbance over seismicallyactive region in southwest China during low solar activity
Using data of Crustal Movement Observation Network of China (CMONC) thisthesis analyzes variations of TEC over southwest China during a period of low solarand geomagnetic activity in April-October2008. During that time, six largeearthquakes with magnitude M≧6.0occurred around the southwest region of China. To determine the abnormal TEC signals, a quartile-based processing is performed. Ateach time point we calculated the median M using the TEC values at the same localtime for the preceding15days. Known that the decisive role in the ionosphere state isperformed by space weather effects, we compared the TEC variations with time seriesof EUV solar radiation, Bz component of the interplanetary magnetic field (IMF),index of geomagnetic activity Dst and planetary index Kp, respectively. It is foundthat the observed anomalies in the regional TEC were in good correlation with thevariations of above solar and geomagnetic activities. We have not detected obviousTEC disturbance before five earthquakes expect for the Wenchuan earthquake. Inorder to additionally check the spatial occurrence of the observed anomalies, wechose a “check-region” with the same geomagnetic latitudes as the epicentral area ofthe six considered earthquakes, and analyzed the TEC changes. The results show thatthe time series of anomalous TEC in the “check-region” is similar to the epicentralarea, which is dominated by space weather. Therefore, it indicates the difficulties inidentifying earthquake precursors in the ionosphere TEC. The seismo-ionosphericcoupling process is complex, and it is necessary to strengthen the observation ofmulti-parameters on the ground, in the atmosphere and ionosphere synchronously.
5. Preliminary study about modeling the pre-earthquake electrostatic effect onthe ionosphere
This thesis introduces the equations of the ionospheric currents and electricalconductivity tensor and summarizes the mathematical model and calculation methodsof the penetration of seisomogenic electrostatic field (SEF) into the ionosphere. Thecaused ionosphere effect by SEF was obtained by the O+continuity equation. It isshown that at night, the horizontally large scale SEF can efficiently penetrate into theionosphere and produce noticeable changes in the horizontal distribution of the Fregion electron density.
全球定位系统（Global Positioning System，GPS）作为当前探测电离层的有效手段已经得到广泛应用，通过其获取的电离层TEC则是揭示电离层时空变化规律和特征的重要物理参量。2000年以来，俄罗斯、美国、法国、乌克兰等国先后发射了地震电磁卫星，尤其是法国DEMETER卫星，其科学数据被广泛的应用于地震电离层扰动研究，取得了极大进展。中国的地震电磁监测试验卫星（ChinaSeismo-Electromagnetic Satellite，CSES）计划已经列入“十二五”立项。
本论文依托地震电磁立体观测体系发展思路，配合中国地震电磁卫星计划的推进，开展了基于DEMETER卫星和GPS数据的地震空间电磁电离层扰动现象的研究。本文研究内容主要可分为三大部分。第一部分，研究GPS测量电离层TEC的原理，掌握电磁卫星数据处理方法和异常提取技术；开展DEMETER卫星各载荷数据处理及异常提取方法研究。第二部分，分别针对汶川地震、日本地震和玉树地震开展了震前电离层扰动的多参数分析，研究和对比了3次强震前电离层异常的特征；分别利用DEMETER卫星数据和GPS数据统计分析了全球7.0级以上强震和中国境内6.0级以上强震期间的电子浓度异常变化规律；针对我国西南地震多发区，开展伴随着多次地震的电离层TEC长时间序列分析，深化对地震电离层异常扰动的研究。第三部分，调研和总结目前地震电离层耦合模拟的最新成果，详细阐述孕震区扰动电场SEF的计算过程，分析其对电离层的效应。
本文研究工作获得以下主要认识：
1．汶川地震的电离层扰动多参数分析结果表明震前多种参量都出现了一定程度的扰动现象，不同参数在时间和空间上的扰动特征并不一致，但这些参数之间存在相应的联系。
利用GPS TEC、DEMETER卫星和NOAA/AVHRR卫星的观测数据，系统梳理了汶川地震前多种参数（包括电离层TEC、Ne、Ni、Tb、Ti以及高能粒子通量等）的变化特征，探索总结了汶川地震前大气层–电离层响应。结果表明，电离层电子浓度在5月6～10日有连续的负异常现象，5月9日则明显增强；异常的范围主要位于震中南部区域，纬向约1100～1670km，经向约1600～3700km，扰动的峰值距震中约700～900km。NOAA/AVHRR卫星数据得到汶川地震前5月7日和8日出现了显著的热红外亮温异常，而DEMETER卫星的原位探测则发现5月9日离子温度有明显增高，幅度约12.5%；以上异常区域都位于震中西北，范围约100～300km。此外，DEMETER卫星IDP探测器高能粒子数据结果显示5月6日夜间100～600KeV能谱段出现明显增强，增幅达6σ；这与前人得到的异常电场增强时间相一致。研究结果表明，不同参数在时间和空间上的扰动特征并不一致，但这些参数之间存在相应的联系，因此有必要联合地基和天基手段共同观测。
2．日本地震和玉树地震的电离层电子浓度变化研究结果表明，震前TEC都出现了不同程度的增强现象，而异常同时可能受到太阳或地磁活动的共同作用。
利用IGS提供的TEC数据和日本NICT提供的电离层测高仪数据分析了2011年3月11日日本Mw9.0级特大地震（Tohoku-oki Earthquake）前上空的电离层变化。研究发现，震前3天即3月8日04～14UT期间赤道异常区电离层TEC出现了明显的增强现象，异常最大幅度达40TECU，并伴随有南半球磁共轭区增强现象。日本境内的4个电离层测高仪数据分析显示，3月8日4个台站同时出现了foF2增强现象，其中Okinawa和Yamagawa两个站增幅明显。需要指出，3月6日～12日太阳活动水平有所增强，可见太阳和地磁活动对3月8日的大幅度的TEC扰动有所贡献。然而，分析表明仅仅是太阳活动增强无法造成如此强度的电离层扰动，即无论从持续时间、空间分布特征以及异常强度等方面都表明，以上TEC变化跟此次大地震有很大的关系。
同时，综合GPS和DEMETER卫星两种观测手段研究了玉树地震前的电离层等离子体参量变化情况。结果表明，GPS观测的电离层TEC在震前20小时左右有明显的增强现象，TEC增强的区域位于震中南部15°N～30°N纬度范围内，扰动的峰值距震中约1000km。TEC增强区域显示为典型的EIA增强现象，与汶川和日本地震前观测到的TEC空间扰动特征较为一致。通过对DEMETER卫星IAP和ISL载荷记录的电离层等离子体参数时序变化和空间纬度变化分析，没有发现明显的跟地震相关的扰动现象。以上也表明了两种观测手段由于测量电子浓度的方式不同，其观测结果存在一定的差别。
3．DEMETER卫星数据统计全球强震期间电子浓度Ne变化结果表明，在赤道和中低纬地区更容易观测到明显的Ne扰动，震级大小则与扰动幅度基本呈正相关。基于GPS数据全国强震期间TEC变化统计分析结果则表明，TEC异常扰动多集中在震前2～6天，同样以增强为主，且集中在地方时12～20LT。
基于法国DEMETER卫星ISL升轨数据，统计分析了2005～2009年全球37个M≧7.0级地震前后电离层电子浓度变化。结果发现，共有19个地震（51%）前观测到了较为明显的电子浓度异常扰动现象，其中大部分表现为异常增强；观测到的电子浓度异常现象一般出现在震前1～5天内，也有部分地震前出现两次及以上扰动现象。分析表明，在赤道和中低纬地区更容易观测到明显的电子浓度扰动，即异常震例基本发生在纬度±40°以内（18次），更是以±20°内居多（13次）。此外，震源深度对电子浓度扰动的影响并不明显，而震级大小则与扰动幅度基本上呈正相关；震前出现的电子浓度异常有时会受到地磁活跃的共同影响，此时的扰动幅度一般较大。
基于GPS数据统计分析了2000～2010年期间中国境内（包括台湾地区）30次Mw6.0级以上地震的TEC异常扰动情况，结果表明，20次地震前（66.7%）观测了较为明显的TEC异常扰动，扰动以增强为主；扰动多集中在震前2～6天，而且具有明显的地方时特点，扰动一般集中在地方时12～20LT，持续一般平均4～6小时，有的地震可以持续几天。震前电离层TEC异常的空间变化主要表现为对电离层赤道异常双峰（EIA）的影响上，不但会增强或降低峰值位置的电子浓度值，而且会造成驼峰最大值位置向磁赤道方向偏移。
4．通过对中国西南地震多发区以及“检验区”电离层TEC变化长时间序列分析结果表明，电离层是一个复杂的系统，受到太阳、地磁以及地震活动等因素影响，其扰动具有多源性。
通过中国地壳运动网络提供的GPS观测数据，获取高精度电离层TEC分布，采用滑动四分位法分析研究了中国西南区域2008年4月～10月（太阳和地磁活动平静时段）6次连续的Mw6.0级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（Global Ionosphere Maps）数据分析了全球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，我们将TEC变化与太阳EUV（Extreme ultraviolet）辐射、行星际磁场（Interplanetary magnetic field，IMF）南向分量Bz以及地磁活动指数Dst和Kp进行了比较。研究发现，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；而除汶川地震外，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。结果表明，电离层是一个复杂的系统，其扰动具有多源性。
5．通过对地震电离层耦合机理研究初探，认识到电离层TEC在SEF的作用下出现明显变化，在赤道向较为明显。
介绍了电离层电流和电导率张量以及电离层O+光化学作用的数学物理方程，总结了孕震区扰动电场（Seisomogenic electrostatic field，SEF）的数学模型及公式推导，结果表明夜间扰动电场更容易传播到电离层，这与观测现象相一致。扰动电场向上传播至电离层F层高度，导致电离层电子浓度变化。结果显示电离层TEC在SEF的作用下出现明显变化，而在赤道向（longitude equatorward）上更为明显，这同样也与观测现象一致。
Recently, there are growing interests in studying the atmosphere-ionosphereresponse prior to earthquakes, including case studies and seismo-ionospheric couplingmechanism analysis. The existing studies have shown that many physical effects suchas piezomagnetism, piezoelectrics and pryromagnetic are associated with anearthquake, due to the extending of original microcrack and the development of theregenerate microcrack in the crust occurring in the earthquake preparation process.Consequently, some seismo-electromagnetic phenomena could then be observed byground based facilities and satellites within a certain range from the seismic source. Abrief list of such phenomena includes:①the phase or amplitude changes of theVLF/ELF/ULF signals,②total electron content (TEC) and critical frequency ofF2-layer (foF2) perturbations in ionosphere,③the anomalies in the electric field,magnetic field as well as plasma parameters,④thermal-infrared emission anomaly,⑤energetic particles bursts.
Global Positioning System (GPS) measurement technique can be used fordetecting ionospheric response to earthquakes, in addition to investigating crustaldeformation characteristics and studying the motion of tectonic. Since the new century,several countries including Russia, USA, France and Ukraine have launched theirseismic electromagnetic satellites. Particularly, the scientific data of FranceDEMETER satellite are widely used to search for and characterize ionosphericperturbations associated with earthequakes. Now, the China Seismo-ElectromagneticSatellite (CSES) has been officially included in the national plans of satellite launch.
This work focuses on the seismo-electromagnetic phenomena based on GPS andDEMETER satellite data. This thesis includes three parts. Firstly, it gives anintroduction about the principle of TEC measurement by GPS technique, theprocessing methods of satellite data as well as the ways to identify the abnormalsignals. Secondly, this thesis analyzes the observations of the multi-parameterseismo-ionospheric anomalies before the Wenchuan, Tohoku-oki and Yushuearthquake, respectively, and compares the characteristics of electron density variations before these three earthquakes. In addition, it presents the statisticalanalysis results of electron density perturbation before major earthquakes observed onDEMETER satellite and GPS measurements. Moreover, aiming to deepen theunderstanding of seismo-ionospheric phenomenon, it analyzes variations of totalelectron content (TEC) over southwest China during a period of low solar andgeomagnetic activity in April-October2008. Thirdly, it presents the research progressof modeling the ionospheric effect of the seismogenic electrostatic field (SEF) seen atthe earth's surface as a perturbation of the vertical atmospheric electrostatic field inthe earthquake preparation zone.
The main results of this work are as follows:
1. Multi-parameter Seismo-ionospheric Anomalies Observation before the2008Wenchuan, China Mw7.9Earthquake
This thesis analyzes the atmosphere-ionosphere response before2008Mw7.9Wenchuan earthquake based on GPS, DEMETER and NOAA/AVHRR satellite data.It is found that GPS TEC above the epicenter continuously decreased in the afternoonfrom6to10May but increased in the afternoon of9May. Contour analysis onionopheric plasma using ISL data onboard DEMETER satellite shows that electrondensity (Ne) and Ion density (Ni) also reduced from6to10May. The anomalousregions mainly lie in the south of epicenter, attesting to the anomalous TEC area. Thebrightness temperature (Tb) from NOAA/AVHRR data, calculated by the K indexmethod, shows a noticeable enhancement on the northwest side of the epicenter on7May, while Ion temperature (Ti) from DEMETER data increased on9May, with theamplitude of~12.5%. Besides, the energetic particle flux exhibited an obviouslyenhancement for the spectrum at100-600KeV energy range on6May. Preliminarystudy suggests the perturbations of these parameters (TEC, Ne, Ni, Tband Ti) beforethe Wenchuan earthquake may be related to the changes of vertical electric field in theatmosphere and ionosphere, induced by the enhancement of stress in tectonic regionsand the electromagnetic signal propagation from the earthquake preparation area.
2. Study of the seismo-ionospheric perturbation before the Tohoku-okiearthquake and Yushu earthquake
The relationship between the behavior in the ionosphere and the2011Mw9.0Tohoku-Oki earthquake was analyzed using the GPS TEC data and ionosonde data. Itcan be found from the GIM TEC maps that during04-14UT on8March2011,3daysbefore the earthquake, there was an enhancement of TEC with an amplitude peak of40TECU appearing in the EIA region, southwest of the epicenter. In addition, thefoF2also recorded an unusual enhancement on8March by the four ionosondestations in Japan, of which the increase was larger in Okinawa and Yamagawa. Thetemporal-spatial distribution of the extreme TEC enhancement within30days beforethe earthquake was particularly pursued. It is found that the extreme enhancement waspersistently located in the region adjacent to the epicenter and the magnetic conjugatepoint for a long time of16h. It is noted that during the same period the solar activityalso had a large increase. The study indicates that only the solar radiationenhancement was not enough to produce the observed significant TEC enhancement.It means that the significant TEC enhancement on March8might be related to boththe M9.0Tohoku-Oki earthquake and the solar activity.
Meanwhile, this thesis presents the ionospheric plasma variations before theYushu earthquake using both GPS and DEMETER data. The result from GPS showsthat TEC began to enhance20hours before the earthquake, appearing in the region15°N～30°N. The spatial characteristics of TEC perturbation were similar to thatbefore the Wenchuan earthquake. On the other hand, based on the DEMETERsatellite IAP and ISL data, this work studies the time series and spatial variations ofthe ionospheric plasma including Ne, Ni, O+, H+and He+before the earthquake, andfinds no disturbance correlated with the shock. It demonstrates that there are certaindifferences over the ionospheric disturbance with the elevation changes before theearthquake.
3. Statistical analysis of electron density perturbation before major earthquakesobserved by DEMETER satellite and GPS measurements
Based on the data of DEMETER satellite, this work attempts to make a statisticalanalysis about the electron density perturbation before37major earthquakes ofmagnitude above7.0during January2005to December2009. The orbits, recorded in local nighttime by satellite, are chosen by a range of longitude20degrees and latitude15degrees during the9days around these earthquakes, with7days before and1dayafter. Among the37earthquakes, electron density perturbations were detected before19earthquakes, nearly accounting for51%. The anomalies were positive before18events and occurred mostly within1-6days before the shocks, part of which appearedmore than one time. Results indicate that it is more easily to observe the electrondensity disturbance around the equatorial and low-latitude regions than thehigh-latitude areas. The disturbance amplitude presents a positive correlation with themagnitude of earthquakes basically, while the depth of earthquakes does not showsignificant influence on the perturbation. It is worth noting that before several events,the anomalies are in the combined effect of earthquake and geomagnetic activity andpresent a higher disturbance level.
This thesis studied the ionospheric anomalies associated with30earthquakesoccurred in China (including Taiwan) during2000to2010by examining theGPS-inferred TEC variations. It is shown that TEC perturbations were detected before20earthquakes among the30events, nearly67%of the total. The TEC anomaliesrepresent increase more often and occurred mostly within2-6days before the shocks,part of them appeared more than one time. And the pre-earthquake ionosphericanomalies appeared during12:00–20:00LT significantly. The analysis indicates thatthe anomalies were not so relevant to the magnitude of the forthcoming earthquakes.Both these upper and lower anomalies were also found in the magnetically conjugatedarea of the seismogenic zone to the geomagnetic equator. The spatial distributions ofthese anomalies are not of an east-west asymmetry and have an equatorward shift ofboth crests of the EIA.
4. Long time series analysis of ionospheric TEC disturbance over seismicallyactive region in southwest China during low solar activity
Using data of Crustal Movement Observation Network of China (CMONC) thisthesis analyzes variations of TEC over southwest China during a period of low solarand geomagnetic activity in April-October2008. During that time, six largeearthquakes with magnitude M≧6.0occurred around the southwest region of China. To determine the abnormal TEC signals, a quartile-based processing is performed. Ateach time point we calculated the median M using the TEC values at the same localtime for the preceding15days. Known that the decisive role in the ionosphere state isperformed by space weather effects, we compared the TEC variations with time seriesof EUV solar radiation, Bz component of the interplanetary magnetic field (IMF),index of geomagnetic activity Dst and planetary index Kp, respectively. It is foundthat the observed anomalies in the regional TEC were in good correlation with thevariations of above solar and geomagnetic activities. We have not detected obviousTEC disturbance before five earthquakes expect for the Wenchuan earthquake. Inorder to additionally check the spatial occurrence of the observed anomalies, wechose a “check-region” with the same geomagnetic latitudes as the epicentral area ofthe six considered earthquakes, and analyzed the TEC changes. The results show thatthe time series of anomalous TEC in the “check-region” is similar to the epicentralarea, which is dominated by space weather. Therefore, it indicates the difficulties inidentifying earthquake precursors in the ionosphere TEC. The seismo-ionosphericcoupling process is complex, and it is necessary to strengthen the observation ofmulti-parameters on the ground, in the atmosphere and ionosphere synchronously.
5. Preliminary study about modeling the pre-earthquake electrostatic effect onthe ionosphere
This thesis introduces the equations of the ionospheric currents and electricalconductivity tensor and summarizes the mathematical model and calculation methodsof the penetration of seisomogenic electrostatic field (SEF) into the ionosphere. Thecaused ionosphere effect by SEF was obtained by the O+continuity equation. It isshown that at night, the horizontally large scale SEF can efficiently penetrate into theionosphere and produce noticeable changes in the horizontal distribution of the Fregion electron density.
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