地震活动和太阳风扰动的电离层响应特征研究
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摘要
本文主要基于DEMETER卫星观测数据,研究了卫星高度电离层场和粒子参量的背景特征,初步建立了全球电离层观测模型,在此基础上对全球299例5级以上“孤立”地震上空电离层变化特征进行了统计分析；此外,还研究了我国几例7级以上典型地震前的电离层异常特征；最后,对卫星高度电离层对太阳风扰动的响应特征进行了分析研究。主要研究成果如下：
1.电离层背景特征及电离层观测模型
(1)全球电离层观测特征
对DEMETER卫星高度电离层的等离子体和VLF电场功率谱特征进行研究发现,除了昼夜变化、年度、半年度、季节和赤道异常等特征外,还发现一些有趣的现象,主要有：
①等离子体偏离宏观电中性?
DEMETER卫星探测到夜侧电离层正离子数密度总和远小于相应电子数密度总和,偏离了等离子体的宏观电中性,可以推断,夜侧电离层应该还有其它相当数量的正离子或正离子团产生,夜侧电离层电子产生机理有待于进一步研究。
②不同参量受太阳辐射的影响也不一样
电离层等离子体来源于太阳紫外辐射及X线辐射离解高层大气,等离子体浓度随太阳辐射增强而升高,从2006年到2008年,太阳活动水平处于下降相太阳辐射减弱,因此,DEMETER卫星观测到电子和O+密度减小,而H+和He+数密度年度变化升高,说明产生这些粒子的主要机理和产生O+的机理不同,季节效应也体现了这一点,电子和O+数密度夏季半球显著高于冬季半球,而夏季和冬季半球的H+和He+数密度无明显差异。电子温度年度间和季节间都没有明显的变化,离子年度变化减小,季节变化也比较明显,说明太阳活动强度对离子温度的影响大于对电子温度的影响。EM和HISS频段的电场功率谱受太阳活动强度的影响较小,2006-2008年期间无明显的年度变化,太阳光照充分的夏季半球和光照较少的冬季半球的功率谱强度也无明显变化。
(2)初步建立卫星高度电离层观测模型
主要利用DEMETER卫星等离子体和VLF电场功率谱数据,从分析这些参量每个月份的全球电离层分布特征着手,研究电子和离子数密度及温度、EM和HISS频段电场功率谱的时间、空间变化特征,总结出应用于地震电离层异常提取的电离层正常观测模型,并对相应的统计误差进行分析。研究发现地磁夏季和地磁冬季季节内月份间的差异较小,可以以整个季节的观测数据为基础建立电离层观测模型,此外,由于电子数密度和温度数据的时间分辨率比较高,可以基于相邻两个月的数据建立更精确的观测模型。对观测模型的统计误差进行分析发现,由于电子温度和数密度的时间分辨率非常高,其模型统计误差最小,相对误差小于10%；离子温度和数密度模型的局部区域统计误差相对较大；电场功率谱除了谱值非常强的北美大陆区域的统计误差较大外,其它区域的统计误差均比较小。
2.地震电离层异常特征研究
(1)地震电离层异常统计研究
针对全球299例5级以上“孤立”地震,研究了地震前后等离子体和VLF电场功率谱的电离层变化特征,主要有以下研究结果：
◇等离子体参量时间-空间域研究的主要结果：
①在我们所关注的地震前后三天的研究区间内,异常主要发生在地震前后40小时内；
②电子(离子)数密度和温度异常进行比较发现,数密度异常出现的频率远大于温度异常的频率,数密度异常占异常总数的80%以上；
③电子参量和离子参量相比较,电子参量出现的频次远高于离子参量的频次,电子异常占78%；
④电子(离子)数密度异常主要以正异常为主,正异常占64%,温度异常出现正负异常的几率相当。
◇VLF电场功率谱
①时间-频率域：通过对地震和随机事件的时-频特征进行对比研究发现,50-300 Hz频段,地震统计结果显著小于随机事件结果,而其它频段二者无明显差异,说明在我们关注的频域里该频段为地震电离层异常的敏感频段。
②时间-空间域：针对50-300 Hz频段研究发现,电离层扰动主要出现在震中距400 km的范围内,正负扰动都有出现。
③空间-空间域：北半球扰动向北偏移,南半球扰动向南偏移,地磁冬季南北半球扰动都出现西向漂移特征。海洋地震震中附近以正异常为主,陆地地震震中附近以负异常为主
(2)典型震例的地震电离层异常
针对我国玉树、汶川和于田三次7级以上大地震,分析研究了震前电离层等离子体和电场的变化特征,发现电场和等离子体参量在震前都有电离层异常出现,VLF电场功率谱信号在震前一个DEMETER卫星重访周期内有电离层扰动发生,即SNR减弱现象。离子参量异常在震前也多次被观测到,其中主要为等离子体数密度异常,三个地震中只观测到一次温度异常,这和统计结果一致；震前3天内为等离子体异常的频发时段；数密度参量中H+异常和地震活动对应得非常好,受地磁活动的影响不明显；O+对地磁活动的响应特征比较显著。
对示范区电离层参量时序特征研究发现,地震引起的VLF电场功率谱异常主要为负异常,磁扰引起的主要为正异常。
3.磁层-电离层系统对太阳风扰动的响应
针对2004年11月7日行星际激波事件,主要利用TC-1卫星和DEME-TER卫星数据,分析了磁层-电离层系统对它的响应,主要是磁尾等离子片及中低纬电离层区域的响应。
(1)磁尾等离子体片振动增强
当激波作用于磁层时,TC-1在近地磁尾观测到等离子体片温度、数密度均突然增加,并且离子流流速突然加快,增强的等离子体流持续一段时间。其中最显著的现象是等离子体准周期振动显著增强,增强的等离子体流与局地磁场近似垂直。这种激波触发的等离子体片准周期对流振动增强的现象迄今为止还没有人报道过。我们推断激波触发的等离子片扰动很有可能是由激波在传播过程中引起磁鞘等离子体动压增强从而对磁尾对称压缩引起的。
(2)中低纬电离层的双相响应
中低纬电离层对激波的响应过程相对比较复杂,首先在激波压缩磁层的同时,夜侧电离层电场和等离子体流扰动增强,可以判断该扰动是由激波压缩磁层-电离层直接作用的结果。在激波作用于磁层顶3 min后,DEMETER再次观测到更为强烈的等离子体扰动,此次扰动伴随着等离子体波动发生,并且扰动发生时间在磁尾扰动发生后～110s,由DEMETER观测到等离子体沿磁场线流动这一特点可以判断,扰动沿磁场线传播,而磁尾扰动以alfven速度传播进电离层需要1-2 min,由此可以判断DEMETER卫星在激波压缩磁层顶3 min后观测到的电离层扰动是由磁尾扰动以磁流体波传播到电离层的结果。到目前为止中低纬电离层对行星际激波的这种双相响应过程还属于首次观测。
In this report, observational study on seismic-ionosphere coupling processes are made mainly based on DEMETER spacecraft observations, with emphasis on background char-acter and establishing model of ionosphere, ionosphere anomalies induced by earthquqke as well as the responsing process of magnetospheric-ionosphere system to interplanetary shocks.
1. The background characteristics and the observational model of the ionosphere
(1) The background characteristics of global ionosphere
Studying the plasma and VLF electric field power spectra at the DEMETER satellite alititude ionosphere we found that there are some intresting phenomena occurred in addi-tion to the conventional aomalies such as day and night differences, annual, semi-annual, season, equtatial anomaly and so on.
①the deviation of palsma from the macro electroneutrality? The total ionos density is much less than that of the electron at nightside ionosphere, which deviating from the palsma macro electroneutrality, so it could be inferred that there should be a considerable number of other types positive ionos or radical ionos generated in nightside ionosphere, and the generation mechanism needs further study.
②The effect of Solar radiation on the different parameters is differential
Ionospheric plasma originated from the solar ultraviolet and X-ray radiation dissoci-ating upper atmosphere, so the plasma density increases with solar radiation enhancement. The solar activity is in the decreasing phase from 2006 to 2008 with the weakening solar radiation, as a result, electron and O+ density reduce correspondingly, however, H+ and He+ density is observed decreasing with the Solar radiation increasing, showing different mechanisms of generating these particles. Moreover, Seasonal effect is also a manifestation of this, electron and O+ density is significantly higher in the summer hemisphere than in the winter hemisphere, but there is no clear different of H+ and He+ density between summer and winter hemisphere.
Solar activity on the temperature of ions and electrons are also not the same, there is no significant change of electron temperature in inter-annual or between seasons, but obvious changes are observed of inter-annual or seasonal ions temperatures, showing the solar activity influence on the ion temperature is greater than the impact on the electronic temperature.
There exists little effect of solar activity on electric field power spectrum in EM or HISS frequency band, no significant change is observed in 2006-2008, also no clear change occurs between summer and winter hemisphere.
(2) The preliminary establishment of ionosphere observational model at satellite alti-tude
Mainly using the DEMETER satellite plasma and VLF electric field power spectrum data, we have studied the temporal and spatial variation of corresponding ionosphere parameters, starting from analyzing of the global ionospheric distribution for each month, summed up the normal observation model of ionosphere which could be used to extract the earthquake ionospheric anomalies, and the corresponding statistical error is analyzed finally.
The results show that there exists little different between months in geomagnetic winter or summer season for almost parameters, this allows us to establish ionospheric observation model based on the whole seasonal data. Moreover, we can establish electron density or temperature model using only two months data in the same geomagnetic season for its relatively high time resolution. The analyzing of model error showing that the statistical error of electron parameters is smallest, its relative error is less than 10%. The statistical error of ion parameter models is relatively large at several local area; The error of Electric field power spectrum model is very small in addition to the North American continental region where the spectrum intensity is very strong.
2.The characteristics of the ionosphere anomalous induced by earthquake
(1)The statistical study
Aiming at the global 299 earthquakes with Ms>5, we have studied the disturbances of plasma and VLF electric field power spectra occurred in ionosphere before and after the seismics, the mainly conclusions are as follows:
0 Plasma parameters
time-space region:
①Paying attention to ionosphere character in the three days time interval before and after the EQs, we found that the anomalies mainly occur in 40 hours before and after EQs;
②The occurrence frequency of electron (ionos) density anomalous is far greater than that of the temperature comparing, density anomales account for more than 80% of the total abnormal;
③By comparing the electron with ionos anomalous, it is found the electron anomalous is far more than that of the ions, electron abnormal account for 78%;
④The positive density anomalies of electron (ionos) are main (comparing to the negative ones), which occupy 64%, and the occurrence frequency of positive and negative temperature anomalies are comparable.
VLF electric field power spectra
①time-frequercy region:By comparing the ionosphere disturbances occurring around the earthquakes and random events in the time-space region, it is found that the electric field power spectra around EQs is much weaker than that around random events in 50-300 Hz band, and there is no clear difference between the two in other band.
②time-space region:Aiming at 50-300 Hz frequency band in time-space region, we found that ionosphere disturbances mainly appear in the 400 km range of distance from epicenter, and the positive and negative anomalous are both observed.
③space-space region:Aiming at 50-300 Hz frequency band in space-space region, it is found that the disturbances in north hemisphere shift northerly, and south hemi-sphere disturbances shift southerly; The phenomenon of disturbances shifting westward is observed in both hemispheres in geomagnetic winter season. positive disturbances are the main ones observed around the epicenter of sea EQs and negative disturbances are the main around the epicenter of land EQs.
(2) The ionosphere disturbances induced by several typical earthquakes Aiming at YuShu, WenChuan and YuTian three earthquakes occurred in our county, we have analyzed the ionosphere disturbance characteristecs of plasma and electric field just before these EQs, respectively. The main results are:
electeric field and plasma anomalouses are both observed before EQs, SNR of VLF electric field power spectra decrease in a DEMETER revisited period before YuShu earth-quake; The ionos anomalies are also observed several times before EQs, the plasma density anomalouses are major ones among all the plasma disturbance observed, and only one temperature anomalous is observed before the three EQs, this is consistent with the sta-tistical results; The time interval of three days before EQs is the frequent period of plasma anomalies occurrence. There is the very good correspondence between the anomalies of H+ density and seismic activity, and H+ density disturbancees is not affected clearly by geomagnetic activities. The characteristics of O+ density responsing to the geomagnetic activities are significant.
The nagative and positive anomalies are mainly induced seismic and geomagnetic activities respectively by checking the time serial character in a typical region.
3. The response of the magnetosphere-ionosphere system to the solar wind disturbance
In terms of the interplanetary shock event occured on November 7,2004, observational study on the response of magnetosphere-ionosphere system to it are made mainly based on TC-1 and DEMETER spacecraft observations, especially the response of the magnetotail plasma sheet (PS) and the low-mid latitude ionosphere.
(1) The convection oscillations enhancement in the magnetotail plssma sheet
When the shock interacts with the magnetosphere, the magnetic field impulsively increases 1～2 min after the geomagnetic field sudden impulse (SI) judged from the Sym-H index change, and the magnetic field line is stretched. On the other hand, all of the ion density, the ion temperature, and the velocity of ion flow in the plasma sheet increase. Interestingly, quasi-periodical oscillations of the ion flow are suddenly enhanced, and the plasma flow is basically perpendicular to the local magnetic field. The responses of the magnetic field and the plasma are nearly simultaneous. The responses in the plasma sheet are probably caused by the lateral compression due to the dynamic pressure enhancement downstream the shock when the shock propagates antisunward in the magnetosheath. As far as we know, the quasi-periodical convection oscillations enhancement directly induced by shocks have never been reported in previous studies.
(2) The two-phase response of low-mid latitude ionosphere
It is a very complicated process for low-mid latitude ionosphere responding to the in-terplanetary shock. Firstly, the enhancements of electric field power spectrum and plasma velocity disturbances are observed at nightside ionosphere simultaneously to the shock compressing the magnetopause, so the reason of the disturbance is regarded as the direct interaction between shock and magnetosphere-ionosphere system. After 3 min of the shock acting on the magnetopause, the stronger electric field and plasma disturbances are ob-served again by DEMETER satellite companied with plasma wave occurring, these second disturbances is observed 110s after the magnetotail disturbance observed by TC-1. The disturbances propagate along with magnetic field which could be judged from the dis-turbing plasma flowing almost along the magnetic field observed by DEMETER satellite. As we know,1-2 minutes are needed for near-Earth magnetotail disturbance propagating into the ionosphere, so the second disturbances in the ionosphere 3 minutes after the shock compressing the magnetopause is propagated from magnetotail as the form of magnetichy-drodynamic wave. As far as we know, this is the first observation of the two-phase response of ionosphere to the interplanetary shock.
1.电离层背景特征及电离层观测模型
(1)全球电离层观测特征
对DEMETER卫星高度电离层的等离子体和VLF电场功率谱特征进行研究发现,除了昼夜变化、年度、半年度、季节和赤道异常等特征外,还发现一些有趣的现象,主要有：
①等离子体偏离宏观电中性?
DEMETER卫星探测到夜侧电离层正离子数密度总和远小于相应电子数密度总和,偏离了等离子体的宏观电中性,可以推断,夜侧电离层应该还有其它相当数量的正离子或正离子团产生,夜侧电离层电子产生机理有待于进一步研究。
②不同参量受太阳辐射的影响也不一样
电离层等离子体来源于太阳紫外辐射及X线辐射离解高层大气,等离子体浓度随太阳辐射增强而升高,从2006年到2008年,太阳活动水平处于下降相太阳辐射减弱,因此,DEMETER卫星观测到电子和O+密度减小,而H+和He+数密度年度变化升高,说明产生这些粒子的主要机理和产生O+的机理不同,季节效应也体现了这一点,电子和O+数密度夏季半球显著高于冬季半球,而夏季和冬季半球的H+和He+数密度无明显差异。电子温度年度间和季节间都没有明显的变化,离子年度变化减小,季节变化也比较明显,说明太阳活动强度对离子温度的影响大于对电子温度的影响。EM和HISS频段的电场功率谱受太阳活动强度的影响较小,2006-2008年期间无明显的年度变化,太阳光照充分的夏季半球和光照较少的冬季半球的功率谱强度也无明显变化。
(2)初步建立卫星高度电离层观测模型
主要利用DEMETER卫星等离子体和VLF电场功率谱数据,从分析这些参量每个月份的全球电离层分布特征着手,研究电子和离子数密度及温度、EM和HISS频段电场功率谱的时间、空间变化特征,总结出应用于地震电离层异常提取的电离层正常观测模型,并对相应的统计误差进行分析。研究发现地磁夏季和地磁冬季季节内月份间的差异较小,可以以整个季节的观测数据为基础建立电离层观测模型,此外,由于电子数密度和温度数据的时间分辨率比较高,可以基于相邻两个月的数据建立更精确的观测模型。对观测模型的统计误差进行分析发现,由于电子温度和数密度的时间分辨率非常高,其模型统计误差最小,相对误差小于10%；离子温度和数密度模型的局部区域统计误差相对较大；电场功率谱除了谱值非常强的北美大陆区域的统计误差较大外,其它区域的统计误差均比较小。
2.地震电离层异常特征研究
(1)地震电离层异常统计研究
针对全球299例5级以上“孤立”地震,研究了地震前后等离子体和VLF电场功率谱的电离层变化特征,主要有以下研究结果：
◇等离子体参量时间-空间域研究的主要结果：
①在我们所关注的地震前后三天的研究区间内,异常主要发生在地震前后40小时内；
②电子(离子)数密度和温度异常进行比较发现,数密度异常出现的频率远大于温度异常的频率,数密度异常占异常总数的80%以上；
③电子参量和离子参量相比较,电子参量出现的频次远高于离子参量的频次,电子异常占78%；
④电子(离子)数密度异常主要以正异常为主,正异常占64%,温度异常出现正负异常的几率相当。
◇VLF电场功率谱
①时间-频率域：通过对地震和随机事件的时-频特征进行对比研究发现,50-300 Hz频段,地震统计结果显著小于随机事件结果,而其它频段二者无明显差异,说明在我们关注的频域里该频段为地震电离层异常的敏感频段。
②时间-空间域：针对50-300 Hz频段研究发现,电离层扰动主要出现在震中距400 km的范围内,正负扰动都有出现。
③空间-空间域：北半球扰动向北偏移,南半球扰动向南偏移,地磁冬季南北半球扰动都出现西向漂移特征。海洋地震震中附近以正异常为主,陆地地震震中附近以负异常为主
(2)典型震例的地震电离层异常
针对我国玉树、汶川和于田三次7级以上大地震,分析研究了震前电离层等离子体和电场的变化特征,发现电场和等离子体参量在震前都有电离层异常出现,VLF电场功率谱信号在震前一个DEMETER卫星重访周期内有电离层扰动发生,即SNR减弱现象。离子参量异常在震前也多次被观测到,其中主要为等离子体数密度异常,三个地震中只观测到一次温度异常,这和统计结果一致；震前3天内为等离子体异常的频发时段；数密度参量中H+异常和地震活动对应得非常好,受地磁活动的影响不明显；O+对地磁活动的响应特征比较显著。
对示范区电离层参量时序特征研究发现,地震引起的VLF电场功率谱异常主要为负异常,磁扰引起的主要为正异常。
3.磁层-电离层系统对太阳风扰动的响应
针对2004年11月7日行星际激波事件,主要利用TC-1卫星和DEME-TER卫星数据,分析了磁层-电离层系统对它的响应,主要是磁尾等离子片及中低纬电离层区域的响应。
(1)磁尾等离子体片振动增强
当激波作用于磁层时,TC-1在近地磁尾观测到等离子体片温度、数密度均突然增加,并且离子流流速突然加快,增强的等离子体流持续一段时间。其中最显著的现象是等离子体准周期振动显著增强,增强的等离子体流与局地磁场近似垂直。这种激波触发的等离子体片准周期对流振动增强的现象迄今为止还没有人报道过。我们推断激波触发的等离子片扰动很有可能是由激波在传播过程中引起磁鞘等离子体动压增强从而对磁尾对称压缩引起的。
(2)中低纬电离层的双相响应
中低纬电离层对激波的响应过程相对比较复杂,首先在激波压缩磁层的同时,夜侧电离层电场和等离子体流扰动增强,可以判断该扰动是由激波压缩磁层-电离层直接作用的结果。在激波作用于磁层顶3 min后,DEMETER再次观测到更为强烈的等离子体扰动,此次扰动伴随着等离子体波动发生,并且扰动发生时间在磁尾扰动发生后～110s,由DEMETER观测到等离子体沿磁场线流动这一特点可以判断,扰动沿磁场线传播,而磁尾扰动以alfven速度传播进电离层需要1-2 min,由此可以判断DEMETER卫星在激波压缩磁层顶3 min后观测到的电离层扰动是由磁尾扰动以磁流体波传播到电离层的结果。到目前为止中低纬电离层对行星际激波的这种双相响应过程还属于首次观测。
In this report, observational study on seismic-ionosphere coupling processes are made mainly based on DEMETER spacecraft observations, with emphasis on background char-acter and establishing model of ionosphere, ionosphere anomalies induced by earthquqke as well as the responsing process of magnetospheric-ionosphere system to interplanetary shocks.
1. The background characteristics and the observational model of the ionosphere
(1) The background characteristics of global ionosphere
Studying the plasma and VLF electric field power spectra at the DEMETER satellite alititude ionosphere we found that there are some intresting phenomena occurred in addi-tion to the conventional aomalies such as day and night differences, annual, semi-annual, season, equtatial anomaly and so on.
①the deviation of palsma from the macro electroneutrality? The total ionos density is much less than that of the electron at nightside ionosphere, which deviating from the palsma macro electroneutrality, so it could be inferred that there should be a considerable number of other types positive ionos or radical ionos generated in nightside ionosphere, and the generation mechanism needs further study.
②The effect of Solar radiation on the different parameters is differential
Ionospheric plasma originated from the solar ultraviolet and X-ray radiation dissoci-ating upper atmosphere, so the plasma density increases with solar radiation enhancement. The solar activity is in the decreasing phase from 2006 to 2008 with the weakening solar radiation, as a result, electron and O+ density reduce correspondingly, however, H+ and He+ density is observed decreasing with the Solar radiation increasing, showing different mechanisms of generating these particles. Moreover, Seasonal effect is also a manifestation of this, electron and O+ density is significantly higher in the summer hemisphere than in the winter hemisphere, but there is no clear different of H+ and He+ density between summer and winter hemisphere.
Solar activity on the temperature of ions and electrons are also not the same, there is no significant change of electron temperature in inter-annual or between seasons, but obvious changes are observed of inter-annual or seasonal ions temperatures, showing the solar activity influence on the ion temperature is greater than the impact on the electronic temperature.
There exists little effect of solar activity on electric field power spectrum in EM or HISS frequency band, no significant change is observed in 2006-2008, also no clear change occurs between summer and winter hemisphere.
(2) The preliminary establishment of ionosphere observational model at satellite alti-tude
Mainly using the DEMETER satellite plasma and VLF electric field power spectrum data, we have studied the temporal and spatial variation of corresponding ionosphere parameters, starting from analyzing of the global ionospheric distribution for each month, summed up the normal observation model of ionosphere which could be used to extract the earthquake ionospheric anomalies, and the corresponding statistical error is analyzed finally.
The results show that there exists little different between months in geomagnetic winter or summer season for almost parameters, this allows us to establish ionospheric observation model based on the whole seasonal data. Moreover, we can establish electron density or temperature model using only two months data in the same geomagnetic season for its relatively high time resolution. The analyzing of model error showing that the statistical error of electron parameters is smallest, its relative error is less than 10%. The statistical error of ion parameter models is relatively large at several local area; The error of Electric field power spectrum model is very small in addition to the North American continental region where the spectrum intensity is very strong.
2.The characteristics of the ionosphere anomalous induced by earthquake
(1)The statistical study
Aiming at the global 299 earthquakes with Ms>5, we have studied the disturbances of plasma and VLF electric field power spectra occurred in ionosphere before and after the seismics, the mainly conclusions are as follows:
0 Plasma parameters
time-space region:
①Paying attention to ionosphere character in the three days time interval before and after the EQs, we found that the anomalies mainly occur in 40 hours before and after EQs;
②The occurrence frequency of electron (ionos) density anomalous is far greater than that of the temperature comparing, density anomales account for more than 80% of the total abnormal;
③By comparing the electron with ionos anomalous, it is found the electron anomalous is far more than that of the ions, electron abnormal account for 78%;
④The positive density anomalies of electron (ionos) are main (comparing to the negative ones), which occupy 64%, and the occurrence frequency of positive and negative temperature anomalies are comparable.
VLF electric field power spectra
①time-frequercy region:By comparing the ionosphere disturbances occurring around the earthquakes and random events in the time-space region, it is found that the electric field power spectra around EQs is much weaker than that around random events in 50-300 Hz band, and there is no clear difference between the two in other band.
②time-space region:Aiming at 50-300 Hz frequency band in time-space region, we found that ionosphere disturbances mainly appear in the 400 km range of distance from epicenter, and the positive and negative anomalous are both observed.
③space-space region:Aiming at 50-300 Hz frequency band in space-space region, it is found that the disturbances in north hemisphere shift northerly, and south hemi-sphere disturbances shift southerly; The phenomenon of disturbances shifting westward is observed in both hemispheres in geomagnetic winter season. positive disturbances are the main ones observed around the epicenter of sea EQs and negative disturbances are the main around the epicenter of land EQs.
(2) The ionosphere disturbances induced by several typical earthquakes Aiming at YuShu, WenChuan and YuTian three earthquakes occurred in our county, we have analyzed the ionosphere disturbance characteristecs of plasma and electric field just before these EQs, respectively. The main results are:
electeric field and plasma anomalouses are both observed before EQs, SNR of VLF electric field power spectra decrease in a DEMETER revisited period before YuShu earth-quake; The ionos anomalies are also observed several times before EQs, the plasma density anomalouses are major ones among all the plasma disturbance observed, and only one temperature anomalous is observed before the three EQs, this is consistent with the sta-tistical results; The time interval of three days before EQs is the frequent period of plasma anomalies occurrence. There is the very good correspondence between the anomalies of H+ density and seismic activity, and H+ density disturbancees is not affected clearly by geomagnetic activities. The characteristics of O+ density responsing to the geomagnetic activities are significant.
The nagative and positive anomalies are mainly induced seismic and geomagnetic activities respectively by checking the time serial character in a typical region.
3. The response of the magnetosphere-ionosphere system to the solar wind disturbance
In terms of the interplanetary shock event occured on November 7,2004, observational study on the response of magnetosphere-ionosphere system to it are made mainly based on TC-1 and DEMETER spacecraft observations, especially the response of the magnetotail plasma sheet (PS) and the low-mid latitude ionosphere.
(1) The convection oscillations enhancement in the magnetotail plssma sheet
When the shock interacts with the magnetosphere, the magnetic field impulsively increases 1～2 min after the geomagnetic field sudden impulse (SI) judged from the Sym-H index change, and the magnetic field line is stretched. On the other hand, all of the ion density, the ion temperature, and the velocity of ion flow in the plasma sheet increase. Interestingly, quasi-periodical oscillations of the ion flow are suddenly enhanced, and the plasma flow is basically perpendicular to the local magnetic field. The responses of the magnetic field and the plasma are nearly simultaneous. The responses in the plasma sheet are probably caused by the lateral compression due to the dynamic pressure enhancement downstream the shock when the shock propagates antisunward in the magnetosheath. As far as we know, the quasi-periodical convection oscillations enhancement directly induced by shocks have never been reported in previous studies.
(2) The two-phase response of low-mid latitude ionosphere
It is a very complicated process for low-mid latitude ionosphere responding to the in-terplanetary shock. Firstly, the enhancements of electric field power spectrum and plasma velocity disturbances are observed at nightside ionosphere simultaneously to the shock compressing the magnetopause, so the reason of the disturbance is regarded as the direct interaction between shock and magnetosphere-ionosphere system. After 3 min of the shock acting on the magnetopause, the stronger electric field and plasma disturbances are ob-served again by DEMETER satellite companied with plasma wave occurring, these second disturbances is observed 110s after the magnetotail disturbance observed by TC-1. The disturbances propagate along with magnetic field which could be judged from the dis-turbing plasma flowing almost along the magnetic field observed by DEMETER satellite. As we know,1-2 minutes are needed for near-Earth magnetotail disturbance propagating into the ionosphere, so the second disturbances in the ionosphere 3 minutes after the shock compressing the magnetopause is propagated from magnetotail as the form of magnetichy-drodynamic wave. As far as we know, this is the first observation of the two-phase response of ionosphere to the interplanetary shock.
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