Ground-based and satellite DC-ULF electric field anomalies around Wenchuan M8.0 earthquake

The ground-based and satellite DC-ULF electric field data were analyzed around Wenchuan M8.0 earthquake on May 12, 2008 in China. The results show that ground electric field anomalies occurred at 3 stations located to the north and south of the epicenter with the amplitude of 3–100 mV/km. The change shapes and their amplitude of ground electric field anomalies are different largely due to their individual underground layer conductivity, water level and so on. The analysis of long time series illustrates that the abnormal geoelectric field started since March 2008. Onboard the DEMETER satellite, the ULF waveforms of electric field were collected and processed by wavelet transform method. The disturbances in the ionosphere were about 3–5 mV/m at a frequency band lower than 0.5 Hz. When the ground and space electric field anomalies were compared, their occurrence time and spatial distribution points are consistent with each other, including the long time anomalies from March 2008 and the short term ones 1–2 days before the Wenchuan earthquake. Finally, the coupling mechanism was discussed.     

Formation mechanism of great positive TEC disturbances prior to Wenchuan earthquake on May 12, 2008

The problems of physical explanation and possible mechanisms of the seismo-ionospheric effects formation are under discussion now. There are proposed different mechanisms of such effects, for example, large- and small-scale internal gravity waves (IGWs), atmospheric electric field, electromagnetic fields and emissions. However, the appearance of local large-scale seismo-ionospheric anomalies in Total Electron Content (TEC) is possible to explain only by two mechanisms: an atmospheric electric field and/or small-scale IGWs. In this paper, the simulation results for reproduction of the observed seismo-ionospheric great positive effects in TEC prior to strong Wenchuan earthquake are presented. The obtained results confirm the proposed mechanism of seismo-ionospheric effects formation by the penetration of the seismogenic electric field from the atmosphere into the ionosphere. It is suggested that so great TEC enhancement observed 3 days prior to Wenchuan earthquake could be explained by combined action of seismogenic vertical electric field and IGWs generated by the solar terminator.     

VLF波从大气层到低电离层的传输特性分析

基于传播矩阵法计算了均匀半空间电离层的反透射系数, 同时解Booker复系数四次方程得到电离层的复折射指数, 分别研究了电离层反射透射系数及折射指数随VLF频段入射电波频率、入射角和地磁倾角、电离层电子浓度及碰撞频率的变化规律. 计算结果表明, 在VLF频段, 垂直电偶极子辐射的横磁(Traverse Magnetic)波更易透射进入电离层, 而水平电偶极子辐射的横电(Traverse Electric) 波易被限制在地-电离层波导内来回反射. 电离层电子密度较低时 (如夜间), 在高纬度地区, 观测到地震电离层VLF异常的概率更大. 当考虑地磁场 的影响时, 电离层将允许地震辐射的超低频(Ultra Low Frequency ,ULF)/甚低频 (Very Low Frequency, VLF)部分的电磁波透射进入电离层, 这一点已有很多卫星观测事实为证, 但其进一步的物理机制尚需深入研究.      

Appearance of VLF and ELF-noises in topside ionosphere under the action of high power radio wave from data of satellite intercosmos-24

Results of a satellite experiment are presented on detection of VLF and ELF-waves excited by irradiation of the night ionosphere F-region by the field of a nonmodulated high-power radio wave. The excited VLF and ELF-waves have been detected at the topside ionosphere heights h=500–1000 km in the frequency bands 8 kHz相似文献   

Model for the VLF/LF radio signal anomalies formation associated with earthquakes

The influence of quasi-static electric field of seismic origin on the characteristics of the internal gravity waves (IGWs) in the Earth’s ionosphere is considered. The electric field in the ionosphere arises due to the injection of charged aerosols into the atmosphere, formation of an EMF in the near Earth atmosphere and perturbation of the conductive electric current in the global electric circuit. Amplification of the electric current in seismic zone is accompanied by the formation of perturbation of the lower ionosphere that affects the amplitude and phase of VLF/LF signals. The action of the electric field on the IGWs is connected with the appearance of the Ampere’s force in the ionosphere. In the spectral range of these waves the latter acts on the neutral component of the ionosphere plasma. As the result of this interaction the ionosphere starts to support the discrete spectrum of oscillations. Periods of their maximums increase as numbers of natural sequence. The existence of such peculiarities of the waves in the ionosphere is confirmed by observations.     

大气行星波影响低频电波吸收变化的机制

本文根据冬季中低纬低电离层中、低频（ＬＦ）电波振幅扰动与高纬平流层中大气行昨波活动密切相关的观测事实，分析研究了可能引起低电离层对ＬＦ电波吸收变化诸因素的作用后，提出了一种能较好地解释观测现象的物理机制，大气行星波可通过两种方式改变大气离化率ｑ，因而引起低电离层中电子密度Ｎ扰动，进而改变由Ｎ大小决定的电离层电波吸收值，结果导致ＬＦ电波振幅发生相应变化。文中给出了描述这一物理计算公式和某些计算结果。     

The ionospheric effect of atmospheric gravity waves excited prior to strong earthquake

We have computed perturbations in the nighttime mid-latitude F2 region ionosphere that could be produced by internal atmospheric gravity waves generated before strong earthquakes through ionospheric Joule heating due to the seismogenic electric field of short duration. There is a strong anisotropy of the atmospheric gravity wave effect with respect to the imminent earthquake epicentre, the electron density changes being maximum poleward and equatorward of the epicentre and being minimum eastward and westward of it. It should be noted that the duration of the electron density perturbation in the F2 region ionosphere is much longer than the duration of the primary precursor of an earthquake – the enhancement of the vertical electric field at the Earth’s surface, which initiates the atmospheric gravity wave generation. This fact is important from the practical point of view of predicting catastrophic earthquakes.     

Modeling of ionospheric scintillation at low-latitude

The presence and movement of plasma density fluctuations in the F-region of the ionosphere are studied by monitoring phase and amplitude of radio waves propagating through the region. In this paper, we have used weak scattering theory and assumed the plasma density fluctuations to behave like phase changing diffraction screen. Appropriate relations for scintillation index S₄, and phase variance δ? are derived and computed for different parameters of the plasma density irregularities of the ionosphere. SROSS-C2 satellite in situ measurements of plasma density fluctuations, which provide direct information about the structure and morphology of irregularities that are responsible for scintillation of radio waves, were used first time to develop a scintillation model for low latitude. It is observed that the scintillation index S₄ and phase variance δ? depends on the strength of the plasma turbulence. Finally, the results obtained from modeling are compared and discussed with the available recent results.     

强震前异常电场激发声重波对电离层的影响

震前地震孕育期地表异常增强的电场,通过大气电导率传输到电离层高度.该异常电场通过非稳态局部加热,可以在电离层高度激发声重波.基于该理论,利用一维时变中纬电离层物理模型,模拟了该扰动源对电离层电子密度的影响.结果表明,重力波引起的中性风速度扰动对电离层电子密度分布影响甚微,该机理无法解释震前电离层异常扰动现象.      

大功率高频无线电波对赤道E区双流不稳定性的影响--PIC静电粒子模拟研究

地面入射的大功率高频无线电波(泵波)和电离层等离子体之间的参数相互作用，能够引起静电波的激发，在一定条件下，产生不稳定性．本文用PIC静电粒子模拟方法，研究泵波与赤道电离层E区等离子体的相互作用．研究结果表明，泵波能够控制双流不稳定性的发生，在不同条件下，泵波对双流不稳定性起着稳定与不稳作用，模拟结果定性地与理论研究结果相符合，这为我们对不规则体产生的地面人工控制提供了依据．     

Partially transmitted lightning signals recorded by pioneer Venus orbiter

Impulsive electric fields appearing on all four frequency channels of the Pioneer Venus electric field detector in the night ionosphere of Venus are characteristic of lightning generated signals. Based on our knowledge of the electron density and magnetic field in the Venus ionosphere, we suggest that lightning waves could be partially transmitted upwards into the ionosphere. The leakage of these lightning waves into the ionosphere on encountering electron density holes may be treated as reradiation into the ionosphere from the hole. Since this radiation pattern is frequency dependent, we should not expect to see all frequency components for every lightning stroke observed.     

On high-altitude electric fields in the auroral downward current region and their coupling to ionospheric electric fields

The downward field-aligned current region plays an active role in magnetosphere–ionosphere coupling processes associated with aurora. A quasi-static electric field structure with a downward parallel electric field forms at altitudes between 800 km and 5000 km, accelerating ionospheric electrons upward, away from the auroral ionosphere. Other phenomena including energetic ion conics, electron solitary waves, low-frequency wave activity, and plasma density cavities occur in this region, which also acts as a source region for VLF saucers. Results are presented from high-altitude Cluster observations with particular emphasis on the characteristics and dynamics of quasi-static electric field structures. These, extending up to altitudes of at least 4–5 Earth radii, appear commonly as monopolar or bipolar electric fields. The former occur at sharp boundaries, such as the polar cap boundary whereas the bipolar fields occur at softer boundaries within the plasma sheet. The temporal evolution of quasi-static electric field structures, as captured by the pearls-on-a-string configuration of the Cluster spacecraft, indicates that the formation of electric field structures and of ionospheric plasma density cavities are closely coupled processes. A related feature of the downward current is a broadening of the current sheet with time, possibly related to the depletion process. Preliminary studies of the coupling of electric fields in the downward current region, show that small-scale structures are typically decoupled from the ionosphere, similar to what has been found for the upward current region. However, exceptions are also found where small-scale electric fields couple perfectly between the ionosphere and Cluster altitudes. Recent FAST results indicate that the degree of coupling differs between sheet-like and curved structures, and that it is typically partial. The electric field coupling further depends on the current–voltage relationship, which is highly non-linear in the downward current region, and still unrevealed, as to its specific form.     

Electric field and ion density anomalies in the mid latitude ionosphere: Possible connection with earthquakes?

The problem of earthquake prediction has stimulated the search for a correlation between seismic activity and ionospherical anomalies. We found observational evidence of possible earthquake effects in the near-equatorial and low latitude ionosphere; these ionospheric anomalies have been proposed by Gousheva et al. [Gousheva, M., Glavcheva, R., Danov, D., Angelov P., Hristov, P., Influence of earthquakes on the electric field disturbances in the ionosphere on board of the Intercosmos-Bulgaria-1300 satellite. Compt. Rend. Acad. Bulg. Sci. 58 (8) 911–916, 2005a; Gousheva, M., Glavcheva, R., Danov, D., Angelov, P., Hristov, P., Kirov, B., Georgieva, K., Observation from the Intercosmos-Bulgaria-1300 satellite of anomalies associated with seismic activity. In: Poster Proceeding of 2nd International Conference on Recent Advances in Space Technologies: Space in the Service of Society, RAST ‘2005, June 9–11, Istanbul, Turkey, pp. 119–123, 2005b; Gousheva, M., Glavcheva, R., Danov, D., Angelov, P., Hristov, P., Kirov, B., Georgieva, K., Satellite monitoring of anomalous effects in the ionosphere probably related to strong earthquakes. Adv. Space Res. 37 (4), 660–665, 2006]. This paper presents new results from observations of the quasi-static electric field and ion density on board INTERCOSMOS-BULGARIA-1300 satellite in the mid latitude ionosphere above sources of moderate earthquakes. Data from INTERCOSMOS-BULGARIA-1300 satellite and seismic data (World Data Center, Denver, Colorado, USA) for magnetically quiet and medium quiet days are juxtaposed in time-space domain. For satellite’s orbits in the time period 15.09–01.10.1981 an increase in the horizontal and vertical components of the quasi-static electric field and fluctuations of the ion density are observed over zones of forthcoming seismic events. Some similar post effects are observed too. The emphasis of this paper is put on the anomalies which specify the mid latitude ionosphere. The obtained results contain important information because they confirm our previous results for near-equatorial and low latitude regions.     

半个太阳活动周期内我国中纬地区上空电离层等效板厚的变化特征

本文用新乡(电离层400km对下点是:32.4°N,115.6°E)、重庆(电离层400km对下点是:27.2°N,108.7°E)接收日本同步卫星ETS-Ⅱ的信标资料,研究了我国中纬地区上空半个太阳活动周期内(1981—1985)电离层等效板厚的变化特征,得到了等效板厚日变化、季变化的二维和三维等值图。由付里叶分析和回归方法得出等效板厚日变化各谐波分量与太阳黑子数12个月滑动平均值之间存在弱的线性关系。同时指出在等效板厚日变化中,于当地时0400—0700LT存在一明显的黎明峰。并对出现这种峰的原因作了讨论。      

Seismoionospheric fountain-effect as analogue of active space experiment

Space investigation includes modelling and initiation of natural processes in ionospheric-magnetospheric plasma by active experimental methods with artificial influence as a specific part. For the first time we present here the possibility of triggering the unique equatorial ionosphere phenomenon known as fountain-effect by lithospheric processes before an earthquake. Anomalous influence of earthquake preparation processes with epicenter close to magnetic equator on latitudinal distribution of f_oF - F-region penetration frequencies and spread-F is discussed and considered by us as an analogue of an active experiment. Using Alouette-1 data it is shown that approximately one day before the earthquake f_oF2 dependence on magnetic latitude looks like a double-crest curve with minimum close to epicenter. Simul taneously oblique and spread traces appeared at topside ionograms. The effect is evident during evening hours. Such kind of f_oF2-dependence is unusual and is not observed during the preceding days, nor some days later. So, a possible reason for the observations might be connected with generation of an anomalous electric field near the epicenter during earthquake preparation stage, which initiates a phenomenon similar to the natural fountain-effect.     

DEMETER卫星探测到的强震前O^＋浓度变化

利用DEMETER卫星观测数据,采用level-2图像目视解译的方法和重访轨道对比方法,对2006年5月3日发生在汤加群岛的7．9级地震进行分析．从震前6天、震前2天、震前1天和震后2天的level-2图像可看到,电场和等离子体参量在震中上空偏北的位置出现扰动信号,震前的扰动信号幅度随地震时间临近有所增强,扰动持续时间也相应增加;震后2天扰动信号依然存在,其幅度并未减弱．通过对比震前1天的轨道在2006年至2008年内的重访轨道,发现O^＋浓度存在三类较规律的变化特征,即在夏季时,北半球的值高于南半球;冬季时则相反;二分季节时,南北半球含量相当．本文给出了地震和磁暴对O^＋浓度的不同影响,地震影响最靠近震中位置轨道上的O^＋浓度,而磁暴影响全球轨道上O^＋浓度分布．同时获取了DEMETER卫星上朗缪尔探针在上述重访轨道同步探测的电子浓度数据,发现电子浓度与O^＋浓度存在同步增加．等离子体浓度增加的现象可能与地震产生异常电场进而影响电离层结构有关．     

Lower ionosphere response to external forcing: A brief review

There are two ways of external forcing of the lower ionosphere, the region below an altitude of about 100 km: (1) From above, which is directly or indirectly of solar origin. (2) From below, which is directly or indirectly of atmospheric origin. The external forcing of solar origin consists of two general factors – solar ionizing radiation variability and space weather. The solar ionization variability consist mainly from the 11-year solar cycle, the 27-day solar rotation and solar flares, strong flares being very important phenomenon in the daytime lower ionosphere due to the enormous increase of the solar X-ray flux resulting in temporal terminating of MF and partly LF and HF radio wave propagation due to heavy absorption of radio waves. Monitoring of the sudden ionospheric disturbances (SIDs – effects of solar flares in the lower ionosphere) served in the past as an important tool of monitoring the solar activity and its impacts on the ionosphere. Space weather effects on the lower ionosphere consist of many different but often inter-related phenomena, which govern the lower ionosphere variability at high latitudes, particularly at night. The most important space weather phenomenon for the lower ionosphere is strong geomagnetic storms, which affect substantially both the high- and mid-latitude lower ionosphere. As for forcing from below, it is caused mainly by waves in the neutral atmosphere, i.e. planetary, tidal, gravity and infrasonic waves. The most important and most studied waves are planetary and gravity waves. Another channel of the troposphere coupling to the lower ionosphere is through lightning-related processes leading to sprites, blue jets etc. and their ionospheric counterparts. These phenomena occur on very short time scales. The external forcing of the lower ionosphere has observationally been studied using predominantly ground-based methods exploiting in various ways the radio wave propagation, and by sporadic rocket soundings. All the above phenomena are briefly mentioned and some of them are treated in more detail.     

Satellite monitoring of anomalous effects in the ionosphere probably related to strong earthquakes

The paper presents simultaneously a satellite monitoring of plasma density disturbances and quasi-static electric fields on the one hand, and a ground earthquake monitoring, on the other. The INTERCOSMOS-BULGARIA-1300 satellite was launched on 7th August 1981. It had a perigee of 825 km, an apogee of 906 km and orbit inclination of 81.2°. For satellite’s orbits over sources of earthquakes M ⩾ 5.5 (seismic data of the World Data Center – Denver, Colorado), in the time period 12.08.1981–30.12.1981, some ionospheric effects are investigated. These effects can be considered eventually as possible earthquake precursors. The paper discusses specific anomalous effects observed in the ionosphere, which cannot be explained by factors of solar-magnetospheric origin. They could be associated with the earthquake growing processes in the lithosphere and troposphere zones above the source. In conclusion the results of ionosphere anomalous effects monitoring are proposed.     

Ionospheric precursors observed at low latitudes around the time of koyna earthquake

On December 11, 1967 at 05:21 LT, an immense earthquake of magnitude 6.7 struck Koyna, the Indian province of Maharashtra. Its epicenter was located at geographic latitude 17.37°N and longitude 73.75°E with depth of about 3 km. Ground based measurements show variation in the critical frequency of ionospheric F2 layer (foF2) before and after the shock. In the present study the behavior of F2-region of ionosphere has been examined over the equatorial and low latitudinal region ionosphere during the month of December 1967 around the time of Koyna earthquake. For this purpose, the ionospheric data collected with the help of ground-based ionosondes installed at Hyderabad (located close to the earthquake epicenter) Ahmedabad, Trichirapulli, Kodaikanal and Trivendrum have been utilized. The upper and lower bound of Interquartile range (IRQ) are constructed to monitor the variations in foF2 other than day-to-day and diurnal pattern for finding the seismo-ionospheric precursors. Some anomalous electron density variations are observed between post midnight hours to local pre-noon hours at each station. These anomalies are strongly time dependent and appeared a couple of days before the main shock. The period considered in this study comes under the quiet geomagnetic conditions. Hence, the observed anomalies (which are more than the usual day-to-day variability) over all stations are likely to be associated with this imminent earthquake. The possible mechanism to explain these anomalies is the effect of seismogenic electric field generated just above the surface of earth within the earthquake preparation zone well before the earthquake due to emission of radioactive particles and then propagated upward, which perturbs the F-region ionosphere.     

GPS电离层反演方法研究及其在地震方面的应用

利用地基GPS数据计算了电离层单球壳模型穿刺点上的垂向总电子含量(VTEC), 根据VTEC和卫星及测站接收机的差分码间偏差DCB的不同时变特性采用了复弧法, 将VTEC作为局部变量(每30 min 一组, 可调), DCB作为一天的全局量进行解算. 在解算的过程中, 充分考虑VTEC的空间分布特性, 利用变异函数通过Kriging插值法建立电离层VTEC的二维格网模型, 并给出了卫星和接收机的差分码间偏差DCB. 通过与IGS结果的比较, 发现其结果可靠, 且时空分辨率和稳定性都有较大提高. 同时, 基于简化的三元样条插值基函数对电离层电子密度进行三维展开, 利用乘型代数重建技术MART算法构建了同批数据的四维层析成像结果, 获得了电离层电子密 度的四维分布. 其结果与CHAMP无线电掩星结果非常一致. 利用上面两种算法又分别对2008年5月长江三角洲地区地基GPS数据进行处理, 简要分析了该时段该地区上空电离层总电子含量和电子密度的变化情况及其对汶川地震的响应.