The equatorial electrojet

The typical quiet day variations of the equatorial electrojet (EEJ) current intensity with time of the day, season, sunspot number, and geomagnetic latitude are presented in terms of the corresponding variations of H which is the deviation of the horizontal component (H) of the geomagnetic field from its steady nighttime level. The observed height structure of the current density in the EEJ as measured in rocket flights is presented, along with the theoretically computed structure. Theoretical model results on the polarization electric fields and east-west currents as generated by the local interactions of height-varying winds in the EEJ show large height gradients and reversals for both currents and electric fields; experimental evidence for the reality of such height structures is also shown. The characteristics of the counter-electrojet events are presented and the possible causative mechanisms are discussed critically.Some typical experimental results are presented on the electric field changes in the EEJ which result from its sensitive response to electrodynamic disturbances in the magnetosphere and the auroral-polar latitude ionosphere during geomagnetic substorms and storms; and their implications are discussed. Possibilities for utilizing the EEJ as a very useful medium for important scientific studies on the larger space domain of ionosphere-magnetosphere system, on plasma waves, and on the earth's conductivity are emphasized.     

Equatorial electrojet as part of the global circuit: a case-study from the IEEY

Geomagnetic storm-time variations often occur coherently at high latitude and the day-side dip equator where they affect the normal eastward Sq field. This paper presents an analysis of ground magnetic field and ionospheric electrodynamic data related to the geomagnetic storm which occured on 27 May 1993 during the International Equatorial Electrojet Year (IEEY) experiment. This storm-signature analysis on the auroral, mid-latitude and equatorial ground field and ionospheric electrodynamic data leads to the identification of a sensitive response of the equatorial electrojet (EEJ) to large-scale auroral return current: this response consists in a change of the eastward electric field during the pre-sunrise hours (0400–0600 UT) coherently to the high-, mid-, and equatorial-latitude H decrease and the disappearance of the EEJ irregularities between the time-interval 0800–0950 UT. Subsequent to the change in hF during pre-sunrise hours, the observed foF2 increase revealed an enhancement of the equatorial ionization anomaly (EIA) caused by the high-latitude penetrating electric field. The strengthening of these irregularities attested by the Doppler frequency increase tracks the H component at the equator which undergoes a rapid increase around 0800 UT. The H variations observed at the equator are the sum of the following components: S_R, DP, DR, DCF and DT.     

地磁场强度对赤道电激流的地方时和经度分布的影响

本文利用秘鲁和印度地磁台站的观测数据,结合全球电离层-热层模型的模拟结果,研究了地磁场强度的减弱对赤道电激流的地方时和经度分布的影响.这对理解地球空间天气和行星演化具有重要的科学意义.结果 表明,当地磁场强度减弱一半时,东向电激流和风发电机电场的峰值时间均提前1h,而低层大气潮汐使峰值时间延后了1h.其主要原因是,随着...     

Letter to the editor Electric field fluctuations (25–35 min) in the midnight dip equatorial ionosphere

Measurements with a HF Doppler sounder at Kodaikanal (10.2°N, 77.5°E, geomagnetic latitude 0.8°N) showed conspicuous quasi-periodic fluctuations (period 25/35 min) in F region vertical plasma drift, V_z in the interval 0047/0210 IST on the night of 23/24 December, 1991 (A_p = 14, K_p < 4^–). The fluctuations in F region vertical drift are found to be coherent with variations in B_z (north-south) component of interplanetary magnetic field (IMF), in geomagnetic H/X components at high-mid latitude locations both in the sunlit and dark hemispheres and near the dayside dip equator, suggestive of DP2 origin. But the polarity of the electric field fluctuations at the midnight dip equator (eastward) is the same as the dayside equator inferred from magnetic variations, contrary to what is expected of equatorial DP2. The origin of the coherent occurrence of equatorial electric field fluctuations in the DP2 range of the same sign in the day and night hemispheres is unclear and merits further investigations.     

Local time and longitude dependence of the equatorial electrojet magnetic effects

An empirical model of the equatorial electrojet (EEJ), including local time and longitude dependence, has been constructed based on the surface magnetic data recorded at 26 stations located in six different longitude sectors that were set up or augmented during the international equatorial electrojet year (IEEY). The model reproduces the characteristic signatures of the EEJ-associated horizontal and vertical magnetic components at ground level. The model-predicted variations at the orbit of the POGS satellite are generally in good agreement with the onboard magnetic signatures, although strong discrepancies are also often seen. The nature of the differences suggests that the global scale magnetospheric or field-aligned current systems may sometimes dominate the satellite data. The nature of the longitudinal inequalities in the EEJ strength indicates that the equatorial electrojet is strongest in South America (80°–100°W) and weakest in the Indian sector (75°E) with a secondary minimum and a maximum centered, respectively, in the Atlantic Ocean (30°W) and in western Africa (10°E). The EEJ strength is shown to be inversely correlated with the main field intensity along the dip-equator.     

Methods and results of monitoring of the natural telluric field in the Baikal rift zone

Variations in horizontal and vertical components of the telluric field are studied within the range of periods from a few tens of seconds to diurnal and longer variations. Variations caused by an external ionospheric source are used for studying the geoelectric heterogeneity of the medium. For this purpose, an electric tipper is introduced by analogy with the geomagnetic field and its frequency response is examined. The magnetotelluric impedance is studied using data of remote geomagnetic observatories. According to the data of monitoring of the electric tipper and magnetotelluric impedance, anomalous effects are revealed that can be related to earthquakes with K ≥ 12. A method is developed for the extraction of electric field anomalies produced by intraterrestrial electric sources.     

大地电场观测地震前兆异常提取技术研究

首都圈地区布设的11个地电场观测台于2001年9月已正式投入观测,现已积累了近7年的资料．使用大地电场观测系统中同一测向的长、短极距测值的比值方法,对其中的10个台和邻区2个台的资料进行了处理,发现该方法能够较好地排除来自大地电场及自然电场远场的变化信息,突出了可识别的地震前兆异常信息．研究结果表明,2006年7月4日河北文安MS5.1震前,多数台站的地电场观测资料出现比值异常变化,从而证明了这次地震前地电场观测存在着短临异常前兆变化信息,只是由于这些信息混杂于正常大地电场变化的背景中,使用常规的分析方法无法识别和提取．研究结果还表明,地电场观测的台址条件和台址到震中区的距离与震前地电场比值异常出现的时间和幅度有关．      

Analyses of zonal atmospheric excitation functions and their correlation with polar motion excitation functions

The atmospheric influence on the Earths, rotation can be described by the effective atmospheric angular momentum (EAAM) functions. In this study we focus on the analysis of short period variations of the equatorial components of the zonal EAAM excitation functions ₁ and ₂ and their influence on similar variations of polar motion. The global objective analysis data of the Japanese Meteorological Agency for the period 1986–1992 were used to compute the EAAM excitation functions in different latitude belts. Time- and latitude-variable amplitude spectra of variations of these functions with periods shorter than 150 days, containing pressure, pressure with the inverted barometric correction, and wind terms were computed. The spectra show distinct latitude and time variations of the prograde and retrograde oscillations which reach their maxima mainly in mid-latitudes. Prograde and retrograde oscillations with periods of about 40–60 days and about 110–120 days are seen in the spectra of pressure terms of the equatorial components of the zonal EAAM excitation functions. Additionally, correlation coefficients and cross-spectra between variations of the geodetic polar motion and equatorial components of the zonal EAAM excitation functions were computed to identify the latitude belts of the globe over which atmospheric circulation changes are correlated mostly with short period variations of the polar motion excitation functions. The correlation coefficients vary in time and latitude and reach maximum values in the northern latitudes from 50°N to 60°N. In the cross-spectra between the polar motion excitation functions and pressure terms of the zonal EAAM excitation functions there are peaks of common prograde oscillations with the periods around 20, 30, 40–50, 60 and 80–150 days and of common retrograde oscillations around 20, 30, 40 and 50–70 days.Paper presented at the IERS Workshop in Paris, March 1994     

Model studies on electro-telluric interpretation problems of some geological inhomogeneities

Summary The rotating nature of telluric field is simulated in an electrolytic model tank. Variations of this rotating field due to the presence of a few two and three dimensional geological models, simulated in the tank, are studied. The telluric ellipses, recorded on a C.R.O. Screen, are found to be useful for qualitative and quantitative interpretation of the non-linearly polarised telluric field data. Relative usefulnesses of the various parameters, e.g., (i)M=ratio of the semimajor and semiminor axis, (ii)K = square root of normalized surface areas, (iii) = linear eccentricity ((a ²/b ²-1) and (iv)e = conventional eccentricity ((1-b ²/a ²)), of a telluric ellipse are tested. Linear eccentricity appears to be the most sensitive parameter. Methods for determining depth and horizontal extent of a structure from a telluric map are suggested. Near surface inhomogeneities, as revealed from the experiment are unlikely to affect the telluric map due to a basement structure.     

唐山地震前后自然电场的周期变化

在唐山地震前两个月至主震后约半年的时段内,西集、宝坻、塘沽等三个地电观测台自然电场断续出现特定周期的变化:其周期为半月、一月及半年;其双振幅每公里达数十乃至一、二百毫伏(超过平时观测标准误差6—20倍);电场呈线性极化,其方向大致平行台址附近断裂带的走向。若把这种变化同复杂的干扰区分开并与理论重力值对比,似与日、月起潮力有所联系。其中半月周期变化的极值点,还与震中周围两口废油井间歇自喷现象同时出现,表明电场的这种变化可能与断裂带內地下液体运动状态的变化有关,故认为是一种过滤速度受起潮力调制的过滤电场.微小的起潮力只在主震前后一段时间内才引起这种过滤电势的变化。      

Typical disturbances of the daytime equatorial F region observed with a high-resolution HF radar

HF radar measurements were performed near the magnetic equator in Africa (Korhogo 9°2463N–5°3738W) during the International Equatorial Electrojet Year (1993–1994). The HF radar is a high–resolution zenithal radar. It gives ionograms, Doppler spectra and echo parameters at several frequencies simultaneously. This paper presents a comparative study of the daytime ionospheric structures observed during 3 days selected as representative of different magnetic conditions, given by magnetometer measurements. Broad Doppler spectra, large echo width, and amplitude fluctuations revealed small-scale instability processes up to the F-region peak. The height variations measured at different altitudes showed gravity waves and larger-scale disturbances related to solar daytime influence and equatorial electric fields. The possibility of retrieving the ionospheric electric fields from these Doppler or height variation measurements in the presence of the other possible equatorial ionospheric disturbances is discussed.     

Non-trough foF2 enhancements at near-equatorial dip latitudes

Fine resolution series from three equatorial ionosondes of the IEEY network in West Africa have revealed small-scale daytime peak F2 structures, superposed on the slowly varying minimum or –trough distribution in the ±5° magnetic latitude zone. We report this new morphology, concentrating on foF2 enhancements of two types: near-equatorial crests (which travel either northwards or southwards) and magnetic field-aligned domes, whose onsets last only tens of minutes. Both types are observed to start at mid-morning or early afternoon hours. We relate their occurrence with the available variations of Vz = E × B upward drift which feeds the equatorial plasma fountain. We suggest the foF2 enhancements to be triggered by brief slow-downs of the Vz velocity near F2 peak altitude in our West African sector. Their short latitude extent differentiates them from the larger-scale tropical crest system. Further analysis of these features should lead to weather-like models of the low latitude ionosphere variations, where unstable local coupling between processes seems to be the trigger.     

MAGSAT data and Curie-depth below Deccan flood basalts (India)

Ground and airborne magnetic data are severely disturbed due to random susceptibility variations in Deccan flood basalts. However, Magnetic Satellite (MAGSAT) data over the Deccan flood basaltic region of the Indian subcontinent exhibit filtering of surficial noise. Three passes over Deccan traps show a low at about 20°N latitude and a high at about 23°N latitude. Spectral analysis of these passes and an available 2-D MAGSAT vertical intensity map indicate a deep (40±4 km) magnetic interface. It is interesting to note that the determination of Curie-depth from MAGSAT matches and confirms the geothermal data model. The estimates correspond to the Moho depth derived from gravity and deep seismic sounding studies. The study suggests a continental shield-like geothermal gradient of about 14°C/km below the area.     

地电场方法在川滇地区地震观测中的应用

利用云南省(5个)和四川省(4个)"十五"期间布设的地电场观测数据,利用大地电场观测系统中,同一测向长短极距测值的比值方法进行分析,发现能够较好地排除来自大地电场及自然电场远场的变化信息,突出可识别的地震前兆异常信息。研究结果表明:在剔除各种环境和空间电流体系的影响因素后,提取了与地震有关的异常信息,在川滇地区5.0级地震前,多数台站的地电场观测资料比值发生异常变化,异常大多出现在地震前0.5—3个月,其中汶川地震和姚安地震前,研究区的9个台站均出现异常。     

Fabry-Perot interferometer measurements of neutral winds and F2 layer variations at the magnetic equator

This letter presents some night-time observations of neutral wind variations at F2 layer levels near the dip equator, measured by the Fabry-Perot interferometer set up in 1994 at Korhogo (Ivory Coast, geographic latitude 9.25°N, longitude 355°E, dip latitude -2.5°). Our instrument uses the 630 nm (O¹D) line to determine radial Doppler velocities of the oxygen atoms between 200 and 400 km altitude. First results for November 1994 to March 1995 reveal persistent eastward flows, and frequent intervals of southward winds of larger than 50 ms^–1 velocity. Compared with the simultaneous ionospheric patterns deduced from the three West African equatorial ionosondes at Korhogo, Ouagadougou (Burkina-Faso, dip latitude +1.5°) and Dakar (Sénégal, dip latitude + 5°), they illustrate various impacts of the thermospheric winds on F2 layer density: (1) on the mesoscale evolution (a few 10³ km and a few 100 minutes scales) and (2) on local fluctuations (hundreds of km and tens of minutes characteristic times). We report on these fluctuations and discuss the opportunity to improve the time-resolution of the Fabry-Perot interferometer at Korhogo.     

基于人工源的甚低频电磁波空间传播特征统计分析

利用2005年1月至2010年11月DEMETER卫星记录的NWC发射站的VLF电场功率谱数据,采用指数拟合的方法,分析了VLF电磁波在卫星高度激发的电场空间分布和衰减特征.研究结果表明:(1)VLF电场在发射站上空及其磁共轭区有着很强的对应关系,存在南、北2个强电场中心涡;(2)相对于发射站的位置,VLF电场中心点具有经度和纬度偏移,日侧地磁经度偏移均值大于夜侧,而地磁纬度偏移均值则小于夜侧;(3)日侧VLF电场强度呈现出周期性的年变化;(4)在VLF电场中心10°范围内,电场强度随距离快速衰减,衰减常数b在长达6年的时间内保持稳定.在以上研究结果基础上初步构建的卫星高度人工源电磁波空间分布特征,将为研究地表-电离层电磁波传播机理提供基础技术支撑.     

基于CM4模型的中国大陆地区地磁场时空分布特征分析

本文利用第四代地磁场综合模型（Comprehensive Model 4,CM4）,计算了1982-2001年中国大陆地区同一经度链和同一纬度链上地磁台站的磁层源磁场及其感应场、电离层源磁场及其感应场的地磁北向分量X、东向分量Y、垂直分量Z的模型值,分析了各场源磁场随时间和空间的变化特征。结果表明:在时间上,经度链和纬度链台站的磁层源磁场及其感应场均呈现出11年和27天周期性变化。电离层源磁场及其感应场具有明显的季节变化,不同年份相同季节变化形态一致但幅度不同。在空间分布上,经度链和纬度链台站磁层源磁场及其感应场的年变化幅度呈现出不同变化特征,电离层源磁场及其感应场在经度链上变化特征不同,而纬度链台站的数值基本一致。日变化分析显示,磁静日和磁扰日期间,模型数据与台站实测数据变化一致性较好,相关性较高。     

Preliminary Results from Paleomagnetic Records on Lake Sediments from South America

The preliminary results of paleomagnetic and radiocarbon dating of late pleistocene-holocene sediments from two lakes of south-western Argentina (41°S, 71.5°W) are presented. The magnetic susceptibility, intensity and direction of the natural remanent magnetisation were measured. The stability of the natural remanent magnetisation was investigated by alternating field demagnetisation. The magnetic parameters allowed the cores within each lake to be correlated. ¹³ C analysis, total organic content measurements and C ¹⁴ dating were carried out. A model of sedimentation is suggested. Using this model and the correlation, curves of variations of magnetic inclination and declination in time are shown.     

Detection Capability of Selected Seismic Stations in Central Europe

The 50% and 90% P-wave detection thresholds of 3 Austrian, 2 Czech, 5 German, 1 Hungarian and 3 Slovak seismic stations have been determined for four intervals of epicentral distances by a direct method using the maximum-likelihood technique. The USGS-NEIC Earthquake Data Reports covering the period from January 1990 to November 1994 were used as the reference system. The seismic stations Kaperské Hory (KHC, Czech Republic) and Collmberg (CLL, Germany) are the stations with the best detection capabilities for all investigated intervals of epicentral distances.