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The complex-image method for calculating the magnetic and electric fields produced at the surface of the Earth by the auroral electrojet 总被引：7，自引：0，他引：7
For studying the auroral electrojet and for examining the effects it can produce in power systems on the ground, it is useful to be able to calculate the magnetic and electric fields that the electrojet produces at the surface of the Earth. Including the effects of currents induced in the Earth leads to a set of integral expressions, the numerical computation of which is complicated and demanding of computer resources. An approximate solution can be achieved by representing the induced currents by an image current at a complex depth. We present a simple derivation of the complex-image expressions and use them to calculate the fields produced by the auroral electrojet at the surface of an earth represented by layered conductivity models. Comparison of these results with ones obtained using the exact integral solution show that the errors introduced are insignificant compared to the uncertainties in the parameters used. The complex-image method thus provides a simple, fast and accurate means of calculating the magnetic and electric fields. 相似文献
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K. Holliger 《Geophysical Journal International》1997,128(1):65-72
Telluric distortion occurs when electric charges accumulate along near-surface inhomogeneities. At low frequencies, the electric currents associated with these charges can be neglected compared to currents induced deeper in the Earth. At higher frequencies, the magnetic fields associated with these currents may be significant. Some parameters describing the distortion magnetic fields can be estimated from measured magneto-telluric impedance matrices. For regional magnetic fields aligned with regional strike directions, parameters associated with the distortion magnetic field component parallel to the regional magnetic field are undeterminable, whereas parameters associated with the distortion magnetic field component perpendicular to the regional magnetic field can be estimated. Optimal estimates are straightforward even for the realistic case of measurement errors that are correlated between elements of a measured impedance matrix. In a simple example of a 1-D anisotropic model with anisotropy direction varying with depth, the modelling of distortion magnetic fields results in regional impedance estimates corresponding more closely to the responses of uncoupled isotropic models, allowing sensible interpretation of an additional one and a half decades of data. 相似文献
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A detailed study has been made of the remanent magnetization of five Pilansberg dykes, by means of measurements on oriented specimens of the rocks. Outcrops of the dykes prove to have suffered magnetic disturbance of their original thermo-remanent magnetization. Specimens from depths of a few thousand feet, taken in Witwatersrand gold mines, show highly consistent magnetisation of the basic parts of the dykes over considerable distances between sampling sites. The mean directions given by the five dykes agree well, and the mean direction from the five dykes gives a North-seeking magnetic pole with inclination +69.3 and azimuth N 24° E. Assuming thermo-remanent magnetisation by a geocentric dipole field, this places a North magnetic pole in latitude 71/2° N, longitude 421/ E at the time of intrusion of the dykes. The age of the dykes is uncertain, but is probably about 300 to 400 million years. Some of the specimens have been subjected to alternating magnetic fields in order to test the stability of their magnetisations. The basic specimens are found to be highly stable, being only slightly affected by fields of 100 to 300 oersteds. The possible implications of the results are discussed, in terms of hypotheses of polar wandering and continental drift. 相似文献
A detailed study has been made of the remanent magnetization of five Pilansberg dykes, by means of measurements on oriented specimens of the rocks. Outcrops of the dykes prove to have suffered magnetic disturbance of their original thermo-remanent magnetization. Specimens from depths of a few thousand feet, taken in Witwatersrand gold mines, show highly consistent magnetisation of the basic parts of the dykes over considerable distances between sampling sites. The mean directions given by the five dykes agree well, and the mean direction from the five dykes gives a North-seeking magnetic pole with inclination +69.3 and azimuth N 24° E. Assuming thermo-remanent magnetisation by a geocentric dipole field, this places a North magnetic pole in latitude 71/2° N, longitude 421/ E at the time of intrusion of the dykes. The age of the dykes is uncertain, but is probably about 300 to 400 million years. Some of the specimens have been subjected to alternating magnetic fields in order to test the stability of their magnetisations. The basic specimens are found to be highly stable, being only slightly affected by fields of 100 to 300 oersteds. The possible implications of the results are discussed, in terms of hypotheses of polar wandering and continental drift. 相似文献
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Alan D. Chave R. P. Von Herzen K. A. Poehls † C. S. Cox 《Geophysical Journal International》1981,66(2):379-406
Summary. We have analysed a thirty-six day recording of the natural electric and magnetic field variations obtained on the deep ocean floor north-east of Hawaii. The electromagnetic fields are dominated by tides which have an appreciable oceanic component, especially in the east electric and north magnetic components. The techniques of data analysis included singular value decomposition (SVD) to remove uncorrelated noise. There are three degrees of freedom in the data set for periods longer than five hours, indicating a correlation of the vertical magnetic field and the horizontal components, suggesting source field inhomogeneity. Tensor response functions were calculated using spectral band averaging with both SVD and least squares techniques and rotated to the principal direction. One diagonal component, determined mainly by the north electric and east magnetic fields, is not interpretable as a one-dimensional induction phenomenon. The other diagonal term of the response function indicates a rapid rise in conductivity to 0.05 mho m−1 near 160 km. No decrease in conductivity below this depth is resolvable. Polarization analysis of the magnetic field indicates moving source fields with a wavelength near 5000 km. Model studies suggest that the two dimensionality in the response function may be caused by motion in the ionospheric current system. 相似文献
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Contemporary research into extratropical cloud systems optimizes the increase in resolution of visible (VIS) and thermal infra‐red (IR) sensors, and the ability to retrieve wind and atmospheric moisture variables at mesoscales using microwave radiometry. These passively‐acquired remote sensing data are used to develop synoptic climatological (conceptual and simple statistical) ‘models’ of mesoscale cyclones in cold‐air outbreaks (mesocyclones, ‘polar lows’) occurring over the otherwise data‐void southern oceans. Mesocyclones present a limitation to successful weather forecasting for New Zealand and coastal Chile, southern Australia and South Africa, during the cold season. The synoptic climatological analyses show that: 1) the patterns of mesocyclone cloud vortex origins, movement and dissipation (‘mesocyclone regimes’), exhibit spatial dependence and have associations with upper‐ocean conditions; 2) mesocyclone ‘outbreaks’ are embedded within characteristic larger‐scale anomaly fields of tropospheric pressure, height, and layer thickness (mean temperature); and 3) composite (statistical average) models of cloud system structure based on the microwave retrievals of marine weather reveal mesocyclones to be relatively dry in comparison with synoptic cyclones, yet very windy. These analyses should permit the development of methods to better predict these important cold‐season storms over southern middle latitudes, and a fuller assessment of their significance for the larger hydroclimatic system. 相似文献
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IGORV POLYAKOV 《Polar research》1994,13(2):219-232
A three-dimensional structure of the M2 tidal currents has been reproduced by mathematical modelling. The possibility of energy transfer from the tidal barotropic motion to large-scale baroclinic circulation of the Arctic Ocean has been estimated. It has been shown that the bottom relief plays an important role in the formation of the large-scale pattern of the baroclinic fields. The hypothesis has been proposed that the M2 tide may support the near-slope convection of the East-Siberian and Laptev seas. Polar Research 13 (2), 219–232 相似文献
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《自然地理学》2013,34(2):156-171
Mean annual snowfall in the Great Smoky Mountains National Park (GSMNP) exhibits considerable spatial variability, ranging from 30 cm in the valleys to 254 cm at higher elevations. Snowfall can be tied to a variety of synoptic classes (e.g., Miller A or B cyclones, 500 hPa cutoff lows), but the frequency and significance of different synoptic classes have not been fully assessed, particularly at higher elevations. In this paper, we manually classify all snowfall events during the period 1991 to 2004 according to a synoptic classification scheme, calculate mean annual snowfall by 850 hPa wind direction and synoptic class, and develop composite plots of various synoptic fields. Hourly observations from nearby first-order stations and 24 hr snowfall totals from five sites within the GSMNP are used to define snowfall events. NCEP/NCAR reanalysis data are used to develop composite plots of various synoptic fields for cyclones passing south and then east of the area (e.g., Miller A cyclones). Results indicate that over 50% of the mean annual snowfall at higher elevations occurs in association with low-level northwest flow, and that Miller A cyclones contribute the greatest amount to mean annual snowfall at all elevations. 相似文献
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沙尘暴天气是毛乌素沙漠边缘内蒙古、甘肃、宁夏、陕西等地区春末夏初特有的一种灾害性天气。应用欧洲中心再分析资料的气压场和各种物理量场,选取了5个沙尘暴个例,分析了沙尘暴时期各层气压场的特征和物理量的空间结构。结果表明：蒙古高压是影响沙尘暴的主要天气系统。在沙尘暴发生时,蒙古高压前部整层都为宽广而强劲的西北气流,从地面可伸展到200 hPa附近,其物理属性是干、冷,其前部近乎于东北—西南向的区域,是锋区的密集带,也是各种物理量表现最为敏感的区域,垂直速度场为上升运动区,涡度场为正涡度区,高—低层散度差值场为正值,这些物理量都为沙尘暴的发生起到了推动和加强作用。 相似文献
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利用2003年在南极中山站考察期间所获取的气象观测资料和气象传真图对中山站地区冬季的低温及增暖天气进行了天气学分析,揭示了这两种天气的风场、温湿场以及天气现象的不同特点和相应的天气形势,为提高极区冬季气温预报水平提供了新的依据。 相似文献
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Risto J. Pirjola Ari T. Viljanen & David H. Boteler 《Geophysical Journal International》2000,140(2):286-294
The relation between the seafloor electric field and the surface magnetic field is studied. It is assumed that the fields are created by a 2-D ionospheric current distribution resulting in the E-polarization. The layered earth below the sea water is characterized by a surface impedance. The electric field at the seafloor can be expressed either as an inverse Fourier transform integral over the wavenumber or as a spatial convolution integral. In both integrals the surface magnetic field is multiplied by a function that depends on the depth and conductivity of the sea water and on the properties of the basement. The fact that surface magnetic data are usually available on land, not at the sea surface, is also considered. Test computations demonstrate that the numerical inaccuracies involved in the convolution method are negligible. The theoretical equations are applied to calculate the seafloor electric fields due to an ionospheric line current or associated with real magnetic data collected by the IMAGE magnetometer array in northern Europe. Two different sea depths are considered: 100 m (the continental shelf) and 5 km (the deep ocean). It is seen that the dependence of the electric field on the oscillation period is weaker in the 5 km case than for 100 m. 相似文献
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Summary. Six submarine rocks dredged from guyots in the western Pacific were dated by means of 40 Ar–39 Ar step heating dating technique. All ages show mid- to late-Cretaceous ages in accordance with the generally supposed age of the ocean crust estimated from the magnetic anomaly pattern.
The back-tracked paths were calculated for the guyots with the use of their absolute ages, for both the Pacific plate movements proposed by Morgan and by Clague & Jarrard. In both cases these show birth places for the guyots, which are located near the equator.
Fossil ages are nearly concordant with the40 Ar–39 Ar ages for the guyots which are older than 95 Myr, whereas the fossil ages are considerably younger than the 40 Ar–39 Ar ages for the guyots younger than 90 Myr. This contrast may be due to the world-wide marine transgression, whose climax is believed to have occurred at about 85 Myr BP. 相似文献
The back-tracked paths were calculated for the guyots with the use of their absolute ages, for both the Pacific plate movements proposed by Morgan and by Clague & Jarrard. In both cases these show birth places for the guyots, which are located near the equator.
Fossil ages are nearly concordant with the
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《Geophysical Journal International》1964,8(4):375-388
Summary. Theoretical calculations have been made for the idealized case in which an ocean is approximated by an infinitely long rectangular conducting sheet. The results indicate that even in the case of magnetic variations with periods as long as 24 hr one should expect a significant and easily observable enhancement in the vicinity of the edge of an ocean. When the inducing field is a function of local time only, it is found that the amplification of the variations is greater at the eastern edge of the ocean than at the western edge.
Laboratory model studies have been used to investigate the effect of irregularities in the shape of a coastline. Results which have been obtained from a model of the Pacific Ocean in the neighbourhood of Japan indicate that it is reasonable to attribute at least a part of the anomalous diurnal variations observed in central Japan to electric currents induced in the ocean. 相似文献
Laboratory model studies have been used to investigate the effect of irregularities in the shape of a coastline. Results which have been obtained from a model of the Pacific Ocean in the neighbourhood of Japan indicate that it is reasonable to attribute at least a part of the anomalous diurnal variations observed in central Japan to electric currents induced in the ocean. 相似文献
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Summary. The Wiener—Hopf technique is used to obtain an exact analytical solution for the problem of H -polarization induction over the edge of a perfectly conducting thin sheet, representing an ocean, electrically connected to a perfectly conducting mantle through a slab of finite conductivity and thickness, which represents the Earths crust. It is shown that the induced currents resulting from this type of induction process are drawn up into the sea from the cust and mantle with the greatest concentration of current near the ocean edge. The surface impedance over the land surface, is calculated for various mantle depths and is shown to increase sharply as the coastline is approached. The magnetic field along the ocean floor is also plotted as a function of distance from the coastline, and the results are found to agree very well with those calculated previously by approximate and numerical methods. 相似文献
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Direct measurements of volume transports through Fram Strait 总被引：9，自引：1，他引：9
Eberhard Fahrbach Jens Meincke Svein Østerhus Gerd Rohardt Ursula Schauer Vigdis Tverberg Jennifer Verduin 《Polar research》2001,20(2):217-224
Heat and freshwater transports through Fram Strait are understood to have a significant influence on the hydrographic conditions in the Arctic Ocean and on water mass modifications in the Nordic seas. To determine these transports and their variability reliable estimates of the volume transport through the strait are required. Current meter moorings were deployed in Fram Strait from September 1997 to September 1999 in the framework of the EU MAST III Variability of Exchanges in the Northern Seas programme. The monthly mean velocity fields reveal marked velocity variations over seasonal and annual time scales, and the spatial structure of the northward flowing West Spitsbergen Current and the southward East Greenland Current with a maximum in spring and a minimum in summer. The volume transport obtained by averaging the monthly means over two years amounts to 9.5 ± 1.4 Sv to the north and 11.1 ± 1.7 Sv to the south (1 Sv = 106 m3 s−1 ). The West Spitsbergen Current has a strong barotropic and a weaker baroclinic component; in the East Greenland Current barotropic and baroclinic components are of similar magnitude. The net transport through the strait is 4.2 ± 2.3 Sv to the south. The obtained northward and southward transports are significantly larger than earlier estimates in the literature; however, within its range of uncertainty the balance obtained from a two year average is consistent with earlier estimates. 相似文献
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海洋表层沉积物的磁学性质逐渐被证明是分析海洋沉积物物源的有效手段之一。选取海南岛周边海域39个站位的表层沉积物进行了磁学特征研究。结果表明：海南岛周边海域表层沉积物的低频磁化率和频率磁化率具有明显的空间分异特征,表现为高值区主要出现在海南岛以南海域,而海南岛以东和以西海域则呈现从岸向海逐渐降低的趋势。磁化率的影响因素分析表明：研究区海洋表层沉积物的磁化率主要由陆源碎屑物质贡献,因此可以认为沉积物低频磁化率和频率磁化率的空间分布特征能指示陆源输入的变化。利用多个环境磁学参数对采样点进行Q型聚类,并结合低频磁化率和频率磁化率的空间分布特征及采样点的地理位置将研究区划分为4个区域：海南岛以东水深超过100 m的海域（Ⅰ区）沉积物主要来源于其东北面的珠江水系,同时粤西沿岸河流携带来的物质也有一定程度的影响;海南岛以东水深小于100 m和海南岛以南海域（Ⅱ区）沉积物可能主要来自于海南岛沿岸侵蚀搬运以及入海河流输入;北部湾北部海域（Ⅲ区）沉积物受琼州海峡和北部湾沿岸河流输入物质的共同影响;海南岛以西海域（Ⅳ区）沉积物受来自海南岛西南侧河流输入、沿岸侵蚀物和由南向北的洋流所携带的沉积物的共同影响。研究表明：低频磁化率和频率磁化率组合可有效指示近岸表层沉积物的物源变化。 相似文献
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Upper layer circulation of the Nordic seas as inferred from the spatial distribution of heat and freshwater content and potential energy 总被引：1，自引：0，他引：1
The spatial distribution of heat and freshwater content and potential energy of a several hundred metre thick surface layer are computed for the Nordic seas and adjacent parts of the northern North Atlantic and the Arctic Ocean using a total of almost 100 000 hydrographic stations. The fields clearly show the major features of the area's circulation, with warm salty water in the eastern part and fresher, colder water in the western part. Comparisons with published estimates show that the potential energy field, representing the baroclinic part of the flow, accounts for about 30 % of the total flow but roughly 100 % of the flow of Polar Water in the northern part of the East Greenland Current, about 50 % of the total flow in the Norwegian Atlantic Current, and just a small fraction of the flow in the eastern part of Fram Strait. This suggests that the barotropic circulation is quite important in many parts of the Nordic seas. The barotropic circulation is also clearly seen by its effects on the integrated fields with isolines following deep bathymetric contours. We speculate that the barotropic circulation in combination with topographic obstacles, like the Greenland–Scotland Ridge and the ridge system in the Jan Mayen area, may have large impact on the spreading of freshwater and heat in the Nordic seas. 相似文献
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summary . An ocean-coast model which consists of a uniformly conducting half-space screened by a perfectly conducting half-plane (the model ocean) is studied. On the land the electric field decreases continuously to zero as the coast is approached. The horizontal magnetic field component is found to vary rapidly, but remains finite; the vertical component on the other hand, increases to infinity at the coast. On the surface of the model ocean as well as on the sea floor, electric field and vertical magnetic field are both nil, but the horizontal magnetic field becomes singular as the seashore is approached. This horizontal magnetic field however, is different on the sea floor and at the ocean surface, because the integrated ocean current is finite, even growing to infinity as the shore is approached. The very large ocean currents near the shore act as an extremely long line antenna, which radiates far afield. This antenna feature explains the very long range of the ocean-coast effects observed under E -polarization induction, compared to the corresponding H -polarization effects where no such antenna-like feature occurs. A similarly large difference of ranges can be expected for all shallow structures with large lateral conductivity contrasts. The present study may therefore be of some interest in relation to geomagnetic depth soundings by the inductive and magnetotelluric methods, as well as in understanding the ocean-coast effect known for some time from records of coastal observatories. 相似文献