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1.
本文将均质的任意二维、三维物体位场的波谱解析表达式的研究成果推广到变密度、变磁化强度的更一般的情形。对密度差随深度呈指数函数衰减或线性变化的模型,获得了任意倾斜多边形质量面、斜平行六面体以及一般的多面体等形体的重力谱的解析表达式。它们的结构与均质体相应表达式一样简单,易于计算。以上结果表明,在很一般的条件下,位场波谱具有指数函数和的形式。 相似文献
2.
Possible configurations of the magnetic field in the outer magnetosphere during geomagnetic polarity reversals are investigated by considering the idealized problem of a magnetic multipole of order m and degree n located at the centre of a spherical cavity surrounded by a boundless perfect diamagnetic medium. In this illustrative idealization, the fixed spherical (magnetopause) boundary layer behaves as a perfectly conducting surface that shields the external diamagnetic medium from the compressed multipole magnetic field, which is therefore confined within the spherical cavity. For a general magnetic multipole of degree n, the non-radial components of magnetic induction just inside the magnetopause are increased by the factor 1 + [(n + 1)/n] relative to their corresponding values in the absence of the perfectly conducting spherical magnetopause. An exact equation is derived for the magnetic field lines of an individual zonal (m = 0), or axisymmetric, magnetic multipole of arbitrary degree n located at the centre of the magnetospheric cavity. For such a zonal magnetic multipole, there are always two neutral points and n – 1 neutral rings on the spherical magnetopause surface. The two neutral points are located at the poles of the spherical magnetopause. If n is even, one of the neutral rings is coincident with the equator; otherwise, the neutral rings are located symmetrically with respect to the equator. The actual existence of idealized higher-degree (n > 1) axisymmetric magnetospheres would necessarily imply multiple (n + 1) magnetospheric cusps and multiple (n) ring currents. Exact equations are also derived for the magnetic field lines of an individual non-axisymmetric magnetic multipole, confined by a perfectly conducting spherical magnetopause, in two special cases; namely, a symmetric sectorial multipole (m = n) and an antisymmetric sectorial multipole (m = n – 1). For both these non-axisymmetric magnetic multipoles, there exists on the spherical magnetopause surface a set of neutral points linked by a network of magnetic field lines. Novel magnetospheric processes are likely to arise from the existence of magnetic neutral lines that extend from the magnetopause to the surface of the Earth. Finally, magnetic field lines that are confined to, or perpendicular to, either special meridional planes or the equatorial plane, when the multipole is in free space, continue to be confined to, or perpendicular to, these same planes when the perfectly conducting magnetopause is present.Also Honorary Research Associate, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK and Visiting Reader in Physics. University of Sussex, Falmer, Brighton BN1 9QH, UK 相似文献
3.
Generalised relative and cumulative response functions for electromagnetic induction conductivity meters operating at low induction numbers 
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下载免费PDF全文 Relative and cumulative analytical response functions have been widely used as a powerful tool for forward modelling and interpretation of measurements obtained by electromagnetic induction conductivity meters operating at low induction numbers for one‐dimensional layered earth models. These well‐known functions were derived and should be used for the instruments laid on the surface of the earth. In this paper, we extended the response functions and obtained new generalised analytical expressions, which can be used for instruments carried at any height from the surface. The proposed new equations were compared with numerically constructed functions, obtained using the full solution of Maxwell's equations, and proved to be in very good agreement at low induction numbers. Quantitative analyses of the behaviour of the relative response and the depth of investigation of electromagnetic induction instruments, when raised from the ground, could also be done using the generalised functions. 相似文献
4.
为计算地球磁极处的磁感应强度,建立地球的磁场是由带电的地球外核的旋转产生的模型.先根据毕奥-萨伐尔定律计算球形模型绕自转轴旋转时在自转轴直径上产生的磁感应强度;再利用已知的地球外核的内外半径及地球半径和磁极处的磁感应强度值,计算出地球外核的电荷体密度及面密度.结果表明:若外核的电荷呈均匀的体密度分布,则其电荷体密度为3.5507 C/m3;若外核的电荷均匀分布在外核的外表面,则其面密度为2.4581×106 C/m2.通过地球表面的磁感应强度信息利用物理规律和地球物理数据推测地球内部难以直接进行探测的相关信息,具有实际意义.根据地震学方法对地球外核厚度、转向等变化的最新研究数据按该文模型可推测地球磁场强度、极性等的变化.而地球磁场的变化对地球上的人类生活颇有影响. 相似文献
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提出了一种基于非结构化四面体以及带地形模型的自适应多层快速多极大规模磁法快速正演算法.该算法弥补了传统积分方法采用FFT加速计算时不能采用非结构化网格的缺陷;同时采用自适应快速多极算法突破积分求和方法求解大规模磁法问题耗时长的突出问题.首先,采用非结构化的四面体网格剖分技术能够更好的模拟复杂模型以及带地形模型,实现磁法模型的高精度模拟;其次,采用一种自适应多层快速多极(AMFM)算法实现大规模磁法正演求解.通过将计算区域划分为近区和远区,对近区采用解析计算高精度求解,对远区采用自适应多层快速多极算法进行加速计算,假设有M个观测点,N个四面体源单元,可将计算复杂度由传统积分求和法的O(MN)减少到O(Mlog N).本文设计了组合体模型以及安徽怀宁地区的实际地形模型,模型计算结果体现了采用该方法进行大规模复杂模型三维磁法正演模拟的高效性和准确性. 相似文献
6.
各向异性双相介质中多极源声波测井理论研究 总被引：4，自引：0，他引：4
将各向异性地层模拟为横向各向同性双相介质 ,使其对称主轴与井孔中心轴线相垂直 .本文针对井孔模型下的多极源声波测井问题提出了一种解析的摄动方法 .将各向异性双相介质看作是由对应的各向同性双相介质加上摄动部分而组成 ,并引入 5个关于弹性模量偏离各向同性时的摄动量 ,对对称主轴与井轴垂直的各向异性双相介质的多极源声波测井问题进行了系统的理论研究和分析 ,严格求解了井孔内外的零级和一级近似声波场 .发现在n级多极源激发下 ,井孔中不仅出现n级多极声场 ,而且还能激发出与各向同性地层时不相同的高于和低于n级的多极声波场 相似文献
7.
各向异性双相介质中多极源声波测井理论研究 总被引：1，自引：0，他引：1
将各向异性地层模拟为横向各向同性双相介质 ,使其对称主轴与井孔中心轴线相垂直 .本文针对井孔模型下的多极源声波测井问题提出了一种解析的摄动方法 .将各向异性双相介质看作是由对应的各向同性双相介质加上摄动部分而组成 ,并引入 5个关于弹性模量偏离各向同性时的摄动量 ,对对称主轴与井轴垂直的各向异性双相介质的多极源声波测井问题进行了系统的理论研究和分析 ,严格求解了井孔内外的零级和一级近似声波场 .发现在n级多极源激发下 ,井孔中不仅出现n级多极声场 ,而且还能激发出与各向同性地层时不相同的高于和低于n级的多极声波场 相似文献
8.
E. L. Stupitsky 《Geomagnetism and Aeronomy》2006,46(1):23-40
The analytical expressions for determining the deceleration region of plasma, produced during large-scale geophysical experiments of explosion type, and taking into account the effect of the rarefied ionosphere and geomagnetic field have been obtained. The possibility of the magnetosphere braking by plasma produced by a powerful explosion is analyzed. The simplified set of equations used to analyze the linear and non-linear stages of flute instability of explosive plasma expanding into the magnetic field has been obtained. The mass overflow between flutes and the viscous force is taken into account. The experimental studies of flute instability during laser plasma expansion into the magnetic field are theoretically substantiated. The mechanism explaining the formation of a jet stream and the main jet inclination of 11° with respect to the axis of symmetry has been proposed. 相似文献
9.
When a vertical fault or dike scatters a normally incident TE-mode plane wave, the horizontal components of the electric and the magnetic fields vary along the direction perpendicular to the strike. This scattering is also responsible for the formation of a vertical component of the magnetic field, which also varies along the direction perpendicular to the strike. Analysis of the lateral variations of the field components permits the identification of the type of geological structure, either fault or dike, as well as an estimate of the position of the geological contacts and the electrical conductivity of each medium. Previous exact analytical solutions have shown that two Fourier cosine integrals can express each field component. A series of functions specified for each model represent the kernel of each integral. From the field components obtained from the first non-zero term of each series, we calculated the following functions: the dip angle and the ellipticity of the vertical polarization ellipse in a plane perpendicular to the strike of the structure, and the azimuth and the ellipticity of the horizontal ellipse. We established master-curves of these functions for the interpretation of vertical faults and dikes for the polarization ellipsoid, for instance VLF or audio-frequency methods. This representation has as variable the induction number , and as parameters σ 2 / σ 1 and , where a denotes the half-thickness of the dike. The interpretation procedure using curve fitting is possible because the induction number is represented on a logarithmic scale. This procedure represents an important step before automatic interpretation is carried out. Two case histories using field data illustrate the effectiveness of the procedure and the type of quantitative interpretation obtained from it. 相似文献
10.
Michael I. Bergman 《地球物理与天体物理流体动力学》2013,107(1-4):151-176
Abstract The stratification profile of the Earth's magnetofluid outer core is unknown, but there have been suggestions that its upper part may be stably stratified. Braginsky (1984) suggested that the magnetic analog of Rossby (planetary) waves in this stable layer (the ‘H’ layer) may be responsible for a portion of the short-period secular variation. In this study, we adopt a thin shell model to examine the dynamics of the H layer. The stable stratification justifies the thin-layer approximations, which greatly simplify the analysis. The governing equations are then the Laplace's tidal equations modified by the Lorentz force terms, and the magnetic induction equation. We linearize the Lorentz force in the Laplace's tidal equations and the advection term in the magnetic induction equation, assuming a zeroth order dipole field as representative of the magnetic field near the insulating core-mantle boundary. An analytical β-plane solution shows that a magnetic field can release the equatorial trapping that non-magnetic Rossby waves exhibit. A numerical solution to the full spherical equations confirms that a sufficiently strong magnetic field can break the equatorial waveguide. Both solutions are highly dissipative, which is a consequence of our necessary neglect of the induction term in comparison with the advection and diffusion terms in the magnetic induction equation in the thin-layer limit. However, were one to relax the thin-layer approximations and allow a radial dependence of the solutions, one would find magnetic Rossby waves less damped (through the inclusion of the induction term). For the magnetic field strength appropriate for the H layer, the real parts of the eigenfrequencies do not change appreciably from their non-magnetic values. We estimate a phase velocity of the lowest modes that is rather rapid compared with the core fluid speed typically presumed from the secular variation. 相似文献
11.
R. W. James 《Pure and Applied Geophysics》1969,74(1):23-24
Summary The force experienced by a multipole when placed in an external magnetic field (which may be the field of another multipole) is formulated in terms of its dependence on the axes of the multipole. 相似文献
12.
We present new analytical three-dimensional solutions of the magnetohydrostatic equations, which are applicable to the co-rotating frame of reference outside a rigidly rotating cylindrical body, and have potential applications to planetary magnetospheres and stellar coronae. We consider the case with centrifugal force only, and use a transformation method in which the governing equation for the “pseudo-potential” (from which the magnetic field can be calculated) becomes the Laplace partial differential equation. The new solutions extend the set of previously found solutions to those of a “fractional multipole” nature, and offer wider possibilities for modelling than before. We consider some special cases, and present example solutions. 相似文献
13.
S. O. Ogunade 《Pure and Applied Geophysics》1982,120(1):136-150
A major problem in electromagnetic induction studies in regions of localized source fields, such as the auroral and equatorial electrojet regions, is the source effect. Using an analytical model, the electromagnetic response of a buried conducting cylinder to sheet current and line current excitations has been studied for the period rangeT=5 s to 24 h. The validity of the numerical results obtained from the analytical model are compared with the numerical results obtained from a finite difference model. The results show that for periods less than 30 min, there is no significant difference in the response of the cylinder to both source fields. However, significant differences are observed at longer periods. It was also observed that the equivalent height at which a uniform sheet current at 100 km above the earth's surface can be approximated by a line current varies as a function of the source period. 相似文献
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The possibility of generation of additional acoustic harmonics in the geomagnetic field after the propagation of an acoustic wave in the conducting medium through the region with a variable magnetic field has been considered. Two possible procedures of such a generation have been analyzed: the parametric procedure and the dynamic procedure related to ponderomotive forces. The expressions have been obtained for three acoustic harmonics generated by an oscillating magnetic field. 相似文献
16.
Variations in the value of the source magnetic moment, responsible for the dipole component of the main geomagnetic field, and Earth rotation velocity fluctuations have been studied comparatively. Three quasi-harmonic components with close periods, for which their own phase shifts were obtained, have been detected in time series of the analyzed processes. The mantle conductivity effect on these shifts has been studied in the scope of a one-layer model; for this purpose, the amplitude-phase characteristic of the mantle as a filter has been constructed. The mantle conductivity has been estimated (σ = 270 ± 20 S m?1) on the assumption that the phase shift mainly depends on the mantle effect. It has been assumed that the variations in the time series of the analyzed processes are almost cophased and can be caused by one external factor, e.g., tides. 相似文献
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The electric and magnetic fields generated by horizontal electric and vertical magnetic dipoles lying on the surface of a conducting medium with horizontal anisotropy are investigated. Full expressions of their Fourier transforms are given, and the fields for a vertical magnetic dipole are calculated numerically. The radial and vertical magnetic components are found to be independent of the receiver-transmitter direction, whereas the other magnetic and electric components strongly vary with this direction. These results give useful criteria for defining the direction and amplitude of anisotropy from ground data; a ground experiment on fissured limestone was found to confirm the expected variations of the various field components. It is believed that this electromagnetic method can be used in order to provide information about the direction and amplitude of electric anisotropy. 相似文献
19.
Summary This paper presents a theory of the transient Bernoulli-Euler beam problem on an elastic foundation which takes into account the effects of axial load and linear damping. An analytical solution of the steady state and the transient components has been obtained due to physically realistic load distributions. With a view to extend its practical applicability, the characteristic features of the solution are explored. Several limiting situations are investigated as special cases. It is shown that the steady state vibration can be achieved as the limit of the solution of the transient problem. 相似文献
20.
The dynamics of the magnetospheric magnetic field during the magnetic storms of different intensity has been studied. The magnetic field variations on the Earth’s surface were calculated using the paraboloid model of the magnetosphere, taking into account the induction currents flowing in the diamagnetically conductive Earth. Dst and its components have been calculated for ten magnetic storms. It has been indicated that relative contributions of magnetospheric sources to Dst change depending on the storm power. For weak and moderate storms, the contribution of the magnetotail current sheet can reach values comparable with the ring current contribution and, sometimes, can even exceed this contribution. For strong storms, the ring current field dominates over the tail current field, the absolute value of which does not exceed 150 nT (also achieved during less intense storms). For storms with minimum Dst exceeding-200 nT, the tail current field is saturated, whereas the ring current can continue developing. 相似文献