摘要:

The physical interpretation which Price [1950] attached to the solutions of the pre‐Maxwell equations is examined and the validity of linear circuit analogies to the problem of geomagnetic induction is questioned. If induced earth currents are solely diffusive, then geomagnetic induction data should be explicable in terms of near‐field scattering theory. However, if the earth's inductive response comprises also a convective current mode similar to a linear circuit, then a more general formulation of the induction problem would be required. It is shown that the internal night‐time Sq field can be attributed more easily to transient induction effects than to the global linear circuits proposed by Ashour and Price [

ISSN:0048-6604    

DOI:10.1029/RS019i004p00949

年代:1984

数据来源: WILEY

摘要:

Ground wave mode coupling coefficients at a discontinuity in ground surface impedance (as at a shoreline) are derived by using the appropriate geometrical theory of diffraction edge—diffraction coefficient. The results agree with previous derivations based on other methods. The discovery that edge diffraction is the mechanism for ground wave mode coupling at a shoreline or other similar discontinuity allows the calculation of ground wave mode coupling by edge diffraction in cases where no easily calculated solution is now availabl

ISSN:0048-6604    

DOI:10.1029/RS019i004p00959

年代:1984

数据来源: WILEY

摘要:

The thermal emission of nonspherical particles is studied by applying the fluctuation dissipation theorem to calculate the Stokes' vector source terms in radiative transfer theory. Generally, all the four Stokes' parameters in the source term are nonzero and are proportional to the absorption coefficient in the backward direction. The measurements of active and passive remote sensing are related by showing that within the framework of radiative transfer theory, the emissivity of vertically and horizontally polarized waves can be expressed in terms of the bistatic scattering coefficients in the usual manner. In addition, the emissions of the third and fourth Stokes' parameters are also expressed in terms of bistatic scattering coefficients.

ISSN:0048-6604    

DOI:10.1029/RS019i004p00966

年代:1984

数据来源: WILEY

摘要:

A new ray approximation for calculating the diffraction attenuation due to several knife edges takes into account the wave‐normal direction of the diffracted wave in the near field, using the Fresnel approximation. The theory is intended for terrestrial radio wave propagation and for other situations where obstructions are nearly collinear. The far‐field form of the wave is adopted, as in asymptotic theories, but the focal line of the diffracted ray pencil coincides with the diffracting edge only in the limit as the field point moves deep into the geometric shadow. In general, the ray passes above the diffracting edges rather than through them. The theory is developed for knife edges only, but may be adaptable to other shapes of diffracting obstruction. In the examples given, the calculated attenuations are much more accurate than if the far‐field approximation is assumed, but less accurate than if the complete Fresnel integration is done. In a comparison with the complete Fresnel integration, the difference turns out to be less than l dB for five or fewer knife

ISSN:0048-6604    

DOI:10.1029/RS019i004p00975

年代:1984

数据来源: WILEY

摘要:

For a prolate spheroid of arbitrary size various components of the scattering mechanism responsible for the backscattered energy (which is of interest to monostatic radar systems) are analyzed. In the case of a spherical structure, due to the symmetry, the conditions for the existence of glory rays and internal surface waves are straightforward. It is shown that for the prolate spheroid with arbitrary direction of incidence, these conditions are more complex, and it is advantageous to resort to a numerical method of ray tracing. The solution yielded by this numerical method, along with some geometrical arguments, shows that for most of the cases of arbitrary scatterers, the single bounce glory ray will not exist and therefore will not contribute to the backscattered energy. For the axial case of incidence, conditions for the existence of internal surface waves have also been developed. Finally, some results for the approximate backscattered signal incorporating the above components, are presented.

ISSN:0048-6604    

DOI:10.1029/RS019i004p00987

年代:1984

数据来源: WILEY

摘要:

It is well known that the geometrical optics approximation, generally valid for high‐frequency fields, fails in the vicinity of a caustic. A systematic procedure of V. P. Maslov that remedies this situation will be reviewed in this paper. Maslov's method makes use of a representation of the geometrical optics field in the phase spaceM=X×K, where a pointm= (x,k) is a pair of a position vectorxεXand a wave vectorkεK. A Lagrangian submanifold ofM, Λ, that lies in the dispersion surface and is a union of the phase space trajectories selected by the initial conditions is constructed. It can be considered as a global representation of the phase. The phase space amplitudes (half densities) satisfy transport equations defined along those trajectories in Λ. Since trajectories inMnever form a caustic, a globally defined amplitude can be established on Λ. The field onXis related to the resultant field on Λ by the “canonical operator,” an operator introduced by Maslov. It generates an integral form of the solution near a caustic that can be evaluated analytically, numerically, or with uniform asymptotic techniques. Away from the caustic it recovers the geometrical optics field. Alternatively, the phase space field can be projected on a hybrid spaceYwhere some of the space coordinates have been replaced by the corresponding wave vector components. For any caustic point inX, one such hybrid spaceYwhere this projection does not encounter a caustic exists. A geometrical optics field results inYthat is related to the original inXby an asymptotic Fourier transform. The solution inXnear a caustic can be represented as the Fourier transform toXof that hybrid space geometrical optics solution. These techniques are illustrated with two simple but revealing problems: continuation of the field through a fold caustic in a linear layer medium and through a caustic with a cusp point in a homoge

ISSN:0048-6604    

DOI:10.1029/RS019i004p01001

年代:1984

数据来源: WILEY

摘要:

The theory of the generalized Doppler effect, concerning the scattering of waves from arbitrary time‐varying objects, is still a vide open class of problems. Over the years, two major approaches have been used for the analysis of the Doppler effect: (1) the “Einstein recipe” of coordinate transformations and (2) the direct solution of the time‐dependent boundary‐value problem. In addition, numerous approximate methods have been devised. Some approaches, e.g., the so‐called quasi‐stationary method, are inherently inconsistent, as explained here. As long as uniform rectilinear motion is involved, we stand on relatively firm ground, and various methods and approximations can be compared and evaluated. For nonuniform motion in the presence of plane waves and plane interfaces (including the logical extension to geometrical optics), some results are available, but the problem already involves heuristic assumptions. Some special problems have been solved for two‐ and three‐dimensional configurations, under very restrictive conditions. The general problem of scattering by a time‐varying obstacle is still an open problem. A delineation of some methods for solving such problems numerically is

ISSN:0048-6604    

DOI:10.1029/RS019i004p01027

年代:1984

数据来源: WILEY

摘要:

Remote determination of the refractive index structure parameter from experimental data by Tatarski's integral equation requires numerical inversion of a compact linear operator. Such problems are known to be ill posed; the consequent ill conditioning of the inversions has led to a large degree of uncertainty in reported reconstructions. In this paper we use the singular value decomposition of a compact operator to define a measure of the maximum amount of recoverable information in such an inversion, terming it the essential dimension of the operator. We propose the use of filtered singular value decomposition as the numerical algorithm that will recover most of the information and minimize uncertainty. A detailed study of the operators appearing in determination of horizontal (wave front is spherically symmetric) and vertical (wave front is plane) profiles of the atmosphere is under‐taken to determine their essential dimensions by both analysis and computation. The results indicate that for typical parameter values, both operators have small essential dimensions, with vertical profiles being harder to reconstruct than horizontal profile

ISSN:0048-6604    

DOI:10.1029/RS019i004p01041

年代:1984

数据来源: WILEY

摘要:

The importance of radio wave scintillations introduced by tropospheric turbulence for both telecommunication and meteorological research is indicated. Amplitude, phase, and phase difference measurements of scintillation were performed at 30 GHz on an 8.2‐km line‐of‐sight path. In situ measurements of temperature and humidity fluctuations and wind velocity and direction were done about midway on the propagation path. The measuring systems are described, and theory is presented to derive the refractive‐index structure parameter C2nand wind velocity v from radio wave scintillations and the in situ measurements. Formulas are given to scale C2nand v with height. Finally, results of the experiments are presented and di

ISSN:0048-6604    

DOI:10.1029/RS019i004p01057

年代:1984

数据来源: WILEY

摘要:

ELF/VLF wave generation by modulation of the auroral electrojet has previously been performed near Tromsø, Norway, with an HF heating beam pointed in a fixed (zenith) direction. By varying the phases of the HF waves transmitted from adjacent rows of antennas in the transmitting antenna array, the beam direction can now be scanned in the north‐south plane at frequencies from zero to approximately 5 kHz. In one type of beam scanning the amplitude of the heating beam is ELF modulated while the beam direction is simultaneously swept slowly, in sawtooth fashion, about the zenith. Variations in the ELF field strength, measured on the ground as the source is swept in this manner, are shown to be associated with spatial variations of the auroral ionosphere in the north‐south direction. We compare these spatial variations with those observed by S

ISSN:0048-6604    

DOI:10.1029/RS019i004p01069

年代:1984

数据来源: WILEY