ISSN:0048-6604    

DOI:10.1029/RS021i004p00539

年代:1986

数据来源: WILEY

摘要:

Using the complete expansions of the fields and imposing the exact boundary conditions at the interfaces of an irregular stratified medium, Maxwell's equations are transformed into a rigorous set of coupled first‐order differential equations for the wave amplitudes. This full wave approach is applied to a large class of propagation problems. Since these solutions account for specular point as well as diffuse scattering in a unified self‐consistent manner, it is not necessary to apply a hybrid physical optics‐perturbation approach to problems of rough surface scattering. The full wave solutions satisfy realizability, duality and reciprocity relationships in electromagnetic theory. They are invariant to coordinate transforma

ISSN:0048-6604    

DOI:10.1029/RS021i004p00543

年代:1986

数据来源: WILEY

摘要:

In previous works, two mode conversion models, based upon slab approximations, were presented for treating lateral inhomogeneity along the path of propagation in the earth‐ionosphere waveguide. One model (termed Full MC in this paper) requires full‐wave determination of a biorthogonal set of height gains as well as numerical evaluation of their associated integrals. In the other model (termed Fast MC in this paper) height gain functions are discarded above some heighthin the guide and are approximated below height h by Airy functions. No adjoint waveguide is introduced, and the associated integrals are performed analytically. Results of the two models are compared in this paper for air to air transmissions in laterally inhomogeneous environments. Results for both the vertical and transverse electric field are given for source electric dipoles of arbitrary orientation. It is found that the Fast MC model gives excellent agreement with the Full MC resu

ISSN:0048-6604    

DOI:10.1029/RS021i004p00551

年代:1986

数据来源: WILEY

摘要:

Expressions for the electric and magnetic vector potentials forz‐directed magnetic and electric current point sources are developed as spectral integrals over longitudinal and azimuthal wave numbers. The representations are valid outside the source region. The construction is systemized, with the aid of Mason's signal‐flow graphs, in order to facilitate numerical implementation. The azimuthal spectral integral form is chosen rather than the azimuthally periodic form so that computations may be carried out in the complex plane associated with angular wave number without recourse to the Watson transform. Some example calculations are provided to demonstrate the practicability of the met

ISSN:0048-6604    

DOI:10.1029/RS021i004p00559

年代:1986

数据来源: WILEY

摘要:

In this work the limitation of using a frozen model to describe a turbulent medium is discussed. Among other assumptions, the frozen model requires the statistically independent velocity fluctuations to have infinitely large scales in comparison with scales of the refractive index fluctuations. When the scale of velocity fluctuations becomes comparable with or smaller than the scale of refractive index fluctuations, the frozen model is no longer valid and has to be modified. Upon three hypotheses, a non‐frozen model is proposed in this paper. These hypotheses are generalizations of the ones used in the past. They include: (1) Both the refractive index and the velocity field of the turbulence are isotropic, stationary, homogeneous, and jointly stationary and homogeneous with each other. (2) The medium is incompressible. (3) The refractive index is conservative. Based on this non‐frozen model several formulas for the space‐time autocorrelation function and the wave‐number frequency spectrum of the refractive index are derived. Electromagnetic wave scattering from a turbulent medium characterized by this non‐frozen model is studied. The width of frequency spectrum and the mean frequency shift have been obtained. The results show that the power spectrum of the scattered field can no longer be regarded as equal to the velocity spectrum of the turbulence in general. The modification is important especially when the scale of velocity fluctuations is small in comparison with the electromagnetic wavelength. In Appendix the effects of finite radar pulse volume and radar beam broadening on the width of frequency spectrum of radar echos from a non‐frozen turbulence drifting perpendicularly to the axis of the radar beam are i

ISSN:0048-6604    

DOI:10.1029/RS021i004p00571

年代:1986

数据来源: WILEY

摘要:

The phase screen‐diffraction layer method is a powerful tool to study the signal scintillation of a wave propagating in a turbulent, stratified medium. Under the forward scattering approximation, the complex amplitude is shown to satisfy a parabolic equation which describes effects arising from phase changes due to irregularities and diffraction due to phase mixing. Below the turning point, these two effects can be computed sequentially. Stepping in altitude, phase changes are imbeded into each phase screen; diffraction between phase screens is accomplished using FFT techniques. This method is equivalent to the split‐step algorithm known in ocean acoustics but generalized to the case of oblique incidence. Near the turning point, the diffraction effects are assumed negligible due to the small vertical thickness of the considered region. The deterministic part of the wave fields is taken to be proportional to the Airy functions. This allows a more accurate evaluation of the phase change near the turning point than the WKB solutions. The coupling between the ascending and descending wave is discussed. The simulation model is described and, as an example, results for a linearly stratified turbulent ionosphere are given, including statistics of the reflected wave such as power spectrum, scintillation index and spatial correlat

ISSN:0048-6604    

DOI:10.1029/RS021i004p00583

年代:1986

数据来源: WILEY

摘要:

During the past two decades, complex image theory techniques have successfully been utilized to determine simple form expressions for the electric and magnetic fields produced by antennas located near the earth's surface, for both single‐layered and multilayered earths. It is the purpose of this paper to provide a tutorial overview of complex image theory. Particular attention will be paid to its range of validity. Specific applications will also be discusse

ISSN:0048-6604    

DOI:10.1029/RS021i004p00605

年代:1986

数据来源: WILEY

摘要:

We employ a simple model to show how the natural electromagnetic field on the surface of the earth, which has a strong horizontal magnetic field component, can be converted to a significant vertical magnetic field at the surface. Such a conversion mechanism will be caused by lateral variations of the subsurface conductivity structure. Our idealized model is a thin conducting sheet with a periodic variation of the conductivity‐thickness product in one horizontal direction onl

ISSN:0048-6604    

DOI:10.1029/RS021i004p00617

年代:1986

数据来源: WILEY

摘要:

We present an analysis for the electromagnetic response of a cylindrical conductor of infinite length which has a uniform coating. We show that subject to the thinness of the coating, an approximate spatially dispersive boundary condition can be developed which holds in a general context.

ISSN:0048-6604    

DOI:10.1029/RS021i004p00623

年代:1986

数据来源: WILEY

摘要:

The physical contents of some integral equations that have been frequently used in recent years in connection with solutions of inverse source problems and of inverse scattering problems are discussed. It is shown that the integral equations can only provide information about a filtered structure of a source or a weak scatterer. The filtering is found to remove all the Fourier components of the source distribution or of the scattering potential that represent details smaller than about a wavelength of the radiated or of the scattered field. The nature of the associated filtered field is also studied, and a comparison of the integral equation method with an older reconstruction method is made.

ISSN:0048-6604    

DOI:10.1029/RS021i004p00627

年代:1986

数据来源: WILEY