bluuebunny/crossref_metadata_2025 huggingface dataset:
A probabilistic theory of electromagnetic wave scattering from a slightly random surface 1. Horizontal polarization
Authors: Nakayama J. , Ogura H. , Sakata M.
Published: February 2008

The scattering of electromagnetic plane waves from a slightly random and perfectly conducting surface is analyzed by the probabilistic method developed in a previous paper. In view of the stochastic homogeneity of the infinite random surface the electromagnetic wave solution proves to be a homogeneous vector random field multiplied by an exponential phase factor. On the basis of the nonlinear theory of the Gaussian random field the electric field is represented in terms of orthogonal functionals associated with the Gaussian distributed surface with unknown coefficients of deterministic functions. Then the boundary condition on the random surface is transformed into a set of equations for such unknown functions, which is approximately solved for horizontal polarization to yield the stochastic solution involving multiple scattering. Unlike the solution by the perturbation method, the stochastic solution never diverges but has such a singularity similar to Wood's anomalies in the diffraction gratings that the partial wave scattered at the Rayleigh wavelength has a finite but large amplitude. The stochastic solution makes it possible to calculate many statistical properties of the scattering, such as the average and variance of the scattered electric field, the intensity of the incoherent scattering, the average power flow of the surface wave, and the scattering cross section per unit area. These quantities as well as the optical theorem are illustrated in the figures.
The accuracy of very long baseline interferometry observables
Authors: Nes Håvard
Published: February 2008

VLBI (very long baseline interferometry) is a powerful technique for measuring high‐precision geometric parameters. Step one in the processing of the received signals from one baseline is to estimate the delay and delay rate. The next step is to estimate the geometric parameters from the delay and delay rate estimates. Expressions for the accuracy of the delay and delay rate estimates are developed. A VLBI system consisting of one master receiving station and one remote receiving station is considered. The system is receiving and processing just one microwave frequency window.
Rays and modes in an acoustic channel with exponential velocity profile
Authors: Niver E. , Cho S. H. , Felsen L. B.
Published: February 2008

High‐frequency sound propagation in a guiding channel is usually analyzed either in terms of a normal mode expansion or in terms of ray acoustics. These formulations lead to difficulties when the number of required modes or rays is excessively large. Moreover, ray acoustics requires corrections near caustics of the ray system, and actually becomes inapplicable in a surface duct where the ray fields may experience an arbitrarily large number of reflections. To alleviate these difficulties, we apply a newly developed theory that utilizes a judiciously chosen mixture of ray fields and modal fields to represent the signal due to an oscillating line source at various depths, observed at different depth and range intervals. The medium is modeled by an exponentially increasing sound velocity between the free surface and a homogeneous semi‐infinite bottom. This environment, which may be encountered in the ocean, combines surface duct and waveguide phenomena. The nature of the hybrid formulation for surface ducted fields and fields guided by top‐bottom interaction is studied in detail, as is the possibility of choosing the ray‐mode mixture so as to avoid corrections near caustics of the ray‐acoustic pressure field.
A hybrid technique combining the mode‐matching method and the geometrical theory of diffraction for edge discontinuities
Authors: Itakura Tokuya , Nishimura Tadashi
Published: February 2008

The mode‐matching method based on the least squares approach and Rayleigh theorem has been put forward as the simple and effective method for the numerical solution of scattering problems. Although this method has a rigorous proof of convergence, it does not always give us a desirable solution for edge discontinuities due to the inevitable limitation of computer memory. The poor convergence prevents more precise computation at higher frequencies. The proposed hybrid technique combining the mode‐matching method and the geometrical theory of diffraction (GTD) is presented to accelerate the convergence of numerical solution for the scattering problem associated with edge discontinuities. To combine GTD with the mode‐matching method, the singular integral equation technique which provides a quasi‐static approximation is introduced in the procedure of formulation. As an example to illustrate the proposed technique, we applied it to the well‐known scattering problem of an infinite plane grating. The numerical results show that the power errors are improved and are much better than those of the mode‐matching method itself.
Scattering from a rough interface
Authors: DeSanto J. A.
Published: February 2008

This paper treats plane wave scattering from a rough interface separating two semi‐infinite fluids of different density occupying regions V1 and V2. The interface is initially considered to be deterministic. A coordinate‐space integral equation is derived for the surface value of the Green's function using Green's theorem in both regions and continuity conditions at the interface. For a source in V1, Green's theorem in V2 when evaluated at a field point in V1 is a nonlocal impedance‐type boundary condition. It is also called an extended or extinction boundary condition. Since there is only one free‐space Green's function, the results of Green's theorem in both regions combine to yield a single integral equation. An integral relation is then used to find the scattered or transmitted field values. In Fourier transform space the T‐matrix for the scattered field satisfies a Lippmann‐Schwinger (LS) integral equation familiar from quantum potential scattering theory. Here the ‘potential’ is noncentral and complex. Several examples and a discussion of the generalization to the electromagnetic problem are listed. When the surface is considered to be a homogeneously distributed Gaussian random variable, this LS equation, Feynman diagram methods, and cluster decomposition methods from statistical mechanics are used to derive an integral equation (Dyson equation) satisfied by the coherent part of T. This is a one‐dimensional equation, and an approximation to it, whose Born term is the result due to Ament, is solved numerically. The result is a multiple scatter theory for coherent specular return which, when compared with experimental measurements, yields good results even for large roughness.
Exact inverse scattering theory
Authors: Bojarski Norbert N.
Published: February 2008

The concepts of reference wave slowness (reciprocal of velocity) and an associated free reference space Green's function slowness spectrum are introduced. A modified Kirchhoff surface integral, containing only the imaginary part of this free reference space Green's function slowness spectrum, is formulated, yielding an integral equation for the unknown fields and sources in the interior of a closed surface on which the (remotely sensed) fields are known. A well‐posed, analytic closed form solution of this integral equation is obtained.
Optimization theory and time‐domain inverse scattering
Authors: Lesselier D.
Published: February 2008

The electromagnetic probing of an inhomogeneous slab is investigated in the time domain by means of optimization techniques. The iterative search of a minimum of a cost function allows us to determine a best approximated conductivity or permittivity profile of such a slab. The principles of this approach and some numerical results are briefly presented. This approach appears particularly suitable in the case of noisy data. However, it does not allow us to overcome the ill‐posed nature of the simultaneous probing of conductivity and permittivity profiles.
Effects of lossy and perfectly conducting ground on the time and frequency behavior of a log‐periodic antenna using the singularity expansion method
Authors: Rech K. D. , Becker K.‐D. , Langenberg K. J.
Published: February 2008

This paper presents an insight into the transient behavior of a vertical log‐periodic dipole antenna (LPDA) above a lossy ground and over a perfectly conducting half space. Mathematically, the problem is formulated in the Laplace domain by a singularity expansion representation of the solution of an electric field integral equation using a Sommerfeld integral kernel. The influence of the feeding network and of the mutual coupling of the elements as well as the influence of the element thickness on the natural resonances of the LPDA is studied. The modes of the antenna elements are presented and the current in frequency and time domain is computed for free space. Dependent on the ground parameters and the antenna height, we show the resonance migration in the complex frequency plane and determine the appropriate modes. Starting with the free space current distribution, we compute the transient far fields of the vertical LPDA over ground for various ground parameters and antenna heights.
Experimental and theoretical analysis of anisotropic trapezoidal antennas
Authors: Popović Branko D. , Djordjević Antonije R.
Published: February 2008

A novel broadband antenna is described, in the form of a comblike trapezoidal anisotropic structure above a ground plane. The base of the comb is mounted close to the ground plane, parallel to it, and is fed at one end by a coaxial line. Such an antenna has uniquely defined polarization of the radiated field and very good aerodynamic properties. An experimental model of the antenna was made using printed‐circuit technique. By trimming the upper edge of the comb, an antenna was obtained experimentally having VSWR less than 2.0 (with respect to 50 Ω) in a frequency range of about two octaves. It was found that the antenna radiation pattern varies with frequency, but that a direction exists in which the antenna radiates well at all frequencies in the range. A mathematical model was also developed, permitting rather accurate numerical analysis of the antenna.
Numerical analysis of thin‐film optical waveguide and directional coupler
Authors: Yasuura K. , Miyamoto T.
Published: February 2008

Thin‐film optical waveguides have been analyzed by several approximate and numerical methods. However, one cannot always expect these methods to give accurate results of analysis near the cutoff frequency. We have proposed a new numerical method, the ‘mode‐matching method,’ for calculating the properties of the thin‐film waveguide. Using this method, we can show results of precise analyses on the rib‐type waveguide and directional coupler. On the diffraction problem for a bounded object, the mode‐matching method is based on the Rayleigh principle for the modal expansion of a wave field: The infinite sum in modal expansion can be available beyond its convergence area. We have always a truncated modal expansion which precisely approximates the wave field in the whole domain under consideration. If we note the correspondence between the Fourier integral representation of the field and the infinite sum of modal expansion, the application of the principle can be extended to the problem for an unbounded object such as the thin‐film waveguide: The wave field can be approximated by the finite Fourier integral, the superposition of plane waves with a band‐limited spatial spectrum. The rib‐type waveguide is considered, which is constructed by the air, film, and substrate media. Each wave field is approximated by the finite Fourier integral, and its spatial spectrum is determined by the mode‐matching technique. Let us fit the approximate fields to the boundary conditions in the sense of least squares; then we have the Fredholm integral equations, with respect to the spatial spectra, to be solved numerically. The dispersion relations near the cutoff frequency and the field distributions of the waveguide are analyzed precisely. The characteristics of the directional coupler are also made clear when the distance between two parallel rib waveguides is altered.
The controlled nonlinear nonstationary evolution of localized wave pulses in dispersive media
Authors: Shvartsburg A. B.
Published: February 2008

The controlled rebuilding of localized intense electromagnetic pulses is described by means of exact solutions of the nonlinear Schrödinger equation. These solutions illustrate the essential dependence of dynamics of such pulses in a dispersive medium upon the details of the initial shape of the pulse; therefore the role of the periphery regions of the pulse may be very important. The threshold values of intensity, divided by the regions of subcritical and supercritical evolution regimes, are calculated for the different pulses. The gradual formation of the spikelike pulses and the electromagnetic shock waves in the supercritical regime and the limited time of such formation are analyzed.
Network loading of thin‐wire antennas and scatterers in the time domain
Authors: Landt J. A.
Published: February 2008

A numerical technique is presented that is capable of calculating the transient response of a thin‐wire antenna or scatterer loaded by a network. The electromagnetic formulation is an adaptation of a well‐validated and widely used code. This code is based on an electric field integral equation and employs point matching and nine‐point Lagrangian interpolation functions. The network is treated by a nodal analysis and can contain a finite number of lumped and possibly nonlinear elements. The formulation is cast in the time domain, and the solution is obtained by a time‐stepping procedure. The numerical approach is outlined, and the responses of several examples are presented.
Mode selecting characteristics of four‐layer dielectric slab waveguide
Authors: Kudo Masaaki , Mushiake Yasuto
Published: February 2008

Mode selecting characteristics of four‐layer dielectric slab waveguide have been studied, considering its practical application to the optical integrated circuits as a mode filter. The dimensions of the waveguides discussed here are restricted to the cases where only TE0 mode or TE0 and TM0 modes can exist and higher modes are in cutoff region. Cutoff characteristics and attenuation characteristics for TE0 and TM0 modes have been investigated for various waveguide parameters. It has been found from the results of this investigation that the four‐layer waveguide can be utilized as a TE0 mode filter where only TE0 mode can propagate without attenuation, provided that the normalized frequency of the waveguide is chosen in the range between two cutoff normalized frequencies for TE0 and TM0 modes. In the latter part of this paper, the four‐layer waveguide joined with a three‐layer waveguide has been treated, and transmittance and reflectance for transmitted and reflected waves at the junction and also the fields of the transmitted wave in the neighborhood of the junction have been studied.
A characterization method of a light source for multimode fiber transmission lines
Authors: Daido Y.
Published: February 2008

A figure of merit for each light source is proposed using the optical power level at the receiving end of a long fiber transmission line, since transmission loss of a multimode fiber depends strongly on launching conditions. A simple measuring method of the figure is also proposed without using a long fiber transmission line. The method enables us to know the optical power level at the receiving end of a fiber transmission line with arbitrary length for each light source.
The phase center of a monopole antenna
Authors: de Jong G.
Published: February 2008

In Doppler satellite positioning techniques the receiver is often equipped with a monopole antenna. The location of the phase center of this antenna is needed if high accuracy positioning is to be achieved. The phase center, however, changes with the direction of incidence of the received electromagnetic wave. In this paper the phase center is calculated for the idealized model of a monopole antenna with an infinitely large, perfectly conducting ground plane. The analysis is carried out by using the boundary integral equation method for determining the surface current density on the conducting parts of the configuration.
Electromagnetic wave scattering by spheroidal conductors for arbitrary polarization and angle of incidence: 1. Theory
Authors: Futterman John A. Hardt , Matzner Richard A.
Published: February 2008

We generalize the technique of using spin‐weighted spheroidal harmonics to calculate electromagnetic scattering cross sections of spheroidal conductors for axially incident radiation. This paper extends the method to waves of arbitrary polarization and angle of incidence, and serves as the foundation for numerical work to be reported in a second paper in this series.
Radiation from coupled parallel‐plate waveguides: Ray‐optical analysis and experimental results
Authors: Driessen Peter F. , Jull Edward V.
Published: February 2008

The radiation pattern of a parallel‐plate waveguide with adjacent guides of finite depth is calculated by ray‐optical methods. The pattern may be adjusted by changing the width and depth of the parasitic guides. Experiments with a long H plane sectoral horn at about 12 GHz show good agreement with the theory.
A numerical method for extending ray trace calculations of radio fields into strong focusing regions
Authors: Warren R. E. , DeWitt R. N. , Warber C. R.
Published: February 2008

This paper presents a practical method for computing radio fields in regions of strong focusing, using ray intercept data provided by a standard ray‐tracing program. The procedure extends the usefulness of the ray trace by allowing fields to be computed near caustics and cusps where ray density calculations fail. Using a plane wave decomposition of the field components, phase integrals are computed by curve‐fitting intercepts of rays traced through ionospheric or tropospheric media whose refractive indices vary arbitrarily with altitude. A numerical algorithm is described for performing the plane wave angular spectral integrations. This procedure avoids the complications associated with higher‐order asymptotic techniques, allowing a broad range of refractive‐index profiles to be analyzed by a single method. It is applied to two sample profiles, and the results agree very closely with higher‐order stationary‐phase estimates in caustic regions. Moreover, the computer code runs efficiently, despite the presence of highly oscillatory integrands. The method is capable of including the effects of weak collisions, the spherical earth, and azimuthally dependent transmitter configurations.
Lateral waves: a new formula and interference patterns
Authors: Wu T. T. , King R. W. P.
Published: February 2008

A new simple formula is derived as an approximation for the general exact integrals for the radial component of the electric field generated by a horizontal electric dipole in a half‐space of water or earth near its boundary with air. The field in the water or earth is investigated as a function of radial distance from the source for εr = 80, 20, and 4, over wide ranges of conductivities and frequencies. Special attention is paid to the ranges in which the direct wave from the dipole produces an interference pattern of standing waves when it interacts with the lateral wave. For selected values of the parameters the radial electric field computed from the new simple formula is compared with the field evaluated numerically from the exact integrals. The agreement is excellent when the ratio of wave numbers characteristic of the denser half‐space and air is large, quite good even when this ratio is as small as 2.
Failure of homogeneous multilayer approximation for transversely nonuniform plasma waveguides
Authors: Adachi S. , Ohnuki S.
Published: February 2008

A homogeneous multilayer approximation is applied to a two‐dimensional transversely nonuniform cold plasma waveguide. As a result, the homogeneous multilayer modeling for an inhomogeneous slab is shown to be invalid for such plasma waveguides. The propagation constant of the TM mode supported by a multilayered plasma slab is found to diverge at a series of the extended Tonks resonance frequencies corresponding to a series of layer boundaries, that is, plasma density discontinuities. It is indicated that the inhomogeneous multilayer modeling having no density discontinuities can eliminate the above difficulty.
Anomalous scattering from a slightly random surface
Authors: Nakayama J.
Published: February 2008

The present paper analyzes the anomalous scattering from a slightly random surface, a phenomenon similar to Wood's anomalies on optical gratings; the anomaly appears as the rapid variations of the coherently reflected and the incoherently scattered power against the angle of incidence. To clarify the physical mechanism of such anomalous scattering, this paper deals with the two‐dimensional case, such that a TM plane wave is incident on the perfectly conductive Gaussian random surface. By a probabilistic method a stochastic wave solution involving multiple scattering is obtained and is shown to have a pole and a zero on the complex wave number plane. The complex pole and zero represent the existence of the guided complex wave (leaky wave) propagating along the random surface. It is concluded that the anomalous scattering is physically caused by the existence of the guided complex wave and can occur strongly in the case of a slightly rough and gently sloping surface. Several properties of the scattering are illustrated in the figures, such as the amplitude and the phase of the coherent reflection, the variance of the wave field, the optical theorem, the angular distribution of the incoherent scattering, and the backscattering cross section.
Application of wave field reconstruction of VHF radio waves to investigate single, isolated ionospheric irregularities
Authors: Tauriainen A.
Published: February 2008

A wave field reconstruction method has been developed and applied to VHF complex scintillation data obtained from the Wideband Satellite. The reconstructed wave fields at the altitude of the ionosphere are displayed in terms of amplitude and phase. They consist of different kinds of structures. In examples of auroral zone data, single, isolated diffraction centers can be clearly identified. But rapid fluctuating structures are encountered also, which are attributed to extended irregularities. In addition, the usefulness and limitations of the method will be discussed.
Radio wave propagation in the atmospheric boundary layer
Authors: Andrianov Vladimir A.
Published: February 2008

A periodic‐nonstationary radiometeorological model of the refractivity of the atmospheric boundary layer based on the concept of turbulent diffusion of physical properties under clear‐sky conditions is developed. The model is applied to predicting the refraction of radio waves in the boundary layer.
A systematic theory of wave propagation over irregular terrain
Authors: Furutsu K.
Published: February 2008

An explicit expression of the attenuation for a radio wave is obtained for a class of terrains consisting, along the wave path, of several sections of different heights and different electrical properties, taking into account the earth's curvature for every section. Here the boundaries of two sections are assumed to be discontinuous, in both height and electromagnetic property, and each boundary may have a thin ridge of arbitrary height. This mathematical model well represents terrains containing ridges, bluffs, coastlines with cliffs, etc., with great varieties of combination along the wave path, and furthermore enables us to systematically obtain an explicit expression of the radio wave intensity over them. The latter expression is obtained in terms of the multiple residue series, in accordance with the number of the terrain sections involved, and, when the class of terrains is reduced to a homogeneous spherical earth, either by letting all the heights and electrical properties of the section be equal or, with one particular section fixed, by letting all the other section lengths equal zero, then the series are uniformly reduced to the well‐known Bremmer series for a homogeneous spherical earth. When the lengths of the internal sections are sufficiently large, the effects of the terminal sections (over which the transmitter and receiver are located) become isolated, and this enables us to evaluate those terminal effects, or ‘terminal gain,’ in detail, independently of those of the other internal sections, to use the terminal gain in just the same way as the ordinary antenna height gain function has been used for the ground wave propagation over a homogeneous spherical earth.
Effect of lateral profile on diffraction by natural obstacles
Authors: Assis Mauro S.
Published: February 2008

A simple approximate mathematical model using Fresnel‐Kirchhoff diffraction theory is proposed to investigate the frequency dependence of propagation attenuation by natural obstacles with lateral profiles having general shapes. Measurements carried out in several links in the frequency band from 270 to 330 MHz have shown variations of ± 10 dB. Results obtained from this model agree reasonably with experimental data. The design reliability of UHF communication systems is therefore improved by its use.
Numerical modeling of geophysical structures excited by an electromagnetic wave
Authors: Cauterman M. , Nguyen Q. T. , Degauque P.
Published: February 2008

The numerical solution for the electromagnetic response of inhomogeneities of any shape in the ground has been developed for a plane wave or electric or magnetic dipole excitation. Two approaches are usually used depending on the structure of the ground in which the inhomogeneity is embedded. For a horizontally stratified ground, the solution is obtained in the form of an integral equation system, while for a heterogeneous medium, the finite element or the finite difference method is used. A brief review of these techniques is made, and examples of application are given. Furthermore, the limitations of each of these models are outlined.
Coherent oscillations in biological systems: Interaction with extremely low frequency fields
Authors: Kaiser F.
Published: February 2008

Frohlich's concept of coherently excited long‐wavelength electric vibrations in the 1011 to 1012‐Hz region associated with metastable states with a very high dipole moment in biological membranes has been the basis for explaining the extraordinary high sensitivity of certain biological systems to extremely weak electromagnetic signals. In this model, long‐range collective interactions within the membranes may lead to oscillatory biochemical reactions (e.g., enzyme‐substrate interactions in the greater membrane of the brain.) The resulting slow chemical oscillation is connected to a corresponding electric vibration by means of the large dipole moments of reaction‐activated enzymes. Thus a macroscopic oscillating polarization is built up, causing large regions to oscillate coherently in the 10 to 100‐Hz region (e.g., EEG activity). The remaining (unscreened) polarization causes the system to exhibit a ferroelectric instability. The nonlinear kinetic equations describing the system are discussed. Extremely low frequency fields interact with the limit cycle oscillation, which is caused to collapse for certain frequencies and intensities of the stimulation. This leads to the onset of propagating pulses even for an extremely weak stimulation. There is increasing experimental evidence for the rather speculative predictions. Furthermore, correspondence between this model and nerve impulse‐generating models has been established.
Irradiation of prolate spheroidal models of humans and animals in the near field of a small loop antenna
Authors: Lakhtakia A. , Iskander M. F. , Durney C. H. , Massoudi H.
Published: February 2008

Analysis of the near‐field irradiation of prolate spheroidal models of humans and animals by a small coaxial loop antenna is described. The near fields of the antenna are known exactly and hence are used to identify the suitable field parameters involved in the near‐field absorption in the spheroidal model. An integral equation is formulated in terms of the transverse dyadic Green's function, and the fields radiated by the current loop are expanded in terms of the vector spherical harmonics. The extended boundary condition method is then employed to solve the integral equation. The power distribution and the average specific absorption rate (SAR) are calculated and plotted, for different human and animal models, as a function of the separation distance from the loop. It is shown that for distances less than 5λ the average SAR values oscillate about the far‐field value. In particular, for d/λ < 0.4 an increase in the average SAR values was generally observed. It is also shown that in spite of the complicated nature of the near fields the absorption characteristics can still be explained in terms of the incident radiation. Furthermore, from the calculated SAR distributions at different frequencies it is shown that at all frequencies, excessive heating occurs at the surface of the spheroid while a limited absorption occurs in the central region around the major axis. This result is of particular importance in hyperthermia, where extensive efforts are being directed toward achieving deep‐tissue heating by a coaxial coil carrying RF power at about 27 MHz.
Millimeter wave irradiation and measurement system for cell suspensions
Authors: Tamburello Carmelo C. , Zanforlin Luigi
Published: February 2008

An interferometric measurement system capable of accurate return loss measurements over the frequency range 65–85 GHz is described. The system exploits the sedimentation rate of a highly aqueous cell suspension. Both fixed frequency and swept frequency systems are described, and the sources of error measurement are discussed. Experimental observations are compared with values predicted from an analytical model.
Biological and behavioral effects of prenatal and postnatal exposure to 2450‐MHz electromagnetic radiation in the squirrel monkey
Authors: Kaplan J. , Polson P. , Rebert C. , Lunan K. , Gage M.
Published: February 2008

Near the beginning of the second trimester of pregnancy, 33 squirrel monkeys were exposed to 2450‐MHz irradiation in a multimode cavity at whole‐body average specific absorption rates equivalent to those resulting from exposure to plane wave irradiation at 0.034, 0.34, and 3.4 W/kg; exposed monkeys were compared with eight pregnant sham‐exposed monkeys. Eighteen of the irradiated mothers and their offspring were exposed for an additional 6 months after parturition, and then their offspring were exposed for another 6 months. No differences were found between irradiated and control adults with respect to the number of live births produced or to measures of locomotor activity, maternal care, urinary catecholamines, plasma cortisol, 3H‐thymidine and 14C‐uridine uptake by phytohemagglutininstimulated blood lymphocytes, or electroencephalographic (EEG) activity. Similarly, no differences were found between exposed and nonexposed offspring on the same blood, urine, and EEG parameters. Growth rate and most aspects of behavioral development were not altered by exposure. The major difference between irradiated and control offspring was the high mortality rate (4/5) before 6 months of age in those exposed at 3.4 W/kg both before and after birth. These results indicate that microwaves at power densities to 3.4 W/kg might have little direct effect on the monkey fetus when exposures occur in utero during the latter half to two‐thirds of pregnancy, but that continued exposure after birth might be harmful.
Scattering from randomly rough surfaces and the far field approximation
Authors: Brown Gary S.
Published: February 2008

This paper derives rigorous results pertaining to the validity of the far‐field approximation for scattering from randomly rough, perfectly conducting surfaces having arbitrary statistics. The methodology employs the stochastic Fourier transform of the current induced on the infinite surface by either a bounded or unbounded incident plane wave. The results are general in that no approximate simplifying forms for the current are employed. Exact expressions are obtained for the mean and variance of the scattered field for unbounded illumination, and they are compared to the far‐field approximations to illustrate how the latter simplifications fail in this limit. Some of the pitfalls of the far‐field approximation in the case of beam illumination are discussed. When the incident plane wave is bounded, the conventional far‐field form for the mean scattered field can be rigorously derived for arbitrary surfaces provided the cross‐sectional area of the incident beam is large in comparison to the square of the electromagnetic wavelength. The conventional far‐field result for the variance of the scattered field is shown to require the additional stipulation that the cross‐sectional area of the incident beam contain many decorrelation intervals of the surface roughness. The results obtained herein are important because they hold for arbitrary surface statistics. Whereas they appear to duplicate previous results, it must be remembered that the earlier results were only valid for a special class of surface statistics, i.e., surfaces for which single scattering theory holds.
On reducing the inherent noise in the double‐pulse Doppler technique
Authors: Whitehead J. D.
Published: February 2008

The double‐pulse technique for measuring the Doppler shift of radar auroral echoes contains inherent noise arising from overlapping echoes. By modifying the method of reducing the data, it is possible to reduce this noise so that either greater precision is achieved or less time is required.
Real axis integration of Sommerfeld integrals: Source and observation points in air
Authors: Johnson W. A. , Dudley D. G.
Published: February 2008

Change of variables and subtraction of asymptotic terms are used to aid in the integration of Sommerfeld integrands along the real axis. The change of variables smooths oscillations which would otherwise become densely packed at the associated branch point. Subtraction of asymptotic terms is used to speed up the truncation process of an otherwise infinite integration interval. Interest is restricted to practical earth parameters and frequencies above 2 MHz. Comparison with a complex contour technique shows the present method to be particularly well suited for the analysis of a vertical, thin‐wire antenna in the presence of a lossy earth.
Dispersion characteristics for decaying or amplifying waves: 2. Analysis of a beam‐plasma system
Authors: Vidmar R. J. , Crawford F. W. , Harker K. J.
Published: February 2008

Dispersion characteristics deduced from a wave packet by observing its amplitude and phase at a distant point are ‘observational’ dispersion characteristics. In this paper, we demonstrate for the example of a cold beam‐plasma system that different observational dispersion characteristics are obtained for excitation by transient and periodic sources. The derivation of these characteristics is presented. Observational dispersion characteristics are of particular value in simplifying the interpretation of dispersion diagrams for absorptive or amplifying media which suggest group velocities that are complex or otherwise nonphysical. A discussion of numerical algorithms extends this analysis to dispersion relations that are difficult to treat analytically.
Solar cycle and seasonal variations of the ionosphere observed with the Chatanika incoherent scatter radar
Authors: Baron M. J. , Heinselman C. J. , Petričeks J.
Published: February 2008

An incoherent‐scatter radar facility was operated at Chatanika, Alaska (geographic and invariant latitudes both ≈ 65°N) for almost 11 years, essentially a full solar cycle. During this period, experiments lasting 24 hours and directed at nearly overhead measurements were made approximately once per month. The seasonal and solar cycle dependences of the ionosphere over Chatanika as deduced from 108 experiments spanning the 11 years are presented. The basic parameters measured were electron concentration, electron and ion temperatures, and line‐of‐sight ion velocity, all as functions of altitude and time. This paper focuses on the ionization in the E and F regions. Empirical relationships were found that enable (1) the daytime maximum E region electron concentrations to be expressed in terms of the 10.7‐cm solar flux and the solar zenith angle, and (2) the daytime maximum F region electron concentration to be expressed in terms of the 10.7‐cm solar flux alone. The dependence of nighttime precipitation‐produced ionization in both E and F regions on solar flux and magnetic activity is also described. A clear distinction between summer and winter F region ionization conditions is evident, as is the rapid switchover near the equinox.
VLF reflection properties of the normal and disturbed polar ionosphere in northern Greenland
Authors: Kossey P. A. , Turtle J. P. , Pagliarulo R. P. , Klemetti W. I. , Rasmussen J. E.
Published: February 2008

This paper describes long‐wave reflection properties of the high‐latitude ionosphere, observed under both quiet and highly disturbed conditions. The data were obtained using a short‐pulse VLF ionosounder located within the polar cap, in northern Greenland. Under quiet ionospheric conditions, substantial diurnal, day‐to‐day, seasonal, and solar cycle variations were observed. During polar cap absorption events the VLF reflection properties of the disturbed ionosphere depended greatly on the solar illumination conditions, as well as the magnitude of the incoming energetic particle flux. The use of the steep‐incidence VLF reflection data to develop and validate models of the lower ionosphere is discussed; and electron density profiles are given, derived from reflection data obtained during quiet and disturbed conditions.
ULF/ELF waves and turbulence in the high‐latitude dayside magnetosphere
Authors: Holtet J. A. , Egeland A. , Doehl J. , Maynard N. C. , Winningham J. D.
Published: February 2008

The magnetospheric polar cusp has been recognized as a region of strong plasma wave activity. The paper briefly summarizes previous observations of waves in the high‐altitude cusp and the dayside magnetosheath. The main part of the work is concerned with low‐frequency waves found in and near the cusp at low altitudes (500–2000 km) and on the ground. Examining the ac electric field measurements made by polar‐orbiting satellites (mainly the DE‐2), the following groups of waves appear to be characteristic for the dayside high‐latitude magnetosphere: (1) ULF/ELF broadband noise extends from the low hertz range to approximately 1 kHz. The region of peak intensity is generally found to be consistent with the latitudinal extension of the cusp. (2) VLF hiss is found to be present in the cusp region, but generally extends outside the established projection of the cusp. (3) Narrow‐band ELF emissions, covering frequencies 200 Hz ≲f ≲ 1 kHz, are observed on the equatorward side of the cusp, in the closed field line regime. Ground‐based low‐frequency wave measurements at Λ = 75.4° show in the daytime the presence of strong micropulsation activity in the frequency range ∼ 1–100 mHz and polar ELF emissions, a narrow band of relatively unstructured noise at frequencies 300 Hz ≲ f ≲ 1 kHz. The latter is believed to correspond to the satellite‐observed narrow‐band ELF noise. The frequency range 1–7 kHz is, in general, characterized in both the satellite and ground recordings by very low emission activity in the time period ∼3 hours on either side of magnetic noon. This classification of the waves is independent of the level of magnetic disturbance. It is discussed how the characteristic appearance of the ULF/ELF noise and the narrow‐band ELF emissions can be used to localize the position of the polar cusp.
Energy flow and dynamical coupling in the solar wind‐magnetosphere‐ionosphere system
Authors: Akasofu S.‐I.
Published: February 2008

An attempt is made first to describe the solar wind‐magnetosphere‐ionosphere system in the simplest way in terms of a directly driven system powered by the solar wind‐magnetosphere dynamo. The dynamo feeds the Pedersen currents to the ionosphere by generating the field‐aligned currents, resulting in the convective motion of the ionosphere and the joule heat production. The field‐aligned currents associated with both the Pedersen and Hall currents introduce a considerable complexity to this simple system by generating a potential drop along the geomagnetic field lines. It is also pointed out that the geometry of the open region, namely the polar cap, changes drastically as the BZ component of the interplanetary magnetic field becomes positive. Therefore, one must be cautious in organizing and analyzing data from the highest‐latitude region.
Long‐line measurements at high latitudes
Authors: MacDougall J. W.
Published: February 2008

The long‐line technique for measuring ionospheric convection was developed for use at high midlatitudes. In this study it was employed near the auroral zone (62°∧). Measurements are very comparable with those from other techniques. The convection patterns were found to be dominated by fluctuation with periods of several minutes to several hours and spatial extent greater than 500 km but less than 1500 km. The convection was quite different on quiet and on disturbed days.
A note on the singular part of the dyadic Green's function in strong fluctuation theory
Authors: Stogryn A.
Published: February 2008

It is shown that the dyadic coefficient S of the delta function part of the effective Green's function arising in strong fluctuation theory may be expressed as a surface integral over a limiting equicorrelation surface when the correlation tensor occurring in this theory may be factored into a constant tensor times a scalar correlation function. A relation between the components of S, which may be used to simplify computations, is also obtained. Examples are given to illustrate the reduction of previously complex problems to elementary integrations when use is made of these formulas.
Mutual coherence function for line‐of‐sight microwave propagation through atmospheric turbulence
Authors: Clifford Steven F. , Lataitis Richard J.
Published: February 2008

Existing expressions for the line‐of‐sight plane and spherical wave mutual coherence function are subject to the restriction that the wavelength λ be much smaller than the inner scale of turbulence lo. It is shown that this restriction can be replaced by λ « ρo, where ρo is the wave coherence length. For weak, path‐integrated turbulence, lo ∼ 5 mm « ρo, indicating that existing expressions for the MCF are valid for wavelengths well into the microwave range.
A comparison of MOF factors and frequency of optimum transmission bands observed using oblique‐incidence sounders with sounder‐updated PROPHET predictions for a one‐month test in the continental United States during the geomagnetically active summer of 1982
Authors: Harnish Lawrence O. , Hagn George H.
Published: February 2008

This paper compares several methods for predicting the frequency of optimum transmission (FOT) during periods of time when the geomagnetic field and the ionosphere are disturbed. The predictions will be made with a relatively simple ionospheric model (PROPHET) updated in near‐real time with information from an oblique‐incidence sounder operating on a different (control) path. Current and previous methods used to predict the FOT are examined along with a new technique which employs additional sounder information from the control path. The use of multipath reduction factors and CCIR‐recommended FOT factors, derived for use with monthly predictions during undisturbed periods, are also examined for the near‐real‐time application with the simple model in the presence of a disturbed ionosphere. Predictions made using each method are compared to measured FOT data scaled from oblique‐incidence ionograms gathered during a 29‐day test in the continental United States during the geomagnetically active summer of 1982. Finally, a comparison is made with data taken during a 1‐week test in the continental United States during the fall of 1984 when the ionosphere and geomagnetic field were not disturbed (Ap ≤ 24).
A quasi three‐dimensional propagation model for subionospherically propagating VLF radio waves
Authors: Tolstoy A. , Rosenberg T. J.
Published: February 2008

A scheme to extend the two‐dimensional VLF propagation model of Pappert to quasi three‐dimensional is presented. The three‐dimensional capability is expected to have application to the prediction of signal effects caused by localized and off‐great‐circle path regions of ionospheric perturbation.
Ionospheric heating via ordinary to extraordinary mode wave coupling
Authors: Seliga Thomas A.
Published: February 2008

The role of mode coupling between ordinary, O, and extraordinary, X, mode waves in the vicinity of the radio wave plasma frequency is examined. For propagation in a stratified inhomogeneous ionosphere the roots of the Booker quartic indicate that coupling is possible over a narrow height range. The amount of coupling depends primarily upon both the launch angle of the radio wave and the vertical variability or gradient of the electron density profile. The basic theory underlying this phenomena is reviewed, and roots of the quartic and full wave solutions are presented to demonstrate the theory. Of particular importance is the fact that most of the energy transferred to the X mode will be deposited within the ionosphere through deviative absorption and plasma wave generation. Implications of this coupling phenomenon for ionospheric high‐power radio wave modification and its relationship to several experimental observations are noted.
50‐MHz auroral Doppler spectra dynamics during the Harang discontinuity
Authors: Sofko G. , Koehler J. , Prikryl P. , McDiarmid D. R.
Published: February 2008

In August 1983, 50‐MHz CW bistatic radars were operated in Canada so as to measure the Doppler spectra from a common volume of aurora near Southend, Saskatchewan. The radar sites were chosen so that complete spectra could be measured simultaneously in roughly the magnetic north‐south and east‐west directions. This paper describes the spectra obtained in a presubstorm period on August 8 (0100–0310 UT) when the magnetometer and radar data showed that the radar viewing area passed from being equatorward of the Harang discontinuity to being directly within the Harang discontinuity. Some unusual spectral features occurred, such as a complete absence of echoes from directions nearly parallel to the eastward electrojet, the simultaneous presence of ion‐acoustic and ion‐cyclotron‐type spectra, and strong zero‐shifted Doppler spikes.
Application of refractive scintillation theory to laser transmission through the atmosphere near ground level
Authors: Booker Henry G. , Vats Hari O.
Published: February 2008

On the basis of the work of Booker et al. (1985) and of Booker and MajidiAhi (1981), laser scintillation near ground level is discussed for divergent terminals at equal heights above a curved earth in an atmosphere possessing rms fluctuations of temperature from 0.03°C to 3°C. Except for short ranges and small temperature fluctuations, refractive scattering by large‐scale irregularities dominates diffractive scattering by small‐scale irregularities. Sample calculations are made of the intensity scintillation spectrum that occurs in the receiving plane. Simple formulae are given for the key scintillation parameters, namely, the outer scale, the Fresnel scale, the mean square fluctuation of phase, the intensity correlation scale, the twinkling scale, the rms fluctuation in the angles of arrival and departure, and the correlation bandwidth. The numerical consequences of these formulae are illustrated.