bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of global and CCIR models for estimation of rain rate statistics
Authors: Crane Robert K.
Published: February 2008

The prediction of the statistics of attenuation by rain on a radio wave propagation path requires first the prediction of rain rate statistics for a point on the path and then the conditional prediction of the path‐averaged attenuation, given the rain rate statistics. In this paper, two models with worldwide applicability, the Global and CCIR models for effecting the first step of the prediction process, are evaluated by comparing model predictions with rain rate measurements. The use of single‐year, measured rain rate statistics as a model for the prediction of the statistics for subsequent years is also evaluated. Surface rain rate distributions for one or more years of observatins at 52 locations were employed for the model evaluations. On the basis of the available data the Global model performed best. The location‐to‐location variations in rain rate at specified probability levels and in probability at specified rain rate levels were found to be well described by lognormal distributions. No significant differences were found between year‐to‐year and location‐to‐location variability estimates relative to the two rain rate prediction models. The data show that the climate models are equivalent to inference from limited‐duration records and suggest that prediction variability can be reduced by subdivision of the rain climate zones into smaller regions.
An application of invariant imbedding to VLF radio wave propagation
Authors: Tolstoy A. , Rosenberg T. J. , Kellogg R. B.
Published: February 2008

The problem of VLF wave propagation in the earth‐ionosphere waveguide is formulated in terms of a two‐point boundary value problem which is subsequently replaced by an initial value problem. As a result we easily derive the ionospheric reflection coefficient of an arbitrarily polarized plane wave reflected off an anisotropic medium.
Practical application of an evaporation duct model
Authors: Paulus R. A.
Published: February 2008

The application of an evaporation duct model in operational and climatological assessments of propagation and the sensitivity of the model to meteorological measurements are examined. The unexpectedly frequent occurrence of evaporation duct heights greater than 40 m is related to thermally stable conditions in the atmospheric surface layer. The existence of stable conditions over the ocean is analyzed in terms of meteorological conditions and temperature measurement accuracies. Air‐sea temperature differences measured by transiting ships are found to be biased toward stable conditions. The evaporation duct model is shown to be sensitive to this bias and yields high duct heights that are not supported by propagation measurements. A modified duct height calculation is proposed and applied to a radiometeorological data set.
Spectral behavior of phase scintillation in the nighttime auroral region
Authors: Fremouw E. J. , Secan J. A. , Lansinger J. M.
Published: February 2008

Enhancement of phase and intensity scintillation, as a radio line of sight scans through grazing incidence on the local L shell in the nightside diffuse‐auroral ionosphere, has been well documented by means of data from the DNA Wideband satellite. In this paper we describe systematic behavior of the phase spectrum found in the enhancement region over Poker Flat, Alaska. Routine Wideband processing included spectral analysis of 20‐s ( ∼ 60‐km) segments of VHF and UHF phase records and log linear fits thereto. Tabulation of the resulting power law spectral indices, p, disclosed increased values in the scintillation enhancement region. It has been established that the increase in p is the signature of a physically real phenomenon and not merely an artifact of statistical nonstationarity arising from the narrowness of the scintillation strength enhancement. Moreover, it has been found that p is not increased in strength enhancements occurring close to the magnetic zenith. Indeed, in some cases, it is substantially decreased. A possible source of these unexpected spectral behaviors is size‐dependent anisotropy, an idealization of the irregularities responsible being small‐scale field‐aligned rods imbedded in large‐scale shell‐aligned sheets. We also present examples of multiple‐regime power law spectra, characterized by an increased spectral index at short structure wavelengths (less than a few hundred meters north‐south) and, more frequently, by an increased index at large wavelengths (more than a few kilometers). These spectral breaks occur both separately and together and both within and outside the scintillation enhancement region.
Terminal properties of a spherical RF electrode in an isotropic Vlasov plasma: Results of a computer simulation
Authors: Calder A. C. , Laframboise J. G.
Published: February 2008

We present results generated using a computer code which has been developed to model an isotropic inhomogeneous Vlasov plasma surrounding a dc‐biased spherical or infinite cylindrical electrode to which a radio frequency potential is applied. The initial Maxwellian velocity distribution of the plasma is approximated by a multiple water bag distribution. The instantaneous response of the plasma to a sinusoidal potential applied to the electrode is calculated in the electrostatic approximation. Transient and nonlinear effects can be modeled in this way. Ions are usually treated as fixed, but some runs include ion dynamics with an ion/electron mass ratio of 16. Data produced by the code include the instantaneous particle flux and electric field at the electrode surface, from which the RF admittance is calculated. Results are presented for spheres in the frequency range 0.1 ωpe to 3 ωpe, RF amplitudes of 1–9 kT/el, and antenna radii of 1 and 10 λD. Here ωpe is the electron plasma frequency and λD is the Debye shielding distance. The code reproduces the admittance behavior expected near the antenna‐plasma series resonance. Some evidence for RF modification of the sheath is seen well above the frequency of the series resonance.
Estimating meteorological wind vector components from monostatic Doppler radar measurements: A case study
Authors: Clark W. L. , Green J. L. , Warnock J. M.
Published: February 2008

During the last decade, great progress has been made in the use of sensitive VHF and UHF Doppler radars (often called MST, ST, or wind profiler radars) to study the dynamics of the clear air in the troposphere and middle atmosphere. Currently used two‐beam and three‐beam metods of transforming observations of radial velocity υr from these radars into wind vector components u, υ, and w were tested for consistency in a five‐beam, 4‐hour case study. The study was made at the Sunset radar field site located 14 km west of Boulder, Colorado, and 17 km east of the continental divide, in the foothills of the Rocky Mountains. A weak jet stream was overhead. Under these conditions, the commonly used two‐beam relation (which assumes horizontal uniform flow) for transforming υr to u and υ gave grossly biased results (systematic discrepancy larger than 5 m/s). The three‐beam relation, based on three‐dimensional uniform flow, greatly reduced the error for much of the observation period. However, localized large errors remained at some heights and times, implying the need for a technique based on a three‐dimensional linearly varying flow field. Examination of a 16‐day comparison of vertical velocity observations made at the Sunset (mountains) and the Platteville (plains, 75 km east of the continental divide), Colorado, radar sites indicated that the flow away from orographic influences is simpler but that the two‐beam transformation may still lead to biased results much of the time.
Geodesy by radio interferometry: Effects of atmospheric modeling errors on estimates of baseline length
Authors: Davis J. L. , Herring T. A. , Shapiro I. I. , Rogers A. E. E. , Elgered G.
Published: February 2008

Analysis of very long baseline interferometry data indicates that systematic errors in prior estimates of baseline length, of order 5 cm for ∼ 8000‐km baselines, were due primarily to mismodeling of the electrical path length of the troposphere and mesosphere (“atmospheric delay”). Here we discuss observational evidence for the existence of such errors in the previously used models for the atmospheric delay and develop a new “mapping” function for the elevation angle dependence of this delay. The delay predicted by this new mapping function differs from ray trace results by less than ∼ 5 mm, at all elevations down to 5° elevation, and introduces errors into the estimates of baseline length of ≲ 1 cm, for the multistation intercontinental experiment analyzed here.
Advanced sounding: 1. The FMCW alternative
Authors: Poole A. W. V.
Published: February 2008

HF radar employing linear frequency‐modulated continuous wave or “chirp” modulation provides an alternative to the more conventional pulsed amplitude modulation technique in advanced ionosphere sounding. An analysis is presented which shows that the phases of the discrete Fourier components of the receiver output can be used in the same fashion as the phases of pulsed, fixed frequency ionospheric echoes to evaluate the standard digital parameters, including group path by the method of stationary phase. This can be used to overcome the group path/Doppler velocity ambiguity of linear frequency modulation. A description is given of a sounding structure from which unambiguous group range, angle of arrival, O/X mode, and Doppler velocity can be evaluated. A simple error analysis is also given.
Solution of a class of imperfect wedge problems for skew incidence
Authors: Senior Thomas B. A.
Published: February 2008

A method is described for solving the problem of a plane electromagnetic wave at nonnormal (skew) incidence on a wedge with one face imperfectly conducting and the other metallic. The method is based on a modified statement of the impedance boundary condition used in conjunction with Maliuzhinets' technique, and is applicable to a wedge whose open angle is mπ/2 with m = 1, 2, 3 or 4. For each wedge angle the general form of the solution is developed, and detailed results are presented for a right‐angled
Measurement and statistical analysis of wideband MF atmospheric radio noise: 2. Impact of data on bandwidth and system performance
Authors: Giordano A. A. , Herman J. R. , DeAngelis X. A. , Marzotto K. F. , Hsu F. M.
Published: February 2008

Measurements of wideband medium frequency atmospheric noise have been made over the past several years in the southwestern United States. In part 1 of a two‐part paper the measurement and data transcription system and the statistical analysis software used to analyze the data are presented. Specific results given in part 1 include the temporal structure of the noise and the distribution and time variation of the measured average noise power. In part 2, representative first‐ and higher‐order statistics and the impact of the data on bandwidth and system performance are described. Specifically, bit error rate simulations of a coherent receiver operating in either modeled or measured atmospheric noise are presented.
Some techniques for the experimental investigation of the ionosphere
Authors: Gledhill J. A. , Rash J. P. S.
Published: February 2008

The availability of limited numbers of modern advanced digital ionosondes offers many opportunities to make progress in our understanding of the physics of the ionosphere and in ionospheric forecasting. Some applications are discussed, with particular reference to the role played by conventional ionosondes. These include travelling ionospheric disturbances and the mid‐latitude trough; ionospheric absorption, especially in the E and F regions; and the uses of oblique incidence ionograms from the aeronomic point of view. It is also pointed out that there is an enormous amount of satellite data lying unused, which could be examined by groups in smaller countries to yield a great deal of useful information about the ionosphere.
ULF/ELF electromagnetic fields generated along the seafloor interface by a straight current source of infinite length
Authors: Inan A. S. , Fraser‐Smith A. C. , Villard O. G.
Published: February 2008

Propagation of ULF/ELF electromagnetic fields along the seafloor interface (assumed to be a plane boundary separating two semi‐infinite conducting media) is considered. Earlier expressions for the electromagnetic fields generated by a straight current source of infinite length are applied to the sea/seabed interface. The field components are calculated numerically and are compared to the field components in seawater of infinite extent. At the seafloor boundary, the fields can propagate longer distances because of the lower seabed conductivities. The new horizontal component of the magnetic field generated as a result of the existence of the sea/seabed interface becomes larger than the vertical component of the magnetic field at large distances; it is also more sensitive to the conductivity of the seabed at low frequencies. The results indicate that there is an optimal frequency at which two of the field components have a maximum field intensity at a certain distance from the source. Some practical applications are discussed.
Low‐frequency sheath admittance of a sphere in a collisionless plasma
Authors: Godard R. , Laframboise J. G.
Published: February 2008

A systematic theoretical study of the low‐frequency sheath admittance of a spherical electrode immersed in a collisionless, nonflowing, Maxwellian plasma with no magnetic field is done for the following ranges of plasma parameters: nondimensional electrode dc bias potential −25 ≤ eϕp/kTe ≤ 25, ion to electron temperature ratio 0 ≤ Ti/Te ≤ 1, electrode radius to Debye length ratio 0 ≤ rp/λD ≤ 200. A surprising feature of the results is the existence of double and triple peaks in the sheath capacitance as a function of dc bias potential at certain values of rp/λD. The same capacitance results also show discontinuous behavior at space potential. Various plasma‐diagnostics applications are suggested.
Summary of Connecticut 76‐Hz vertical electric, transverse magnetic, and radial magnetic field‐strength comparisons
Authors: Bannister Peter R.
Published: February 2008

The results of 60 days of vertical electric‐field and 136 days of radial horizontal magnetic‐field measurements taken in Connecticut are summarized in this article. The main results are (1) the vertical electric field‐strength behavior usually is very similar to the transverse horizontal magnetic field‐strength behavior (in both amplitude and relative phase) during both normal and disturbed‐propagation conditions, (2) during normal‐propagation conditions, the radial magnetic field‐strength amplitude behavior usually is similar to the transverse magnetic field‐strength amplitude behavior, while the radial magnetic field‐strength relative‐phase variation usually is somewhat greater than that for the transverse magnetic field strength, and (3) during disturbed‐propagation conditions, the daily radial and transverse magnetic field‐strength plots, versus GMT, usually are dissimilar (in both amplitude and relative phase). In particular, the radial magnetic field‐strength variations usually are greater than the transverse magnetic field‐strength variations.
A phase modulation technique for a sevenfold statistical improvement in incoherent scatter data‐taking
Authors: Sulzer Michael P.
Published: February 2008

A new technique for improving the quality of data in incoherent scatter measurements is described. The technique is useful under high signal to noise ratio conditions; the number of independent samples in the estimates is increased a factor of 7 by the simultaneous transmission of multiple frequencies. This either increases the time resolution by a factor of 7, or decreases the statistical errors by a factor of the square root of 7. Three important factors concerning the implementation of the technique are discussed: (1) the generation of the multiple frequencies, (2) the required frequency spacing to assure independence of the estimates, and (3) a practical technique for combining the independent estimates. A phase coded pulse is used to generate the seven frequencies with nearly equal power. Specially designed baseband filters are used to avoid distortion in the temperature and velocity measurements. Data demonstrating the improvement are presented, and the application of the technique to other radar uses, such as tracking radars, is discussed.
Two‐frequency intensity cross‐spectrum
Authors: Codona Johanan L. , Creamer Dennis B. , Flatté Stanley M. , Frehlich R. G. , Henyey Frank S.
Published: February 2008

The intensity cross‐spectrum (spatial Fourier transform of the two‐frequency intensity correlation) for scintillations caused by a plane wave passing through a random phase screen is considered. Two series solutions (one valid for low and the other for high spatial frequencies) are obtained which are the generalizations of previous results for the monochromatic intensity spectrum. We show that the Gaussian‐field approximation (modeling the cross‐spectrum as the transform of the square of the second moment) breaks down when the outer scale is large compared with the diameter of the scattering disk.
Modeling very large array phase data by the Box‐Jenkins method
Authors: Basart John P. , Zheng Yi
Published: February 2008

The quality of radio astronomical images made with an antenna array depends upon atmospheric behavior. As baselines and frequencies increase, phase variations become increasingly erratic. The phase fluctuations are time dependent and we found them to be correlated in time order in each baseline. We can represent these correlations by stochastic models. Models obtained by the Box‐Jenkins method are referred to as autoregressive integrated moving average processes (ARIMA). ARIMA models of VLA phase provide good short‐term predictions that may be useful for improving present calibration techniques. ARIMA models of VLA phase are data dependent and can be used in a variety of situations. A technique that works in all cases can be programmed into a software package such that modeling can be accomplished with no operator interactions. Another important application of ARIMA models involves the use of Kalman filtering to reduce the atmospheric effects when self‐calibration does not work well. The performance of the Kalman filter critically depends upon the models of the processes. An ARIMA model of the phase fluctuation can be represented in a state space form as noise in the Kalman filter equations.
High‐frequency scattering by a resistive strip and extensions to conductive and impedance strips
Authors: Herman Martin I. , Volakis John L.
Published: February 2008

An asymptotic solution is presented for the diffraction by a resistive strip which is useful in the simulation of thin dielectric layers. Up to third‐order diffraction terms are derived which include the surface wave field effects in a uniform manner. Extensions to the case of conductive and impedance strips are also given in Appendix C. The derivation of the higher order terms is based on the extended spectral ray method. New first‐order diffraction coefficients for the impedance, resistive and conductive half planes are also presented. The last are uniform everywhere, including the surface wave field boundaries.
High‐frequency expression for the field in the caustic region of a cylindrical reflector using Maslov's method
Authors: Hongo Kohei , Ji Yu
Published: February 2008

High‐frequency expressions in the caustic region are derived for the wave reflected by a circular and a parabolic cylindrical reflector using Maslov's method when a plane wave is incident obliquely. Maslov's method is a systematic procedure for predicting the field in the caustic region combining the simplicity of ray and generality of the transform method. Numerical computations are made for the field pattern around the caustic or cusp region and the variation of peak positions of the reflected field with respect to the incident angle.
Relationship between long‐term phase advances in high‐latitude VLF wave propagation and solar energetic particle fluxes
Authors: Sauer Herbert H. , Spjeldvik Walther N. , Steele F. Kenneth
Published: February 2008

Anomalous advances of OMEGA VLF navigation system (10.2‐kHz) signals during three extended periods (totaling 60 days) of solar cosmic ray event activity during 1982 are examined and compared with energetic proton, alpha particle, and electron precipitation into the polar cap ionosphere. Time profiles of the OMEGA 10.2‐kHz Hawaii‐to‐Norway path phase advance are found to be almost congruent with the time profiles of the logarithms of the energetic proton fluxes precipitating into the polar cap as determined from instruments aboard the polar orbiting NOAA‐6 satellite and the GOES‐2 geostationary satellite. Although both energetic alpha particles and electrons contribute to the anomalous phase advance experienced by the 10.2‐kHz Hawaii‐to‐Norway OMEGA signal, a simple, empirical algorithm reasonably relates the observed phase advance to the observed differential fluxes of 6‐MeV solar cosmic ray protons.
Distributions of the cross‐polarization discrimination and the copolarized attenuation due to multipath in clear air
Authors: Jin Hui‐Qun , Sha Zong , Xie Yi‐Xi
Published: February 2008

For multipath propagation in spherically stratified clear air, the formulas are derived for the phase difference between the direct ray and the multipath ray reflected from an atmospheric layer, and for their arrival angles. With the earth's curvature set to zero, the above formulas will reduce to those currently in use for a plane‐stratified atmosphere. With the formulas here derived and the empirical cumulative distribution of refractive index gradients, theoretical expressions for the statistical distributions of the cross‐polarization discrimination (XPD) and copolarized attenuation (CPA) are obtained. Furthermore, the relationship between equiprobable values of XPD and CPA is found to be linear. All the above formulas are related to the frequency, the path length, meteorological parameters and cross‐polar antenna patterns. The theoretically predicted results for XPD and CPA agree well with experimental data for several paths.
A new modulation principle for incoherent scatter measurements
Authors: Lehtinen Markku S. , Häggström Ingemar
Published: February 2008

A new method for measuring the ionosphere plasma autocorrelation function with an incoherent scatter radar is described. This method can be used in similar situations where multipulse methods have been used previously. In situations where the design of the transmitted signal is limited by a maximum allowed modulation time and a maximum radar peak power and where the signal to noise ratio is small, this method provides significantly more accurate ACF estimates than can be obtained by frequency commutated multiple pulse measurements. A simple explanation of the method is given, as well as a precise definition based on ambiguity functions. Computational algorithms are discussed, and it is shown that there are several possibilities to modify present algorithms so that this method can be implemented.
Characteristics of mid‐latitude sporadic E observed with a wideband HF channel probe
Authors: Wagner L. S.
Published: February 2008

Wideband HF channel probe measurements were used to study the structure, time development, and reflection mechanism involved in mid‐latitude sporadic E. A system bandwidth of 125 kHz was used for evaluating the sporadic E reflection coefficient, while a bandwidth of 1 MHz was used to describe the structure and time variation of the reflecting layer. Evidence linking the maximum observed frequency for sporadic E, ƒtEs, to total reflection from discrete, overdense electron density patches is presented. This observation appears to be true independently of blanketing conditions and time of day or night. Evidence is also presented showing the presence of scatter type signals at levels of the order of 30 dB below the maximum level of the reflected signal. The scatter signals frequently influence the observed value of ƒtEs. Finally, evidence is presented purporting to show sporadic E returns from rapidly rising and descending layers. Some of the vertically moving layers appear to originate within the E layer and could be an indication of physical processes contributing to the time variability of the layer.
Incoherent diffuse scattering by irregular shaped particles of finite conductivity
Authors: Bahar Ezekiel , Fitzwater Mary Ann
Published: February 2008

The incoherent diffuse scattering intensities (modified Stokes parameters) are evaluated for infrared and optical excitations at oblique incidence upon a layer consisting of random distributions of finitely conducting irregular shaped particles. The full wave approach is used to determine the elements of the phase matrix and the extinction matrix appearing in the equation of transfer. The rough surface height of the particles is characterized by its surface height spectral density function.
Leakage from higher modes on microstrip line with application to antennas
Authors: Oliner Arthur A.
Published: February 2008

Some confusion in the literature is clarified regarding the properties of microstrip‐line higher modes in the neighborhood of cutoff. It is shown that those modes become leaky in that range; with the aid of the steepest‐descent plane, one finds that the continuous spectrum can be characterized in a highly convergent manner by essentially a single leaky mode. The leakage occurs in two forms: a surface wave and a space wave. For structures without a top cover, it is found that almost all of the leakage is in the form of a space wave, so that an efficient leaky‐wave antenna of particularly simple configuration may be designed that consists of just a length of uniform microstrip line fed in its first higher mode. An accurate leaky‐wave analysis is developed that explains quantitatively the performance features and the limitations of this class of antennas.
Numerical analysis of surface current distribution on an infinite plane grating
Authors: Uchida Kazunori , Noda Takeaki , Matsunaga Toshiaki
Published: February 2008

A spectral domain method combined with the sampling theorem is described for evaluation of induced surface current densities concerning an infinite plane metallic grating loaded with a dielectric layer. Two different polarizations, TM and TE excitations, are treated here. Edge condition is analytically considered, and final results show a good convergence.
Real and complex rays for scattering from a target with inflection points
Authors: Ikuno H. , Felsen L. B.
Published: February 2008

Transient scattering by smooth concave‐convex targets reveals certain features that cannot be interpreted as conventional specular reflection and creeping wave contributions. The explanation can be found with an extended ray theory that includes complex specularly reflected incident and creeping rays in addition to the conventional real rays, and furnishes quantitatively accurate values for the scattered field when the incident pulse has a weak low‐frequency spectrum. The theory has been tested previously on a periodically deformed cylinder. Now, the cylindrical test object is peanut shaped. The excellent agreement between the extended ray solution and a numerical reference solution serves as further confirmation of the validity and utility of the extended ray technique for quantitative treatment of scattering from smooth convex‐concave shapes.
On sources of complex rays at complex locations: Higher‐order beam waves
Authors: Hashimoto Masahiro
Published: February 2008

The sources of complex rays at complex locations are discussed. It is shown that non‐Gaussian beam waves can also be represented in terms of these sources. However, such a representation is not unique, because the same field which is valid in the Fresnel diffraction region is represented in term of different sources which are located on a complex plane. Several examples are given and an application to the problem of diffraction is also shown.
Complex space multipole theory for scattering and diffraction problems
Authors: Lindell Ismo V. , Nikoskinen Keijo I.
Published: February 2008

Classical multipole theory can be extended to multipoles located in complex space and applied in scattering and diffraction problems with the advantage that, if the point of the multipole is correctly chosen, the first term may give an order of magnitude better approximation to the source than when the multipole is in real space. The basic theory, given elsewhere, is presented here in a more straightforward manner and the improvement in radiation pattern is demonstrated for sources of constant polarization. Applications on scattering by spheroidal dielectric bodies and diffraction by a dielectric half‐space are discussed.
Three‐dimensional electromagnetic half‐plane diffraction at the interface of different media
Authors: Ciarkowski Adam
Published: February 2008

Diffraction of an electromagnetic plane wave by a perfectly conducting half‐plane placed at the interface of two dissimilar dielectric media is considered. The incident direction is assumed oblique to the edge of the screen. The problem is solved exactly by means of the Przeździecki method. Uniform asymptotic solution valid in the optically thinner medium is also given.
Full wave analysis of complex planar microwave structures
Authors: Pascher Wilfrid , Pregla Reinhold
Published: February 2008

For the efficient analysis of discontinuities in planar circuits a fast computer algorithm is needed based on an uncomplicated theory. In the past the method of lines was successfully applied to planar waveguiding structures and simple two‐dimensional cases. In this paper we adapt the method of lines to structures requiring a high number of discretization lines. The two‐dimensional discretization and transformation of the Helmholtz equation into the spectral domain are reformulated in an elegant way using the Kronecker product of two matrices. A fast algorithm for the solution of the characteristic equation is developed for periodic structures employing the inversion of block Toeplitz matrices. Any microstrip, finline, or slotline circuit or discontinuity which is composed of several rectangular patches of metallization can be treated in this way. The current distribution of a periodic microstrip step discontinuity is given.
Coupled beam analysis of curved planar Bragg gratings
Authors: Jacob J. , Lange J.
Published: February 2008

When a laser or a fiber is butt joined to a single mode film waveguide it launches a two‐dimensional nearly Gaussian beam in the fundamental film mode. A curved Bragg grating may deflect and even refocus such an incident Gaussian beam. A coupled beam analysis has shown that the distortions of the refocused field may be kept low if the distance between the beam waists and the curved grating is sufficiently long. The greater this distance, the more the curved grating behaves like a concentrated reflector, which has the same wavelength dependence of reflectivity as would result for a plane wave incident upon a corresponding plane grating with straight lines.
A rigorous analysis of a thin‐film dielectric waveguide with a sinusoidal surface corrugation
Authors: Yasumoto K. , Murayama M.
Published: February 2008

The problem of wave guidance in thin‐film dielectric waveguides is formulated using the modematching method in the sense of least squares. The result is applied to analysis of the first‐order Bragg interactions in a three‐layered dielectric waveguide having a sinusoidal surface corrugation. The precise numerical results of the dispersion relation near the first‐order stopband are presented for both of the lowest TE and TM modes and compared with those obtained by a conventional perturbation approximation.
Ionospheric heater beam scanning: A mobile source of ELF radiation
Authors: Barr R. , Rietveld M. T. , Stubbe P. , Kopka H.
Published: February 2008

The heating facility at Ramfjordmoen near Tromsø, Norway, is used to modulate the auroral electrojet at extremely low frequencies (ELF) and the radiation of low‐frequency waves by the system is now well documented. In most reports the heater has been operated with a stationary heating beam, modulated at ELF, directed toward the zenith. This paper describes the signals recorded at Lycksele, 500 km from the heating site, when the HF heating beam was modulated at an ELF frequency and simultaneously scanned up to 37° from the zenith in the north‐south direction. ELF phase measurements at Lycksele confirm that the moving heated patch of ionosphere acts as a mobile source of ELF radio waves in the Earth‐ionosphere‐waveguide and that this mobile source may be used to determine the propagation parameters of the waveguide. Using square wave modulation of the beam position, of a constant amplitude HF beam, two simultaneous sources of ELF radiation have been produced in the ionosphere and a directional ionospheric ELF radio source synthesized.
Measuring and modeling the backscattering cross section of a leaf
Authors: Senior T. B. A. , Sarabandi K. , Ulaby F. T.
Published: February 2008

Leaves are a significant feature of any vegetation canopy, and for remote sensing purposes it is important to develop an effective model for predicting the scattering from a leaf. From measurements of the X band backscattering cross section of a coleus leaf in varying stages of dryness, it is shown that a uniform resistive sheet constitutes such a model for a planar leaf. The scattering is determined by the (complex) resistivity which is, in turn, entirely specified by the gravimetric moisture content of the leaf. Using an available asymptotic expression for the scattering from a rectangular resistive plate which includes, as a special case, a metallic plate whose resistivity is zero, the computed backscattering cross sections for both principal polarizations are found to be in excellent agreement with data measured for rectangular sections of leaves with different moisture contents. If the resistivity is sufficiently large, the asymptotic expressions do not differ significantly from the physical optics ones, and for naturally shaped leaves as well as rectangular sections, the physical optics approximation in conjunction with the resistive sheet model faithfully reproduces the dominant features of the scattering patterns under all moisture conditions.
A priori application of results of electromagnetic theory to the analysis of electromagnetic interaction data
Authors: Baum Carl E.
Published: February 2008

Since the introduction of the singularity expansion method (SEM) for the representation of transient and broadband electromagnetic interaction with general objects, there has been considerable attention given to associated analysis of electromagnetic response experimental data to find the natural frequencies. Usually, this has considered only single waveforms of frequency spectra for a parameter such as the surface current density at a particular position on the object under some particular excitation such as a particular direction of incidence with a particular polarization. This paper explores several concepts for advancing the analysis of interaction data to obtain the various SEM and eigenmode expansion method parameters. Basically, the various properties of natural modes and eigenmodes are explored for application to the problem of the taking and analyzing of experimental data. Various techniques are explored, including the enforcement of the SEM pole factors in multiple data records, separation of the modes by object symmetry, separation of the modes into E and H modes by measurement of surface charge density and equivalent magnetic charge density, and separation of the natural modes and natural frequencies by association with eigenmodes and use of eigenmode orthogonality. Basically, these concepts involve application of a priori physics to the design of experiments and data analysis.
Reciprocity and the formulation of inverse profiling problems
Authors: Blok H. , Zeylmans M. C. S.
Published: February 2008

The reciprocity theorem for scalar wave functions in its global form is used to formulate inverse profiling problems in a systematic way in terms of domain integral equations in the time‐Laplace domain (s domain) as well as in the time domain. The resulting equations relate the field in an “actual” configuration, containing the medium to be probed, and the field in an auxiliary background configuration. Four typical transmitter‐receiver situations, important in inverse profiling experiments, are considered. In addition, the formulation of the Porter‐Bojarski integral equation for “exact“ inversion is discussed.
Small holes and leaky feeders
Authors: Delogne P.
Published: February 2008

The theory of scattering of electromagnetic waves by a small hole in a conducting screen is reviewed in order to identify the key elements of the problem. Then attention is turned to holes in the shield of a coaxial cable and to leaky feeders. The engineering model currently used is described. Integral equations for the electric field in the apertures are established. The cases of a single hole and of a periodic distribution of holes are examined, and suggestions are made for the numerical solution of the problem.
Green's function for the Fourier spectra of the fields from two‐dimensional sources or scatterers in uniform motion
Authors: de Zutter D.
Published: February 2008

The moving sources are represented in terms of a vector and a scalar potential, in a reference frame in which the sources are at rest. Subsequently, Green's functions pertaining to the Fourier spectra of the radiated fields are found. The spectra of the fields detected by a stationary observer are given as space integrals over the volume occupied by the sources, of the product of these Green's functions with the source current density. The method is also applied to the spectra of the fields scattered by a two‐dimensional moving body. In that case the scatterer is replaced by equivalent induced currents and/or charges which are the sources of the scattered field. For the scattering case, the theory is not developed in its full generality but the method is illustrated by the canonical case of a moving perfectly conducting circular cylinder immersed in an incident plane E wave.
Some essential formulas in dyadic analysis and their applications
Authors: Tai C. T.
Published: February 2008

Some commonly used formulas in dyadic analysis are reviewed and summarized in this article. In particular, we have derived the dyadic‐dyadic version of two vector‐dyadic Green's theorems. The application of these theorems to electromagnetics is illustrated by two examples.
An image method to compute the static magnetic field due to currents injected into a homogeneous, conducting, and magnetically polarizable half‐space
Authors: Sezginer A. , Habashy T. M. , Wait J. R.
Published: February 2008

An image method for computing the static magnetic field due to an arbitrary current injected into a conducting and polarizable half‐space is derived. The advantage of this approach is that the magnetic field is given in terms of a Biot‐Savart integral over only the impressed current sources and their finitely many images. Integration over the conducting currents, which are distributed in the entire conductive half‐space, is avoided. The magnetic field is evaluated in closed form when the conductors that feed current to the electrodes are laid out in a rectilinear path.
Simultaneous VHF and UHF radar observations of the mesosphere at Arecibo during a solar flare: A check on the gradient‐mixing hypothesis
Authors: Rastogi P. K. , Mathews J. D. , Ying W.‐P. , Röttger J.
Published: February 2008

Simultaneous UHF and VHF radar observations of the 60–90 km mesosphere at Arecibo were made during the occurrence of a January 1981 type 4 solar X ray flare. The observations involved use of the UHF incoherent scatter technique at 430 MHz and the VHF turbulent scatter technique at 46.8 MHz. These observations provided a unique opportunity to test the basic premises of the turbulent gradient mixing hypothesis. UHF measurements show that enhanced electron concentration gradients were established with the flare onset. Two turbulent layers were observed with the 46.8 MHz radar before, during and after the flare. Enhanced scattering from both layers peaked within 3–6 min of the beginning of the flare. Only a slight variability in Doppler width of VHF returns from both layers was observed over the pre‐ to post flare period indicating that the power dissipation associated with turbulence remained essentially constant. We find that almost all the enhancement of VHF signals can be attributed to the flare induced increase in electron concentration gradients. We also conclude that the observed delay of 3–6 min between flare onset and the peak of VHF returns is related to an eddy overturning time required for mixing‐in of gradients to the Bragg scale.
Traveling ionospheric disturbances (TIDs) at mid‐latitudes: Solar cycle phase dependence
Authors: Soicher Haim
Published: February 2008

Faraday observations of total electron content (TEC) at Haifa, Israel (32.87°N, 35.09°E), during periods near the maximum (1980) and minimum (1984) phases of the current solar cycle have yielded information about the structure and variability of the ionosphere during both epochs in general, and about traveling ionospheric disturbances (TIDs) in particular. The TEC is characterized by the large differences in the seasonally dependent absolute values, by the generally occurring spatially confined postsunset secondary maxima during solar maximum which are absent during solar minimum, and by the regularly appearing modulations of the structure due to TIDs. There is a definite seasonal and solar phase dependence in the characteristics of the TIDs. This is expressed in terms of their frequency of occurrence, likely phase of the diurnal variation during which the occurrences take place, their absolute magnitudes, and the time rate of change of these magnitudes.
Amplitude and phase scintillation at high latitudes over northern Europe
Authors: Kersley L. , Pryse S. E. , Wheadon N. S.
Published: February 2008

Amplitude and phase scintillation measurements have been made at a high‐latitude site over northern Europe during a 2‐year period. Transmissions from the polar‐orbiting multisatellite Navy Navigation Satellite System have been used to achieve both latitudinal and temporal coverage of the observations, with more than 12,000 satellite passes being monitored to yield in excess of 400,000 values of each of the S4 and σф indices for amplitude and phase scintillation respectively. Results are presented of scintillation occurrence for season, time of day and geomagnetic activity. A premidnight maximum and more scintillation activity in summer and autumn than in winter and early spring are apparent. The results demonstrate the importance of propagation geometry to high‐latitude scintillation measurements, and estimates of the geometrical factors in both S4 and σϕ, computed for a model using different irregularity anisotropies, are discussed.
Ionospheric constraints on VHF/UHF communications links during solar maximum and minimum periods
Authors: Basu Santimay , MacKenzie E. , Basu Sunanda
Published: February 2008

An extensive VHF/UHF scintillation data base covering the frequency range of VHF to a few gigahertz has been utilized to determine the magnitudes of phase and intensity scintillations and their temporal/spatial structures during the sunspot maximum and minimum periods. The equatorial portion of the study has been based on geostationary satellite observations at Huancayo, a station on the magnetic equator, and at Ascension Island, which is an equatorial anomaly station having an extremely disturbed irregularity environment. The high‐latitude part of the study is based on quasistationary satellite measurements at a polar cap location (Thule) and two auroral locations (Goose Bay and Tromsø). The Tromsø observations are augmented with the Defense Nuclear Agency HiLat satellite beacon measurements during the solar minimum period. The data indicate a strong solar cycle control of scintillation activity at all locations, resulting in a drastic reduction of the magnitudes and occurrence of scintillations during the current solar minimum period. This pattern is consistent with both a reduction of F region ionization density and a reduction of irregularity generation in the solar minimum period. At the magnetic equator the magnitude of scintillations at 1.5 GHz seldom exceeds 3 dB with the percentage occurrence > 2 dB varying from 70% during high sunspot conditions to 30% during low sunspot conditions. At the crest of the equatorial anomaly, on the other hand, during the solar maximum in 1979, fades of 20 dB at 1.5 GHz are observed 30% of the time. At a decreased level of solar activity in 1982, a similar occurrence level is obtained at 1.5 GHz for fade levels of only 5 dB. During the solar minimum period, 1.5‐GHz scintillations are virtually absent. Phase scintillation measurements made at Ascension Island indicate that the median value of rms phase deviation is about 5 rad for detrend intervals of 100 s. In the auroral region, during the solar maximum period under magnetically disturbed conditions, the median values of scintillation fades and rms phase deviation (82‐s detrend) at 250 MHz are observed to be 15 dB and 3 rad, respectively. At Thule, located deep within the polar cap, the median values of scintillation fades and rms phase deviation at 250 MHz attain values as large as 20 dB and 4 rad during the sunspot maximum period. Unlike Ascension Island the scintillation activity at high‐latitude stations exhibits a threshold effect and does not decrease until 1983. However, in 1986 with sunspot numbers in the vicinity of 10, fade levels as low as 5 dB at 250 MHz are recorded in the polar cap and auroral stations only 5% of the time. It is noted that at auroral locations the most prominent feature, namely the existence of magnetic L shell‐aligned irregularity sheets, is equally evident at both sunspot maximum and minimum.
Rain attenuation ratios on 30/20‐ and 14/12‐GHz satellite‐to‐Earth paths
Authors: Kozu Toshiaki , Awaka Jun , Fukuchi Hajime , Nakamura Kenji
Published: February 2008

Up‐ and down‐link attenuations were simultaneously measured at 30/20 and 14/12 GHz for more than 10 rainfall events from 1979 to 1981 with two Japanese geostationary satellites, Communication Satellite (CS) and Broadcasting Satellite for Experimental Purposes (BSE). Statistical analyses of the ratio of up‐link attenuation to down‐link attenuation, the “attenuation ratio,” show that attenuation ratios of both CS and BSE do not necessarily agree with theoretical values assuming the Marshall and Palmer raindrop size distribution and that attenuation ratios vary widely from one event to another. Raindrop size distributions measured by a disdrometer are employed for the analysis of measured attenuation ratios. It is found that disdrometer‐derived attenuation ratios agree well with measured ones for BSE but that the agreements are not so good for CS. A model calculation of bright band attenuation and a data analysis with rain type classification suggest that the above discrepancy between measured and disdrometer‐derived attenuation ratios is caused by bright band attenuation. It is concluded that disdrometer data are useful for the estimation of slant path attenuation ratio for the heavy rainfall rate range.