bluuebunny/crossref_metadata_2025 huggingface dataset:
Oscillation models for predicting raindrop axis and backscatter ratios
Authors: Beard K. V.
Published: April 2008

Raindrop oscillations have been modeled using a potential flow, ellipsoidal variation about an equilibrium distortion along with a potential energy function that includes surface and gravitational energy. Two degenerate modes were investigated, the axisymmetric (vertical) oscillation and the ellipsoidal (horizontal) oscillation, and found to have different time‐averaged axis ratios and oscillation frequencies. Averaged axis and backscatter ratios were calculated for oscillations in a steady state balance between collisional energy and viscous dissipation. The results indicate that the polarization properties of rain are significantly affected by the nonequilibrium distortion of large drops in heavy rainfall.
The effect of charge and electric field on the shape of raindrops
Authors: Zrnić D. S. , Doviak R. J. , Mahapatra P. R.
Published: April 2008

Closed form theoretical solutions have been obtained that relate the eccentricity of raindrops to charge and electric fields. The dependence of drop shape on electric field is highly nonlinear; it appears that commonly observed electric fields in clouds have little effect on drop shape whereas for somewhat stronger fields the growth in distortion is explosive. This is true irrespective of the charge on the drops. In the absence of an electric field the natural oblateness of drops is enhanced by the presence of charge. This increase in eccentricity might be detected as an enhancement of the differential radar reflectivity. With increasing vertical electric field, drops elongate vertically, starting from an oblate shape and passing through spherical and prolate shapes before breaking up. The role of charge in this process is to enhance preexisting oblateness or prolateness.
The ability of dual‐polarization radar (copolar linear) to predict rainfall rate and microwave attenuation
Authors: Goddard John W. F. , Cherry Stephen M.
Published: April 2008

Dual‐polarization radar measurements of rain (using horizontal and vertical polarizations with copolar transmit and receive) are compared with simultaneous measurements using a Joss‐Waldvogel distrometer and a rapid response rain gauge. The radar pulse volume was situated 200 m above the ground‐based instruments, at a range of 8 km. The parameters compared are absolute and differential radar reflectivity factors and rainfall rate. A correlation of 0.95 was found between the radar and distrometer measurements of absolute and differential reflectivity factor, with the radar estimate of absolute reflectivity exceeding the distrometer estimate by 1.6 dB on average, and the distrometer estimate of differential reflectivity exceeding the radar estimate (assuming Pruppacher and Pitter drop shapes) by 0.1 dB on average. The latter difference is small but significant and is possibly attributable to drop canting or oscillation. A new relationship between drop shape and drop size is proposed, reducing the mean difference in differential reflectivity to ±0.02 dB. Radar estimates of rainfall rate, assuming an exponential drop‐size distribution, tend to produce significant overestimates compared with the direct measurements of the two ground‐based devices. This is shown to be largely attributable to departure from a strictly exponential distribution of drop sizes, and a correction factor is derived to compensate for this.
Highlights of the Symposium on the Multiple‐Parameter Radar Measurements of Precipitation: Personal reflections
Authors: Atlas David
Published: April 2008

This paper summarizes some of the highlights of the symposium, synthesizes them in the context of prior observations and understanding, and attempts to resolve apparent conflicts. The differential reflectivity ZDR technique has shown excellent results in the quantitative measurement of rainfall. Systematic differences were found to be due to deviations from the originally assumed exponential drop size distribution. The consensus was that the circular depolarization ratio (CDR) method is more versatile and powerful but that ZDR is simpler and cheaper. Precautions in the use of ZDR have been noted. The apparent conflict between CDR and ZDR measurements near the melting layer seems to have been resolved. A spectrum of scientific and practical applications has been identified. Most importantly, the ZDR method is recommended for further development and use wherever accurate rainfall measurements are required.
Propagation of a geometrical optics field in an isotropic inhomogeneous medium
Authors: Bremmer H. , Lee S. W.
Published: April 2008

The cross section of an infinitesimally small ray tube (ray pencil) expands or contracts as rays propagate. The square root of the cross‐section ratio between two points on a ray is defined as the divergence factor DF. In a homogeneous medium, rays are straight lines. Consequently, (DF)2 is simply equal to the ratio of Gaussian curvatures of the wave fronts. In an inhomogeneous medium, rays are curved, the DF becomes more complex. In this paper we derive several integral expressions for calculating DF and the related formulas that govern the field and energy variations along a ray.
Transient electromagnetic response from a thin dyke in the earth
Authors: Howard A. Q. , Nabulsi Khalid
Published: April 2008

An electromagnetic solution for a thin tabular conductor in a lossy medium is presented. This is an extension of “Maxwell's theorem.” Both impulsive and time harmonic electric line sources are analyzed. The orientation of the source is parallel to the sheet conductor. This assumption renders the problem two‐dimensional. The solution and associated analytical results have application to geophysical probing. For example, the sheet can model a conducting ore dyke or a fluid‐filled crack in an induction logging application. Transient numerical results are seen to be diagnostic of the conductivity thickness product W = σ2d.
Statistical characterizations of equatorial scintillation in the Asian region
Authors: Fang D. J. , Liu C. H.
Published: April 2008

The morphological aspect of gigahertz equatorial scintillations in the Asian region has been reported. As a follow‐up, this paper studies the statistical aspect of equatorial scintillations in the same region. Power spectra of prominent scintillation events collected over a 16‐month period during the solar maximum years in sunspot cycle 21 were analyzed. The slope ν of the spectrum at the high‐frequency end was analyzed with respect to the strength of scintillation as indicated by scintillation index S4. The values of S4 were compared with theoretical calculations based on either phase screen formulation or Rytov approximation. These comparisons were used to assess ionospheric parameters such as the height and thickness of the irregularities layers, rms fluctuation of the total electron content, and the axial ratio of the irregularities. Autocorrelation calculation provided a characteristic coherent interval τ. The study suggests that equatorial ionospheric irregularities in the F region, with subkilometer sizes in the evening hours after local sunset, are the main cause of the gigahertz scintillations observed. The spectra of the irregularities appear to be the power law type, with spectral indices p that generally have values greater than 4. The characterizations of S4, ν, and τ for both the Pacific and Indian Ocean region links provide an important basis for improved modeling of ionospheric scintillation phenomena in the Asian region.
HF‐enhanced plasma lines in the lower ionosphere
Authors: Djuth F. T.
Published: April 2008

The ionospheric modification facility at Arecibo, Puerto Rico, has been used to study Langmuir waves excited in the lower ionosphere by a high‐power HF radio wave. Measurements of HF‐enhanced plasma lines have been made in the lower F region (< 160 km altitude) and in the E region using the 430‐MHz radar at Arecibo Observatory. These measurements complement observations made in the past at higher altitudes. Enhanced plasma line spectra observed in the lower F region peak at the so‐called “decay line” and contain additional spectral structure similar to that found in the upper F region. In the E region the spectra exhibit narrow peaks located at 430 MHz ±ƒHF, where ƒHF is the frequency of the modifying HF wave. While strong plasma line enhancements are commonly observed in sporadic E, only weak enhancements have been detected in the normal daytime E layer. Candidate mechanisms for the E region enhancements include the oscillating two‐stream instability and direct conversion of the HF wave into Langmuir waves.
Radiometric correction of atmospheric path length fluctuations in interferometric experiments
Authors: Resch George M. , Hogg David E. , Napier Peter J.
Published: April 2008

To support very long baseline interferometric experiments, the Jet Propulsion Laboratory has developed a system for estimating atmospheric water vapor path delay. The system consists of dual microwave radiometers, one operating at 20.7 GHz and the other at 31.4 GHz. The measured atmospheric brightness temperatures at these two frequencies yield the estimate of the precipitable water present in both vapor and droplets. To determine the accuracy of the system, a series of observations were undertaken comparing the outputs of two water vapor radiometers with the phase variation observed with two connected elements of the very large array (VLA). The results show that (1) water vapor fluctuations dominate the residual VLA phase and (2) the microwave radiometers can measure and correct these effects. The rms phase error after correction is typically 15° at a wavelength of 6 cm, corresponding to an uncertainty in the path delay of 0.25 cm. The residual uncertainty is consistent with the stability of the microwave radiometer but is still considerably larger than the stability of the VLA. The technique is less successful under conditions of heavy cloud.
Wave emissions from dc and modulated electron beams on STS 3
Authors: Shawhan Stanley D. , Murphy Gerald B. , Banks Peter M. , Williamson P. Roger , Raitt W. John
Published: April 2008

The first active beam‐plasma experiments utilizing the space shuttle were carried out in March 1982 as part of the NASA Office of Space Science mission on the third space shuttle flight. A fast pulse electron generator emitted a 1‐keV, 100‐mA electron beam in either a continuous (dc) mode or an on/off modulated mode in the ELF to HF frequency range. Stimulated electrostatic and electromagnetic waves and associated plasma effects were measured with the plasma diagnostics package as it was maneuvered through and near the beam by the remote manipulator system. For the dc beam the wave spectrum was electrostatic, was peaked in the 300‐ to 500‐Hz range with a spectral density of 4 × 10−3 V2 m−2 Hz−1, and was unpolarized. Above the nominal lower hybrid resonance frequency the intensity decreased but was polarized. Strong emissions near the electron gyrofrequency and the plasma frequency were occasionally detected. The background spectrum was similar to the dc case for VLF and ELF modulations, but the emissions at the modulation frequency were more electromagnetic and more intense with field strengths of up to 1 Vm−1.
Computer simulations of beam injection experiments for SEPAC/Spacelab 1 mission
Authors: Omura Yoshiharu , Matsumoto Hiroshi
Published: April 2008

Computer simulations are performed to investigate beam‐plasma physics in the Space Experiment With Particle Accelerators, which is planned for active and interactive experiments in the earth's upper atmosphere and magnetosphere using a high‐power electron gun on board the space shuttle. Using a two‐dimensional electromagnetic particle simulation code (EM2), we study the divergence and propagation of the electron beam through the ambient magnetoactive plasma as well as the plasma wave excitation by the electron beam. First, we apply the EM2 code to a one‐dimensional model where the x axis is taken perpendicular to the static magnetic field. We find a strong excitation of a slow extraordinary wave around the electron beam and a propagation of a pulselike ordinary wave packet excited as an impulse response to the onset of the beam injection. Second, we apply the EM2 code to a two‐dimensional model where the x‐y plane is taken transverse to the static magnetic field. The detailed behavior of the beam as well as the background cold plasma is studied. The electron beam is locally injected in the x‐y plane with a drift velocity Vz. The electron beam initially expands in the x‐y plane owing to an electrostatic field produced by the excess charge of the beam electrons, and it subsequently oscillates with the frequency of the slow extraordinary wave which is excited around the beam. Uniformity in the z; direction is assumed, producing a self‐consistent two‐dimensional simulation as a preliminary to a later three‐dimensional study where this condition can be relaxed.
A kinetic cyclotron maser instability associated with a hollow beam of electrons
Authors: Wu C. S. , Freund H. P.
Published: April 2008

A kinetic cyclotron maser instability associated with a hollow‐beam distribution function is studied. The instability differs from that discussed for the gyrotron device in two respects: in the present case the momentum dispersion is substantial, and furthermore there exists a low‐energy background plasma. On the basis of physical arguments we can demonstrate that the hollow‐beam distribution is far more unstable than the loss cone distribution which has been extensively investigated in recent years. A criterion for maximum growth rate is established on the basis of consideration of the resonance ellipse. The validity of this criterion is supported by the results of numerical calculation.
Ordinary mode auroral kilometric radiation, with harmonics, observed by ISIS 1
Authors: Benson Robert F.
Published: April 2008

ISIS 1 topside sounder receiver observations that reveal examples of O mode auroral kilometric radiation (AKR) are presented. They correspond to locations outside the low‐density source region of intense AKR X mode emission. The propagation modes are identified by comparing the natural radiation wave cutoffs with the local resonant and wave cutoff phenomena stimulated by the sounder transmitter. The O mode AKR is the dominant emission in these regions of relatively high electron density, but it is considerably weaker than the intense X mode AKR observed to emanate from low‐density cavities above the auroral regions. In addition to the fundamental O mode, second and third harmonic bands of radiation have also been detected. These harmonics associated with O mode AKR are less intense than the harmonics associated with X mode AKR. It is difficult to explain the variety of harmonic AKR observations (X as well as O mode) on the basis of present AKR theories.
Further studies of auroral roar
Authors: Kellogg Paul J. , Monson Steven J.
Published: April 2008

Extended observations of auroral roar, a weak radio emission near the second harmonic of the electron cyclotron frequency, are reported. A total of 69 events were recorded and analyzed. Data for frequency band, duration, and local time of occurrence are presented. The height of the generation region is estimated to be 275–575 km.
On the shuttle glow (the plasma alternative)
Authors: Papadopoulos K.
Published: April 2008

It is shown that the anomalous glow observed in the recent space shuttle flights can be attributed to a combination of beam plasma discharge and critical ionization phenomena. While the shuttle velocity (∼8 km s−1) is below the critical ionization velocity, which for oxygen is 12.7 km s−1, specular reflection of a small fraction (2–3%) of ambient ions forms an ion beam with velocity 16 km s−1, making up for the energy deficit in the critical ionization velocity phenomenon. The wave activity observed is consistent with this model.
Spectroscopic consequences of Papadopoulos' discharge model of spacecraft ram glows
Authors: Kofsky I. L.
Published: April 2008

The gaseous and surface impact‐excited ultraviolet, visible, and infrared emissions that would result from plasma interactions near the windward directed surfaces of satellites orbiting in the earth's F region are reviewed. No process in the electrical discharge that would produce the intense orange‐red component of the glows seen on space shuttle and Atmospheric Explorer vehicles is identified. Spectroscopic and spatial‐distribution measurements to determine the contribution to the excitation of the collisionlessly energized suprathermal electrons and ions are suggested.
Analysis of propagation characteristics of radio waves in tunnels using a surface impedance approximation
Authors: Yasumoto K. , Shigematsu H.
Published: April 2008

A simple approximate method to analyze the propagation of radio waves in tunnels is proposed, based on a surface impedance assumption. This method is used, as a basic application, to determine the propagation constants of normal modes in circular tunnels bounded by a uniform lossy dielectric or bounded by two lossy dielectric layers. From the comparison of numerical results it is shown that our approximate solutions agree with the exact ones obtained from the general dielectric waveguide theory, with enough precision for practical purpose.
A closed form physical theory of diffraction solution for electromagnetic scattering by strips and 90° dihedrals
Authors: Michaeli A.
Published: April 2008

The physical theory of diffraction is applied to the high‐frequency scattering of a plane electromagnetic wave by a perfectly conducting strip. Closed form expressions are obtained for the farscattered field in the TE and TM cases. These expressions are continuous at grazing incidence and uniform in the overlapping transition regions. They extend the closed form results of Tiberio and Kouyoumjian from strictly grazing to nearly grazing incidence. The comparison with numerical results based on the moment method shows that these expressions furnish an excellent high‐frequency approximation to the exact solution of the strip problem. The results for a strip are then used in a ray tracing analysis of the backscattering from a 90° dihedral.
Comparison of measured cross‐polarization isolation and discrimination for rain and ice on a 19‐GHz space‐earth path
Authors: Cox D. C. , Arnold H. W.
Published: April 2008

Rain and ice in a radio path can couple power from a wave transmitted with a given polarization into the orthogonal polarization. At any point along a path, the ratio of the cross‐polarized signal component to the copolarized signal component is defined as depolarization or as cross‐polarization discrimination. Cross‐polarization isolation is the ratio of the cross‐polarized signal component from one transmitted polarization to the copolarized signal component for the orthogonal transmitted polarization. The two signal components that determine cross‐polarization isolation are received on the same polarization. In contrast, the two signal components that determine cross‐polarization discrimination are received on orthogonal polarizations. Conventional single‐polarization propagation experiments can only measure cross‐polarization discrimination. However, cross‐polarization isolation is the quantity that limits the performance of dual‐polarized communication systems. Simultaneous measurements of copolarized and cross‐polarized signal components for two orthogonal linear polarizations were made at Crawford Hill, New Jersey, by using polarization‐switched 19‐GHz beacons on COMSTAR satellites. Data from this experiment are used to show that instantaneous values of cross‐polarization discrimination and cross‐polarization isolation are equal. This is the first known experimental demonstration of this equality for a space‐earth propagation path.
Modeling and scientific application of scintillation results
Authors: Fremouw E. J. , Secan J. A.
Published: April 2008

Efforts continue to update and improve empirical models of the mean morphological behavior of ionospherically produced scintillation. While often intended for engineering application, these models also can be scientifically useful to the extent that they describe not only the signal fluctuations but also the plasma density irregularities that cause them. Such a model is contained in the computer code called WBMOD, which employs the phase screen propagation theory to calculate the power law spectral index and strength (at a fluctuation frequency of 1 Hz) of phase scintillation and the standard deviation of phase, for a user‐specified system. Explicit provision is made for an ionospheric outer scale, but the relevant phase spectrum is cut off by a system parameter in most applications. The intensity scintillation index S4 is calculated from weak‐scintillation theory and corrected for the saturating effect of multiple scatter on the basis of Rice statistics. The irregularity model in WBMOD describes three‐dimensionally anisotropic (sheetlike) irregularities on the nightside of the high‐latitude scintillation zone and axially symmetric (rodlike) irregularities elsewhere. The current status of WBMOD, which is undergoing continued refinement, is described in this paper for all latitude regimes. Plans for future development of its irregularity model are stated, and it is suggested that such models should now move in the direction of incorporating irregularity dynamics.
Spatial variability of total electron content in the eastern Mediterranean region
Authors: Soicher Haim , Klobuchar John A. , Doherty Patricia H.
Published: April 2008

Faraday rotation observations were conducted at Haifa, Israel (32.87°N, 35.09°E), and Athens, Greece (37.97°N, 23.72°E), during the maximum phase of the current solar cycle using the VHF beacon of the SIRIO satellite. The subionospheric points (at 420 km) are (29.9°N, 27.9°E) and (34.5°N, 18.4°E), and the subionospheric L shell values are 1.24 and 1.37, respectively. Expected latitudinal and local time differences in total electron content (TEC) for the two locales are observed. However, the Haifa data are characterized by generally occurring, seasonally independent, large postsunset electron content maxima which are absent for the Athens data. Furthermore, the postsunset increases would appear to be a solar maximum phenomenon, as they are not observed during the minimum phase of the solar cycle. The postsunset increases are attributed to electron fluxes arriving from the equatorial regions along the magnetic lines of force. The correlation coefficients of hourly TEC at the Haifa/Athens locales exhibit a seasonally independent diurnal variation with minimum values at night and maximum values generally at the end of the buildup phase of TEC variation. The daytime ratios of the standard deviation of TEC to the average TEC are generally seasonally independent and behave quite similarly at the two locales, with daytime values below ∼25%.
Comparison of NAVSTAR satellite L band ionospheric calibrations with Faraday rotation measurements
Authors: Royden H. N. , Miller R. B. , Buennagel L. A.
Published: April 2008

Using the Faraday rotation technique, the Deep Space Network of the Jet Propulsion Laboratory (JPL) monitors ionospheric total electron content (TEC) at its three complexes at Goldstone, California, Canberra, Australia, and Madrid, Spain, to correct data used in the orbit determination process for ionospheric effects. With the number of earth satellites suitable for Faraday rotation measurements steadily decreasing, JPL is investigating other ways to determine ionospheric TEC. One promising method has been developed by MacDoran and Spitzmesser. This method exploits the coherent P code modulation of the two L band signals (1575.42 and 1227.60 MHz) transmitted by satellites of the NAVSTAR Global Positioning System (GPS), but does not require knowledge of the code. It has been named SLIC, for satellite L band ionospheric calibration. With the full complement of 18 GPS satellites in operation in 1987, continuous ionospheric sampling in at least four directions will be feasible with SLIC. In this paper, TEC data measured at Goldstone using SLIC are compared with the results of mapping Goldstone Faraday TEC to the five GPS satellite lines of sight. The results of the two methods are in reasonable agreement.
Nonspectral representation for the field of a horizontal wire above ground
Authors: Carpentier M. P. , dos Santos A. F.
Published: April 2008

A representation in error functions for the current and field of a horizontal wire above ground excited by a delta function voltage generator is studied. This representation involves contributions from both proper and improper solutions of the modal equation and makes it possible to assess the relative importance of proper and improper solutions in the radiation mechanism. The new representation requires the analytic continuation of the field tranforms into the whole complex plane of the transverse wave number which is studied for the first time as far as the improper half plane is concerned. An asymptotic expression for the radiation field is obtained, and it is shown that the principal improper mode provides a good approximation to the far field.
Some features of ground rain patterns measured by radar in north Italy
Authors: Pawlina A.
Published: April 2008

The radar records of rain observed during 1980 by the weather radar facility of Spino d'Adda in north Italy were used to generate a large data base composed of about 16,000 ground rain cells characterized by a reflectivity factor greater than 34 dBZ. This large sample of rain elements, collected at the low altitude of 1.5 km from the ground, was analyzed in order to supply the statistical information required for rain attenuation modeling. The point rainfall rate distribution calculated against this data base, as well as the cumulative distributions of maximum, mean, and root‐mean‐square of rain rate referred to an individual rain area (cell), were found to be power law functions of the rain rate. The study of the horizontal extent of cells, based on the entire cell collection, leads to the conclusion that the probability distribution of the linear extent D of a rain cell with a given rate intensity R exceeded over the cell is an exponential function of D, while a power law relates the probability distribution of cells with a given size D to the rain rate exceeded in the cell. The empirically derived formula for calculating the absolute probability of a rain cell with a given intensity and size is proposed and found adequate at least for the observed region and period.
All‐weather millimeter wave imagery in the marginal ice zone
Authors: Hollinger James P. , Troy Ballard E. , Hartman Michael F.
Published: April 2008

In a recent series of flights over the marginal ice zone in the Greenland Sea the Naval Research Laboratory demonstrated that millimeter wavelength radiometry can be used for the remote determination of sea ice edge properties under all weather conditions. The imaging measurements, obtained at 90 GHz (3.3 mm) and 140 GHz (2.1 mm) from high altitude through heavy cloud conditions, can be used to delineate the sea ice edge, to locate polynyas and leads, to measure ice concentration, to determine ice type, age, and thickness, and to quantify the sea surface wind and atmospheric properties. Sea ice images were also obtained at 220 GHz (1.4 mm) but only under clear atmospheric conditions. The location, movement, and concentration of the sea ice edge greatly affects the dynamics of ice‐ocean circulation and air‐sea interaction. This information is fundamental for accurate long‐term weather prediction and for ship operations. These are the first measurements of sea ice ever obtained at 140 and 220 GHz.
Multiple‐frequency Doppler sounding of the ionosphere: Theory and experimental comparison with incoherent scatter results
Authors: Bertel L. , Guyader P. , Lassudrie‐Duchesne P.
Published: April 2008

The Doppler frequency shift of HF radio waves vertically reflected from the ionosphere is related to the apparent velocity profile of the reflecting layer through an integral equation. An algorithm is proposed to solve this equation when simultaneous measurements of the Doppler shift at several frequencies are available. This permits one to recover the apparent velocity profile from Doppler data. In order to test the theory, a vertical Doppler sounder has been built which can explore sequentially several preselected probing frequencies in the HF band. This sounder is composed of currently available laboratory equipment controlled by a desk top computer. A comparison has been performed between the Doppler results and the St‐Santin incoherent scatter data. Though obtained from a limited amount of data, experimental evidence seems to support the theory. Conditions for the applicability of the method are discussed.
Numerical simulations of intensity scintillation using the power law phase screen model
Authors: Rino C. L. , Owen J.
Published: April 2008

Simulations of radio wave scintillation in power law media were performed to study the structure of the intensity and phase spectra when the Fresnel radius is much smaller than the outer scale or the low‐frequency limit of the power law regime in the in situ irregularity spectrum. The results verify the critical dependence of intensity scintillation on the power law index as predicted by both asymptotic and more refined theoretical computations. Because of more recent findings, the scintillation characteristics of propagation in media characterized by a two‐component power law were also investigated. The results reconcile the interpretation of scintillation data from the Wideband satellite which showed a shallowly sloped power law phase spectrum with more recent intensity scintillation data.
Complex cross correlators with three‐level quantization: Design tolerances
Authors: D'Addario L. R. , Thompson A. R. , Schwab F. R. , Granlund J.
Published: April 2008

Digital correlators have been analyzed in considerable detail for cases in which the components behave ideally. Here we consider the effects of the major deviations from ideal behavior which are encountered in practice. Attention is restricted to three‐level quantization, but the fairly general case of the complex cross correlator is studied. (The real autocorrelator, for example, is a special case of this.) The errors analyzed include quantization threshold errors, quantizer indecision regions, sampler timing skews, quadrature network errors, and numerical errors in algorithms for converting digital measurements to equivalent analog correlations.
Theory of the Doppler effect: Fact, fiction and approximation
Authors: Censor Dan
Published: April 2008

The theory of the generalized Doppler effect, concerning the scattering of waves from arbitrary time‐varying objects, is still a vide open class of problems. Over the years, two major approaches have been used for the analysis of the Doppler effect: (1) the “Einstein recipe” of coordinate transformations and (2) the direct solution of the time‐dependent boundary‐value problem. In addition, numerous approximate methods have been devised. Some approaches, e.g., the so‐called quasi‐stationary method, are inherently inconsistent, as explained here. As long as uniform rectilinear motion is involved, we stand on relatively firm ground, and various methods and approximations can be compared and evaluated. For nonuniform motion in the presence of plane waves and plane interfaces (including the logical extension to geometrical optics), some results are available, but the problem already involves heuristic assumptions. Some special problems have been solved for two‐ and three‐dimensional configurations, under very restrictive conditions. The general problem of scattering by a time‐varying obstacle is still an open problem. A delineation of some methods for solving such problems numerically is given and discussed.
On the solution of vector Wiener‐Hopf equations occurring in scattering problems
Authors: Daniele V.
Published: April 2008

Recently, the author proposed a unified method to factorize a class of matrices involved in diffraction problems leading to coupled Wiener‐Hopf equations. In this paper, as an example of application, a new canonical problem has been considered and solved by this method. This problem considers the diffraction of a plane wave incident upon a pair of parallel half planes where the field satisfies arbitrary impedance conditions with opposite impedances on the two faces of each half plane. In addition, the factorization procedure is extended to all matrices commuting with polynomial matrices.
Focusing in lossy media
Authors: Andersen J. Bach
Published: April 2008

The spot size of the focus in a very lossy medium is discussed with relevance to creation of local temperature increase in muscle tissue by means of electromagnetic waves. A slot in a cylinder is used as a source for heating a lossy cylinder and the internal fields are analyzed by a modal method and a fast Fourier transform (FFT) method.
Surface diagnosis of large reflector antennas using microwave holographic metrology: An iterative approach
Authors: Rahmat‐Samii Y.
Published: April 2008

Recent investigations have shown that it is quite possible to accurately characterize the surface profile of large reflectors using microwave holographic techniques. In these techniques the complex (amplitude and phase) far‐field pattern of the antenna is measured first. The surface profile is then constructed using the Fourier transform relationship which exists between the far field and a function related to the induced surface current. In this paper the concept of the Fourier transform relationship is first investigated to demonstrate that it is, in general, a summation of many Fourier transforms. However, for large reflectors with small beam widths, only the first term of the series has the major contribution. Furthermore, an iterative scheme is described to analytically/numerically continue the far‐field pattern outside the measurement window. This, then, results in an improved resolution of the surface map data and, in particular, reduces the amplitude artifacts outside the boundary of the reflector. A novel and efficient simulation model has been developed to properly evaluate the accuracy of the technique in recovering the simulated surface errors. Finally, results of a recent measurement are summarized.
Analysis of open optical and microwave guides of arbitrary transverse permittivity profiles
Authors: Davies J. B. , Rahman B. M. A.
Published: April 2008

A full vector H field finite element formulation has been developed for the analysis of a wide range of uniform guided wave problems. It deals with microwave or optical structures where the permittivity has arbitrary variation in the transverse cross section. The permittivity can have a tensor of arbitrary anisotropy, limited only to being loss‐free. Infinite elements have been introduced to represent well the fields in an open type of waveguide. A matrix algorithm is used which takes complete advantage of the matrices' sparsity. Computed results are illustrated with optical channel and rib guide, image guide, and waveguide containing a skew uniaxial dielectric.
New physical effects on periodically grooved open dielectric waveguides
Authors: Oliner A. A. , Peng S. T.
Published: April 2008

When a surface wave strikes a structural discontinuity on an open dielectric waveguide at oblique incidence, instead of normal incidence as most studies have assumed, the resulting boundary value problem becomes vector, and one finds a rich variety of interesting and sometimes unexpected new physical effects. Among the new physical effects described here for periodically grooved open dielectric waveguides, of importance for integrated optics and for millimeter waves, are the following: four stop bands instead of the usual two in the Bragg reflection region, beam steering and focusing, radiation at peculiar skew angles, and surprisingly large cross‐polarization effects, none of which are present for normal incidence.
Numerical analysis of open waveguide discontinuities
Authors: Ray S. , Mittra R.
Published: April 2008

The spectral‐iterative technique, a numerical method recently developed for scattering applications, is extended to open waveguide discontinuity problems. In this method, the fields in the region near the discontinuity are calculated numerically with the spectral‐iterative technique. Fields in the regions removed from the discontinuity are treated analytically using waveguide modes with unknown complex amplitudes. Field matching along the boundaries between these regions gives an iterative solution for the reflection and transmission coefficients on the guide. Use of the spectral iterative technique provides numerical efficiency due to the use of the Fast‐Fourier Transform. Results are presented for transverse electric waves on a truncated dielectric slab waveguide and also for step discontinuities. Several methods for improving the convergence of the technique are discussed.
The application of the conjugate gradient method to the solution of transient electromagnetic scattering from thin wires
Authors: Rao Sadasiva M. , Sarkar Tapan K. , Dianat Soheil A.
Published: April 2008

Previous approaches to the problem of computing scattering by conducting bodies have utilized the well‐known marching‐on‐in‐time solution procedures. However, these procedures are very dependent on discretization techniques and sometimes lead to instabilities as the time progresses. Moreover, the accuracy of the solution cannot be verified easily, and usually there is no error estimation. In this paper we describe the conjugate gradient method for solving transient problems. For this method, the time and space discretizations are independent of one another. The method has the advantage of a direct method as the solution is obtained in a finite number of steps and also of an iterative method since the roundoff and truncation errors are limited only to the last stage of iteration. The conjugate gradient method converges for any initial guess; however, a good initial guess may significantly reduce the computation time. Also, explicit error formulas are given for the rate of convergence of this method. Hence any problem may be solved to a prespecified degree of accuracy. The procedure is stable with respect to roundoff and truncation errors and simple to apply. As an example, we apply the method of conjugate gradient to the problem of scattering from a thin conducting wire illuminated by a Gaussian pulse. The results compare well with the marching‐on‐in‐time procedure.
Transient scattering from periodic deformed cylinders
Authors: Ikuno Hiroyoshi
Published: April 2008

The approximate backscattered impulse response waveforms of perfectly conducting periodic deformed cylinders for both parallel and perpendicular polarizations are analyzed by a Fourier synthesis technique, in which we use the band‐limited scattering data calculated by the mode‐matching method. The normalized approximate impulse response waveforms from the nonconvex body become more complicated than those from the convex body, and directly reflect the surface character of the scatterer. In fact, we observe three kinds of specular‐type reflections and the reflected creeping waves from concave‐to‐convex transitions on the shadowed part of the surface in addition to the conventional creeping wave for the perpendicular polarization. The high‐frequency spectral contributions in the numerical results can be interpreted by the physical optics method. They contain both contributions from the complex stationary points with real parts located near the (nonspecular) concave‐to‐convex inflection points and from the conventional stationary points on the illuminated part of the surface.
Propagation through an assembly of anisotropic electric and magnetic dipole scatterers
Authors: Bennett J. A.
Published: April 2008

The propagation of electromagnetic waves through a random assembly of electric and magnetic dipole scatterers is considered. The results may be taken as the leading term of the solution for more general multipole scatterers. Multiple forward scattering and the coherent‐wave approach of J. B. Keller are compared. To the first order, the theories differ only in that they predict different wave polarizations. Particular attention is given to the case in which the distribution of scatterer orientations is axisymmetric. It is found that this case can be treated as propagation through an assembly of uniformly aligned equivalent scatterers. The application of these results to propagation through rain is discussed briefly.
Some factors affecting the remote sensing of rain by polarization diversity radar in the 3‐ to 35‐GHz frequency range
Authors: Holt A. R.
Published: April 2008

Polarization diversity radars, both CDR and ZDR, are currently being used to investigate precipitation environments for meteorological and radio science purposes. Various frequencies are being used. This paper investigates the use of such radars for the remote sensing of rain, including the dependency of the various measurements on frequency, as well as on various model assumptions about the rain, such as drop shape, drop size distribution, water temperature, and canting angle distribution. The effect of rain occurring on the path between the radar and the rain target is also considered, and at S band propagation effects are clearly worse for CDR than for ZDR radars. It is clear that the use of frequencies above about 20 GHz for the purpose of rain measurement is very problematical.
Coherent electromagnetic wave propagation through randomly distributed and oriented pair‐correlated dielectric scatterers
Authors: Varadan V. K. , Ma Y. , Varadan V. V.
Published: April 2008

Coherent attenuation of electromagnetic waves by randomly distributed and oriented pair‐correlated dielectric scatterers is studied as a function of frequency and volume concentration of scatterers. Average frequency dependent dielectric properties are also studied. The results indicate that the attenuation and hence the effective properties differ considerably from those of aligned scatterers.
Scattering of electromagnetic waves from a half space of nonspherical particles
Authors: Tsang Leung
Published: April 2008

The scattering of a plane electromagnetic wave obliquely incident on a half space of nonspherical particles is studied. Coherent wave transmission and reflection are investigated with the quasicrystalline approximation. Generally, there can be two characteristic waves that are excited in the scattering medium. The propagation constants and the polarization properties of the two characteristic waves are calculated from the generalized Lorentz‐Lorenz law. The amplitudes are then obtained by solving the equations of the generalized Ewald‐Oseen extinction theorem. In the limit of low concentration of particles, the results are identical to those of Foldy's approximation. The incoherent scattered field is calculated with the distorted Born approximation and generally gives a nonzero depolarization return in the backscattering direction. Numerical results are illustrated as a function of incidence angle and properties of the scattering medium.
The validity of shadowing corrections in rough surface scattering
Authors: Brown Gary S.
Published: April 2008

A rigorous examination of the validity of the shadow corrected Kirchhoff approximation is initiated for the purpose of validating this result for use as an asymptotic check on complicated multiple scattering theories. The magnetic field integral equation is iteratively solved in the quasi‐optical limit to determine if it leads to incident and scatter shadowing functions. The incident shadowing function can be rigorously derived in the absence of multiple scattering and, although it represents something more than single scattering, its true purpose is the conversion of mathematical rays to real rays which obey Snell's law. The scattering shadowing function is fundamentally flawed because it forces the current on the surface to be a function of the point of observation for the scattered field. The possibility of focusing from a point of specular reflection on the surface to a shadowing point causes further difficulties with the scatter shadow function. In general, shadowing functions are shown to be either inadequate or insufficiently rigorous as to provide a valid asymptotic check on complex multiple scattering theories when multiple scattering exists on the surface.
Impedance of vertical electric and magnetic dipole above a dissipative ground
Authors: Alanen Esko , Lindell Ismo V.
Published: April 2008

Self‐impedance formulae for a small horizontal wire loop and for a short vertical dipole above the earth are derived by using the exact image principle. The resulting integrals appear extremely well behaved for low antenna elevations and give good results with the simplest numerical approximations.
Coherence limits in VLBI observations at 3‐millimeter wavelength
Authors: Rogers A. E. E. , Moffet A. T. , Backer D. C. , Moran J. M.
Published: April 2008

Very long baseline interferometry (VLBI) experiments at a frequency of 89 GHz (3.4‐mm wavelength) using hydrogen maser frequency standards show that under good atmospheric conditions coherence can be maintained for times up to 700 s, corresponding to an Allan standard deviation of approximately 10−14. The stability appears to be largely limited by the phase noise resulting from fluctuations in delay through the troposphere. Methods of estimating the interferometer fringe amplitudes and closure phases in the presence of large phase fluctuations are examined. The spectrum and the Allan standard deviation of the observed phase fluctuations are derived.
Findings From an Accelerated in vivo Corrosion Model of Magnesium
Authors: Yuen Chi Keung , Ip Wing Yuk
Published: April 2008

In this article, we are presenting some findings of magnesium corrosion in an in vivo model. While there is consensus that subcutaneous bubbles will occur, it was found in our corrosion-accelerated model that magnesium corrosion is more severe in areas under the gas bubbles. This finding may bring insights to novel methods in reducing the uneven corrosion.
Adsorption of random copolymers by a selective layer: Monte Carlo studies
Authors: Kłos J. S. , Sommer J.-U.
Published: April 2008

We use scaling arguments and computer simulations to investigate the adsorption of symmetric AB-random copolymers (RC) from a diluted solution onto a selective ABA layer. Depending on the ratio between the layer thickness and the size of excess blobs, d∕ξ, three regimes of RC adsorption are predicted. For large values of the layer thickness RC adsorption can be understood as adsorption on two selective interfaces where sequences of RC chains form bridges. When the layer thickness is of the order of ξ, excess blobs are trapped in the layer and localize the copolymer chain strongly. If the layer thickness is very small a weak adsorption scenario is predicted where large loops are formed outside the layer. Our simulations using the bond fluctuation model are in good agreement with the scaling predictions. We show that chain properties display non-monotonous behavior with respect to the layer thickness with optimal values for d≃ξ. In particular, we discuss simulation results for density profiles, statistics of bridges, loops and tails formed by the adsorbed chains, as well as for the adsorption order parameter and free energy.