bluuebunny/crossref_metadata_2025 huggingface dataset:
F region tilts and ionogram analysis
Authors: Paul Adolf K.
Published: February 2008

Modern digital ionosondes with capability of angle of arrival measurements provide information about tilts in the ionosphere. In the F region, most of those tilts are coupled with the temporal variation of this layer. Well‐known examples for this behavior are the sunrise and sunset periods where the rapid temporal increase (decrease) of the electron density causes a strong horizontal gradient and therefore a significant deviation of the layer from horizontal stratification. Zenith angles close to 20° were recorded during the sunrise period in winter time. Less is known about the tilts produced by gravity waves, since periodicities of the order of a fraction of an hour are undersampled by the standard hourly recordings of ionograms. We found that such short period gravity waves are present very frequently and can produce local tilts comparable in magnitude with the sunrise/sunset tilts. Partial analysis of some observations during the winter 1980/1981 at Brighton, Colorado, will be presented and the consequences of F region tilts for the analysis of ionograms, especially the electron density calculations, will be discussed.
The 11.9‐GHz rain attenuation and site diversity effect on the earth‐space paths of low elevation angles
Authors: Yasukawa Koji , Yamada Matsuichi
Published: February 2008

In order to study rain attenuation characteristics and site diversity effects on earth‐space paths of elevation angles lower than 10°, sky noise measurements by radiometers at 11.9 GHz and 1‐min rain rate observation were conducted with several site diversity configurations in the western part of Japan. Measured statistics such as the cumulative distributions of 11.9‐GHz single‐site rain attenuation and related joint probabilities at diversity sites are presented. It is shown that extensively spread rain regions associated with typhoons or frontal lines have considerable effect on both rain attenuation and site diversity distributions. As a guide to prediction of site diversity effects for low elevation angle conditions, joint probability of rain attenuation as a function of site spacing from 17 to about 200 km is derived from the measurement results. Also shown is the site diversity effect dependence on elevation angle.
Measurement of anisotropic permittivity structure of upper troposphere with clear‐air radar
Authors: Waterman A. T. , Hu T. Z. , Czechowsky P. , Röttger J.
Published: February 2008

When radar echoes are obtained from the upper troposphere, tropopause and lower stratosphere, one characteristic reported by several groups is a variation in echo strength with pointing angle. These observations are interpreted in terms of an anisotropic structure to the permittivity of the air in that region. The present paper attempts to make some quantitative estimates of this anisotropy. The measurements analyzed were obtained with the SOUSY 50 MHz radar installed in the 1000‐foot‐diameter antenna at Arecibo. Observations were made at eight zenith angles, from 0° to 11.7°, and at two azimuths, north and east. Echo strength as a function of zenith angle was obtained for these two azimuths and for many heights, with a range resolution of 150 m. These data were then fit to an anisotropic scattering model, and the possible anisotropy parameters were found. Horizontally, there was a range of uncertainty between the degree of anisotropy and its orientation. This uncertainty was inherent in the original measurements, since only two azimuths were used. However, despite this latitude, the vertical anisotropy was quite closely determined. For most heights, the ratio of vertical to horizontal correlation distance of the permittivity field was 0.2 or less.
Characteristics of reflection of electromagnetic plane waves from a semibounded plasma with a diffuse scattering boundary
Authors: Yasumoto K. , Banfuku K.
Published: February 2008

The reflection of electromagnetic plane waves incident obliquely upon a semibounded, maxwellian plasma is investigated based on the kinetic theory. The perturbed velocity distribution function is assumed to satisfy the boundary condition of a diffuse scattering at the surface of the plasma. The numerical results of the reflection coefficients are presented for both cases of incidence of TM and TE waves. It is shown that as the temperature of plasma increases, the reflected energy from the diffuse scattering boundary is significantly reduced, especially within the frequency range of usual total reflection. This phenomenon is discussed at some length and is brought out to be closely related to a collisionless energy absorption by the surface charges which couple resonantly to the incident wave.
Monitoring ionospheric irregularities in the southern auroral region by means of a satellite beacon
Authors: Lambert Malcolm , Cohen E. A.
Published: February 2008

During 1984 total electron content (TEC) data were gathered from Melbourne, a mid‐latitude station, and from Macquarie Island in the southern auroral zone, using the Faraday rotation of geostationary satellite signals. Large‐scale fluctuations in the TEC due to auroral activity were observed from Macquarie Island, and some of these disturbances were seen to propagate towards Melbourne with speeds of 200 to 1,000 m/s. Large‐scale rapid decreases in TEC were observed during the evening hours of about one third of autumn equinox days studied. These decreases occurred at a geomagnetic latitude of approximately 60°S and could be associated with the main ionospheric trough travelling through the ray path from the satellite to the receiving station.
Experimental and theoretical studies of ionospheric irregularities using scintillation techniques
Authors: Liu C. H. , Franke S. J.
Published: February 2008

Theoretical and experimental aspects of using radio scintillation to study ionospheric irregularities will be reviewed in this paper. Both weak and strong scintillation theories will be presented. Theoretical results will be compared with typical experimental data to demonstrate the procedure. Discussions will be concentrated on what information about ionospheric irregularities can be extracted from scintillation data and how. Also, a recently developed simulation technique for radio wave propagation through ionospheric irregular structures will be discussed. It will be shown that by applying this technique, one can obtain information about the properties of the irregularities that are of either statistical or deterministic nature. Specific examples of applying this technique to study equatorial ionospheric bubbles will be reviewed.
Polarization rotation in meteor burst communication systems
Authors: Cannon Paul S.
Published: February 2008

The problems of polarization rotation losses, in meteor burst communication systems, are examined using a theoretical model developed for the purpose. The paper takes into account both the polarization changes due to ionospheric Faraday rotation and the rotation of the wave polarization that takes place as a result of scattering from underdense meteor trails. Linearly polarized systems, employing copolar transmitting and receiving antennas, and hybrid systems, employing a linearly polarized transmitting antenna and a circularly polarized receiving antenna, are studied. It is shown that, particularly in linearly polarized systems, polarization rotation may introduce unexpected diurnal performance variations in systems operating at frequencies ∼40 MHz. For the two 40‐MHz linearly polarized links investigated in detail the model predicts that, for noon summer solstice conditions during periods with a high solar sunspot number, these losses cause a reduction in data throughput to between 15 and 70% of the value expected had no polarization rotation occurred. By using a cross polarization approach, qualitative experimental confirmation of the predictions is also given.
Very long baseline interferometry
Authors: Schilizzi R. T.
Published: February 2008

A general review is given of very long baseline interferometry, touching on basic principles and applications, current instrumentation, organization and future developments. Some astronomical high‐lights are discussed.
Sounder updates for statistical model predictions of maximum usable frequencies on HF sky wave paths
Authors: Reilly Michael H. , Daehler Mark
Published: February 2008

A method is presented for the short‐term prediction of maximum usable frequencies (MUFs) in a large communications region, It is shown how ionospheric measurements from a network of ionospheric sounders can be used to update sunspot number or solar 10.7 cm flux inputs to a climatological MUF prediction model, MINIMUF in this case, which is then used to predict MUFs on paths throughout the region. Analysis of mid‐latitude oblique‐incidence sounder data sets indicates the advantage gained from single‐path sounder updates of flux for MUF predictions on adjacent paths. Under specified conditions a further dramatic improvement in MUF prediction accuracy is found from spatial interpolation of sounder‐updated flux values. MUF prediction accuracies within 0.5 MHz are obtained for fairly modest sounder network deployments, in which the sounder midpath point distributions and updating frequency satisfy particular requirements.
Surface currents induced on a circular strip
Authors: Williams Jeffery T. , Dudley Donald G. , Butler Chalmers M.
Published: February 2008

The problem of the induced surface currents on a perfectly conducting circular strip mounted on a perfectly conducting ground plane is considered. The induced currents are due to the radiated fields of an electric dipole source on the axis of the circular strip. An electric field integral equation is formulated in terms of the induced surface current and solved in closed form in the quasi‐static limit as an expansion of Chebyshev polynomials. In addition, the integral equation is solved for the general case by the method of moments. Numerical results are given which show the excellent agreement between the quasi‐static and method of moments solutions, and results are given which show the effects of varying the radius and the height of the strip, and the location of the dipole source.
Interference detection and correction applied to D region incoherent scatter radar power spectral measurements
Authors: Ying W.‐P. , Mathews J. D. , Rastogi P. K.
Published: February 2008

A three‐stage interference detection and correction data processing scheme has been applied to Arecibo D region incoherent scatter power spectra. Individual power spectral estimates at a particular frequency are, in the absence of interference, Gaussian distributed about the mean for that frequency. The first processing stage involves rank ordering or median filtering techniques to estimate the parameters of the underlying Gaussian process at each spectral frequency. The second processing stage uses this information to construct a “window” which excludes the upper and lower 1% of the Gaussian distributed (interference free) power spectral data. Since interference power is additive, this window also excludes any interference contaminated spectral points that might also be present. The final processing stage involves weighted “fitting” of a model spectrum to the final corrected and averaged spectra. This three‐stage signal processing scheme has yielded significant improvements in final data quality.
Inverse solution modeling of the complex‐valued, frequency dependent electrical properties of natural geologic materials
Authors: Baranyi E. , Jensen O. G. , LaFleche P. T.
Published: February 2008

Nature demands realistic models of the electrical behavior of (geo)physical materials in which the basic electromagnetic parameters are complex‐valued and frequency dependent. The empirical Cole‐Cole model for the permittivity and resistivity of materials is convenient as it presents a relatively simple mathematical formulation which can be modeled by a simple equivalent circuit. This model is observed to reliably fit several sets of electrical measurements. The inversion fitting of several broadband data sets yields electrical parameters which accurately describe the electrical properties of a material.
Propagation of electromagnetic pulse with a Gaussian envelope through an inhomogeneous anisotropic plasma layer
Authors: Zhang D. Y. , Tschu K. K.
Published: February 2008

Effects of an inhomogeneous anisotropic plasma layer on the propagation of a Gaussian envelope carrier pulse are studied numerically. It is found that the magnitude of the right‐handed circularly polarized component of the transmitted wave increases with increasing magnetostatic field in the direction of propagation. However, the magnitude of the left‐handed circularly polarized component of the transmitted wave first decreases with increasing magnetostatic field, up to a critical point, then increases with the increasing magnetostatic field; the critical point is dependent on the carrier frequency.
Analytic description of the electromagnetic wave field for effective refractive indices with an infinity
Authors: Lundborg Bengt
Published: February 2008

General analytical formulas describing the wave pattern in the linear regime of an electromagnetic wave impinging vertically upon a horizontally stratified ionosphere, where the effective refractive index squared has an infinity, a first‐order pole, are derived within the uniform approximation employed in wave propagation problems in previous papers by the author and B. Thidé. A great advantage of this approximation is that the formulas are valid also at the reflection and transition points of the problem. Particular attention is paid to the case of monotonically increasing electron density with height in the relevant region of the ionosphere. Important applications of the formulas derived are the X mode wave when the wave frequency is higher than the electron gyrofrequency and the 0 mode wave when the wave frequency is lower than the electron gyrofrequency. The type of wave equation studied here may occur also in other wave propagation problems.
Relativistic scattering by a random rough Kirchhoff surface in uniform motion
Authors: Chrissoulidis D. P. , Ivrissimtzis L. P. , Kriezis E. E.
Published: February 2008

The physical optics approach and Lorentz transformations are combined to formulate scatter from a random rough surface in uniform motion. The perfectly conducting scatterer is excited by an elliptically polarized plane electromagnetic wave and the analysis ends in the velocity‐dependent expressions for the scattering cross section per unit area, the depolarization factor and the radial tilt. The direction of motion is considered parallel as well as normal to the scatterer's mean plane. The combined effect of roughness and motion on scattering and depolarization is investigated through a numerical example. The calculations reveal that parallel motion modifies scatter through aberration while normal motion results in scatter amplification. The depolarization factor and/or radial tilt might be used for motion estimation.
A method for synthesis of small arrays of printed dipole antennas backed by a ground plane
Authors: Popović Branko D.
Published: February 2008

A method is proposed for synthesis of antenna arrays consisting of narrow strip‐dipole antennas printed on a thin dielectric substrate. Approximately antiresonant dipoles (i.e., dipoles about one wavelength long) are considered, because by moderate variation of their width and length their impedance can be varied in a wide range. A method for analysis of such arrays is described first, in which the printed strip dipoles are replaced by approximately equivalent circular‐cylindrical dipoles with coaxial magnetic coating, using recently proposed generalization of equivalent radius of thin cylindrical antennas. An interactive optimization procedure is next applied for synthesis, which enables arrays to be obtained having array radiation pattern and, particularly, dipole impedances close to the desired.
A review and examination of results on uniqueness in inverse problems
Authors: Stone W. Ross
Published: February 2008

Inverse source and inverse scattering problems represent distinct situations with regard to the uniqueness of their solutions, although they are often treated the same. This paper examines the reasons why the distinction between these two problems is critical in determining uniqueness. What is known about sufficient (and, in some cases, necessary) information to insure uniqueness for real‐world inverse problems is reviewed. Finally, although Hoenders' result is clear for the inverse scattering problem, there is some controversy over what set of constraints are necessary and sufficient to insure uniqueness in the inverse source problem. This is examined, and the origins of questions which arise from the usual derivations of the fields associated with nonradiating sources are explained.
Systematic behavior of signal statistics of MST radar echoes from clear air and their interpretation
Authors: Kuo Fu‐Shong , Chen Chih‐Chen , Liu Shu‐Ing , Röttger Jurgen , Liu Chao‐Han
Published: February 2008

Signal statistics of data from the SOUSY‐VHF radar were analyzed. Mean, variance, skewness and kurtosis of the distribution of the quadrature components of the signals as well as Nakagami m coefficient of the amplitude distribution were calculated. We found a systematic behavior in the correlation between kurtosis and Nakagami coefficient, and we propose a stochastic model for its explanation. These stochastic effects on the signal are generated in the variable echoing process itself. Numerical simulation and analytical calculation of the model were carried out for comparison with the data, and the results are encouraging.
On the flow of plasma around a dipole magnetic field
Authors: Hill Jacqueline L. , Seyhoonzadeh Amir , Chang Hong‐Young , Lonngren Karl E.
Published: February 2008

A series of experiments intended to examine the flow characteristics of a plasma incident on a localized magnetic dipole field in a low β plasma is described. Both the electron and ion densities are measured upstream and downstream from the magnetic structure, and the results are compared with a nonflowing plasma of the same density. Significant differences are detected which can be attributed to the flowing plasma.
Note on low‐frequency waves
Authors: Jones D. S.
Published: February 2008

The low‐frequency behavior of perfectly conducting bodies is investigated with the aim of determining the first‐order correction. The relevant governing equations are obtained, and the form of the distant field is evaluated. The reduction that occurs for discs and apertures is considered separately.
A high‐frequency spectrum utilization model
Authors: Rush C. M. , Washburn J. S. , Berry L. A.
Published: February 2008

In preparation for the second session of the High‐Frequency Broadcasting Conference the Institute for Telecommunication Sciences developed a computer program to assess the impact of planning the frequency bands allocated to the high‐frequency broadcasting service on U.S. broadcasting objectives. The program provides a powerful tool to numerically simulate the performance of high‐frequency sky wave systems operating in both noise‐limited and interference‐limited environments. This paper describes the philosophy behind the design of the program and its various components, such as propagation model, antenna routines, and interference assessment. Examples are given of the application of the program for frequency planning and frequency assignment. Areas in which the program can be expanded to include other services (fixed, mobile, amateur, etc.) operating in the frequency range 3–30 MHz are also discussed.
Multipath propagation experiments on 8.2‐km line‐of‐sight path at 30 GHz
Authors: Herben M. H. A. J.
Published: February 2008

The importance of event analysis of radio meteorological phenomena and relations is indicated. Experimental results of copolar amplitude, phase, and amplitude distortion measurements obtained on an 8.2‐km line‐of‐sight path at 30 GHz are reported. The occurrence of multipath propagation is compared with the occurrence of temperature inversions, measured with two vertically spaced thermometers. The results of both long‐term temporal and short‐term spectral analyses of the signals measured during multipath propagation events are described. The measured long‐term copolar phase variations are compared with those computed from simultaneously measured refractive index variations. The spectra of copolar amplitude and phase are compared with those obtained from tropospheric turbulence‐induced scintillations.
Monostatic observation of the F region structure
Authors: Paul Adolf K.
Published: February 2008

F region parameters can change by relatively large amounts over short periods of time (fraction of an hour) under so‐called “undisturbed” conditions. There is evidence that these changes are due to propagating acoustic gravity waves. While the periods of these variations and other local parameters (Doppler velocity and angle of arrival) can be measured with a high‐resolution digital ionosonde, other quantities like wavelength or magnitude and direction of the propagation velocity cannot be observed. The Clarke Lake radio astronomical antenna array which can be operated in the HF band allows tracking the motion of medium scale (100–500 km) disturbances by observation of the changes in the apparent position of radio stars. Some first data samples were recorded and analyzed (R. L. Higgins and B. McManus, private communication, 1987).
Nonlinear signal enhancements of medium‐frequency electromagnetic transmissions in the ionosphere
Authors: James H. G.
Published: February 2008

During a close rendezvous of the ISIS I and ISIS II topside sounder spacecraft, medium‐frequency signals transmitted from the latter to the former exhibited two kinds of anomalous enhancement, one at about twice the electron gyrofrequency 2fc and another at a separation below the RF carrier approximately equal to fc. Signal enhancements of up to 50 dB above those calculated with classical theories of transmitting and receiving antennas were observed. The sounder data clearly show that these signals propagated in the normal cold plasma upper branch O and X modes. Natural radio emissions in the form of whistler mode and Z mode hiss were recorded at the same time, implying the presence of energetic particles. Second‐harmonic cyclotron amplification along the ray path connecting the transmitter and receiver has been evaluated and found unsatisfactory for explaining the 2fc enhancement. The postulate that a plane wave decay nonlinearity is enhanced by soft electron fluxes that also produce the observed hiss has also been considered and rejected. Nonlinear electron motion in the near field of the transmitting antenna might produce both types of enhancement.
Electromagnetic scattering by spheroidal objects with impedance boundary conditions at axial incidence
Authors: Sebak A. , Shafai L.
Published: February 2008

An analytic solution of a plane electromagnetic wave scattering by coated and impedance prolate spheroidal bodies is obtained, for axial incidence, by expanding the incident and scattered fields in terms of prolate spheroidal vector wave functions. The unknown expansion coefficients are determined by a doubly infinite system of equations derived using the impedance boundary condition, which relates the tangential components of the electric field to those of the magnetic field through an effective surface impedance of the coating. To solve for the unknown coefficients, the system of equations is truncated by retaining only the first N equations in N unknowns, where N depends on the size of the body and the desired accuracy. Numerical results for the scattering cross section are presented to show the effect of different coatings on the magnitude of the scattered field.
A local oscillator system for millimeter wave interferometry
Authors: Padin S. , Woody D. P. , Scott S. L.
Published: February 2008

The design details for a millimeter wave local oscillator for interferometry in the 115‐GHz and 230‐GHz bands are presented. The system uses a low phase noise 5‐GHz synthesizer to provide the phase reference to phase lock a millimeter wave source. Both the 5‐GHz synthesizer and the millimeter wave source phase lock circuits use digital phase‐frequency detectors. Design tolerances for the local oscillator system are presented, and a coherence test arrangement for evaluating the performance of the system is described. The local oscillator has an interreceiver phase jitter of 7° at 115 GHz and the long‐term stability is typically better than 20° pp in 24 hours. The system is currently being used in the Owens Valley three‐element millimeter wave interferometer.
Three different kinds of Fraunhofer approximations: 1. Propagation of the field amplitude
Authors: Carter William H.
Published: February 2008

The Fraunhofer approximations as usually used in diffraction, radiation, and antenna theories have some important and very useful differences. This paper examines three kinds of Fraunhofer approximations in applications to the propagation of a scalar field amplitude from sources to detectors. Each of the three approximations is examined to make clear the differences in their conditions for validity and mathematical structure. Some differences in their applications are also discussed.
An analysis of coupled strip antennas and of the strip transmission line
Authors: King Ronold W. P. , Wu Tai T.
Published: February 2008

The integral equations for the current in isolated and coupled strip antennas are derived and related to the corresponding equations for the circular tubular antenna. The integral equations for the two‐element array and the N‐element circular array of strip antennas are formulated. The equations are solved specifically for the closely spaced two‐strip array and the properties of the two‐strip transmission line are obtained. It is noted that mutual coupling can be evaluated more accurately among strips than among tubular cylinders.
A model for propagation of ELF waves throughout the lateral extent of the inhomogeneous Earth‐ionosphere waveguide
Authors: Shellman C. H.
Published: February 2008

A formulation is described for finding the surface vertical E field throughout the lateral extent of the spherical Earth‐ionosphere waveguide at extremely low frequencies (ELF) below 100 kHz for a completely general model of values of the sine of the waveguide eigenangle. These values are specified at elements of a mesh point array that extends throughout the lateral extent of the spherical shell guide. The solution is based on the condition that the vertical E field at the surface must satisfy the reduced two‐dimensional wave equation at every localized group of mesh points. It has been found practical to use a mesh size of 312.5 km halfway to the antipode, which implies a mesh size of 50 km at a distance 1 Mm from the transmitter. Eigensolutions are found for any far range from the transmitter for which there is no transverse variation of properties, and these are combined with the more general solution which applies to the region of transverse inhomogeneity. The latter may include the entire waveguide. An example is given for illustration. It is expected that use of the formulation will make it possible to describe the combined effects of inhomogeneities such as land‐sea boundaries around continents and large seas and islands, the day‐night terminator, and particle precipitation on the amplitude and phase of the vertical E field.
Dispersive microwave fading and lower atmospheric structure: An observational study
Authors: Lee Jiin‐Lang
Published: February 2008

Simultaneous measurements of microwave fading and lower atmospheric conditions were carried out in Georgia in 1986 as a part of the continuing effort to study the line‐of‐sight digital radio performance. Acoustic sounding was a principal means of exploring the mechanism that causes dispersive fading. It was found that dispersive multipath fading occurs during nights with strong radiation inversion accompanied by a subsiding air mass. Subsidence inversion causes dispersion, and the origin of the air mass affects the mean received power level under multipath propagation conditions. When the height of the nocturnal boundary layer coincides with the height of the ground‐based radiation inversion layer and is at 100–150 m, fading is heavy, active, and dispersive for a 57‐km path with both transmitting and receiving antennae at 60 m above grade. Case studies of the significant episodes are presented, and the statistics of the different types of fading associated with the types of meteorological conditions are summarized for the 5‐month study period.
Experimental probing of multipath microwave channels
Authors: Sylvain M.
Published: February 2008

It is very difficult to relate the behavior of a multipath microwave channel to its physical description. Much information, however, can be gained from radioelectrical probing of the channel, and models can be built independent of the complex underlying physical situation. In this paper we review the various experimental techniques used to investigate the channel, their successes, and their limitations. We conclude by stressing the directions in which progress has to be made.
LORAN‐C timing errors caused by propagation over finitely conducting ground
Authors: Cooray Vernon , Orville R. E.
Published: February 2008

Timing errors caused by the propagation of the LORAN‐C pulses over finitely conducting earth are investigated. The results show that timing errors of 1–4 μs could be expected due to 100‐ to 400‐km propagation over finitely conducting ground.
Numerical investigations of two‐frequency mutual coherence functions of an ionospheric reflection channel
Authors: Wagen J.‐F. , Yeh K. C.
Published: February 2008

The ionospheric communication channel is characterized by its transfer function. Each frequency component of the transfer function is given by the ratio of the wave field received to that transmitted. Many works exist which deal with the computation of the ionospheric transfer function when electron density varies smoothly. As frequently happens, the ionosphere is permeated by random irregularities; this paper is concerned with the computation of transfer functions in such a complex ionosphere. Each frequency component of the transfer function can be computed by using the phase‐screen‐diffraction‐layer method, also called the split‐step algorithm, in underwater acoustics. Stepping along the ray path, the random phase fluctuations in each slab are lumped into a phase screen. Diffractions between the phase screens are taken into account by assuming the waves are propagating in the background medium. Since different frequencies follow different paths between the transmitter and the receiver, care must be exercised by taking proper correlations between the different ray paths. The simulation scheme presented briefly in this paper introduces the concept of correlated phase screens to achieve proper correlations between ray paths at different frequencies. Although this concept is not limited by the channel bandwidth, the computations presented here are restricted to a bandwidth of the order of the coherence bandwidth of the channel. The result of the simulation is used to compute the two‐frequency mutual coherence function, which is widely used to characterize a random communication channel. One very important parameter of the two‐frequency mutual coherence function is the coherence bandwidth. This parameter can also be derived from a much simpler treatment of the phase screen theory. Comparisons are made and show the need for further investigations.
HF sideband generation in the ionosphere
Authors: Noble S. T. , Gordon W. E. , Duncan L. M. , McCoy J. E.
Published: February 2008

The temporal development of sidebands excited near sunrise by two strong HF waves separated by a few hertz is presented. Sidebands are not observed before sunrise when the ionospheric critical frequency is less than the heater frequency. As the ionospheric density increases following sunrise and overdense conditions are established, strong sidebands emerge. Even though these results favor a mechanism which phase modulates the reflected HF wave over one which first downconverts the HF power to ULF before exciting sidebands, it is possible that either mechanism could at times contribute to sideband production.
An impedance boundary condition approach to radiation by uniformly coated antennas
Authors: Huddleston P. L. , Wang D.‐S.
Published: February 2008

It is shown that the impedance boundary condition may be used to model radiation by uniformly coated antennas. Radiation by slot antennas on uniformly coated conducting bodies is studied. It is shown how to choose a surface impedance, a pseudoaperture location, and the impressed pseudoaperture fields to best represent the original problem.
Diffraction of an H‐polarized electromagnetic wave by two equal infinite circular strips
Authors: Jain Sarita , Jain D. L.
Published: February 2008

We solve here the two‐dimensional problem of diffraction of an obliquely incident low‐frequency H‐polarized electromagnetic plane wave by two equal coaxial perfectly conducting infinite circular strips. Approximate expressions are derived for the far‐field amplitude of the magnetic intensity associated with the scattered wave and the scattering cross section.
Techniques to obtain Faraday electron content from polarimeter data at low and middle latitudes
Authors: de Adler Nieves Ortiz , Ezquer Rodolfo G.
Published: February 2008

The technique used at present to obtain Faraday electron content from polarimeter ground measurements of beacon signals is briefly reviewed and its basic assumptions are pointed out. A new alternative method is described. This method obviates the knowledge of the orientation of the transmitted electrical field and removes the so‐called ambiguity problem and may be useful in the case of interrupted polarimeter records. It makes use of electron density profiles obtained from an ionosonde located near the subionospheric point. Numerical comparisons of this method with the conventional one give good agreement. This was made using the combination of data from geostationary satellites with ionogram records. The relative advantages of both techniques are pointed out.
The refractive index structure parameter, C2n, for a year over the frozen Beaufort Sea
Authors: Andreas Edgar L.
Published: February 2008

The Arctic Ice Dynamics Joint Experiment (AIDJEX) produced a year of continuous, 6‐hourly observations of the surface fluxes of momentum and sensible and latent heat over the drifting ice of the Beaufort Sea. From this data set I compute the refractive index structure parameter C2n at four electromagnetic wavelengths—one each in the visible (0.55 μm), the mid‐infrared (10.6 μm), the near‐millimeter (0.337 mm or 890 GHz), and the radio regions. I present the resulting C2n values as histograms sorted according to the four seasons, spring, summer, autumn, and winter. Because each histogram derives from at least 300 C2n values, this is one of the largest studies of C2n climatology ever reported. For all four wavelengths the winter C2n histograms are the narrowest and have the highest peaks; the summer histograms are the broadest and most uniform. For the shorter wavelengths (0.55 and 10.6 μ) the mode value of C2n shows a seasonal progression; the maximum is in winter, the minimum in summer. The longer wavelengths (0.337 mm and radio) do not show a clear seasonal trend in the mode C2n value; although the C2n distributions change shape with season, the mode value is fairly constant. The beta distribution is a useful probabilistic model for the C2n histograms.
Enhanced imaging of reflector antenna surface distortion using microwave holography
Authors: Gilmore Sean W. , Rudduck Roger C.
Published: February 2008

Two signal processing techniques are discussed that improve the accuracy of a microwave holographic measurement by removing unwanted signals from the aperture distribution: pattern simulation and subtraction, and time domain filtering. Pattern simulation and subtraction involves modeling unwanted scattering mechanisms and then removing them from the measured far‐field data. Time domain gating requires a swept frequency measurement of each far‐field point to separate undesired scattering mechanisms from the desired response. Measurements taken on a focal point geometry and a Cassegrain geometry at 11 GHz in the Ohio State University ElectroScience Laboratory Compact Range provided excellent amplitude and phase stable data to be processed by the holographic analysis system. Pattern simulation and subtraction was applied to both geometries. Surface deformation profiles generated for the Cassegrain antenna by this system were compared to an optical measurement of the main reflector surface. Good agreement was obtained. Because of system limitations, time domain filtering could not be applied.
Diffraction by a partially coated perfect electric conducting half plane
Authors: Rojas R. G. , Chou L. M.
Published: February 2008

The high frequency diffraction (transverse electric case) by a partially dielectric‐coated, perfect electric conducting (PEC) half plane is considered. The coating is of finite length and is the same on both sides of the half plane. The length of the two dielectric strips can be different; however, to simplify the analysis and without loss of generality, it is assumed to be the same. This problem requires the solution of two canonical problems which are solved via the Wiener‐Hopf method. The coating is assumed to be thin, and it is modeled by a generalized impedance boundary condition of O(t), where t is the thickness of the coating on each face of the PEC half plane. The interactions between the edges of the coating, i.e., surface wave diffraction, doubly and triply diffracted fields, are also considered. These multiply diffracted fields, including two new terms, are obtained via an extended spectral ray method which is related to the spectral theory of diffraction. The backscattered and bistatic echo widths are computed with the solutions developed here and compared with an independent moment method solution. The agreement between the two solutions is very good.
Signal frequency dependence of ionospheric amplitude scintillations
Authors: Bhattacharyya A. , Rastogi R. G. , Yeh K. C.
Published: February 2008

The S4 index, which is a measure of the strength of amplitude scintillations, is assumed to vary with signal frequency f according to S4 ∝ f−n, and the frequency exponent n is derived from observed equatorial amplitude scintillations on 40‐ and 140‐MHz signals. A large data base has been used to study the variation of this exponent n(40/140) with the S4 index on the 140‐MHz signal for both daytime and nighttime scintillation events. Although the equatorial E and F region irregularities, which cause the observed scintillation events, have different characteristics, the variation of n(40/140) with S4(140) is found to be almost universal. Theoretical computation of n(40/140) for different values of the irregularity parameters yields results which are in good agreement with these observations.
Numerical simulation of solar wind density fluctuations and their effects on VLF radio interferometry
Authors: Williamson Robert S. , Jones Dayton L.
Published: February 2008

A mission consisting of an array of small satellites has been proposed to investigate radio sources at frequencies from approximately 2 to 20 MHz, a range unobservable from the ground because of ionospheric absorption and aberrations. Such a mission seems feasible and relatively inexpensive, but further investigation is still necessary. The solar wind is a major concern because it contains turbulence‐induced random density variations that cause a fluctuating phase shift in any electromagnetic signal passing through, sometimes making radio source imaging by interferometry impossible. A numerical simulation has been developed which attempts to accurately characterize solar wind density fluctuations. Results have allowed more concrete conclusions to be drawn about observing constraints and have shown that interferometric imaging at radio frequencies below 10 MHz with baselines up to 100 km is possible at solar elongation angles of about 90° or more.
Some comments on seabed propagation of ULF/ELF electromagnetic fields
Authors: Chave Alan D. , Flosadóttir Agústa H. , Cox Charles S.
Published: February 2008

A reference model for the electrical conductivity structure beneath the deep seafloor is proposed and justified using a variety of geophysical evidence. The model consists of relatively conductive sediment and crustal layers of 6.5 km extent overlying a resistive (≈10−5 S/m) subcrustal channel of 30 km thickness and terminated in a deeper conductive layer and half‐space. Its seafloor‐to‐seafloor response to a horizontal electric dipole source is explored as a function of frequency and range, showing that, compared with the response for a half‐space with the lowest conductivity in the reference model, significant enhancement of the field amplitude can occur at long ranges (>100 km) and low frequencies (<1 Hz). At the same time, marked attenuation relative to the half‐space response is seen at higher frequencies. The field enhancement is due to trapping of electromagnetic energy in a leaky subcrustal waveguide, as demonstrated by computing the complex Poynting vector. The attenuation occurs in the relatively conductive sedimentary and crustal layers overlying the lithospheric waveguide when their electrical thickness exceeds a skin depth. The results indicate that attempts either to model controlled electromagnetic sources or to interpret controlled source data using half‐space models for the Earth can be badly misleading. The practicality of lithospheric communications in the real Earth is also investigated. Using measured receiver noise figures and the reference model, the receiver bandwidth necessary to achieve a given signal‐to‐noise ratio as a function of range and frequency is estimated for a seafloor horizontal source of strength 105 A‐m. The results indicate that significant (≈100 km) ranges can be achieved only around 1 Hz with a bandwidth of ≈1 Hz at a SNR of 10, yielding a very low data rate of <3.5 bits/s. Longer ranges and higher frequencies are precluded by attenuation in the sediment and crustal layers and because the conductivity in the resistive channel is too large.
Comparison of thermal advection measurements by clear‐air radar and radiosonde techniques
Authors: Crochet M. , Bazile E. , Rougier G.
Published: February 2008

Vertical profiles of the horizontal wind have been measured every 4 min by a clear‐air radar (stratosphere‐troposphere radar), and vertical profiles of temperature have been obtained every 2 hours by three radiosonde soundings in the same zone in Brittany during the Mesoscale Frontal Dynamics Project “FRONTS 87” campaign. Radar thermal advection ATR is deduced from the thermal wind equation using the measured real horizontal wind instead of the geostrophic wind. Radiosonde thermal advection ATS is determined directly from the sounding station data sets of temperature gradients and also approximated from the thermodynamic equation by the temperature tendency ∂T/∂t\p. These approximations, applied during a frontal passage, show the same general features and magnitude of the thermal advection, giving a preliminary but encouraging conclusion for a possible real‐time utilization of clear‐air radars to monitor thermal advection and to identify its characteristic features.
Simultaneous observations of the troposphere and lower stratosphere by the Flatland and Urbana ST radars
Authors: Dester G. D. , Liu C. H. , Franke S. J. , Green J. L.
Published: February 2008

Simultaneous VHF radar observations of the troposphere and lower stratosphere were carried out on August 17–19, 1988, near Urbana, Illinois, using the Flatland radar and the Urbana stratosphere‐troposphere (ST) radar, which are operated by the Aeronomy Laboratory of the National Oceanic and Atmospheric Administration and the University of Illinois Department of Electrical and Computer Engineering, respectively. The two radar sites are located on the vast plain area of central Illinois, separated by approximately 25 km. The geographical and observational configuration is considered most suitable for investigating mesoscale structures in the troposphere‐stratosphere region. One horizontally stratified layer at about 15.5 km related to the tropopause, and another in the troposphere at about 8 km were observed at both radar sites for a long period of time. Velocity fluctuations with periods of 100 min < T < 200 min were observed to be well correlated during a period of quiet horizontal wind. A convective thunderstorm on the evening of August 18 coincided with the dissipation of the tropospheric layer observed at 8 km. We present here a qualitative comparison of low‐pass and band‐pass filtered echo power and radial velocities, and a comparison of vertical power profiles from the two radars.
Kinematic history of the Narragansett Basin, Massachusetts and Rhode Island: Constraints on Late Paleozoic plate reconstructions
Authors: Mosher Sharon
Published: February 2008

The formation of the Narragansett Basin of Rhode Island and Massachusetts and the polyphase deformation geometry of the Pennsylvanian‐age basin sediments can be explained by collision and strike slip interactions between an Avalonian microcontinent, the European‐North American plate, and the African plate. During the late Paleozoic, sinistral movement between an Avalonian microcontinent and the European‐North American plate resulted in formation of pull apart basins. Subsequently, Africa moved toward North America along an east‐west trending, dextral megashear zone and collided with the eastern margin of southern North America. The Narragansett Basin is located on the Avalonian microcontinent near the suture with the North American continent and in the transition zone between the megashear and the collision tectonic regimes. The basin shape, stratigraphy, and sedimentary relationships can be explained by the coalescence of two composite grabens which formed at different times. The thicker northern graben is only affected by one deformation, whereas the southern graben shows evidence of four distinct phases of postrifting deformation. The first two coaxial folding deformations of the basin sediments are attributed to the closure of the basin complex during collision of Africa and North America. The third and fourth deformations are interpreted as sinistral R′ riedal and dextral shears, respectively, associated with the east‐west trending megashear system. A prograde Barrovian metamorphic front started in the western portion of the southern graben slightly after the first deformation and migrated northward and eastward, where it is synchronous with the second deformation.
Epeirogeny on a short geologic time scale
Authors: Officer Charles B. , Drake Charles L.
Published: February 2008

Evidence is presented that vertical movements of an oscillatory or episodic nature are an important feature of plate tectonics. The vertical movement within presumably stable intraplate regions are in the range of 0.1–1.0 cm/yr, approximately an order of magnitude less than horizontal plate velocities. The temporal scale of the movements is 10,000–100,000 years, a relatively short geologic time interval, and the spatial scale is of the order of 100–1000 km. The supporting data for these conclusions come from recent movements, the Quaternary geologic record, and the Phanerozoic sedimentary record.