bluuebunny/crossref_metadata_2025 huggingface dataset:
Interpretation of ionograms in the vicinity of the dayside auroral oval by ray tracing
Authors: Høeg P. , Ungstrup E.
Published: February 2008

A ray‐tracing program based on the Haselgrove equations and the Appleton‐Hartree formula has been developed and used to synthesize sweep‐frequency ionograms. We have looked at ionograms taken in summer periods at high‐latitude ionosonde stations in the vicinity of the dayside auroral oval. Especially the traces on ionograms from oblique reflections from an electron density enhancement have been studied. The enhanced ionization is simulated either by a two‐dimensional Gaussian distribution or by a double exponential function similar to the bottom part of a Chapman function. The enhancements are aligned with the earth's magnetic field direction. Ray paths in the magnetic meridian plane are calculated and discussed in connection with computed ionograms and observed ionograms from Canada and Greenland. The ray‐tracing exercise was done in order to estimate characteristics of the electron density enhancement from observed ionograms. By varying the parameters in the models of the ionosphere to obtain agreement between observed and synthesized ionograms we have been able to analyze temporal and spatial progress of an electron density enhancement that could be a feature of the auroral oval like the field‐aligned current sheets in the cusp region.
Poker Flat MST radar measurement of winds and wind variability in the mesosphere, stratosphere, and troposphere
Authors: Balsley Ben B.
Published: February 2008

The capability of the mesosphere‐stratosphere‐troposphere (MST) technique for measuring winds and wind variability in the mesosphere, stratosphere, and troposphere is demonstrated using recent results obtained from the MST radar at Poker Flat, Alaska. Mesospheric results include mean flow, tidal motions, and shorter‐term fluctuations. Stratospheric‐tropospheric results include, in addition, high‐frequency (≃15m) wavelike fluctuations observed in the vertical wind field.
Seasonal and day‐to‐day variability of total electron content at mid‐latitudes near solar maximum
Authors: Soicher H. , Gorman F. J.
Published: February 2008

Faraday rotation observations were conducted at Fort Monmouth, New Jersey (40.15°N, 74.01°W), utilizing beacon transmissions from a geostationary satellite during the period just following the maximum phase of solar cycle 21. Seasonal and day‐to‐day variabilities are observed. Unique representation of the data has permitted the study of the day‐to‐day variability of derived total electron content (TEC) values. For example, winter data indicate uniformity of day‐to‐day TEC values during the buildup and decay phases of the diurnal variation, and summer data indicate uniformity during the predawn and sunrise phases, whereas equinox data indicate wide variability during all phases. In addition, winter and summer data indicate that during non‐magnetically‐disturbed periods, TEC values at the diurnal peak, which exceed monthly mean values, tend to occur on consecutive days. During the equinoxes the peak values may occur randomly. The response of TEC to magnetic activity is also discussed. Since day‐to‐day variability is a most difficult quantity to predict, the results have implications for prediction improvement.
Admittance of a cylindrical antenna coated with an inhomogeneous cold plasma sheath with a resonance
Authors: Kamp L. P. J. , Weenink M. P. H. , Hagebeuk H. J. L.
Published: February 2008

In this paper a perfectly conducting, infinitely long, cylindrical antenna is considered. The antenna is driven by an alternating voltage V applied uniformly around an infinitesimally thin, circumferential gap. Furthermore, the antenna is surrounded by a sheath that goes smoothly over into the surrounding plasma so that the electron density increases smoothly with the distance from the antenna. For this two‐parameter family of electron‐density profiles the admittance of the antenna is calculated for all values of the driving frequency ω > 0 and as a function of the parameters, i.e., of the radius of the antenna and the plasma frequency far away from it. The calculated admittance is further compared with that of the same antenna in vacuum for which Papas' (1949) results have been generalized. It is shown that a measurement of the admittance of the antenna at only one frequency is, in general, not sufficient to determine uniquely the plasma frequency far away from the antenna. At least two measurements at two different frequencies or with two antennas with different radii are necessary to determine uniquely the electron density far away from the antenna.
A decoder for a 30‐m height resolution ST radar
Authors: Petitdidier M. , Desautez A. , Glass M. , Penazzi G.
Published: February 2008

A new hardware decoder‐coherent integrator, which will allow a 30 m height resolution with a UHF radar, is described. By using quasi‐complementary codes, the decoding is performed in real time before the coherent integration. This device consists of 20 identical PC boards, as many as the number of subpulses in the transmitted pulse, and is able to process simultaneously 20 gates; 40 additions or substractions and memory accesses are performed in 200 ns. This decoder‐coherent integrator is very modular and simple in its design, with its 20 identical boards which enhance the overall reliability of the system.
Pulse compression hardware decoding techniques for MST radars
Authors: Sulzer M. P. , Woodman R. F.
Published: February 2008

The techniques for decoding in hardware received signals transmitted by phase‐coded mesosphere‐stratosphere‐troposphere (MST) radars are reviewed. The designs consist of digital and analog types which resemble filters or correlators in their operation. A new analog design is presented, and a discussion of the choice between hardware and software decoding is given. The number of bits required for digital coherent integrators and decoders is discussed, and the use of fast Fourier transforms for decoding is described.
Variability in doppler slant‐beam measurement of horizontal wind: A case study
Authors: Waterman A. T. , Hu T‐z , Czechowsky P. , Röttger J.
Published: February 2008

A set of clear‐air Doppler radar measurements is analyzed in which the radar beam was pointed at several different directions from the zenith, in each of two different azimuths. The data were then examined to assess the variability in inferred horizontal wind, for each azimuth, and in horizontal vector wind, for the combined azimuths. Root‐mean‐square errors in determination of horizontal wind are estimated to be around 1 m/s. Possible reductions in this error are projected for greater off‐zenith angles. The application to measurement of the vertical shear of horizontal wind is discussed.
A new asymptotic high‐frequency analysis of electromagnetic scattering by a pair of parallel wedges: Closed form results
Authors: Michaeli A.
Published: February 2008

A new asymptotic analysis of the high‐frequency scattering by a pair of parallel wedges is presented, resulting in a uniform, closed‐form approximation for the doubly diffracted field. The analysis is based on an extension of the physical theory of diffraction, in which the radiation integral over the actual induced currents is replaced by the Helmholtz integral over a surface enclosing the wedges. A term‐by‐term relationship is established between the geometrical theory of diffraction ray decomposition of the scattered field and the asymptotic decomposition of the radiation integral into endpoint contributions. The closed form expressions obtained for the various endpoint contributions to the singly diffracted field are used in an extended spectral theory of diffraction analysis of the doubly diffracted field to cast the latter in the form of a steepest descent path integral satisfying the principle of reciprocity. An Ansatz based on the uniform theory of diffraction in combination with a singularity‐matching procedure is proposed to transform the initial nonuniform approximation for the doubly diffracted field into a uniform one. The latter includes terms to order k−1 and applies to all angles of incidence and scattering, including the overlapping transition regions. The limitations on this closed‐form solution are that the incident wave be approximately plane throughout the gap between the edges, the observation point be in the far scattered field, and the gap be large in terms of the wavelength.
Scattering by a conducting strip with a randomly serrated edge
Authors: Eftimiu C. , Huddleston P. L.
Published: February 2008

The coherent monostatic response of a strip with a randomly serrated edge is evaluated by considering the serration a realization of a stochastic process. Results are given for various values of the average width, variance, and correlation length that characterize the strip statistically.
Measurements of heat flux in the atmospheric boundary layer by sodar and RASS: A first attempt
Authors: Peters G. , Hinzpeter H. , Baumann G.
Published: February 2008

By combined measurements with two ground‐based remote sensing techniques, sodar and radioacoustic sounding system (RASS), the covariance of vertical wind and temperature perturbations can be determined. The estimation of errors indicates that this method is probably so accurate that profiles of the atmospheric turbulent heat flux can be determined. This is supported by experimental results.
Measurement and statistical analysis of wideband MF atmospheric radio noise: 1. Structure and distribution and time variation of noise power
Authors: Herman J. R. , Giordano A. A. , DeAngelis X. A. , Marzotto K. F. , Hsu F. M.
Published: February 2008

Wideband measurements (100 kHz) of medium frequency atmospheric noise have been made over the past several years in the southwestern United States. In part 1 of a two‐part paper the measurement and data transcription system and the statistical analysis software used to analyze the data are presented. In part 2, representative first‐ and higher‐order statistics and the impact of the data on bandwidth and system performance are described. Measurement results presented in part 1 include the temporal structure of atmospheric noise, the distribution and time variation of the measured average noise power, and comparisons with predictions by the International Radio Consultative Committee (CCIR).
Simultaneous measurements of atmospheric emissions at 10, 33, and 90 GHz
Authors: Costales James B. , Smoot George F. , Witebsky Chris , De Amici Giovanni , Friedman Scott D.
Published: February 2008

As part of a larger experiment to measure the cosmic microwave background radiation spectrum, frequent simultaneous measurements of microwave thermal emissions from the earth's atmosphere have been made at 10, 33, and 90 GHz. We have performed these measurements at two separate locations, Berkeley, California, and the White Mountain Research Station located near Bishop, California, which differ greatly in altitude and climatic conditions. Typical values of the atmospheric antenna temperature measured in Berkeley (250‐m elevation) during good weather are 2.54±0.29, 12.4±0.3, and 34.3±0.5 K for 10, 33, and 90 GHz, respectively. Corresponding values measured at White Mountain (3800 m) are 1.11± 0.04, 4.48±0.18, and 11.0±0.1 K. Because the measurements are simultaneous in nature, correlations between the measurements taken at the various frequencies provide constraints on models of the microwave emission of the earth's atmosphere, especially models describing atmospheric emission as a function of precipitable water content.
A simple graphical method for dealiasing digital ionosonde echo location data
Authors: Dudeney J. R. , Jarvis M. J.
Published: February 2008

A simple user interactive technique that allows digital ionosonde sky map echo location data to be dealiased uniquely is described. Unambiguous tracking of large‐scale ionospheric features is thus made possible. The method utilizes the frequency dependence of the relationship between obliquity of propagation and signal phase.
Broadside excitation of ELF by a horizontal dipole beneath a sporadic E environment: Theory
Authors: Pappert Richard A.
Published: February 2008

A simple surface propagation model is commonly used to estimate the effects of ionospheric disturbances on extremely low frequency (ELF) propagation. Though often adequate, one shortcoming of the model is its failure to allow for excitation by the broadside component of a horizontal electric dipole. That excitation can be quite significant in a sporadic E environment because the modal polarization can then contain a substantial mixture of transverse electric component. This is demonstrated by considering a flat earth geometry with a vertical geomagnetic field superimposed on a laterally homogeneous, cylindrically symmetric disturbance centered over the transmitter. The method utilizes normal mode decomposition. Matching equations at the boundary of the disturbance are developed by evaluating height gain integrals, and it is shown that this method is tantamount to simply matching the ground vertical electric and azimuthal magnetic fields at the boundary. Results for a sporadic E type environment indicate substantial departures from other model predictions.
On a unified interpretation of the polar Slant E Condition (SEC) and other high E field related phenomena
Authors: Olesen J. K. , Stauning P. , Tsunoda R. T.
Published: February 2008

Recent measurements by the incoherent scatter radar and by several other instruments in the polar cusp region at Sondre Stromfjord, Greenland, have shown features similar to previously published occurrences at lower, auroral‐zone latitudes of high electric field related electron heating events ascribed to an E region plasma instability effect. Our results further substantiate previous suggestions on a plasma instability and high electric field intensities in the E region being closely related to the ionogram Slant E Condition (SEC) and its two main signatures: the slant sporadic E (diffuse E region backscatter) and the lacuna (abnormal E region radiowave damping). Our observations suggest E region absorption as a souce of weak riometer deflections, and as a possible basis for the creation of the lacuna‐damping. The slant Es diffuse backscatter, most likely created by instability irregularities, has been found identical to diffuse radar aurora at HF and probably at higher frequencies too. It is suggested that the four phenomena, Slant E Condition, diffuse radar aurora, riometer E region absorption and E region electron heating events, are different, but often simultaneous effects related to a plasma instability and to high electric field and conductivity in the polar E region. It is emphasized that the routinely observed ionogram SEC phenomenon may be useful as a diagnostic tool for the study of the various associated phenomena mentioned above.
Comparison of calculated and measured height profiles of transverse electric VLF signals across the daytime earth‐ionosphere waveguide
Authors: Field E. C. , Warber C. R. , Kossey P. A. , Lewis E. A. , Harrison R. P.
Published: February 2008

Airborne VLF antennas radiate energy that propagates via both transverse electric (TE) and transverse magnetic (TM) modes in the earth‐ionosphere waveguide. In order to compare the structure of such signals, measurements were made using rocket probes launched from Wallops Island, Virginia. The probes measured TE and TM fields at all altitudes between the ground and the base of the ionosphere. The nearly horizontal airborne transmitting antenna radiated a TE signal that was stronger than its TM signal at altitudes above about 10 km. The signals comprised one or more well‐defined TE or TM waveguide modes. Calculated height profiles agree well with the measured ones and correctly reproduce details of profile structure caused by interaction between two or more modes.
Strong scintillations during atmospheric occultations: Theoretical intensity spectra
Authors: Hinson David P.
Published: February 2008

Atmospheric dynamics, such as turbulence or buoyancy waves, modulate the quiescent refractive index and cause radio‐wave scattering during occultations of spacecraft by planetary atmospheres. The resulting rapid signal fluctuations are a common feature observed during occultation experiments, especially in the outer solar system, and provide an opportunity for remote sensing of the underlying atmospheric dynamics. However, the absence of a theory that accounts for strong scattering during atmospheric occultations has obstructed progress in this area, and has restricted data interpretation to the weak‐scattering regime. We have removed this limitation of the theory by deriving a formula for the spatial spectrum of intensity fluctuations due to strong scattering in the occultation geometry. In obtaining this new result, we generalized an existing formula for strong scattering from an irregular thin screen to include the effect of a spatially varying average refractive index in the scattering region. In the model phase screen, the refractive irregularities are anisotropic with a size distribution that follows a power law. The strength, axial ratio, orientation, and power‐law exponent which characterize the irregularities, as well as the scale height and radius of curvature of the background refractive index, are included in the theoretical expression as free parameters. Unfortunately, the final result takes the form of an intractable double integral. To obtain specific results for the theoretical intensity spectrum, we developed an accurate algorithm for numerical integration. Representative results are given, showing the dependence of the spectrum on each of the free parameters.
Solution for the fourth moment of waves propagating in random media
Authors: Codona Johanan L. , Creamer Dennis B. , Flatté Stanley M. , Frehlich R. G. , Henyey Frank S.
Published: February 2008

A series expression is developed for the fourth moment of a beamed field incident on a random phase screen or an extended medium. The series has a symmetry that allows its first few terms to generate useful approximations at both low and high spatial frequency. The parabolic wave equation, the Markov approximation, and Gaussian refractive index fluctuations are assumed. The result for the phase screen is obtained by Green's function techniques. The extended‐medium result is derived in an analogous manner by using path integral methods. The same results are also derived by moment‐equation methods. The behavior of the leading terms is compared to previous results for plane‐wave and point‐source geometries.
Simulation studies of tidal analysis using meteor echo returns
Authors: Tetenbaum D. , Avery S. K. , Bowhill S. A.
Published: February 2008

A computer simulation of the Urbana, Illinois, meteor radar is performed to estimate the accuracy of the data analysis procedures in determining upper mesospheric tides. Various wind models such as planetary waves, gravity waves, and turbulence are tested and it is found that for reasonable values of atmospheric variance the two‐dimensional standard deviation of the tidal estimator is at most 8 ms−1. The observed variability of the tides at Urbana during March 1980 exceeds the variability of a modeled steady‐state tide, and the nature of the observed variability suggests a non‐linear interaction of the tide with gravity waves and/or planetary waves. The semidiurnal tide also exhibits phase fluctuations indicating the possible presence of a transient S2,3 mode.
Vertical‐10 rocket measurements of mid‐latitude, nighttime F region irregularities
Authors: Wernik A. W. , Natorf L. , Gdalevich G. L. , Ozerov V. D.
Published: February 2008

The results of a rocket experiment carried out in the mid‐latitude, nighttime ionosphere revealed an isolated plasma density depletion between 700 and 1100 km. A substantial enhancement of intensity of irregularities coinciding with the depletion was observed over a broad band of irregularity sizes from tens of meters to several hundred kilometers. The power spectra of irregularities show a break in slope occurring at irregularity size 10–15 km. At larger sizes the spectrum is very steep with the spectral index close to 4. Smaller irregularities have a shallow slope, and the spectral index is close to 1. No correlation between the spectral index and irregularity amplitude is found. It is suggested that the plasma depletion and large‐scale irregularities are slow mode hydromagnetic waves stimulated by gravity waves in the lower atmosphere and propagating along magnetic field lines. Medium‐scale irregularities observed inside the depletion could be produced by the
gradient drift instability mechanism. Steep spectrum, however, indicates that the nonlinear evolution of the mid‐latitude, topside irregularities can be quite different from those in other regions of the ionosphere. A potential production mechanism of small‐scale irregularities could involve a high velocity stream of cold plasma from the plasmasphere to the ionosphere, as indeed observed in our experiment.
Radar cross section of an open‐ended circular waveguide: Calculation of second‐order diffraction terms
Authors: Lee C. S. , Lee S. W.
Published: February 2008

The radar cross section of an open‐ended, semi‐infinite circular waveguide is calculated by the geometrical theory of diffraction and the equivalent‐current (EC) method. We include both single‐ and double‐diffraction terms. It is found that the double‐diffraction term is stronger than the singlediffraction term for the horizontal polarization at wide‐angle incidence. Its inclusion is necessary in order to check with experimental data and the asymptotic expansion of the exact Wiener‐Hopf solution. The EC method is used for the single diffraction. On the other hand the double‐diffraction term is obtained by using the EC method for axial incidence and geometrical theory of diffraction for wideangle incidence. These two solutions are matched with a proper matching function. This technique is computationally more efficient than the matching technique of integration over the equivalent currents.
Bandwidth expansion effects on the voltage deviation parameter (Vd) of MF and HF atmospheric radio noise
Authors: Herman John R. , DeAngelis Xavier A.
Published: February 2008

Medium frequency atmospheric radio noise measured during periods in 1980 and 1981, and recorded on digital tape in 100‐kHz bandwidth has been analyzed to model effects of bandwidth on the noise envelope voltage deviation parameter (Vd). It has been found that the magnitude of this impulsiveness parameter Vd for a given noise sample increases with increasing bandwidth at a slower rate than predicted by CCIR report 322 (International Radio Consultative Committee, Geneva, 1964). Moreover, the rate (slope) is not constant, but rather becomes steeper as the initial (narrower bandwidth) Vd increases due to the impulsive character of the noise at the time of measurement. A simple model based on multiple regression analysis of the MF data allows estimation of Vd in a new bandwidth from a measured Vde using the equation Vd = Vde + (0.4679 + 0.2111 Vde) log BWR, where BWR is the ratio of the new bandwidth to the measured bandwidth. The reason for the unexpectedly small slope lies in the fact that the average envelope voltage, formerly assumed to be independent of bandwidth, increases with increasing bandwidth, but at a slower rate than the rms envelope voltage. Preliminary analysis of a few HF (4–8 MHz) noise recordings indicates a similar behavior in bandwidth effects on Vd.
On the relationship between the degree of preferred orientation in precipitation and dual‐polarization radar echo characteristics
Authors: Hendry A. , Antar Y. M. M. , McCormick G. C.
Published: February 2008

The relationship between preferred orientation of atmospheric precipitation particles and the radar echoes obtained with two‐channel polarization diversity radar systems is considered. An experimental technique in which measurements are made sequentially at circular and linear polarization for the purpose of determining the degree of preferred orientation is described. This technique, which requires rotation of the direction of linear polarization during the observations, provides a means for separating the effects of preferred orientation from the effects of particle shape distributions. It is shown that two orientation parameters are required to describe the measurements made with the differing polarization base vectors. Representative experimental results on various types of precipitation are reported. The results show that both orientation effects and shape effects are significant in remote sensing of precipitation.
Instability of the roots of the modal equation for the tropospheric waveguide
Authors: Levy Mireille F.
Published: February 2008

The waveguide modes obtained in the study of electromagnetic wave propagation in the troposphere depend spectacularly on the smoothness of the refractivity profile. The trilinear profile always yields an infinite number of nondiffractive waveguide modes, whereas holomorphic profiles only give a finite number of nondiffractive modes. If the trilinear profile is approximated by a holomorphic profile, which is always possible in the continuous case, then most of the nondiffractive modes disappear. This instability of the modes suggests using holomorphic profiles for field calculations.
The effects of ionospheric dispersion on the operation of satellite phase links
Authors: Cannon W. H.
Published: February 2008

The use of communication satellites to synchronize the phase of remotely sited oscillators has been recently demonstrated. This technique, which may find extensive applications in various areas of scientific research and engineering, will be corrupted and limited in its ultimate precision by the corrupting effects of the ionosphere. In this paper explicit formulae expressing the corrupting effects of the ionosphere on a variety of satellite phase link configurations are derived. For each type of phase link implementation studied, conditions on the frequency arrangements of the link are derived which, if satisfied exactly, will cancel the corrupting effects of ionospheric dispersion. In cases where preexisting link parameters do not permit these conditions to be satisfied exactly, they are useful as a design guide for minimization of the effects of ionospheric dispersion on the performance of the link.
Phase and frequency stability of Cassegrainian antennas
Authors: Cha A. G.
Published: February 2008

Phase and frequency stability of Cassegrainian antennas is important in radio astronomy, geodesy, and planetary sciences. This paper presents a rigorous approach, exact definitions, and simple algo‐rithms for computing these characteristics. Such a consistent and rigorous treatment of phase and frequency stability does not appear to exist in the literature.
Scintillation from extended incoherent sources
Authors: Codona Johanan L. , Frehlich R. G.
Published: February 2008

The Green's function formalism is used to derive a series solution for the correlation of the intensity fluctuations of waves propagating through a random phase screen or an extended random medium from a spatially extended, incoherent source. The satistics of the random medium are assumed to be homogeneous in the direction transverse to the propagation direction. The spatial incoherence of the source produces two terms for the intensity correlation. One is the “standard” intensity correlation obtained by summing the intensity contributions from different regions of the source. The second describes the correlation of the rapid temporal fluctuations observed by Hanbury Brown and Twiss with their intensity interferometer. The introduction of a model narrowband receiver clarifies the role of these two contributions in a typical observation. The intensity spectrum of each contribution is expressed as a high and low spatial‐frequency series. When possible, these expressions are compared with previous results.
Nonlinear sidebands of two powerful waves at closely spaced frequencies in the ionosphere
Authors: Huang Z. H. , Fejer J. A.
Published: February 2008

Observations of nonlinearly produced sidebands in the ionospherically reflected wave, using two powerful incident HF waves at closely spaced frequencies, are described. The sidebands are separated by multiples of the difference frequency of the incident waves. A theory for the production of the sidebands is proposed in terms of a modulation of the phase of the reflected wave at the difference frequency. The results of this theory are in reasonable agreement with the observations.
Scattering of electromagnetic waves by conducting periodic surfaces: A comparison of exact integral equation methods
Authors: Eftimiu C. , Huddleston P. L.
Published: February 2008

Exact, integral equation formulations of electromagnetic scattering by corrugated conducting surfaces, resulting from the extinction theorem, electric field boundary condition, and magnetic field boundary condition, are examined from the viewpoint of practical, numerical calculations.
Ionospheric wave and irregularity measurements using passive radio astronomy techniques
Authors: Erickson W. C. , Mahoney M. J. , Jacobson A. R. , Knowles S. H.
Published: February 2008

The low‐frequency radio telescope at the Clark Lake Radio Observatory (CLRO) near Borrego Springs, California, provides a unique and powerful method for studying ionospheric structure at mid‐latitudes. By observing the refractive offsets that ionospheric gradients impose upon the signals from natural radio sources, we are able to measure these gradients simultaneously at many points in the ionosphere. The telescope is a highly sophisticated system which operates in the 10‐ to 125‐MHz frequency range—a range in which ionospheric refraction completely dominates tropospheric refraction. It is sensitive enough to detect many thousands of natural radio sources and, because it is electronically steered, it can be pointed anywhere in the sky in less than a millisecond. The telescope can be programmed to rapidly cycle over a grid of sources whose astronomical coordinates are well known and can be used to determine the refractive displacements of the apparent positions of these sources. Thus the ionosphere can be “punctured” at separations of a few kilometers to a few hundreds of kilometers by the lines of sight to these sources and ionospheric gradients can be measured at each of these puncture points. Ionospheric waves are observed; their amplitude, wavelength, direction of propagation, and speed of propagation can be measured. The two‐dimensional shapes and sizes of ionospheric irregularities are determined as well as large‐scale ionospheric gradients.
Analysis and interpretation of spaced receiver scintillation data recorded at an equatorial station
Authors: Spatz D. E. , Franke S. J. , Yeh K. C.
Published: February 2008

Ultrahigh frequency satellite signals from the Pacific FleetSat (53° elevation angle) and L band signals from the Pacific Marisat (50° elevation angle) were recorded in Guam, an equatorial station, from April 1982 through May 1, 1983. Three receivers aligned in the magnetic east‐west direction were used, with the most widely separated receivers (457.2 m separation) recording the UHF signal amplitudes and the middle receiver recording the L Band signal amplitudes. These data are analyzed for the drift characteristics of ionospheric irregularities. The digitized spaced receiver data are processed to yield the scintillation index, the cross‐correlation function and the self‐power spectrum. By assuming a locally frozen flow, several methods have been devised to compute the drift velocity, V0, and the characteristic random velocity, VC. These methods seem to show internal agreement. They also show that the velocity fluctuations decrease markedly during the first few hours after local sunset. The spectral index of the self‐power spectrum roll‐off was also determined during times of both weak and strong scintillation. For the weak scintillation cases, the slopes were observed to gradually steepen as the night progressed into the postmidnight period. This steepening suggests the decaying of smaller‐sized irregularities. There were a few exceptions to this observed decay. For the strong scintillation cases, the shape and slope of the self‐power spectrum were observed to be influenced by the drift velocity and scattering strength of the irregularities.
Ionospheric distortion of HF signals
Authors: Basler Roy P. , Price Gary H. , Tsunoda Roland T. , Wong Terrence L.
Published: February 2008

The channel transfer characteristics of high‐frequency (HF) radio signals propagating through the disturbed ionosphere were measured in a series of experiments conducted in the polar cap, auroral, and equatorial regions. Transmissions over one‐hop F layer paths were observed to be distorted in the range (propagation time) and Doppler domains on the order of hundreds of microseconds and tens of hertz, respectively. A frequency‐modulated continuous wave (FMCW) waveform was used for oblique sounding purposes, and a pseudorandom noise phase modulation with a 20‐kHz bandwidth was used to measure range and Doppler spreads. The oblique sounding data were used to define a three‐layer quasi‐parabolic model of the ionospheric electron density profile, which in turn was used in the calculation of the channel scattering function to compare with the observed range and Doppler spreads.
Plasma instabilities stimulated by HF transmitters in space
Authors: Benson Robert F. , Viñas Adolfo F.
Published: February 2008

Diffuse incoherent signal returns are often observed on Alouette and ISIS topside ionograms in addition to coherent echoes of electromagnetic and electrostatic waves. These diffuse signals, which at times can be the dominant features on topside ionograms, have been attributed to sounder‐induced temperature anisotropies which drive the Harris instability. Previous theoretical investigations were based on the electrostatic approximation to the dispersion equation. The present paper will present calculations indicating that when the electromagnetic terms are retained in the dispersion equation and when the sounder‐stimulated perpendicular electron temperature approaches 1 keV, then the whistler mode can have a temporal growth rate larger than the electrostatic electron cyclotron harmonic wave mode central to the diffuse resonance problem. Present sounders lack the power and antenna lengths to generate whistler mode waves in this manner. In addition, such waves would have large group velocities and would quickly leave the vicinity of the sounder. Experiments to investigate the wave growth, propagation, and damping of such stimulated waves are planned for the 1990s using a highly flexible sounder on the space shuttle and a receiver on a subsatellite.
High‐frequency beam wave propagation in a stratified random ionosphere
Authors: Lin Jyh‐Chang , Kiang Yean‐Woei
Published: February 2008

High‐frequency (HF) beam wave propagation in a turbulent stratified ionosphere is investigated. At oblique incidence the beam wave is refracted in the ionosphere with its amplitude and phase calculated by geometrical optics approximation. The diffraction from the bottom of the ionosphere down to the ground is taken into account by solving a parabolic equation. Under appropriate assumptions, numerical results for the two‐frequency mutual coherence function and the mean field are obtained. It turns out that the random medium does have effects on the beam wave propagation. As the turbulence strength increases, the coherence bandwidth of the wave is greatly reduced. Thus the width of a pulsed wave will be broadened if it propagates in the random ionosphere.
A theory for high‐frequency electromagnetic sideband generation by two pump waves
Authors: Huang Z. H.
Published: February 2008

Sidebands on the high‐frequency (HF) waves received from two transmitted waves separated by small frequency intervals were first observed in January 1984 at the Arecibo Observatory (Ganguly and Gordon, 1986). Papadopoulos proposed that the nonlinear mechanism for the HF sideband generation is a parametric decay of a high‐frequency radio wave into a low‐frequency compressional Alfven wave and a high‐freqeuncy sideband (Papadopoulos et al., 1982). In 1984, Fejer proposed another nonlinear mechanism (phase modulation) for the HF sideband generation (Fejer et al., 1985; Huang and Fejer, 1987). A modulation index theory using the WKB approximation is proposed here, showing that the amplitude of the sidebands are related to (1) the incident HF power, (2) the HF frequency, (3) the temperature of the electrons, (4) the scale height of the ionosphere, and (5) the difference frequency of the two HF pump waves.
Auroral backscatter observed at HF from Ottawa
Authors: Montbriand L. E.
Published: February 2008

Bistatic HF radar recordings of auroral scattering sources north of Ottawa, Canada were made during 1980–1981 using the sampled aperture receiving array of the Communications Research Centre, Ottawa and swept‐frequency CW transmissions from the Rome Air Development Center facilities, Rome, New York. Doppler frequency spreads in excess of ±100 Hz (corresponding to scatter motions of 2200 m/s) were obtained at least 35% of the time. The peak of the Doppler frequency distribution was sometimes shifted from zero by as much as 50 Hz, and significant contributions often occurred at Doppler frequencies greater than 150 Hz. Signals simultaneously received on both arms of a two‐arm direction‐finding array were used to identify the specific elevations and bearings of the backscatter signals. A detailed study of a particular hour‐long period is reported. On the basis of elevations and bearings obtained from data recorded at different times (minutes apart), for different Doppler frequencies and different delay times, a large number of “apparent” auroral backscatter sources are identified. All of these sources were probably located in the F layer. The source regions were elongated mainly in the north‐south direction and extended over at least 3° of latitude. Between such sources, north‐south corridors were found in which echo returns were either absent or very weak.
Simulation of HF propagation and angle of arrival in a turbulent ionosphere
Authors: Wagen J.‐F. , Yeh K. C.
Published: February 2008

A numerical study of waves propagating in a turbulent, stratified medium, such as the ionosphere, is performed by using the phase‐screen‐diffraction‐layer method. In this method, phase fluctuations due to irregularities and diffraction effects are computed sequentially along the ray path. Stepping in altitude, phase changes are imbedded into phase screens, while diffraction between phase screens is computed using fast Fourier transform techniques. In the reflection region, a careful examination of the propagation is required. The derivation presented in this paper leads to improvements in the computational scheme and gives some physical insights. The resulting scheme is used to compute the angle of arrival and some of its statistics as an example of practical applications.
A numerical approximation to weak electromagnetic scattering
Authors: Caorsi S. , Gragnani G. L.
Published: February 2008

This paper addresses the problem of computing, by the moment method, the electromagnetic field induced inside a weakly scattering object. The main feature of the method is that starting from the well‐known Born approximation, a matrix power series expansion can be developed, so that the solution of the problem doesn't require that any linear system be solved, unlike the usual solution by the moment method. A theoretical analysis is made in order to obtain some criteria for the convergence of the power series expansion. Finally, some simulations are performed in order to assess the capabilities and limitations of the proposed method.
An analysis of the induced polarization dilution factors for a spherical anomaly
Authors: Peters Richard Alan
Published: February 2008

In the theory of induced electrical polarization, a dilution factor, B, is a measure of the sensitivity of the apparent resistivity of an object to changes in the apparent resistivities of other objects. Intuitively, one would expect B to exhibit two characteristics: When measured far away from an object, B should be small. And, when measured inside an object, away from all interfaces, B should be close to unity. Surprisingly, the second expectation is not always met. The actual behavior becomes clear upon consideration of the action of charges within the object.
Vertical ionograms and dispersive bandwidth for an oblique path
Authors: Lin K. H. , Yeh K. C. , Soicher H. , Reinisch Bodo W. , Gamache R. R.
Published: February 2008

The ionosphere is known to be a dispersive medium. Additionally, for a fixed ground distance inan oblique propagation geometry, the ray of each Fourier component of a pulse follows its own path in a phenomenon known as frequency splitting. As a consequence of these two factors distortion results. Under the approximation of a quadratic phase channel, this distortion is most conveniently characterized by a quantity, known as the dispersive bandwidth. Formulas for the dispersive bandwidth have been derived both for the linear layer and the parabolic layer and are given without showing derivations. A technique has been developed to analyze the vertical ionogram for dispersive bandwidth. This technique has been applied to several days of digital ionosonde data. The resulting values of dispersive bandwidth are reasonable and plotted over a diurnal cycle. Such a technique should be further developed as a step toward eventual application to ionospheric informatics.
Application of the bymoment method to electromagnetic scattering from multiple cylinders
Authors: Lee Robert , Cangellaris Andreas C.
Published: February 2008

We apply the bymoment method to the problem of electromagnetic scattering from multiple cylinders. The bymoment method uses Green's theorem to couple the finite element solution inside and around each of the cylinders to the exterior region. This method allows us to enclose each cylinder by its own individual mesh, resulting in an enormous computational and memory savings. Numerical results for several cases are presented and compared to series solutions available in the literature.
Fresnel‐Kirchhoff theory applied to terrain diffraction problems
Authors: Whitteker J. H.
Published: February 2008

Fresnel‐Kirchhoff theory is adapted to the problem of finding the diffraction attenuation at VHF and UHF over terrain profiles of arbitrary shape. Approximations are based on the assumptions of small wavelength and small diffraction angles. As a preliminary step, the theory is applied to the multiple‐knife‐edge problem. The field is found as a function of height above each knife edge in turn. In an application of Huygens' principle, an integration over the field above one knife edge provides the field at any point above the next. This formulation is equivalent to knife‐edge formulations used in the past. Then each pair of neighboring knife edges is bridged with an imperfectly reflecting plane surface, representing the terrain. Huygens' principle is used again for the reflected wave, neglecting backscatter. The field found in this way is accurate for a good reflector but not a poor one. An analytical comparison shows agreement with rigorous diffraction theory for the problem of a plane wave incident on a perfectly reflecting wedge. Numerical comparisons with rigorous diffraction theory for a spherical Earth and for a small‐radius hill (approximated with linear segments) show agreement to within 1.5 dB for the parameters chosen.
VHF radar observations of mesoscale motions in the troposphere: Evidence for gravity wave Doppler shifting
Authors: Vincent R. A. , Eckermann S. D.
Published: February 2008

A VHF radar at Adelaide (35°S, 138°E) has been used to study small‐scale horizontal velocity fluctuations in the troposphere. The measurements have been analyzed in the frequency domain and show that the motions are often “polarized,” such that they show a preference to be aligned along the direction of the mean background wind. This azimuthal anisotropy is especially apparent at high observed frequencies and becomes more pronounced as the wind speed increases. The cause of the polarization is analyzed in terms of both the gravity wave and vortical mode explanations of the mesoscale wind fluctuations. It is shown that the observed behavior is very consistent with the predicted effects of Doppler shifting on a spectrum of atmospheric gravity waves by non zero background winds. The results, however, are inconsistent with the predicted behavior of a spectrum of vortical modes advected past a ground‐based observer. It is concluded that internal gravity wave motions dominate the energy of the tropospheric mesoscale wind field in these observations. These findings have a number of implications for studies which use ground‐based techniques to study gravity wave wind fluctuations in the frequency domain because Doppler shifting of wave energy can produce significant biases, especially if narrow frequency bands are used. This problem and possible solutions are discussed.
Small‐scale deformation structures and physical properties related to convergence in Japan Trench slope sediments
Authors: Carson Bobb , von Huene Roland , Arthur Michael
Published: February 2008

Diatomaceous mudstones, cored on DSDP legs 56/57 off Japan, contain numerous postdepositional veins, healed fractures, and microfaults. Beneath the forearc basin, these secondary structures occur only at depths greater than 620 m (Miocene) in a zone of normal faulting. Beneath the landward slope of the trench, however, they occur at depths as shallow as 250 m (Pliocene) in hemipelagic sediments which are inferred to blanket tectonically accreted deposits. The intensity of deformation in the trench slope deposits increases in a seaward direction. Bulk densities indicate that the trench slope deposits have not dewatered normally but are, above 400 m, more consolidated than sediment deposited on the deep sea terrace. A decline in sediment bulk density with increasing depth suggests that open fractures may exist in situ, perhaps induced and/or maintained by excess pore pressure. Below about 550 m, a normal pattern of increasing density with increasing depth may reflect fracture closure and consolidation under lithostatic load. We suggest that tectonic stress related to plate convergence has been communicated to the sediment on the landward Japan Trench slope, causing tectonic dewatering within the subduction complex and inducing pervasive but small‐scale veining, fracturing, and faulting in near‐surface (≤700 m) hemipelagic deposits. This deformation occurs in sediment which has not been subject to large tectonic movement and does not eradicate larger (50–100 m thick) structural/depositional units, which are apparent on seismic reflection records. The brecciation induced by deformation may reflect natural hydrofracturing under abnormally high pore pressures or opening of fractures formed earlier (at shallower depths) during the dewatering process. The overpressures may, in turn, locally reduce the sediment shear strength at shallow levels in the sediment column and induce downslope mass sediment movement.