bluuebunny/crossref_metadata_2025 huggingface dataset:
On the spectrum of atmospheric velocity fluctuations seen by MST/ST radar and their interpretation
Authors: Gage K. S. , Nastrom G. D.
Published: February 2008

We consider the observations of the spectrum of atmospheric motions over the range of periods from a few minutes to many hours that have been made with ST/MST radars in the past 5 years. This range of periods includes the periods associated with buoyancy waves and the scale of atmospheric motions often referred to by meteorologists as the mesoscale. We consider the spectra of both horizontal and vertical velocities and examine their interpretation in terms of buoyancy wave theory and turbulence theory. To help in interpreting these spectra we present some recently determined aircraft wavenumber spectra. It is found that radar and aircraft horizontal wind spectra are in reasonable accord with expectations from quasi two‐dimensional turbulence theory. The vertical velocity spectra are believed to be due to waves. Comparison of the energy levels and shapes of the horizontal and vertical velocity spectra are not consistent with existing models of internal wave spectra. However, it is possible that these inconsistencies arise from Doppler‐shifting effects that are not taken into account in existing internal wave models. Nevertheless, we stress that all the observations we have examined support the hypothesis that quasi two‐dimensional turbulence coexists with a nearly universal spectrum of buoyancy waves.
Postset beam steering and interferometer applications of VHF radars to study winds, waves, and turbulence in the lower and middle atmosphere
Authors: Röttger J. , Ierkic H. M.
Published: February 2008

Spaced antenna VHF radar measurements are utilized for the postset beam steering technique and for interferometer applications to study winds, waves, and turbulence in the stratosphere and mesosphere. Following a brief description of the basic instrumental setup and the off‐line analysis method, some results are presented which demonstrate the consistency and applicability of these methods. Vertical velocity oscillations due to gravity waves are analyzed by means of the cross‐spectrum technique. It is shown that phase differences of gravity wave oscillations, measured between vertical and two opposite, off‐vertical postset beams, change their sign when the beam is swung back and forth. The same features of change of velocity sign are noticed when the mean wind is observed. The mean wind deduced with the postset beam steering technique is similar to the wind deduced with the spaced antenna drift technique and the independent radiosonde winds. Taking these results as justification for the applicability and feasibility of the postset beam steering method, the total wave vector and the intrinsic angular frequency of gravity waves in the stratosphere are deduced. Reasonable arguments are discussed to support the suggestion that these waves were generated in lower stratosphere wind shear regions and were attenuated little when propagating up to the middle stratosphere. Turbulence blobs in the mesosphere are tracked with the interferometer technique. It is shown that these blobs moved horizontally as well as vertically, a feature which cannot unequivocally be detected with conventional Doppler methods.
The imaging Doppler interferometer: Data analysis
Authors: Adams G. W. , Edwards D. P. , Brosnahan J. W.
Published: February 2008

Imaging Doppler interferometry has been developed as a pulsed‐radar technique for obtaining three‐dimensional images of mesospheric scattering. Here we discuss the analysis procedure and algorithms and show the results of processing synthetic data records for a variety of situations.
Extraction of sea state from HF radar sea echo: Mathematical theory and modeling
Authors: Lipa Belinda J. , Barrick Donald E.
Published: February 2008

We present mathematical methods for the interpretation of high frequency radar sea‐echo measured by narrow‐beam and CODAR systems. (CODAR is a small transportable radar with a scanning broad beam). These methods are based Barrick's equations for the ocean radar cross section in terms of the directional ocean waveheight spectrum and surface current, using a dimensionless mathematical formulation. In this paper, we describe the simulation of narrow‐ and broad‐beam radar sea‐echo from both deep and shallow water, discussing the effect of ocean surface currents, including vertical and horizontal current shear. This paper provides the mathematical tools for the modeling of common experimental situations in HF radar oceanography; such models are used in subsequent papers that describe the extraction of sea‐state information from HF radar sea‐echo data.
Theoretical considerations for time‐dependent transient scattering
Authors: Censor Dan
Published: February 2008

Transient scattering, particulary the associated pole configuration, is nowadays intensively studied for target identification purposes. At least theoretically, it is interesting to understand the effects introduced by time dependence. However, these heretofore neglected phenomena might be even of practical importance for cases where the velocities are close to the wave velocity, e.g., acoustics, or where the distances between scatterer and source are small. Although the simple Doppler effect becomes ambiguous for short pulse scattering, the velocity effects are present, manifesting themselves in virtual migration of poles and virtual creation of poles of higher multiplicity. The new ideas are explored by using the spectral approach, i.e., the wealth of studies available on Doppler effects and harmonic scattering is exploited, and by complex integration the conclusions pertinent to pulse scattering can be drawn. Inasmuch as this is a theoretical study, focusing mainly on electromagnetic waves, the exact relatlvistic formalism is used. The first‐order velocity effects, which are of more practical interest, are emphasized.
Simultaneous CW radio measurements of meteor and auroral drifts
Authors: Prikryl P. , Koehler J. A. , Sofko G. J.
Published: February 2008

A bistatic 50‐MHz CW radar was used to simultaneously observe the horizontal components of E region auroral drift motions and the underlying D region meteor trail drift motions over northern Saskatchewan (L ∼ 6.5). About 1800 meteor trail echoes were analyzed over 5 separate days, 3 of which were geomagnetically quiet and 2 of which were disturbed. For the quiet days, the observed meteor trail drifts were consistent with the normal D region motions observed at these latitudes. For the disturbed days, however, the meteor trail drifts near local midnight were much larger than normal, were predominantly southward and were well correlated with the E region auroral drifts measured at some 10 km higher altitude. This suggests that meteor trail drift motions may not be representative of the neutral medium motions at auroral latitudes during periods of geomagnetic disturbance.
The effect of small amplitude wave irregularities on radio wave observations of the ionosphere
Authors: Bennett J. A. , Dyson P. L.
Published: February 2008

Variational theory is used to derive the effects of wavelike ionospheric irregularities on group path, phase path, Doppler shift and angle of arrival of radio waves when the earth's magnetic field is neglected. General results are obtained showing the relationship between these quantities for perturbations to vertical incidence rays. In the case of irregularities with relatively large vertical wave numbers, asymptotic results are obtained for different model ionospheric layers. In the case of an exponential layer, the accuracies of the first‐order and asymptotic results have been checked and found to be very acceptable. Examples of observations of topside and bottomside irregularities which can be analyzed by using the results are discussed.
Scattering from a conducting strip with rough edges
Authors: Huddleston P. L.
Published: February 2008

Electromagnetic scattering from a perfectly conducting strip with rough edges is studied using the electric field integral equation. The serrated edges are treated as a realization of a stochastic process and are given a statistical characterization. The average induced surface current density and average monostatic scattering cross section are studied as functions of the statistical parameters characterizing the strip. The results are compared with the case of a strip with a single randomly serrated edge.
A wideband correlator employing a single‐bit digital by analogue multiplication scheme
Authors: Padin S. , Davis R. J.
Published: February 2008

The design details for a 768‐Mbit/s single bit digital by analogue correlator for a wideband interferometer system are given. Design tolerances for a high‐speed 1‐bit digitizer realized using several lower speed converters are presented, and it is shown that the arrangement may be extended to 1200 Mbits/s if a 5% loss in sensitivity can be tolerated. A variable path compensating delay using a high‐speed cyclic memory is described. The operation of a 1‐bit by analogue multiplier is considered in detail and design tolerances for this device are also presented. The measured sensitivity of the correlator, relative to that of a perfect analog system, is 0.77±0.02, which compares well with the predicted value of 0.78. The correlator has been used with the 76‐m Mk 1A and 26‐m MK II radio telescopes at Jodrell Bank to provide a high‐sensitivity, single‐baseline inerferometer.
Standing wave pattern of HF radio waves in the ionospheric reflection region: 2. Applications
Authors: Lundborg Bengt , Thidé Bo
Published: February 2008

General analytical formulas derived within a uniform approximation in a previous paper by the present authors are utilized for determining the wave pattern of a vertically propagating HF wave that is totally or partially reflected from the ionosphere. The full three‐dimensional wave is calculated accurately, also at the reflection point. Parameter values typical of ionospheric modification experiments at Arecibo and Tromsø are chosen. The influence of the geomagnetic field and collisions are properly taken into account but nonlinear and coupling effects are excluded. It is shown that the geomagnetic field affects the wave pattern strongly, especially in the Tromsø case, and leads to a very high swelling in the first wave maximum below the reflection point.
Integral equations for the first two statistical moments of the field in a high‐density medium of strongly scattering particles
Authors: de Wolf David A. , Begum Syeda R.
Published: February 2008

Integral equations are derived for the coherent field and for the field autocorrelation in the very general case of electromagnetic wave propagation in a medium of densely packed nontenuous particle scatterers. All orders of Ntuple particle correlations are included. The resulting equations are a generalization of those derived recently by Tsolakis et al. (1985); the four lowest‐order terms of each equation include those of the earlier work which only incorporate two‐particle correlations.
Scattering of a Hermite‐Gaussian beam mode by a ferrite‐coated conducting cylinder
Authors: Yokota M. , Takenaka T. , Fukumitsu O.
Published: February 2008

The scattering of a beam mode by a circular conducting cylinder coated with an axially magnetized ferrite is considered. A complex‐source‐point method introduced by Deschamps and developed by Felsen and his collaborators is used. Numerical results are presented for the scattering patterns of a lowest‐order mode by a conducting circular cylinder covered with an unmagnetized ferrite. The effects of the spot size and the location of the waist of the beam on the scattered field are discussed. A comparison between the scattering of a beam mode and that of a plane wave is presented. The scattering patterns for a magnetized ferrite‐coated conducting cylinder are also calculated and compared with those for an unmagnetized ferrite coating. Finally we calculate the scattering patterns in the resonance region of a magnetized ferrite.
Rough surface propagation and scatter: 1. General formulation and solution for periodic surfaces
Authors: Walsh J. , Srivastava S. K.
Published: February 2008

A general formulation of the electromagnetic propagation and scattering problem for rough surfaces is presented. The method is based on a decomposition of both the physical properties and the various electromagnetic field quantities associated with two media in terms of characteristic (or step) functions determined by the geometry. Maxwell equations are invoked and are interpreted in a generalized function (or distributional) sense. The results of the analysis are two coupled integral equations for the fields above and below the surface separating the media, together with a boundary condition. The boundary condition is generated as a by‐product of the analysis. The integral equations are simplified by the use of a spatial Fourier transform. It is shown that when certain assumptions are made on the refractive index and spatial spectra of the surface profile and fields, an approximate solution of one integral equation may be obtained. This last result is of the same nature as the “surface impedance” boundary condition. Using the formulation, a series solution is proposed for the surface electric field in the spatial and temporal Fourier transform domain for a good conducting two dimensional periodic surface. The choice of a finite source is kept arbitrary. As an application of the series solution in ground wave propagation and scattering we have then considered the source to be an elementary vertical pulsed electric dipole. It is assumed that the surface slopes are small compared to unity. For this source, the zero‐, first‐ and second‐order approximations of the vertical component of the surface field are asymptotically transformed back into coordinate space. These solutions are in the form of ground waves with modified surface impedances.
Effects of nonspherical statistics on EM wave propagation in discrete random media
Authors: Varadan V. V. , Varadan V. K. , Ma Y. , Steele W. A.
Published: February 2008

Earlier results for electromagnetic wave propagation in discrete random media assumed spherical statistics for describing the spatial distribution of even nonspherical scatterers. The appropriate pair correlation function for nonspherical scatterers can, in general, be obtained by the Monte Carlo method which is essential in analyzing nonspherical statistics. This paper presents new results using nonspherical statistics in the investigation of multiple electromagnetic wave scattering by aligned dielectric prolate as well as oblate spheroids randomly distributed in space. Comparison between previous results using spherical statistics and present calculations show that approximating the spatial distribution of nonspherical scatterers using spherical statistics will yield effective medium characteristics that differ quite widely. Of all approximations using spherical statistics for non‐spherical scatterers that using an equal volume sphere appears to be the best if the actual statistics are not available.
Numerical errors in the real‐height analysis of ionograms at high latitudes
Authors: Titheridge J. E.
Published: February 2008

The accuracy of real heights in the ionosphere, calculated from sweep‐frequency virtual height records, is limited by numerical errors at dip angles greater than about 65° (corresponding to magnetic latitudes greater than 49°). For high‐order modes of analysis, using five‐point Gaussian integration, errors increase by a factor of about 10 at dip 70°, a factor of 100 at 80° and a factor of 500 at 86°. Allowance for the effect of low‐density ionization, or of ionization in the valley between two layers, requires the use of combined ordinary and extraordinary ray data. These calculations become unusable at dip angles greater than 80° with normal integration procedures. A simple dual‐range integration procedure is developed for use at high latitudes. This maintains accuracy in the analysis of ordinary ray data up to a dip angle of 89°. For the start and valley calculations, numerical errors are effectively zero at all dip angles up to 89.9°. Alternatively, the extreme errors which occur at high latitudes can be avoided by analyzing the ionograms using a decreased value for the dip angle. An expression is derived giving the optimum dip angle to use with different integration orders and frequency intervals.
A spectral view of wave propagation
Authors: Felsen L. B.
Published: February 2008

Wave propagation in environments characterized by weakly nonseparable conditions can be described by adiabatic modification of spectra pertaining to the unperturbed separable case. Recent developments are reviewed here, with emphasis on approximate adiabatic transform relations and adiabatic invariants, which permit reduction of the direct weakly nonseparable wave problem to spectral form and, conversely, reconstruction of wave spectra from measured data.
Comparison between measured and calculated contributions of multiply scattered radiation to the transmittance of a light beam through a turbid medium
Authors: Bruscaglioni P. , Zaccanti G. , Ismaelli A. , Pili P.
Published: February 2008

Results are presented of measurements pertaining to multiple scattering effects on the propagation of a HeNe laser beam through suspensions of latex spheres in water. Both spheres with a large radius in comparison with the radiation wavelength and spheres with a small radius were used. The data are compared with numerical results obtained by means of a Monte Carlo method employing scaling relations. The effect of varying the distance between the diffusing medium and the receiver was examined.
Whistler wave propagation in plasma ducts
Authors: Karpman V. I. , Kaufman R. N.
Published: February 2008

Analytical theory of whistler wave propagation in axially symmetric ducts with plasma enhancements as well as rarefactions, directed along the constant external magnetic field B, is given. The consideration is based on Maxwell equations in WKB‐approximation with the account of the leakage caused by the tunnel transformation of one branch of the perpendicular wave number into another. The conditions of waveguide propagation and the equations for eigenmodes are given. The equations for maximum number of trapped modes are obtained.
Singular system analysis for temperature retrieval in microwave thermography
Authors: Bardati F. , Bertero M. , Mongiardo M. , Solimini D.
Published: February 2008

This paper refers to the inversion of microwave radiometric data for retrieving the temperature distribution in the human body. The singular system of the radiative integral operator furnishes an appropriate basis, provided some a priori information is included. The relevant features of the technique are discussed and numerical simulations of the retrievals are reported.
A new free‐wave method for ferrite measurement at millimeter wavelengths
Authors: Cullen Alex L.
Published: February 2008

It is shown that by measurement of the complex Fresnel reflection coefficients of a thick slab of ferrite for both parallel and perpendicular polarization, the complex permittivity and permeability of the ferrite can be found. In case the reflection coefficients are too small to be measured accurately, it is also shown that a slab of arbitrary but known thickness, backed by a metal plate, may be used. The necessary formulas are given for both cases. They are simple and explicit and do not involve the solution of transcendental equations, as might have been expected in the case of the second method proposed.
On the roles of physical intuition, computation, and mathematical analysis in electromagnetic diffraction theory
Authors: Petit R. , Cadilhac M.
Published: February 2008

Some difficulties frequently encountered when dealing with the theory of electromagnetic waves are illustrated with the help of examples taken from the work of our laboratory. We also lay stress on some recent results and the appendix contains two propositions concerning the completeness of some sequences of functions often used to expand the electromagnetic fields.
Sheet simulation of a thin dielectric layer
Authors: Senior Thomas B. A. , Volakis John L.
Published: February 2008

To facilitate the computation of the field scattered by a thin non‐magnetic dielectric layer, it is customary to model the layer as an infinitesimally thin resitive sheet, but the simulation becomes increasingly inaccurate at oblique angles of incidence when the electric vector has a component normal to the layer. By starting with a volume integral formulation of the scattered field, it is shown that the accuracy is greatly improved when a “modified” conductive sheet is included in addition to the resistive one. The boundary conditions for the new sheet differ from those of a standard conductive sheet by the presence of a second normal derivative, and the combination of two coincident sheets yields results for a thin layer that are virtually indistinguishable from those provided by a volume integral equation. The advantages of this type of simulation are discussed, and the extension to a layer of arbitrary shape and composition is described.
Basic features of the null field method for dielectric scatterers
Authors: Ström Staffan , Zheng Wenxin
Published: February 2008

The null field approach to electromagnetic scattering from a dielectric body is presented in its general form, i.e., for a general choice of surface field expansions. Vector spherical harmonics are used to construct tangential fields which are shown to be complete for star‐shaped surfaces. The numerical performance of these sets is compared to that obtained when expansions in terms of different kinds of spherical vector waves are used. A demonstration of the uniqueness of the solutions to the null field equations for a permeable body is included in an appendix.
Propagation over an inhomogenous irregular surface
Authors: Wu Z. , Maclean T. S. M. , Bagwell D. J. , Mehler M. J.
Published: February 2008

A new integral equation has been obtained for the attenuation factor of a radio wave launched by an elevated element, as the wave propagates over a radially inhomogeneous surface of varying height, with an elevated current element as a receiver. In the analysis use is made of the vector form of compensation theorem, using the concept of a real surface impedance on the irregular surface together with that of an equivalent normalized surface impedance in air, at points which follows the contour of the irregular surface. The new integral equation generalises previous work to treat arbitrary terrain, so that both gradual and sharp changes of terrain height, such as a cliff and a knife edge are considered. The validity of the integral equation has been tested theoretically by comparison with alternative solutions for particular cases, and also by experiment.
A systematic error in MST/ST radar wind measurement induced by a finite range volume effect: 2. Numerical considerations
Authors: Fukao Shoichiro , Inaba Motoyuki , Kimura Iwane , May Peter T. , Sato Toru , Tsuda Toshitaka , Kato Susumu
Published: February 2008

A finite range volume effect causes a systematic error in MST/ST radar wind measurement when a thin turbulent layer is simultaneously located in several adjacent range volumes. This error appears as a false vertical shear of horizontal wind or as a false temporal wind variation at some ranges even if the ambient wind field is uniform with height and does not change at all. Also, because of this effect the observed Doppler power spectrum becomes asymmetric, and a noticeable error is induced in estimation of echo power, mean Doppler velocity, and spectral width. The present investigation will show that these errors are well explained by a simple numerical model which is made to simulate an actual observational situation. The observed wind velocity is more reliable at ranges where the echo intensity is relatively larger compared with adjacent ranges. The finite range volume effect is negligibly small for weak ambient wind velocities less than approximately 10 ms−1 and/or for an antenna beam width less than one degree.
Oblique transmission of electromagnetic waves through a magnetized ferrite slab
Authors: Mueller R. S.
Published: February 2008

New transmission properties of a magnetized ferrite slab are investigated theoretically for an oblique incident H plane wave. When the direction of magnetization is normal to the plane of incidence, there is single refraction. For magnetization parallel to the plane of incidence, then double refraction leads to a double beam of microwaves at the exit surface. If the electromagnetic field vectors of these two beams can be made orthogonal then after the beams exit the ferrite slab they can be independently modulated and subsequently analyzed and demodulated at a remote receiver. It was found that the two transmitted beams are contrarotating and elliptically polarized. The required interrelationship of the transmission coefficients was determined for cross polarization. The analysis not only explains the double refraction effect but also defines new transmission coefficients that can be used to evaluate the components of the transmitted waves.
Study of ionospheric models for satellite orbit determination
Authors: Bilitza D. , Rawer K. , Pallaschke S.
Published: February 2008

A study of empirical ionospheric models was undertaken to improve the accuracy of the ionospheric correction that is needed in satellite orbit determination. Our evaluation concerns the two most widely used empirical ionospheric models (Bent and international reference ionosphere (IRI)) and suggests improvements based on comparisons with satellite and ground‐based data. In both models, the topside electron density profiles decrease too sharply with increasing altitude. Our improved IRI topside formula compares well with AEROS satellite data and incoherent scatter measurements of the Jicamarca, Peru, radar facility. For the bottomside, a model for the effective parabolic thickness is introduced, based on the detailed IRI predictions. Recent progress in modeling the F2 peak altitude and electron density is reviewed. The increase in prediction accuracy compared to older F2 models is evaluated with Ionosphere Sounding Satellite b results (for density) and incoherent scatter measurements from Millstone Hill, Massachusetts (for altitude). The plasmaspheric electron content is also taken into account. Finally, results of a METEOSAT tracking exercise are used to check the ionospheric delays calculated with the different models. The changes indicated above were found to be helpful and resulted in an overall improvement.
Efficient ray tracing through a realistic ionosphere
Authors: Reilly Michael H. , Strobel Eric L.
Published: February 2008

An algorithm for accurate three‐dimensional ray tracing is presented for realistic, nonspherical ionospheric electron density distributions, ignoring effects of the magnetic field. At the start of each ray path increment a truncated Taylor series expansion of the electron density results in a computationally efficient, closed‐form solution of the Euler‐Lagrange equations for the increment. The technique is applied to computations of ray paths in a nonspherical ionosphere which is specified by a climatological model used for HF over‐the‐horizon (OTH) radar studies. The effects of tilts in a sunrise transition region on signal losses and ray path deviations are found to be substantial. Results of various ionospheric approximations used for single‐site transmitter location are calculated.
E‐polarized scattering by a resistive half plane with linearly varying resistivity
Authors: Yang Seung‐In , Ra Jung‐Woong , Senior Thomas B. A.
Published: February 2008

The Kontorovich‐Lebedev transform is used to obtain an exact solution for the scattering from a linearly tapered resistive half plane for E polarized plane wave illumination. Uniform and nonuniform asymptotic results for the scattered field are obtained, which provides the ray optical interpretation of the calculated scattered field including the edge diffraction pattern for various values of resistivity gradient. The edge diffraction approaches that of the conducting half plane as the gradient approaches zero and that of an isolated wire as the gradient approaches infinity. Calculated interference fringes of the scattered field in the illuminating region for the varying resistive half plane differ significantly from those of either the conducting or uniform resistive half plane.
High‐frequency propagation at long ranges near a concave boundary
Authors: Ishihara T. , Felsen L. B.
Published: February 2008

High‐frequency propagation along and near a concave boundary excited by a source close to the boundary exhibits anomalous effects attributable to the failure of ray theory for the high‐order multiply reflected fields. For moderate values of the universal parameter γ = (ka/2)1/3s/a, where k is the wave number, a is the boundary radius and s is the propagation distance measured along the boundary, suitable alternative field representations can be established by hybrid forms that combine legitimate ray fields self‐consistently with whispering gallery (WG) mode fields and/or integral remainders. These previously established formulations become problematic where γ gets to be large, as happens for long propagation distances or very weak surface curvature. Modified hybrid options are developed, which can cover all phenomena near and far from the boundary, for small through large γ. Of special interest is a new ray‐parabolic equation (PE) scheme wherein the WG modes in the boundary layer near the surface are replaced by a beam‐type propagator. PE can be implemented numerically by a forward marching algorithm and can be adapted to variable curvature boundaries, including concave‐convex shapes for which the other options fall. Extensive numerical comparisons reveal the validity, utility and limitations of the various alternative field representations.
Finite length helical sheath antenna in a general homogeneous medium
Authors: Casey John P. , Bansal Rajeev
Published: February 2008

The sheath helix is an anisotropically conducting cylindrical tube that represents a good approximation to an actual helix consisting of many turns per wavelength. In this paper an integral equation for the current distribution along a finite length, axisymmetrically driven helical sheath antenna in a general homogeneous medium is solved by the method of moments to obtain the current distribution and radiation pattern of the antenna. The characteristics of various helical sheath antennas in both dissipative and nondissipative media are presented. Comparisons are made of the current distributions along multifilar thin‐wire helical antennas and helical sheath antennas in free space.
An induction gradiometer for conductivity contrast detection: Theoretical considerations
Authors: Lovell John R. , Habashy Tarek M.
Published: February 2008

We present a full wave theoretical model for an induction gradiometric sensor employed for detection of conductivity contrast. The sensor is composed of a wire antenna radiating from a rectangular waveguide, with receivers in the waveguide positioned symmetrically on either side of the transmitter. We first solve the canonical problem of an open‐ended rectangular waveguide in a planar stratified medium and then apply reciprocity and the Born approximation to deduce the response of the sensor in the presence of an arbitrary external medium. We compare our results against experimentally measured data and find excellent agreement. We present a discussion of these results together with an analysis of the numerical considerations involved in the computation.
Spatial interferometer measurements with the Chung‐Li VHF radar
Authors: Röttger J. , Liu C. H. , Chao J. K. , Chen A. J. , Pan C. J. , Fu I.‐J.
Published: February 2008

The Chung‐Li VHF radar consists of three phase‐coherent transmitting‐receiving antenna systems, which allow an optimum application of interferometer and incidence angle measurements of radar echoes from the troposphere and the stratosphere. We briefly describe the applications of the spatial domain interferometry and present here the first measurements of cross spectra and the corresponding phases, which vary as function of frequency due to the mean wind. We also present the first incidence angle measurements and show that the scattering‐reflecting structures in the troposphere can be inclined to the horizontal by a tilt or inclination of l°–2°. We also notice a periodic change of this inclination as well as the radial velocity measured with vertical antenna beam as a function of altitude and time which we attribute to long‐period waves or mountain waves. There are clear indications that the radial velocity measured with the vertical beam is diluted by the projection of the horizontal wind in the presence of inclined or tilted reflectivity structures. We also describe observations of long‐period waves seen in the horizontal wind pattern, which could be of the same origin as the quasi‐periodic pattern observed in the inclination and the reflectivity.
Pulse compression and frequency domain interferometry with a frequency‐hopped MST radar
Authors: Franke Steven J.
Published: February 2008

The use of a coherent frequency‐hopped radar for pulse compression and frequency domain interferometry is considered. An analytical model for the two‐frequency correlation at the output from a matched filter receiver is developed that includes the effects of finite beam width. The analysis is restricted to the case of vertically directed radar beam. This result is used to derive the effective height weighting function that can be obtained by appropriate superposition of the receiver output from multiple frequencies. The result is also used to study the normalized correlation, or coherence, which is used in the frequency domain interferometry technique. The influence of finite beam width on the altitude resolution and coherence function is illustrated, and practical applications and limitations of the technique are discussed.
A poststatistics steering technique for MST radar applications
Authors: Kudeki Erhan , Woodman Ronald F.
Published: February 2008

A new interferometric data analysis procedure equivalent to software beam synthesis and scanning is introduced. It is shown that the self‐ and cross‐spectra of interferometric multiple‐receiver signals can be linearly combined to obtain the self‐ and cross‐spectra of radar returns from selected viewing directions within the two‐way radar beam pattern.
Application of computerized tomography to the investigation of ionospheric structures
Authors: Raymund T. D. , Austen J. R. , Franke S. J. , Liu C. H. , Klobuchar J. A. , Stalker J.
Published: February 2008

Ionospheric total electron content (TEC) measurements, obtained simultaneously at several locations, can be processed using computerized tomography (CT) algorithms to obtain two‐dimensional images of ionospheric electron density. Using TEC data, computerized ionospheric tomography (CIT) reconstructs an image of the electron density structures in a vertical slice above the receiving stations. We successfully applied this technique to realistic simulations of ionospheric density variations over 16° of latitude and a height range of 50 to 1000 km. A method for approximating the peak height and scale height of the electron density profile will be discussed as well as a reconstruction technique based on the multiplicative algebraic reconstruction technique algorithm and a back projection based initial guess. The quality of reconstructions is considered for two geometries and image resolutions. In particular, the image of a mid‐latitude trough with background horizontal density gradient and large‐scale irregular structures has been reconstructed from TEC data generated from a model based on an incoherent scatter radar observation. The CT reconstructed image was compared with the original contour map obtained by the incoherent scatter radar. Good agreement has been achieved. The CIT technique has also been applied to a modeled ionosphere to calculate the range rate corrections for a Doppler‐tracking radar.
Refraction of 50‐MHz radar waves in a realistic ionospheric model
Authors: Watermann Jurgen
Published: February 2008

The role of ionospheric refraction in observing coherent radar backscatter from the auroral E region has recently come into the discussion again when it was proposed that for 50 MHz radars refraction efficiently brings the radar beams to virtually perpendicular intersection with the geomagnetic field at the scatter location and thus explains a number of otherwise puzzling results. However, without a thorough calculation of the actual refraction under the given ionospheric conditions its effect cannot be assessed accurately since it is not simply the maximum electron density and layer tilt but rather the product of density gradient and beam propagation distance measured along the contours of constant density which determines the refraction. In order to study typical refraction features a ray tracing computer program was applied to an ionospheric model consisting of an auroral arc with high electron density imbedded in a horizontally stratified ionosphere. It turns out that, when adapted to the Canadian BARS Red Lake radar, a sufficiently enhanced electron density within the arc can bring the radar beam to virtually perpendicular intersection with the geomagnetic field at ranges exceeding the distance from the radar to the centre of the arc by about 100 to 150 km.
A polarimetric radar calibration technique with insensitivity to target orientation
Authors: Whitt M. W. , Ulaby F. T.
Published: February 2008

A convenient polarimetric radar calibration technique is presented and used to calibrate an X band polarimetric radar. The technique uses a distortion matrix model for the errors introduced by the transmitter and receiver, and it is applicable to any polarimetric radar system where the concept of an invariant distortion matrix is valid. A sphere and two arbitrary passive targets are used to calibrate the radar with respect to the unknown polarization transmitted when the v‐polarized channel is energized. The unknown transmitted polarization is then recovered by measuring any nondepolarizing target. Because knowledge of the scattering matrices for the two arbitrary targets is not required, the technique is insensitive to errors in the orientation of calibration targets. Experimental results are presented, and they indicate magnitude and relative phase measurement errors of less than 0.2 dB and 2°, respectively.
Ionogram inversion for a tilted ionosphere
Authors: Wright J. W.
Published: February 2008

Digital ionosondes such as the Dynasonde disclose that the ionosphere is seldom horizontal even when it is plane stratified to a good approximation. The local magnetic dip does not then determine correctly the radiowave propagation angle for inversion of the ionogram to a plasma density profile. The measured echo direction of arrival can be used together with the known dip for an improved propagation angle. The effects are small for simple one‐parameter laminae but become important when differential (ordinary, extraordinary) retardations are used to aid correction for “valley” and “starting” ambiguities. The resulting profile describes the plasma distribution along the direction of observation, rather than the vertical; it thus conveys information about horizontal gradients. Observations suggest that advances in inversion methods may be practicable for application to modern ionosonde recordings, by which local lateral structure can be described in greater detail.
Computer simulation of ionospheric radio frequency heating
Authors: Close R. A. , Bauer B. S. , Wong A. Y. , Langdon A. B. , Kruer W. L. , Mjølhus E.
Published: February 2008

Linear and nonlinear absorption of electromagnetic waves in a warm, magnetized, nonuniform plasma is studied using an electromagnetic particle simulation code (ZOHAR) with either one or two spatial and three velocity dimensions. With fixed ions, finite temperature, and finite wave amplitude, transmission of the ordinary mode to the slow extraordinary wave (Z mode) is found to agree with cold plasma theory for linear conversion, and interference of mode converted waves is observed. With mobile ions and a parabolic density profile which peaks at the critical layer (ωp = ω), the wave electric field is enhanced (factor of 10 or more) near the critical layer where the wave becomes electrostatic, and small‐scale density fluctuations (cavitons) are observed (δl ≈ 10λD; δn/n ≈ 0.1), typical of strong Langmuir turbulence. Wave damping and particle acceleration are also observed. Application to ionospheric heating is discussed.
Pressure solution lithification as a mechanism for the stick‐slip behavior of faults
Authors: Angevine C. L. , Turcotte D. L. , Furnish M. D.
Published: February 2008

Many major faults, including a large fraction of the San Andreas, appear to be virtually quiescent between great earthquakes. The locked sections of the San Andreas near San Francisco and Los Angeles have little or no seismic activity on the primary fault trace, although secondary faults may be active. Stick‐slip behavior on a fault can be explained in terms of a static coefficient of friction, which is larger than the dynamic or sliding coefficient. In this paper we propose a modification of the friction hypothesis in which chemical lithification (cementation) occurs on the fault between earthquakes. As the stress on the fault increases it becomes large enough to break the cemented bonds between particles causing slip on the fault. We treat this problem quantitatively, assuming that pressure solution is responsible for the cementation. An advantage of this model is that the hydrostatic pressure approaches the lithostatic pressure during cementation so that low fault strengths are predicted.
Subduction of the Caribbean Plate and basement uplifts in the overriding South American Plate
Authors: Kellogg J. N. , Bonini W. E.
Published: February 2008

The new tectonic interpretations presented in this paper are based on geologic field mapping and gravity data supplemented by well logs, seismic profiles, and radiometric and earthquake data. The present Caribbean‐South American plate boundary is the South Caribbean marginal fault, where subduction is indicated by folding and thrusting in the deformed belt and a seismic zone that dips 30° to the southeast and terminates 200 km below the Maracaibo Basin. The Caribbean‐South American convergence rate is estimated as 1.9 ± 0.3 cm/yr on the basis of the 390‐km length of the seismic zone and a thermal equilibration time of 10 m.y. The Caribbean‐South American convergence has produced a northwest‐southeast maximum principal stress direction σ1 in the overriding South American plate. The mean σ1 direction for the Maracaibo‐Santa Marta block is 310° ± 10° based on earthquake focal mechanism determinations, and structural and gravity data. On the overriding South American plate, basement blocks have been uplifted 7–12 km in the last 10 m.y. to form the Venezuelan Andes, Sierra de Perija, and the Colombian Santa Marta massif. Crystalline basement of the Venezuelan Andes has been thrust to the northwest over Tertiary sediments on a fault dipping about 25° and extending to the mantle. In the Sierra de Perija, Mesozoic sediments have been thrust 16–26 km to the northwest over Tertiary sandstones along the Cerrejon fault. A thrust fault dipping 15° ± 10° to the southeast is consistent with field mapping, and gravity and density data. The Santa Marta massif has been uplifted 12 km in the last 10 m.y. by northwest thrusting over sediments. The basement block overthrusts of the Perijas, Venezuelan Andes, and the Santa Marta massif are Pliocene‐Pleistocene analogs for Laramide orogenic structures in the middle and southern Rocky Mountains of the United States. The nonmagmatic basement block uplifts along low‐angle thrust faults reveal horizontal compression in the overriding plate over 500 km from the convergent margin. Present‐day east northeast‐west southwest (080°) compression is indicated by earthquake focal mechanisms and strike slip motion on the Bocono fault. These earthquakes are intraplate deformation associated with east‐west (080°) Nazca‐South American convergence.
Paleomagnetism of the Paleocene Ghost Rocks Formation, Prince William Terrane, Alaska
Authors: Plumley Peter W. , Coe Robert S. , Byrne Tim
Published: February 2008

The mean paleomagnetic inclination of Paleocene volcanics from two regions on Kodiak Island in the Ghost Rocks Formation suggests that the Prince William terrane originated at mid latitudes, about 25° south of its ‘expected’ Alaskan latitude in Paleocene time. The remanent magnetization of these rocks passes both the fold and reversal tests and is well constrained in age. The mean declinations of the two regions, however, differ by approximately 120°, suggesting they have rotated with respect to each other, perhaps during emplacement. We suggest that, despite the lack of evidence for a major Tertiary suture zone between the Prince William terrane and central Alaska, the Prince William and perhaps adjacent terranes may have lain substantially south of their present position in the Early Tertiary.
Mesozoic aseismic ridges on the Farallon Plate and southward migration of shallow subduction during the Laramide Orogeny
Authors: Henderson Laurel J. , Gordon Richard G. , Engebretson David C.
Published: February 2008

We propose that an aseismic ridge of Late Jurassic and Early Cretaceous age on the Farallon plate was subducted beneath the Unites States Cordillera during the Laramide orogeny. The relative buoyancy of the aseismic ridge caused shallow subduction which resulted in a magmatic lull 70–40 Ma in the existing near‐trench magmatic arc and Laramide uplift and faulting of crystalline basement 1000–1500 km inland from the trench. The timing and latitudinal limits of the subduction of the aseismic ridge are similar to the timing and latitudinal limits of both the Laramide orogeny and the magmatic lull in the western Cordillera. Because the north–northeast trending aseismic ridge was moving northeast relative to the North American plate, the point of collision of the aseismic ridge with North America, and therefore the locus of shallow subduction, migrated southward with time, causing the well‐documented southward migration of the magmatic lull.
Post‐12 million year clockwise rotation in the Central Columbia Plateau, Washington
Authors: Reidel Stephen P. , Scott Gary R. , Bazard David R. , Cross Randal W. , Dick Brian
Published: February 2008

The Pomona flow (12 m.y. B.P,) of the Columbia River Basalt Group was sampled for paleomagnetic analysis at 32 sites in the Yakima Fold Belt of the central Columbia Plateau to assess the pattern of deformation. Field directions from anticlinal ridges are found to be rotated clockwise relative to sites from synclinal areas. Two geographic patterns of rotation are present. The primary pattern is one in which the greatest amount of rotation occurs in the crest‐hinge area of the anticlines and decreases into synclines. A secondary pattern, in which the amount of rotation is consistent within a geometric segment but varies among them, is superimposed on the primary pattern. The pattern of clockwise rotation is interpreted to be the result of local tectonic rotation occurring along a closely spaced north‐northwest trending primary shear system and northeast trending secondary shear system. The shear system and concurrent rotation are interpreted to have developed during the growth of the Yakima folds. The relationship between the paleomagnetic data and the geologic structures can be used to constrain tectonic models for the Columbia Plateau. The data reported in this study indicate that clockwise rotation can not be attributed to rigid block or microplate rotation, but support models that predict localized dextral shear. Furthermore, it is postulated that this mechanism may contribute to clockwise rotation observed in other parts of the western United States.