bluuebunny/crossref_metadata_2025 huggingface dataset:
Reflection of VLF radio waves from the Canadian Rockies
Authors: Barr R. , Armstrong T. R.
Published: February 2004

Wideband (2–53 kHz) VLF recordings have been made on an aircraft flight from Honolulu, Hawaii, to St John's, Newfoundland, via Comox on Vancouver Island. Clear periodic variations have been observed in both the amplitude and phase of the VLF signals from both navigation and communications VLF transmitters, and these oscillations reached maximum amplitude in the region above the Coast and Rocky Mountains. We suggest these oscillations were produced by the reflection of the VLF signals from the mountains. The oscillations were seen on signals typical of single‐mode propagation and on those undergoing significant modal interference. We measured reflection coefficients of up to 18% at the Omega navigational frequencies and 43% at the frequency of the NPM transmitter (23.4 kHz). The effect of the reflections at the Omega frequencies was seen more than 3000 km before the mountains, but the effect of reflections at the higher VLF frequencies, although greater in magnitude, was much more localized to the immediate region of the mountains.
A general solution of the second‐order difference equation occurring in diffraction theory
Authors: Demetrescu C.
Published: February 2004

The second‐order difference equation, frequently encountered in diffraction of electromagnetic waves by a wedge‐shaped region, is studied, and the solution is given for an arbitrarily angled wedge. The solution is suitable for the Sommerfeld‐Maliuzhinets representation of the diffracted field. It is shown that the diffracted field from an arbitrarily angled wedge with anisotropic impedance boundary conditions on its faces can be calculated when the electromagnetic plane wave is obliquely incident. The solution can also be used to find the scattered field when the scatterer is penetrable, for example, the hollow dielectric wedge. By this solution, the matrix Riemann problem for analytic vectorial functions is solved, and the solution is presented in closed form.
Relationship of theoretical patch climatology to polar cap patch observations
Authors: Bowline M. D. , Sojka J. J. , Schunk R. W.
Published: February 2004

During a southward orientation of the interplanetary magnetic field (IMF), patches are often observed moving antisunward across the polar cap. In saying “patches” we refer to structures in which the F region electron densities are enhanced relative to lower background levels; we do not in this paper consider patches which are observed optically (see J. J. Sojka et al., Ambiguity in identificiation of polar cap F region patches, submitted to the Journal of Atmospheric and Terrestrial Physics, 1995). The patches can be modeled by a process which involves the “chopping up” of the tongue of ionization (TOI) [Sojka et al., 1993a]. Various mechanisms for chopping the TOI have been suggested; our preferred method is to introduce temporal changes in the convection electric field pattern. In any case the present study is quite independent of any particular mechanism, so long as the TOI is considered to be the source of the patches. In this study we have used the Utah State University Time‐Dependent Ionospheric Model (TDIM) to model the TOI for various IMF By orientations. In our simulations the location of the TOI in the polar cap is mainly determined by the IMF By component, and hence the patch locations are also expected to be By dependent. This suggests that a polar ground‐based instrument may not see patches even when they are present in the polar ionosphere. This is because the typical field of view of a ground‐based instrument, such as an all‐sky camera, covers less than 10% of the polar region. The TDIM simulation results were used to predict the By dependence of patches that different ground‐based sites would observe. Eureka (Canada) at the magnetic pole is predicted not to observe patches for southward IMF conditions if the By component is strongly negative. Sondrestrom (Greenland) and NyAlesund (Svalbard), although at similar cusp latitudes, are expected to see quite different diurnal responses to patches. At Sondrestrom, patches are seen at noon in winter; both sites should see them in the premidnight sector. These model predictions are the “groundwork” for detailed patch observation‐model comparisons at all three sites.
Periodic structure analysis using a hybrid finite element method
Authors: McGrath Daniel T. , Pyati Vittal P.
Published: February 2004

A numerical solution for the time harmonic electromagnetic fields in a unit cell of an infinite, planar periodic structure has been developed and validated. It uses a variation of the hybrid finite element method that includes periodic radiation conditions at the exterior surfaces and periodicity conditions at the unit cell walls inside the structure. This technique allows the method to deal with structures that include inhomogeneous dielectrics and conductors with arbitrary shape and orientation. This paper presents the formulation of the finite element problem and shows the results of test cases involving an inductive screen, an artificial dielectric, and a layered‐dielectric bandgap medium. The results show that the new method is accurate and versatile.
Backscatter gain of aperture antennas
Authors: Hagen Jon B. , Baumgartner Hans A.
Published: February 2004

It is well known that the conventional on‐axis gain of an aperture‐type antenna is maximized when the aperture illumination pattern has uniform amplitude and phase. For backscatter radar measurements, however, the distributed target brings the off‐axis gain into play, and the sensitivity is proportional to the square of the gain averaged over the solid angle of the beam. It has been assumed that this backscatter gain, like on‐axis gain, is maximized when the illumination is uniform. We show that this is not quite the case; for a fixed aperture area the backscatter gain can be increased at least 6% by using the kind of amplitude taper that reduces sidelobe levels. It is also well known that the maximum on‐axis gain is proportional to the area of the aperture but independent of the shape. We show that this is not the case for backscatter gain.
Calculation of radiation characteristics of cavity‐backed aperture antennas using a combined finite element method, method of moments, and geometrical theory of diffraction technique
Authors: Reddy C. J. , Deshpande M. D. , Fralick D. T. , Cockrell C. R. , Beck F. B.
Published: February 2004

Radiation pattern prediction analysis of elliptically polarized cavity‐backed aperture antennas in a finite ground plane is carried out using a combination of the finite element method (FEM), method of moments (MoM), and geometrical theory of diffraction (GTD). The magnetic current on the cavity‐backed aperture in an infinite ground plane is calculated using the combined FEM‐MoM analysis. GTD is used to calculate the diffracted fields due to both soft and hard polarizations at the edges of the finite ground plane. The slope of the incident magnetic field at the diffraction points is derived and analytical expressions are presented. Numerical results for the radiation patterns of a cavity‐backed coaxial cavity and also a cavity‐backed circular spiral microstrip patch antenna excited by a coaxial probe in a finite rectangular ground plane are computed and compared with measured results.
Fast steepest descent path algorithm for analyzing scattering from two‐dimensional objects
Authors: Michielssen E. , Chew W. C.
Published: February 2004

A novel algorithm for accelerating the iterative solution of integral equations governing scattering from surfaces is presented. The proposed fast steepest descent path algorithm (FASDPA) complements previously developed fast solvers, notably the fast multipole method (FMM) and the matrix decomposition algorithm (MDA). Whereas the computational complexity per iteration of a two‐level FMM and MDA is O(N3/2), the FASDPA permits a matrix‐vector multiplication in O(N4/3) operations.
Finite array scan impedance Gibbsian models
Authors: Hansen R. C.
Published: February 2004

Oscillations in scan impedance over a finite array have been shown previously; here they are modeled by a Gibbs‐type standing wave. The wave period derived from the transform of a single pulse with phase shift is .5λ/(1 − sin θ0), which closely fits simulated array data. Gibbsian models containing sine integrals provide fair to excellent matches to computer‐simulated dipole arrays, with and without screen, and for E and H plane scans. The E plane scan results are new, as are those for the H plane scan with screen.
Feedback neural networks for ARTIST ionogram processing
Authors: Galkin Ivan A. , Reinisch Bodo W. , Ososkov Gennadii A. , Zaznobina Elena G. , Neshyba Steven P.
Published: February 2004

Modern pattern recognition techniques are applied to achieve high quality automatic processing of Digisonde ionograms. An artificial neural network (ANN) was found to be a promising technique for ionospheric echo tracing. A modified rotor model was tested to construct the Hopfield ANN with the mean field theory updating scheme. Tests of the models against various ionospheric data showed that the modified rotor model gives good results where conventional tracing techniques have difficulties. Use of the ANN made it possible to implement a robust scheme of trace interpretation that considers local trace inclination angles available after ANN completes tracing. The interpretation scheme features a new algorithm for ƒ0F1 identification that estimates an α angle for the trace segments in the vicinity of the critical frequency ƒ0F1. First results from off‐line tests suggest the potential of implementing new operational autoscaling software in the worldwide Digisonde network.
Seasonal variations of globally measured ELF/VLF radio noise
Authors: Chrissan D. A. , Fraser‐Smith A. C.
Published: February 2004

Our laboratory has been conducting a global survey of extremely low frequency (ELF) and very low frequency (VLF) radio noise since February 1985. Eight measurement stations around the world record the instantaneous noise amplitude in each of sixteen narrow‐frequency bands in the 10‐Hz to 32‐kHz frequency range, and we have calculated the monthly averages of these amplitudes for the four stations with the longest times of operation. The period, amplitude, and phase of temporal variations in the averages are important indicators of the sources and propagation characteristics of the noise in the various frequency bands. Furthermore, since the principal source of ELF/VLF radio noise is lightning, long‐term variations of the noise must relate to changes in global thunderstorm activity. We find that the noise amplitudes vary seasonally by up to a factor of 4 in some of the sixteen frequency bands; in addition, many of the variations correlate quite well with global lightning flash rates.
Improved short‐term predictions of ƒ0F2 using GPS time delay measurements
Authors: Houminer Z. , Soicher H.
Published: February 2004

Reliable HF communications along short‐, medium‐ and long‐range paths require propagation assessment. Such assessment could be facilitated with the monitoring of ionospheric characteristics by continuously available passive means, i.e., measurement of the total electron content (TEC) using satellite‐emitted signals without a need for burdening the electromagnetic spectrum. With ubiquitous Global Positioning System (GPS) providing instantaneous time delay, or equivalently, TEC, values when needed, an assessment of HF propagation conditions may be available on a near‐real‐time basis. Both TEC and the peak electron density of the ionosphere, which determines the ordinary upper frequency limit (ƒ0F2( for HF sky wave vertical propagation, vary strongly with solar and geomagnetic parameters. Their ratio, the equivalent slab thickness, may vary to a lesser degree and hence be modeled with greater accuracy. A slab thickness model combined with real‐time TEC measurement anywhere on the globe may possibly yield an improved HF parameter prediction algorithm. To test the efficacy of the hypothesis, one has to ascertain the correlation, as exhibited by the correlation coefficient, between the TEC daily variability about the monthly mean and the ƒ0F2 variability. To determine such correlation, a study compared Faraday TEC data as well as GPS‐generated TEC data collected in Israel and with corresponding ƒ0F2 values obtained from vertical sounder measurements near the appropriate subionospheric location in Cyprus. The analysis shows that for large percentages of the time, very good correlation exists between TEC and ƒ0F2 short‐term variations. The correlation coefficient varies between 0.7 or better during winter and summer months to about 0.5–0.6 during equinox months. A study of the diurnal dependence of the correlation indicates that a better correlation exists during daytime than nighttime. There was no indication that the coefficient is dependent on geomagnetic activity or on protonospheric electron content during the period of this study.
Derivation of horizontal ionospheric gradients from variable azimuth and elevation backscatter ionograms
Authors: Caratori Julien , Goutelard C.
Published: February 2004

It is well known that the electron density in the ionosphere varies with the time of the day and the geographical location, giving rise to horizontal ionization gradients responsible for asymmetric ray paths, out of the great circle plane reflections and other tilted propagations (i.e., ducted modes and chordal modes). Simple ionospheric models, assuming spherical symmetry, must then be modified in order to take into account, at least, gross horizontal gradients. A monostatic backscatter sounder, with angle‐of‐arrival (azimuth and elevation) measurement capability, is very well suited for horizontal gradients measurements. This paper describes a new real‐time inversion method, using constant frequency, variable azimuth, and elevation angle backscatter ionograms for the derivation of mesoscale, or average, horizontal ionization gradients. The inversion process which has been developed for that purpose is fully explained. Tests on model data demonstrate the potential accuracy of the method and its stability to measurement errors. Finally, limitations and possible improvements to the current method are discussed.
A comparison of meteor radar systems at Buckland Park
Authors: Valentic T. A. , Avery J. P. , Avery S. K. , Cervera M. A. , Elford W. G. , Vincent R. A. , Reid I. M.
Published: February 2004

This paper describes a comparison of two meteor radar systems operated simultaneously from June 29 to July 15, 1994, at the Buckland Park field station near Adelaide, Australia (35°S, 138°E). Both meteor systems operate on a narrow‐beam VHF wind profiler. The first meteor system was developed by the atmospheric physics group at the University of Adelaide. The second meteor system was the University of Colorado's meteor echo detection and collection (MEDAC) system. The goal of the campaign was to determine how closely the two similar meteor systems performed with regards to the detection of meteor trail echoes and the estimation of the Doppler frequencies. Classification of the signals in the resulting data set showed that a number of the echoes, including a class of echoes that appear to be from meteors traveling straight down the beam, were from sources other than under dense meteor trails. When the nonunder dense echoes were operated on by the Doppler frequency estimators, widely varying estimates between the two systems were produced. Only when taking into careful consideration the details of the detection routines, the signal composition of the data set, and performance characteristics of the Doppler estimators was the comparison satisfactory.
Numerical evaluation of plane‐wave diffraction by a doubly periodic grating
Authors: Matsuda Toyonori , Okuno Yoichi
Published: February 2004

A numerical algorithm is presented for analyzing plane‐wave diffraction by a doubly periodic grating whose surface profile is corrugated periodically in two directions. The algorithm is based on the Yasuura method: diffracted fields are expanded in terms of Floquet modes for doubly periodic structures, and the expansion coefficients are determined so that approximate solutions satisfy the boundary conditions in the sense of least squares. A numerical technique for solving the resultant large‐sized least squares problem is developed. As a result, the present algorithm can be successfully implemented on a desktop computer such as an engineering workstation. This is demonstrated in numerical examples. Further, it is numerically confirmed that the present algorithm enables us to solve the problems of diffraction by a doubly periodic grating made of dielectric or metal in the resonance region (i.e., the wavelength and the size of a unit cell of the grating are of the same order); the wavelength can range from visible to infrared. The algorithm is applicable also to problems in the microwave region.
The diffraction of an inhomogeneous plane wave by a wedge
Authors: Kouyoumjian R. G. , Manara G. , Nepa P. , Taute B. J. E.
Published: February 2004

The diffraction of an inhomogeneous plane wave by a wedge is investigated. An integral representation for the total field is obtained and then evaluated by a uniform asymptotic procedure. The solution is expressed in the form of the uniform geometrical theory of diffraction (UTD) so that it can be applied to calculate the scattering from more complex shapes. The shadow and reflection boundaries of the geometrical optics field are found to be displaced from their conventional locations. The extent of the transition regions is also described. The solution is then extended to account for dissipative losses in the medium surrounding the wedge. To demonstrate the accuracy of the UTD solution, numerical results are presented and compared with those calculated from an eigenfunction solution.
Directions for space‐based low frequency radio astronomy: 1. System considerations
Authors: Basart J. P. , Burns J. O. , Dennison B. K. , Weiler K. W. , Kassim N. E. , Castillo S. P. , McCune B. M.
Published: February 2004

Although observations at the low end of the radio astronomy spectrum were the precursor of all work in radio astronomy, this portion of the spectrum has languished for decades while research at the upper radio frequencies has fluorished. Previous work at low frequencies (below 30 MHz) has clearly shown that sensitive high‐resolution ground‐based observations are extremely difficult to make, if not impossible. Observation quality at low frequencies can leap forward using space‐based interferometers. Radio telescopes such as these can be built principally from “off‐the‐shelf” components. A relatively low cost space program can make great strides in deploying arrays of antennas and receivers that would produce data contributing significantly to our understanding of galaxies and galactic nebulae. This paper discusses the various aspects of low‐frequency telescopes such as past history and significant issues like sensitivity, interference, baseline calibration, wave scattering, and mapping. All aspects of the first stages of space‐based, low‐frequency radio telescopes can be accomplished with no dependencies on new types of hardware. The time has come to open the final electromagnetic frontier in astronomy.
Accurate modeling of transparent boundaries in quasi‐optics
Authors: Popov Alexei V.
Published: February 2004

A general approach to reduction of the infinite computational domain to a strip or a cylinder is developed for two‐ and three‐dimensional narrow‐angle diffraction problems. Our method is based on constructing exact solutions of the parabolic wave equation in open subdomains free of nonuniform diffractive objects. As a result, an exact nonlocal boundary condition is derived providing full transparency of the artificially introduced computational border. This technique leads to an essential reduction of the computing time without any loss of accuracy, which is illustrated by practical examples from radio propagation and X ray diffractive optics. Extension of the method onto quasi‐stratified environments and modified parabolic equations is discussed.
Eigenvector approach for solving nonisotropic mixing formulas
Authors: Sihvola Ari , Olyslager Frank
Published: February 2004

This paper deals with mixtures of nonisotropic media. The mixing formulas to describe the effective material parameters of the mixture are complicated expressions where the different parameter dyadics are coupled. Using six‐vector notation, the different mixing rules become more manageable. However, for certain effective theories the matrix equations have been tractable only through numerical approach. Here we present a way to solve the effective material parameters in closed form, using an approach where the eigenvectors of the material six matrix are exploited. As a numerical example, the effective parameters of a mixture are calculated where the inclusion material is a mirror image of the background medium.
Traveling ionospheric disturbances observed in digitized polarimeter data
Authors: Daniell R. E. , Brown L. D. , Strickland D. J. , Barnes R. P. , Klobuchar J. A. , Thompson R. H.
Published: February 2004

Faraday rotation data from a pair of polarimeters located in Tucson, Arizona during the first half of 1990 have been examined for evidence of traveling ionospheric disturbances. With the use of digital filtering techniques, oscillations in total electron content (TEC) with periods ranging from 20 to 90 minutes and amplitudes of 1–5 × 1016 m−2 have been detected in the data. When present, the oscillations persist for varying time intervals (usually several hours) and show considerable variation from day to day. Although the viewing geometry (nearly along the local magnetic field line) is not ideal for observation of TIDs, these oscillations do appear to be real ionospheric oscillations. We have examined the possibility that the oscillations are not caused by actual changes in electron content but rather are due to changes in the Faraday rotation resulting from the motion of the ionospheric plasma along a magnetic field line and the resulting change in effective magnetic field strength. Our calculations indicate waves with displacement amplitudes of a few tens of kilometers would be sufficient to explain the observations. We also examined the possibility that the oscillations could be due to actual changes in TEC by using a one‐dimensional F region model and imposing a plausible wind oscillation (simulating a gravity wave). We found that a wave with a velocity amplitude of 15 m s−1 was also sufficient to explain the observations. We expect that the observations are best explained by a combination of the two effects, that is, an actual oscillation in electron content and an oscillation in Faraday rotation, both caused by gravity waves in the thermosphere. We estimate that for the 90‐min oscillations, 25% of the amplitude is due to the Faraday rotation effect and 75% is due to an actual change in TEC.
Recurrence relations for spherical waves
Authors: James Geoffrey C.
Published: February 2004

This paper derives new recurrence relations for scalar spherical waves in Cartesian coordinates and compares the performance of these relations with that of traditional recurrence relations. They are numerically stable for travelling waves and some standing waves, and are faster for evaluating scalar spherical waves up to order 40 on a nonspherical surface. Because of their algebraic simplicity, they will be especially useful for symbolic‐algebra computer programs, allowing explicit expressions for scalar and vector spherical waves to be obtained using a succinct procedure.
Likelihood estimation of tropospheric duct parameters from horizontal propagation measurements
Authors: Rogers L. Ted
Published: February 2004

A Bayesian estimation technique using an embedded electromagnetic parabolic equation model is implemented to solve the inverse problem of determining the atmospheric refractivity from measurements of beyond‐line‐of‐sight radio‐frequency propagation factors. The inverted refractivity structure is associated with the trapping layer that can be formed by the capping inversion of the stable marine boundary layer. Why this inverse problem is of interest is established by describing the refractivity structure associated with the stable marine boundary layer in the context of its effect on horizontal propagation and then describing the difficulty of obtaining representative estimates of the tropospheric refractivity structure using existing sensing methods. The implementation is then described. A three‐parameter model of the marine boundary layer is used; two of the three parameters are inverted, while the third is assumed to be a random variable. The inversion method is applied to radio data from the Variability Of Coastal Atmospheric Refractivity Experiment (VOCAR). A matrix of propagation factors corresponding to the two parameters to be inverted is calculated for the path geometry and the frequencies of VOCAR. Parameter likelihoods are determined for the entire parameter space, and mean estimates of refractivity parameters are computed from the distributions. The parameter estimates are compared with values obtained from radiosondes launched during VOCAR. The remotely sensed refractivity parameters for multiple‐frequency sensing show good agreement with directly sensed observations.
Numerical and experimental studies of a helical antenna loaded by a dielectric resonator
Authors: Hui H. T. , Yung Edward K. N. , Leung K. W.
Published: February 2004

A helical antenna loaded by a dielectric resonator (DR) is rigorously studied. Effects due to the dielectric core are analyzed by replacing it with an equivalent polarization current density so that only the homogeneous dyadic Green's function is required. Special interest is paid to the handling of the singularity of the dyadic Green's function in the source region. Two coupled integral equations are obtained by enforcing the boundary condition on the surface of the helical antenna and by equating the total electric field inside the dielectric resonator. The method of moments is used to solve the coupled equations and hence to obtain other properties of the antenna, such as the input impedance, the radiation pattern, the beam width characteristics, and the axial ratio. Experimental measurements were conducted and compared with the theoretical results. It is found that excellent agreement is observed between theoretical values and experimental measurements. Various characteristics of the dielectric resonator loaded antenna are also discussed.
Impact of antenna errors on the radiometric accuracy of large aperture synthesis radiometers
Authors: Camps A. , Bará J. , Torres F. , Corbella I. , Romeu J.
Published: February 2004

A classification of system errors in aperture synthesis radiometry applied to Earth observation is presented. A general procedure to quantify the impact of antenna errors on the radiometric accuracy is developed and is then particularized to an L‐band Y‐shaped interferometer called MIRAS (microwave imaging radiometer by aperture synthesis) currently under study at the European Space Agency. This work analyzes in detail the impact of antenna errors on the radiometric accuracy of the instrument. These antenna errors are grouped into amplitude and phase antenna pattern errors, antenna position errors and antenna cross polarization errors. Special attention is paid to antenna coupling effects because of their importance in the selection of a suitable inversion algorithm for large aperture synthesis interferometers: the G‐matrix techniques or the Fourier techniques proposed for MIRAS.
Application of finite integral technique to electromagnetic scattering by two‐dimensional cylinders: Transverse magnetic case
Authors: Gothard Griffin K. , Rao Sadasiva M. , Wilton Donald R.
Published: February 2004

In this work, the frequency domain current distributions induced on infinitely long cylinders of arbitrary cross section illuminated by transverse magnetic plane wave are obtained using the finite integral technique (FIT). These bodies may be composed of conducting, dielectric, or inhomogeneous materials. In the FIT, the Maxwell's equations in the integral form are solved directly, thus generating a sparse system matrix. The problem region is subdivided into triangular elements, and distinct constitutive parameters may be assigned to each element, thus modeling the inhomogeneous materials accurately and efficiently. Next, the measured equation of invariance (MEI) concept is used to truncate the mesh which preserves the sparsity of the system matrix. Further, MEI allows the termination of the grid close to the structure, in fact, only two layers, increasing the computational efficiency of the method. Several numerical examples are presented to illustrate the applicability of the method.
Analytic ray parameters for the quasi‐cubic segment model of the ionosphere
Authors: Norman R. J. , Dyson P. L. , Bennett J. A.
Published: February 2004

Models of the ionosphere giving analytic ray solutions are very useful for the study of ionospheric propagation. Probably the most useful is the quasi‐parabolic model which gives analytic solutions for a spherical ionosphere when the effects of the Earth's magnetic field are ignored. Furthermore, its use in analytic ray tracing has been extended recently by developments in which real ionospheres are described by fitting quasi‐parabolic segments (QPS) to measured vertical profiles. While numerical ray tracing is required to take account of the range of horizontal gradients that occur in the ionosphere, in some real‐time applications, analytic results are still preferred because of the shorter computation time. Even though the QPS approach has greatly extended the utility of analytic ray tracing, it produces model ionospheres which are continuous in only the first derivative of the refractive index. The lack of continuity of the second derivative can lead to relatively abrupt changes in ray quantities with, for example, elevation angle. This drawback has been addressed by developing a model based on quasi‐cubic segments (QCS) which also provide analytic ray solutions. In this paper the QCS model is described, and analytic solutions are derived for range, group, and phase paths. An example of an application is presented in which the QCS model was used to check the performance of a numerical ray tracing code.
Interferometry observations of three‐dimensional spatial structures of sporadic E irregularities using the Chung‐Li VHF radar
Authors: Chu Yen‐Hsyang , Wang Chien‐Ya
Published: February 2004

By using the interferometry technique implemented at the Chung‐Li VHF radar, the striated echoes with quasi‐periodic characteristics in the range‐time‐intensity plot generated from the electron density irregularities associated with sporadic E layer are investigated. It is shown that the Es irregularities above 110 km drifting mostly westward along a stationary path of a few kilometer's width are responsible for the striated echoes. Considering the field‐aligned property of the Es irregularities and the geometry of the echoing region over the Chung‐Li radar site, it indicates that this stationary path is the cross section of a tilted layer which has a sharp electron density gradient in the direction across the layer parallel to the magnetic field line in the E region and orients geographically 72°NW. The observations also demonstrate that the echoing regions of the Es irregularities over the Chung‐Li radar station are confined on the right side of a tilted thin plane with the thickness of a few kilometers at the elevation angle of 52° in the radar viewing region. These characteristics can be explained by using the radar backscatter from field‐aligned targets in the field‐perpendicular direction. The behavior of the sporadic E layer in the equatorial anomalous region is also investigated and discussed, and a descending sporadic E layer modulated by the gravity waves is observed. The descent rate of the layer is about 3.6 m/s, considerably larger than that reported by other investigators. The primary gravity wave modulating the sporadic E layer has a period of 12–15 min and propagates upward in phase with a vertical wavelength of about 50 km. Moreover, a positive correlation between the peak intensity of radar returns from Es irregularities below 110 km and the vertical shear of their horizontal drift velocity is seen. This feature, combined with the positive correlation between radar backscatter and the Doppler spectral width, strongly suggests that the crucial role the neutral wind plays in the excitation of the Es irregularities below 110 km cannot be ignored.
Two‐dimensional tomography in a statistically homogeneous medium
Authors: Tereshchenko E. , Hagfors T.
Published: February 2004

The problem of reconstruction of the moments of a function from its central projections is discussed when the range of one of the variables is limited. The theory is then considered in the context of incoherent scatter radar observation of the ionosphere with a chirped radar system. The combination of the chirp technique with special phase coding schemes is also considered but is found unattractive in most cases because of the strict requirements on time stationarity of the medium, a situation which often does not exist.
Effect of periodic horizontal gradients on the retrieval of atmospheric profiles from occultation measurements
Authors: Belloul M. B. , Hauchecorne A.
Published: February 2004

Spherical symmetry is generally assumed during the retrieval process of atmospheric profiles from occultation measurements. The existence of periodic horizontal gradients, occurring on scales comparable to the distances traveled by the rays around their tangent point, is produced by gravity waves. These waves can introduce density perturbations of up to 1 or 2% in amplitude and affect the retrieved parameters accordingly. We show the consequences of ignoring these gradients in the retrieved refractivity profiles when spherical symmetry is assumed. We find that only the waves, with horizontal wavelengths close to the horizontal distance that rays travel in their final 6 or 8 km in the vertical before they are tangent in the atmosphere, will have an influence on the retrieved profiles by introducing a phase delay in the local profile of the retrieved refractivity. The horizontal wavelength of these waves corresponds to the minimum horizontal resolution associated with the retrieved profiles. We also find that smaller scale waves do not have any significant impact on the retrieved profiles, as their contribution cancels out by averaging through the periodic perturbation, while waves with very long horizontal wavelengths are in good agreement with a local spherical symmetry assumption.
Cross correlations and cross spectra for spaced antenna wind profilers: 2. Algorithms to estimate wind and turbulence
Authors: Holloway Christopher L. , Doviak Richard J. , Cohn Stephen A. , Lataitis Richard J. , Van Baelen Joel S.
Published: February 2004

In part 1 of this paper we developed analytic relationships linking the cross correlation and cross spectrum of the echoes from a spaced antenna system to the properties of a horizontally isotropic scattering medium (e.g., clear‐air refractive index irregularities) and the background flow (e.g., laminar or isotropic turbulent flow). Using these analytic expressions, in the present paper, part 2, we construct algorithms (for both the time domain and frequency domain) for extracting unbiased wind and turbulence estimates. We derive a condition under which one can ignore turbulence when computing winds from the time delay to the peak of the cross‐correlation functions. We show profiles of the horizontal wind and turbulence based on these algorithms using data from the unique 33‐cm wavelength spaced antenna wind profiler developed by the National Center for Atmospheric Research.
Triassic plutonism in southern California: Southward younging of arc initiation along a truncated continental margin
Authors: Barth A. P. , Tosdal R. M. , Wooden J. L. , Howard K. A.
Published: February 2004

Earliest Cordilleran magmatism in the southwestern United States is recorded by a belt of Triassic plutons that intrude Proterozoic basement of the Mojave crustal province and its cratonal/miogeoclinal cover. The belt extends from the western Mojave Desert through the Transverse Ranges to the Colorado River trough. Triassic plutons are predominantly alkali‐calcic, Fe‐ and Sr‐enriched quartz monzodiorites and monzonites. The northern part of the belt is composed of two older plutonic suites (241–231 Ma) which are high K to shoshonitic; the southern part of the belt is a younger (218–213 Ma), sodic‐alkalic suite. The plutonic record in southern California suggests a short‐lived, southward younging continental margin arc setting for emplacement of Triassic plutons, superimposed on a continental margin modified by sinistral transform faulting. Triassic plutonism in this region was followed by a magmatic lull prior to the onset of voluminous Middle to Late Jurassic Cordilleran arc magmatism.
Tectonic control of the subducting Juan Fernández Ridge on the Andean margin near Valparaiso, Chile
Authors: von Huene R. , Corvalán J. , Flueh E. R. , Hinz K. , Korstgard J. , Ranero C. R. , Weinrebe W.
Published: February 2004

Near the latitude of Valparaiso, Chile, a fundamental change in configuration of the Benioff Zone, volcanic arc activity, and the structure of the continental margin occurs opposite the subducting Juan Fernández Ridge. Upper plate tectonics related to subduction of the ridge were studied by an international group of geoscientists in the two‐degree segment offshore Valparaiso, extending from the shelf edge seaward across the eastern end of Juan Fernández Ridge. Near the O'Higgins group of seamounts, the Juan Fernández Ridge strikes northeast rather than continuing its east‐west trend across the Pacific Basin. The ridge uplifts the upper plate, and sediments of the Valparaiso Basin are deformed against its southern flank. This deformation is consistent with the southward migration required by an oblique trending ridge and the nearly trench‐normal vector of plate convergence. In the trench axis, the ridge forms a basement barrier behind which sediments 2.5 km deep have ponded. The lower slope over the ridge appears eroded, whereas the margin not yet affected by ridge subduction is fronted by an accretionary prism about 25 km wide. Nazca Plate relief clearly influences tectonism of the margin where it subducts beneath thin continental crust; its relation to deeper processes segmenting the Andean Orogen appears to involve prior tectonic events.
Successive orthogonal and oblique extension episodes in a rift zone: Laboratory experiments with application to the Ethiopian Rift
Authors: Bonini Marco , Souriot Thierry , Boccaletti Mario , Brun Jean Pierre
Published: February 2004

Small‐scale modeling was performed to examine the effects of the superposition of two successive extensional phases from orthogonal to oblique (type 1) and from oblique to orthogonal (type 2). In both the type 1 and type 2 models, faults produced during the first stage strongly control fault development during the second stage. In type 1 models, the oblique faults developed during the second oblique phase are confined within a first‐phase graben, whereas in type 2 models the oblique faults, produced during the first phase, continue to develop during orthogonal extension and connect with each other to give sigmoidal fault blocks. Type 1 models are compared with the structural setting of the Ethiopian Rift; the evolution of the rift is related to a recent extensional event, whose principal direction of stretching trends at around 50° to preexisting major normal faults. Type 1 laboratory models are fairly comparable to the northern sector of the Ethiopian Rift, referred to here as MER. They account for both the development of the en echelon oblique faults of the Wonji Fault Belt and the sinistral shear gradient running parallel to the eastern border of the MER, which formed during an oblique rifting extension. The statistical analysis of the whole Ethiopian Rift fault pattern by reference to the experimental data allows the determination of a N100°–N110° mean direction of stretching.
Thrust kinematics and internal deformation in basement‐involved fold and thrust belts: The eastern Orobic Alps case (Central Southern Alps, northern Italy)
Authors: Carminati Eugenio , Siletto Gian Bartolomeo , Battaglia Daniele
Published: February 2004

A new model, which integrates original detailed structural data and published geological data, is proposed for the Alpine (Cretaceous to present) convergent evolution of the northern portion of the Central Southern Alps. The Central Southern Alps are believed to have developed from the very beginning of their convergent evolution as a critical wedge, characterized by plastic deformation at depth. Clear evidence for the existence of a brittle‐plastic transition at depth within the Central Southern Alps wedge during pre‐Oligocene evolutionary stages comes from detailed structural data (original 1∶10000 form surface mapping). A regionally verified fabric sequence manifests the superposition of brittle to plastic to brittle structures developed during the path followed by the same volume of rocks within the wedge. On the other side no direct evidence exists in the rear part of the wedge for the occurrence of plastic deformation at depth during post‐Oligocene stages. However, considerations driven by the comparison of timing and amount of deformations in the Central Southern Alps and brittle wedge theory predictions strongly support this hypothesis. Different deformation mechanisms operating at different structural levels have been controlling a crustal scale strain partitioning between deep plastic and shallow brittle portions of the Central Southern Alps wedge. Plastic deformations have strongly controlled the overall mechanical behavior of the Central Southern Alps wedge, reducing sensitively the amount of internal deformation required in order to attain criticality of the wedge.
A Reach‐Averaged Model of Diurnal Discharge Wave Propagation Down the Colorado River Through the Grand Canyon
Authors: Wiele S. M. , Smith J. D.
Published: February 2004

As part of the Glen Canyon Environmental Studies, we have developed a discharge model that routes daily discharge waves released from Glen Canyon Dam to Diamond Creek, 386 km downstream. Owing to the length of the diurnal discharge wave and the sparseness of the available topographic data, the latter were averaged over the entire length of the system. Terms too small to be significant in the momentum equation were identified by scaling arguments based on data from past dam releases and on channel hydraulic geometry. Channel friction results primarily from form drag on large topographic elements and from variations in cross‐sectional area and flow depth, rather than bed roughness, producing a stage‐dependent friction that is not well represented by a constant value of standard channel roughness parameters, such as Manning's n. Channel friction as a function of stage was determined from field data available at high discharge (792 m3/s) and intermediate discharge (425 m3/s) and by using simple kinematic wave theory together with wave speed measurements to determine channel friction at low discharge (about 142 m3/s). Model predictions of wave speed and shape agree well with data from five streamflow gaging stations and 42 stage gaging stations located along this segment of the Colorado River.
Walter Langbein and the emergence of scientific hydrology
Authors: Dooge James C. I.
Published: February 2004

Walter Langbein (1907–1982) contributed to the growth of scientific hydrology in a number of ways. His research publications introduced original concepts of both theoretical interest and practical importance. He served on a number of committees of the American Geophysical Union and played a key role in promoting new approaches in hydrological thinking. He played a major role in the launching of the International Hydrological Decade of 1965–1974 and in the establishment of the International Association of Hydrological Sciences Committee on Mathematical Models in Hydrology in 1967. He was a source of inspiration to many young hydrologists who have since made their own important individual contributions to that discipline. It is timely and useful to look back at his personal contribution to hydrology and to draw lessons on it for our future work.
Dynamics of heat exchange between sediment and water in a lake
Authors: Fang Xing , Stefan Heinz G.
Published: February 2004

Traditionally, it has been stated that heat is transferred from water to lake sediments during the summer and released from lake sediments into the water during the winter (ice cover) period. While this is true for seasonal averages, the direction of the heat flux reverses frequently on shorter, for example, daily timescales. Therefore the lake sediments not only provide seasonal heat storage but also add significant thermal inertia to the water column. Sediment heat fluxes are most important in shallow lakes. In this study the sediment heat exchange is calculated separately for every layer of a one‐dimensional, deterministic, unsteady year‐round lake water temperature model and then included as a source/sink term in the lake water temperature simulation. The importance of sediment heat transfer has thus been identified, and the dynamics of sediment heat exchange have been discovered. Heat can transfer into or out of the lake sediment during both the open water season and the winter ice cover period. The influence of the sediment heat exchange on water temperature simulations is quantified.
Aggregate Diffusion Model Applied to Soil Vapor Extraction in Unidirectional and Radial Flows
Authors: Ng Chiu‐On , Mei Chiang C.
Published: February 2004

An aggregate diffusion model for soil vapor extraction is applied in this paper. The model accounts for the diffusion within water‐saturated aggregates and convective diffusion in air‐filled macropores. Focusing only on the dominant and measurable mechanisms, we represent the kinetic term for the gas aggregate exchange by a convolution integral. The nondimensional transport equation involves only three basic parameters: (1) Peclet number, (2) ratio of advection time to aggregate diffusion time, and (3) ratio of mass within aggregate to mass in gas. Two examples of soil vapor extraction based on the model are studied. The first problem is concerned with vapor transport in a one‐dimensional column, and the theory is compared with published experiments. The second example deals with vapor extraction by a well centered at a radially symmetric contaminated zone. Effects of the physical parameters on contaminant removal are discussed.
Influence of Valley Type on the Scaling Properties of River Planforms
Authors: Beauvais Anicet A. , Montgomery David R.
Published: February 2004

Scaling properties of 44 individual river planforms from the Cascade and Olympic Mountains of Washington State were defined using the divider method. Analysis of the standardized residuals for least squares linear regression of Richardson plots reveals systematic deviations from simple self‐similarity that correlate with the geomorphological context defined by valley type. A single fractal dimension describes rivers flowing through bedrock valleys. Those flowing in inherited glacial valleys exhibit two distinct fractal dimensions, with a larger fractal dimension at small scales. Rivers flowing in alluvial valleys are also described by two fractal dimensions, but with a larger dimension at large scales. We further find that the wavelength of the largest meander defines an upper limit to the scaling domain characterized by fractal geometry. These results relate scaling properties of river planforms to the geomorphological processes governing valley floor morphology.