bluuebunny/crossref_metadata_2025 huggingface dataset:
The North Atlantic Current and surrounding waters: At the crossroads
Authors: Rossby T.
Published: February 2004

The North Atlantic Current is a well‐defined western boundary current that flows north along the east side of the Grand Banks from 40° to 51°N, where it turns sharply to the east and begins its journey across the ocean. The current is unique in transporting warm tropical waters to much higher latitudes than any other western boundary current and thus plays a crucial role in ameliorating the climate of the European subcontinent. The North Atlantic Current originates in the Gulf Stream when the latter curves north around the Southeast Newfoundland Rise, a major submarine ridge that stretches SE from the Grand Banks. A well‐defined front delineates the path of the current as long as it flows north as a western boundary current. After the current turns east in the north, it broadens into a widening band of eastward drift without a sharp or permanent front in the sense of the eastward flowing Gulf Stream after it separates from Cape Hatteras. The North Atlantic Current transports more than 40 Sv (1 Sv = 106 m³ s−1) in the south and about 20 Sv by the time it flows east across the Mid‐Atlantic Ridge. The currents along the northward flowing front are quite swift, with typical maximum average speeds in the upper 300 m near 1 m s−1 (= 2 knots). The current meanders almost as wildly as a “snaking” river, but unlike steep meanders in the Gulf Stream these meanders appear to be stable, and with one exception have not been observed to break off to form pools of warm and/or cold waters as frequently occurs in the Gulf Stream. The meanders appear to be induced by major topographic features along the path of the current, namely, the Southeast Newfoundland Rise, the Newfoundland Seamounts, and Flemish Cap. Strong recirculations develop on the concave side of the meanders. One of these, the “Mann eddy” at the first meander crest of the North Atlantic Current, should be regarded as a permanent feature of the North Atlantic circulation. Other meanders also contain recirculations that can persist for months. Under certain conditions these can merge together to form an extended SW flow (recirculation) just east of the North Atlantic Current.
Spectral structure of auroral F layer patches
Authors: Erukhimov L. M. , Muravjeva N. V. , Myasnikov E. N. , Evstafjev O. V. , Kosolapenko V. I.
Published: February 2004

Analysis of the power law spectral form for the differential phase and amplitude scintillation spectra has been performed for the orbital satellite beacons at 150–400 MHz receiving at the site Verkhnetulomsky (68.4°N, 31.85°E). It is shown that inside of the local scintillation patches, which are seen during ground‐based observations in the vicinity of the east‐west direction, a significant increase of power law slope occurs at the scales l⊥ > 0.5 km. A model of plasma density spectrum combining both the small‐scale rod‐like and large‐scale sheet‐like irregularities is proposed.
Volume scattering by tropospheric turbulence at midlatitudes: Frequency validity range
Authors: Ravard O. , Chevrier F.
Published: February 2004

A theoretical propagation model is developed to quantify the probability density function of the power received on tropospheric transhorizon radio links at midlatitudes. Experimental radiosonde data are used to build a new random model of the refractive index structure constant. This model takes into account the turbulence strength for both long‐term and altitude variations. A method is developed to evaluate the received power probability density function by numerical integration through the common volume allowing the consideration of antenna radiation patterns and ground effects. After numerous comparisons between our new theoretical propagation model and the semiempirical one, NBS 101, we have set up an upper limit for the validity of the volume scattering theory. The limit is found near 3 GHz for beyond line of sight radio links in the case of a turbulence described by the Kolmogorov's spectrum.
Direct Cerenkov excitation of waveguide modes by a mobile ionospheric heater
Authors: Borisov N. , Gurevich A. , Papadopoulos K. , Chang C. L.
Published: February 2004

A comprehensive theoretical analysis of direct Cerenkov excitation of the Earth ionosphere waveguide using ionospheric heating is presented. The model relies on transient ionospheric heating with a heater spot moving horizontally at the bottom of the waveguide with speed close to the speed of light. The cases of isotropic ionospheric conductivity, corresponding to heating altitudes below 70 km, and of anisotropic conductivity, corresponding to higher heating altitudes, are examined separately. It is found that enhanced radiation coupling requires that the speed of the heater approaches the speed of light. For the anisotropic case, such enhancement occurs independently of the direction of motion, while for the isotropic case, motion parallel to the ambient electric field is required.
Numerical solution to Maxwell's equations in the time domain on nonuniform grids
Authors: Palaniswamy Sampath , Hall William F. , Shankar Vijaya
Published: February 2004

In most integration schemes for the Maxwell's equations, damping and distortion errors are strongly dependent on the size of the time step in relation to the size of the spatial discretization Δx. The disadvantage of strong dependence on this ratio becomes evident when one computes the solution on nonuniform meshes. A systematic way for arriving at a scheme that can operate accurately on nonuniform meshes is presented here. Performance of a higher‐order scheme is compared with that of another recently developed scheme on a ramp grid.
Solenoidal edge elements
Authors: Jevtić J. O. , Lee R.
Published: February 2004

In this paper we introduce a new element called solenoidal edge element for the finite element method. This element is specially designed for electromagnetic modeling. The basis functions are chosen to automatically satisfy the wave equation within each element. The main advantage of such elements is that they have very low numerical dispersion relative to more commonly used nodal and edge elements based on polynomial approximations. The accuracy of the elements is demonstrated for boundary value, eigenvalue, and scattering problems.
Scattering properties of dense media from Monte Carlo simulations with application to active remote sensing of snow
Authors: Zurk L. M. , Tsang L. , Winebrenner D. P.
Published: February 2004

Monte Carlo simulations are used to derive the phase matrix, effective permittivity, and scattering coefficient for a random medium consisting of densely packed spheres up to 5000 in number. The results include correlated scattering and coherent wave interaction among the scatterers. The Monte Carlo simulations are based on a multiple‐scattering formulation of the Foldy‐Lax equations. It is shown that the derived phase matrix is in good agreement with dense media radiative transfer theory for copolarized scattering. The depolarization, however, can be substantially larger than conventional theory. Two methods are used to analyze the behavior of the coherent wave to obtain the real part of the effective permittivity. For the small particle case both methods yield values of permittivity that agree with the results of mixing formulas such as the Clausius‐Mossoti mixing formula. The phase matrix and scattering coefficient obtained by simulation are used in a second‐order radiative transfer model to predict the amount of backscatter from a layer of snow. It is also shown that sticky spheres, which can be used to model metamorphosed snow, produce high levels of copolarized and depolarized backscatter that can exceed the independent scattering model.
Hybrid spectral‐spatial method for the analysis of printed antennas
Authors: Vecchi Giuseppe , Matekovits Ladislau , Pirinoli Paola , Orefice Mario
Published: February 2004

A technique is presented combining the advantages of the spatial and spectral versions of the method of moments for a general electromagnetic problem involving a printed structure. The metalizations are of arbitrary shape and are described by a shape‐conforming mesh with subdomain basis functions. The technique is based on the extraction of the leading singular terms of the Green's function, in which space and frequency dependences are separated. The part of the impedance matrix associated with these terms is efficiently evaluated in the space domain, once for all frequencies. The eigenfunctions of these singular parts (expressed in terms of the subdomain basis functions) are then used as entire‐domain basis functions for the remaining regular part, whose associated impedance matrix is evaluated in the spectral domain. This appears to be the generalization to arbitrary patch shapes of the use of entire‐domain functions (like orthogonal polynomials or cavity modes) and drastically reduces the number of unknowns, showing potential for the full‐wave analysis of printed arrays.
Near–focus active optics: An inexpensive method to improve millimeter–wavelength radio telescopes
Authors: Greve A. , Baars J. W. M. , Peñalver J. , LeFloch B.
Published: February 2004

The application of active and adaptive optics allows the construction of large diameter lightweight optical telescopes for observations below the seeing limit of the atmosphere. Active wavefront correction in a Cassegrain/Gregorytype radio telescope can be made with a deformable main reflector or deformable subreflector. Here we suggest the possibility of correcting spatially large‐scale wavefront deformations with a small size corrector located near the focus of the telescope. Using representative examples of the Institut de Radioastronomie Millimetrique (IRAM, Spain) 30‐meter diameter, millimeter‐wavelength telescope, we calculate the improvement expected from the correction of (1) the systematic component in homology deformations, (2) the large‐scale residual errors of a reflector adjustment, and (3) the beam degradation experienced in observations with a wobbling subreflector. The improvement in surface/wavefront precision obtained from piston correction with a corrector of some 50 elements is of the order of 30 – 40%. We investigate in particular the systematic component of homology deformations, their representation by low‐order Zernike polynomials, and their elimination by near‐focus correction. We study in detail the homology deformations of the IRAM 30‐m reflector.
Relative impact of meteor scatter and other long‐distance high‐latitude propagation modes on VHF communication systems
Authors: Cannon Paul S. , Weitzen Jay A. , Ostergaard Jens , Rasmussen John E.
Published: February 2004

We have analyzed the duty cycle, due to ionospheric propagation, of very high frequency sounding signals for both polar cap and auroral paths. We find that at 35 and 45 MHz the propagation is often sustained by sporadic E layers and other nonmeteoric modes rather than by meteor scatter. At the higher frequencies of 65 and 85 MHz we find that the path is generally dominated by meteor scatter modes. These results have important ramifications for frequency reuse and security in meteor burst communications systems and for the development of extended frequency range HF systems (above 30 MHz) with a capability to operate on any available propagation mode. The diurnal, seasonal, and geomagnetic variations of the nonmeteoric duty cycle have been examined. A polar cap path model is presented for the nonmeteoric duty cycle as a function of geomagnetic activity.
On the eikonal function
Authors: Cornbleet S.
Published: February 2004

A survey of the various mathematical derivations of the eikonal equation of Bruns is given, each having its own geometrical approximation. These include the Sommerfeld‐Runge ansatz and the variational principle applied to the action integral. The apparent arbitrariness of the ansatz is justified by its giving the same form of eikonal equation as does the more rigorous extremal solution of the Euler‐Lagrange equations. A two‐scalar representation of the optical part of the total field is presented, termed Bateman potentials, which, for the null field, also derives the eikonal equation. It is shown that the ansatz is a form of Bateman potential, and the derivation explicitly defines rays, phase fronts, and the field polarization. When applied to ray congruences in nonuniform media, the Bateman potential description is in general agreement with known results. In the case of the Maxwell fish‐eye, however, a discrepancy occurs. The reasons and consequences of this suggest the possibility of using the Bateman potential method to form an improvement to the basic geometrical optics approximation.
Measurement of dielectric constant and conductivity of samples using guarded electrodes
Authors: Darayan Shahryar , Shattuck David P. , Shen Liang C. , Liu Richard C.
Published: February 2004

A laboratory apparatus is designed to measure the complex permittivity of both liquid and solid samples in the frequency range from 2 to 13 MHz. This device uses a guarded‐electrode sample holder to significantly reduce fringe effects. A synthetic focusing method is utilized to ensure that the potentials at both the main and the guard electrodes are equal. Extensive measurements on NaCl solutions of different salinities are made to study the accuracy of the guarded‐electrode method. The measured complex permittivities are in good agreement with those reported in the literature. The deviation is less than 3% for the conductivity measurement and depends on the salinity of the solution for the dielectric constant measurement. Various measurements conducted on Berea sandstones and glass beads saturated with saline water showed the accuracy and feasibility of the experimental set up. This measurement method might be used to obtain accurate values for the conversion of measured data from the measurement while drilling (MWD) resistivity tool.
Transient response of a wide‐angle cone with dielectric loading
Authors: Nusseibeh F. , Bansal R.
Published: February 2004

Introducing a dielectric material near a conical antenna affects the transmitted and received signals. In this paper, wide‐angle cones with dielectric loading are analyzed in both the transmitting and the receiving modes. A Gaussian pulse is used as the exciting signal.
Nonlinear dynamics of a domain wall in the periodic magnetic field
Authors: Ostrovskaya N. V. , Popkov A. F.
Published: February 2004

Self‐localized domain wall (DW) oscillations in a uniaxial ferromagnet in the presence of dissipation and a periodic homogeneous magnetic field are under investigation. On the basis of perturbation theory and the direct numerical solution of magnetodynamic equations, the stabilization conditions of soliton‐like domain wall oscillations have been found. For a given frequency of pumping field, the narrow range of field magnitude, where the effects of self‐organization on the domain wall appear, has been discovered. The threshold of the external field amplitude, when the generation of Bloch lines in the domain wall starts, has been obtained approximately. Such generation is the initial stage of chaotization of the domain wall under the parametric pump, which has been observed in the numerical experiments.
Moment method and iterative reconstruction of two‐dimensional complex permittivity by using effective modes with multiple sources in the presence of noise
Authors: Park Cheon‐Seok , Park Seong‐Kil , Ra Jung‐Woong
Published: February 2004

An iterative hybrid algorithm combining the steepest descent algorithm of Levenberg‐Marquardt and the simulated annealing algorithm is used for reconstructing the complex permittivity of a two‐dimensional, high‐contrast, large object. The scattered fields calculated in a cost function are expanded in angular spectral modes. The “illposedness” in the inversion due to the noise in the scattered fields is regularized by keeping only the effective modes and filtering out the higher angular spectral modes. The object is discretized into N square cells of unknown permittivities and N or larger number of effective modes (P) are needed for this spectral domain iterative method to reconstruct the shape and the permittivity distribution of the object. When N > P, multiple view (K) and effective modes of each view (P) make the reconstruction possible if N ≤ PK.
Direct comparison between in situ and VHF oblique radar measurements of refractive index spectra: A new successful attempt
Authors: Luce Hubert , Dalaudier Francis , Crochet Michel , Sidi Claude
Published: February 2004

A direct comparison between in situ and radar measurements of temperature spectra of atmospheric turbulence was performed by Dalaudier et al. [1989]. The widely accepted hypothesis of scattering from isotropic turbulence in the inertial subrange was used for the oblique reconstructions. However, the results were “puzzling” because neither the shape, the dynamic range, nor the power level was correctly reproduced. Other investigations were thus needed in order to determine the causes of these disagreements. A new attempt was performed using essentially the same method based on the same model and a more extensive and improved data set. The results, presented here, are now very satisfying for most of measurements. This agreement allows us to understand that some errors and technical problems were at the origin of the discrepancies described by Dalaudier et al. [1989]. From the new results it can be concluded that the method used, based on the isotropic model in the inertial subrange, is adequate to fairly reproduce most of the VHF radar measurements at 15° from the zenith.
The operator expansion formalism for electromagnetic scattering from rough dielectric surfaces
Authors: Smith Ralph A.
Published: February 2004

Milder's operator expansion formalism for scattering from surfaces of moderate slope is extended to the case of electromagnetic scattering from an interface between media of arbitrary isotropic electromagnetic properties, i.e., dielectric, magnetic, or imperfectly conducting materials. A numerical implementation of the second‐order formulas is described. It computes scattering amplitudes for complicated two‐dimensional surfaces in a few minutes on a workstation. Computations of oblique scattering from sinusoidal gratings demonstrate that the scheme is very accurate for moderate values of Fresnel number, surface curvature, and grazing angle. Practical applications include radar scattering from the ocean and optical scattering from rough metals and insulators.
Application of modal expansion and method of moments to study electromagnetic scattering from a rectangular cavity recessed in a three‐dimensional conducting surface
Authors: Deshpande M. D. , Reddy C. J. , Beck F. B. , Cockrell C. R.
Published: February 2004

The problem of electromagnetic (EM) scattering from an aperture backed by a rectangular cavity recessed in a three‐dimensional (3‐D) conducting body is analyzed using the modal expansion inside and the integral equation method outside the cavity. Assuming (1) an equivalent electric surface current density flowing on the 3‐D conducting surface of the object including the cavity aperture and (2) an equivalent magnetic surface current density flowing over the aperture, only the scattered fields outside the cavity are determined . The EM fields inside the cavity are determined using the waveguide modal expansion functions. Making the total tangential electric and magnetic fields across the aperture continuous and subjecting the total tangential electric field on the outer conducting 3‐D surface of the object to zero, a set of coupled integral equations is obtained. The equivalent electric and magnetic surface currents are then obtained by solving the coupled integral equation using the method of moments (MoM). The numerical results on scattering from rectangular cavities embedded in various 3‐D objects are compared with the results obtained by other numerical techniques.
Modal solution to the plane wave two‐frequency mutual coherence function in random media
Authors: Oz Jasmin , Heyman Ehud
Published: February 2004

Pulse propagation in a random medium is mainly determined by the two‐frequency mutual coherence function which satisfies the parabolic equation. It has been shown recently that this equation can be solved by separation of variables, thereby reducing the solution for any structure function into solutions of ordinary differential equations. Via a proper modal‐expansion theorem, this representation may also be applied to any source problem. The modal approach also provides new physical interpretations for relevant physical parameters. This new solution approach is being reviewed here within the simplified framework of plane wave initial conditions. In particular, a general power law structure function is investigated, and the results are compared with the known exact solution for quadratic medium and a numerical solution for a Kolmogorov medium. Using the new modal approach, we present two alternative representations: a “mode series” and a “collective mode solution.” Both representations are suitable for extension into the time domain, giving a series of “wave front arrivals” and “collective resonance” contributions respectively.
Directions for Space‐Based Low‐Frequency Radio Astronomy: 2. Telescopes
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

Astronomical studies of celestial sources at low radio frequencies (0.3 to 30 MHz) lag far behind the investigations of celestial sources at high radio frequencies. In a companion paper [Basart et al., this issue] we discussed the need for low‐frequency investigations, and in this paper we discuss the telescopes required to make the observations. Radio telescopes for use in the low‐frequency range can be built principally from “off‐the‐shelf” components. For relatively little cost for a space mission, great strides can be made in deploying arrays of antennas and receivers in space that would produce data contributing significantly to our understanding of galaxies and galactic nebulae. In this paper we discuss an evolutionary sequence of telescopes, antenna systems, receivers, and (u, v) plane coverage. The telescopes are space‐based because of the disruptive aspects of the Earth's ionosphere on low‐frequency celestial signals traveling to the Earth's surface. Orbiting antennas consisting of array elements deposited on a Kevlar balloon have strong advantages of nearly identical multiple beams over 4π steradians and few mechanical aspects in deployment and operation. The relatively narrow beam width of these antennas can significantly help reduce the “confusion” problem. The evolutionary sequence of telescopes starts with an Earth‐orbiting spectrometer to measure the low‐frequency radio environment in space, proceeds to a two‐element interferometer, then to an orbiting array, and ends with a telescope on the lunar farside. The sequence is in the order of increasing capability which is also the order of increasing complexity and cost. All the missions can be accomplished with current technology.
Signatures of polar cap patches in ground ionosonde data
Authors: James H. G. , MacDougall J. W.
Published: February 2004

The interpretation of localized density structures in the high‐latitude ionosphere as observed by ground ionosondes is a task that is complicated by the complexity of those structures and by the variety of processes that ionosonde waves undergo. To aid interpretation, we have performed a first‐order analysis of the shapes of traces seen in ionosonde displays. This has been done using an idealized model of a particular structure of current interest, a polar ionospheric patch. The patch is modeled as a spheroidal enhancement of density superposed on a horizontally stratified F layer. Ray tracing is used to find O‐ and X‐mode rays that reflect back to a ground ionosonde after total reflection. Theoretical curves are calculated for swept‐frequency (h'ƒ), fixed‐frequency (h't), and Doppler‐frequency (ƒDt) operational modes. The computed h'ƒ curves have U shapes, the h't curves are quasi‐hyperbolic, and the Doppler curves have reverse‐S shapes. By moving the patch through a series of horizontal separations from the ionosonde, the sequence of curves is calculated for a patch passing by an ionosonde. The theoretical traces have been compared with data from the Canadian advanced digital ionosondes (CADIs) at Resolute Bay, Eureka, and Alert. The CADI h'ƒ traces are observed usually to have incomplete U shapes, missing their lower‐frequency arms. In the light of the ray tracing the CADI results are consistent with the hypothesis that HF‐scattering irregularities are found on both the leading and trailing edges of patches but are stronger on the trailing edge. Similar results have been reported for scintillation‐causing irregularities. This points to the dominance of the gradient‐drift instability. The hyperbolic h't curves are asymmetric, having slower downsweeps than upsweeps. This can be caused by horizontal asymmetries in the patch density distribution. Often, polar patches seen by CADI are not single, isolated enhancements. Rather, they appear as composites of substructures, each of which mimics certain aspects of the single patch described by the present theoretical model.
HF radars: Multifrequency study of refraction effects and localization of scattering
Authors: André R. , Hanuise C. , Villain J.‐P. , Cerisier J.‐C.
Published: February 2004

Monostatic HF radars take advantage of the refraction of radio waves in the ionosphere to achieve perpendicularity between the wave vector and the Earth's magnetic field at high latitude. The location of the backscattering region therefore depends on the horizontal and vertical ionospheric electron density profiles. Multifrequency radar data are used to infer estimates of differences in propagation path introduced by refraction without knowledge of the ionospheric plasma distribution. Because the plasma velocity due to the electric field mapped from the magnetosphere is a specific property of the ionosphere, the Doppler velocity measured on the same field line but at different probing frequencies should be identical. Differences in the radial velocity versus group delay profiles obtained at different frequencies can be attributed to differences in the propagation path. These effects are quantified by computing the correlation function between velocity profiles obtained at different frequencies. The shift corresponding to the maximum of the correlation function is a direct measure of the relative delay introduced by differences in ionospheric propagation between the two correlated frequencies. A statistical study has been conducted for several frequencies pairs using data from the Système HF d'Etude Radar Polaires et Aurorales radar. The statistical differences in group path are respectively 20 km for the 11/12‐MHz pair and 60 km for the 11/14‐MHz pair. The observed distribution of propagation path shifts have also been modeled with a ray‐tracing program. It has been shown that the observed experimental values are overestimating the shift. Computed differences, for example, 10 km for the 11/14‐Mhz pair, are lower than experimental ones. The discrepancy is shown to be a consequence of experimental conditions. Finally, errors on the location of scatters made by assuming straight‐line propagation and fixed altitude are quantified. With a judicious choice of the scattering altitude, the maximum error is of the order of 20 km.
Kilometer scale kinetic energy perturbations in the mesosphere derived from EISCAT velocity data
Authors: Hall C. M.
Published: February 2004

Traditionally, radar instruments which obtain coherent echoes from structures delineated by sharp gradients in refractive index have been used as a means of determining parameters describing neutral turbulence in the middle atmosphere. In these cases the very characteristics of the coherent signals are used. An alternative, somewhat akin to using velocity estimations from meteor trails and trails of artificially introduced racers, is to determine the kinetic energy perturbation from the velocity profiles alone, the latter being available from incoherent scatter radar. Here we describe a method which assumes that the velocity fluctuations are caused by perturbations of a 1‐km scale, in the buoyancy subrange, but which still may be indicative of energy flux at the outer scale, illustrated by the remarkable similarity to energy dissipation rates determined by other measurements.
A two‐dimensional analytic ray tracing technique accommodating horizontal gradients
Authors: Norman R. J. , Cannon P. S.
Published: February 2004

A new two‐dimensional analytic ray tracing method, known as segmented method for analytic ray tracing (SMART), which automatically segments the ionosphere along the ray path is described. Unlike other techniques, SMART is able to accurately ray trace through horizontal gradients which vary with altitude along and in the direction of the ray path. Computer run times are approximately 10 times faster than those of numerical ray tracing packages, and ground range errors are typically less than 2.5%.
Effects of anisotropy and power spectrum of refractivity irregularities on the determination of layer thickness and layer position using the frequency domain interferometry technique
Authors: Chen J. S. , Liu J. Y. , Chu Y. H.
Published: February 2004

In this article we study the effects of the wave number power spectrum of refractivity irregularities with anisotropic structures on the determination of the layer thickness and position by using the frequency domain interferometry (FDI) technique of MST radar returns. Numerical results show that both the power spectral form and the anisotropy of the refractivity irregularity contribute remarkably to the magnitude and phase of a two frequency cross‐correlation function, provided that the radar beamwidth is not too narrow. By approximating the power law spectral form in the scattering integration equation to a modified Gaussian function, we propose a more realistic FDI expression for estimating the layer thickness and position. Furthermore, an FDI experiment at the Chung‐Li coherent scatter 52‐MHz radar was carried out on November 10, 1995. The radar data are used not only to calculate FDI coherence and phase but also to estimate the horizontal correlation length of the refractivity irregularities with conventional space antenna drift (SAD) method. Observational result shows that the horizontal correlation lengths are mostly within 10–40 m, indicating the anisotropic behavior of the irregularities. Substituting the observed values into the theoretical FDI expressions, we can obtain the layer thickness and position accordingly. By comparing the results of the proposed FDI expression with those of the existing FDI equations, we show that a large 50% difference in layer thickness estimation and tens of meters difference in layer position estimation are seen when vertical correlation length is assumed to be 3 m. These results suggest that the effects of the power spectral form and the anisotropy of the refractivity irregularity on the determination of layer thickness and layer position are significant and cannot be ignored.
Electron density and height at the F1 region minimum gradient at low solar activity for an equatorial station
Authors: Adeniyi J. O. , Radicella Sandro M.
Published: February 2004

The presence of a ledge of ionization between the E and F2 laver maximum electron density, either with a distinct critical frequency (fOF1) or in the form of an inflection point, is found to be a persistent feature of the equatorial ionosphere from 0700 to 1800 LT at low solar activity. The electron density NF1, and the height hF1, at the position of minimum gradient in ionospheric profiles were investigated for Ouagadougou, Burkina Faso, an equatorial station. NF1 was found to exhibit a clear diurnal variation, which depends on solar zenith angle (x). The cos(x) power was found to be 0.34. An obvious seasonal variation in NF1 was not found. The height hF1 shows neither a diurnal nor seasonal dependence, and has an average value of (166 ± 6) km. NF1 and hF1 were compared at 1000, 1200 and 1400 LT in the months of January and July, for Ouagadougou and Ibadan, Nigeria, another equatorial station. The comparison shows that there is no significant difference in hF1 for both stations, but some differences were found in NF1.
Electromagnetic coupling to conducting objects behind apertures in a conducting body
Authors: Kabalan Karim Y. , El‐Hajj Ali , Khoury Shahwan , Rayes Asaad
Published: February 2004

A procedure for computing characteristic modes for the problem of electromagnetic coupling to arbitrary shape conducting objects behind arbitrary shape apertures in a conducting body is developed starting from the operator formulation of the currents. The mode currents are obtained from the admittance and impedance operators. This formulation is general and characterized by its simplicity and efficiency. This method is applied to a finite thin wire backed by a rectangular aperture in a conducting plane in an unbounded medium. The integral equations in terms of the magnetic and electric current distributions are derived for the magnetic and electric fields. The moment method is used to obtain matrix equations approximating the integral equations. The expansion functions are chosen to satisfy the continuity equations and to vanish at the wire endpoints. Finally, modal solutions are computed to illustrate the convergence of the solution as the number of modes increases.
Changing magmatic and tectonic styles along the paleo‐Pacific margin of Gondwana and the onset of early Paleozoic magmatism in Antarctica
Authors: Encarnación John , Grunow Anne
Published: February 2004

Basement rocks of the Transantarctic Mountains are believed to record a change in the paleo‐Pacific margin of Gondwana from a passive to a tectonically active margin. Widespread emplacement of calc‐alkaline batholiths (Granite Harbor intrusives) occurred during the active margin phase. We present new concordant zircon and titanite U‐Pb ages for these magmatic rocks in southern Victoria Land and the Scott Glacier area. Most magmatic rocks previously associated with a pre‐late Early Cambrian (>530 Ma) deformational event(s) (Beardmore orogeny) have yielded younger crystallization ages. The lack of definite arc magmatism prior to ∼530 Ma suggests that deformation may have been associated with a strike or oblique‐slip regime, although shallow subduction without significant arc magmatism cannot be ruled out. Local transpressional and transtensional domains may account for compressional deformation and rare alkaline and carbonatite magmatism during this early period. The oldest and most voluminous magmatic rocks were emplaced after ∼530 Ma. This magmatism has been associated with active subduction, and suggests a fundamental change in the plate boundary at ∼530 Ma. Ductile shearing of plutons and contractional deformation of supracrustal rocks after ∼530 Ma (Ross orogeny) may have been due to transpressional tectonics in an oblique subduction setting and/or a collision. Compressional deformation associated with the Ross orogeny may have ceased by ∼500 Ma along the southern Victoria Land‐Scott Glacier segment of the Antarctic margin, as indicated by undeformed magmatic rocks of this age, although magmatic activity continued to at least ∼485 Ma.
Tectonic affinity of Nisutlin and Anvil assemblage strata from the Teslin tectonic zone, northern Canadian Cordillera: Constraints from neodymium isotope and geochemical evidence
Authors: Creaser Robert A. , Erdmer Philippe , Stevens Robert A. , Grant Steven L.
Published: February 2004

We present geochemical and isotopic data for Nisutlin assemblage metasedimentary rocks and Anvil assemblage greenstones from the Teslin tectonic zone of the northern Canadian Cordillera. This study aims to establish the tectonic setting of formation for the sedimentary and basaltic protoliths of these highly deformed and metamorphosed rocks and thereby place constraints on the origin of these enigmatic rocks for which differing tectonic models have been proposed. For the Nisutlin assemblage metasedimentary rocks, the geochemical and isotopic data show that two widely different source regions contributed detritus to the original sediments. One source region was felsic, upper crustal material with Nd isotopic compositions compatible with ultimate derivation from the North American continent (depleted mantle model age (TDM) 2.5–2.8 Ga). The second source region is deduced to be chemically primitive crust (basaltic‐andesitic) with a short crustal residence history (<0.9 Ga). For the Anvil assemblage greenstones, immobile trace element abundances are dissimilar to within‐plate, oceanic arc and normal mid‐ocean ridge basalts and similar to calc‐alkaline basalts in active continental margin settings. We interpret the paleosetting for the Nisutlin assemblage to be at the outermost margin of the ancestral North American continent, in an area which received detrital input from the distal North American craton in the early Paleozoic. However, this area also received detritus from a chemically and isotopically juvenile magmatic arc source, a source type not known from Paleozoic metasedimentary rocks from the miogeoclinal sequence. On the basis of similar Nd isotopic relationships recorded elsewhere in deformed Paleozoic rocks of the orogen, we infer that these geochemical signals reflect tectonic processes of regional extent. The trace element geochemistry of the Anvil assemblage greenstones does not support a correclation with known Paleozoic greenstones of the Slide Mountain terrane, which some tectonic models have advocated.
Age Dating of Shallow Groundwater with Chlorofluorocarbons, Tritium/Helium: 3, and Flow Path Analysis, Southern New Jersey Coastal Plain
Authors: Szabo Z. , Rice D. E. , Plummer L. N. , Busenberg E. , Drenkard S. , Schlosser P.
Published: February 2004

Groundwater age dating through the combination of transient tracer methods (chlorofluorocarbons (CFCs) and tritium/helium 3 (3H/3He)) and groundwater flow path analysis is useful for investigating groundwater travel times, flow patterns, and recharge rates, as demonstrated by this study of the homogeneous shallow, unconfined Kirkwood‐Cohansey aquifer system in the southern New Jersey coastal plain. Water samples for age dating were collected from three sets of nested observation wells (10 wells) with 1.5‐m‐long screens located near groundwater divides. Three steady state finite difference groundwater flow models were calibrated by adjusting horizontal and vertical hydraulic conductivities to match measured heads and head differences (range, 0.002–0.23 m) among the nested wells, with a uniform recharge rate of 0.46 m per year and porosities of 0.35 (sand) and 0.45 (silt) that were assumed constant for all model simulations and travel time calculations. The simulated groundwater travel times increase with depth in the aquifer, ranging from about 1.5 to 6.5 years for the shallow wells (screen bottoms 3–4 m below the water table), from about 10 to 25 years for the medium‐depth wells (screen bottoms 8–19 m below the water table), and from about 30 to more than 40 years for the deep wells (screen bottoms 24–26 m below the water table). Apparent groundwater ages based on CFC‐ and 3H/3He‐dating techniques and model‐based travel times could not be statistically differentiated, and all were strongly correlated with depth. Confinement of 3He was high because of the rapid vertical flow velocity (of the order of 1 m/yr), resulting in clear delineation of groundwater travel times based on the 3H/3He‐dating technique. The correspondence between the 3H/3He and CFC ages indicates that dispersion has had a minimal effect on the tracer‐based ages of water in this aquifer. Differences between the tracer‐based apparent ages for seven of the 10 samples were smaller than the error values. A slight bias toward older apparent ages, found not to be statistically significant, was noted for the 3H/3He‐dating technique relative to the CFC‐dating technique. This result may be caused by enrichment of local air in CFC‐Il and CFC‐12 from urban and industrial sources in the northeastern United States and minor contamination from sampling equipment. The demonstrated validity of the combined tracer‐dating techniques to determine the age of water in the Kirkwood‐Cohansey aquifer system indicates that groundwater flow models can be refined when apparent ages based on 3H/3He‐ and CFC‐ dating are used as calibration targets.
Effects of Farmer Response to Nitrogen Fertilizer Management Practices on Groundwater Quality
Authors: Kim C. S. , Sandretto Carmen , Hostetler John
Published: February 2004

This research investigates the validity of efforts to protect groundwater quality for a variety of beneficial uses by using a widely accepted nitrate‐accounting procedure as the mechanism to obtain reductions in farmer‐applied nitrogen fertilizers. A dynamic model of nitrogen fertilizer use, incorporating critical elements of the nutrient cycle, is employed to demonstrate that recommended nitrogen fertilizer application rates based on this nitrate‐accounting procedure underestimate nitrogen fertilizer use. These results occur because the nitrate‐accounting procedure inadequately represents farmer responses to nitrates available from irrigation water. An optimal tax rate on nitrogen fertilizer use is derived which, if implemented, would lead to the reduced levels of nitrogen fertilizer use intended by the nitrate accounting procedure.
A Nonparametric Wet/Dry Spell Model for Resampling Daily Precipitation
Authors: Lall Upmanu , Rajagopalan Balaji , Tarboton David G.
Published: February 2004

A nonparametric wet/dry spell model is developed for resampling daily precipitation at a site. The model considers alternating sequences of wet and dry days in a given season of the year. All marginal, joint, and conditional probability densities of interest (e.g., dry spell length, wet spell length, precipitation amount, and wet spell length given prior to dry spell length) are estimated nonparametrically using at‐site data and kernel probability density estimators. Procedures for the disaggregation of wet spell precipitation into daily precipitation and for the generation of synthetic sequences are proffered. An application of the model for generating synthetic precipitation traces at a site in Utah is presented.
Plume development of a nonlinearly adsorbing solute in heterogeneous porous formations
Authors: Bosma W. J. P. , van der Zee S. E. A. T. M. , van Duijn C. J.
Published: February 2004

Transport of nonlinearly adsorbing solutes in homogeneous and heterogeneous porous formations is considered. Initially, a fixed amount of solute is assumed to be present in the domain. Transport is characterized in terms of the first and second spatial moments. Nonlinear equilibrium adsorption is described by the Freundlich isotherm, with the Freundlich exponent n, 0 < n < 1. By asymptotic balancing we derive first‐order approximations of the limiting behavior of the plume position and plume growth as a function of time for the homogeneous case. In the heterogeneous case we consider random variation of a physical (log conductivity) and a chemical (log adsorption coefficient) parameter, both with an isotropic exponential covariance function. Expected behavior of the relevant spatial moments is obtained by applying a Monte Carlo approach. Individual realizations are solved numerically with a particle‐tracking scheme in which nonlinear adsorption is accounted for by a time‐dependent retarded velocity. We assess the effects of varying certain transport parameters, such as the degree of physical and chemical heterogeneity, degree of nonlinearity, the adsorption coefficient, and the degree of correlation between hydraulic conductivity and adsorption coefficient. Results of the homogeneous case for a strong degree of nonlinearity show good agreement between the numerical calculations and the limiting analytical expressions. Nonlinear adsorption is shown to have a strong effect on the shape of the plume, especially in longitudinal direction. For the heterogeneous case the analytical expressions predict well the time dependence of the growth rate of the spatial moments. Variation of the transport parameters demonstrates a dominating effect of the degree of nonlinearity on the plume dimensions, which is hardly affected by the degree of heterogeneity, correlation, and the adsorption coefficient. The variance of the mean plume position is affected by all parameters. Although the degree of heterogeneity has a strong impact, the longitudinal and transverse variances are reduced with respect to the linear adsorption case, due to the large size of the plumes.
Transport of conservative chemical through an unsaturated two‐dimensional Miller‐similar medium with steady state flow
Authors: Roth Kurt , Hammel Klaus
Published: February 2004

Numerical simulation of water flow in a two‐dimensional, macroscopically homogeneous, Miller‐similar medium showed the existence of a network of flow channels with two complementary states separated by a critical point [Roth, 1995]. The consequences of this for solute transport are explored by numerical simulations using particle tracking. It is found that many experimentally observed features of transport through soil are reproduced qualitatively by these simulations. Analyzing the results reveals that in the corresponding effective medium the travel distance for the transition to convection‐dispersion and the effective dispersivity depend on the water flux. In particular, the effective longitudinal dispersivity, which is often assumed to be a material constant of the porous structure, is found to vary by more than an order of magnitude with a minimum near the critical point. The simulations further demonstrate that the local structure of the velocity field and the subscale hydrodynamic dispersion are of minor importance for the field‐averaged transport process.
Water Pathways and Chemistry at the Groundwater/Surface Water Interface to Lake Skjervatjern, Norway
Authors: Norrström A. C. , Jacks G.
Published: February 2004

Macropore flow at the groundwater/surface water interface to a humic lake and the seepage fluxes through the lake bottom were measured, as well as was the groundwater chemistry. With a 100‐fold difference in discharge from the lake, the seepage rates through the lake bottom differed by only 15%, indicating that macropores provided a major part of the inflow during high‐flow events. Two larger macropores had flow rates between 0.3 and 0.7 L s−1, corresponding to velocities of 1–3 m min−1. The flow rates of four smaller macropores were recorded to only 0.02–0.1 × 10−4 L s−1. One of the larger macropores had water with high concentration of alkalinity, Ca and Si in dry periods, whereas during wet periods dissolved organic carbon was elevated and pH was depressed. This is a indication of different source areas depending on the rainfall rate. As the alkalinity/chloride ratios were lower in the macropores than in the peat water, acid groundwater seemed to find a shortcut directly into the lake.