bluuebunny/crossref_metadata_2025 huggingface dataset:
Metastable CaCO3 dissolution at intermediate water depths of the Caribbean and western North Atlantic: Implications for intermediate water circulation during the past 200,000 years
Authors: Haddad Geoffrey A. , Droxler André W.
Published: February 2004

We present late Quaternary records of metastable carbonate dissolution determined for sediment cores recovered from intermediate water depths on the Nicaragua Rise (Caribbean Sea, 1000–1894 m) and near the Bahama Banks (western North Atlantic Ocean, 655 and 1934 m). Upper North Atlantic Deep Water is believed to dominate these two regions at present. Both areas are predicted to be good locations to study past variations of average middepth Atlantic chemistry and circulation. However, statistical analyses of metastable carbonate dissolution indices (% Mg calcite, pteropod abundance, % whole pteropods, and % clear pteropods) yielded a composite dissolution index (CDI) which displays different carbonate dissolution histories for Bahama and Nicaragua Rise sediments. The Bahama records resemble deep Atlantic carbonate records with dissolution during glacial oxygen isotope stages 6 and 4 and with preservation during interglacial stages 5 and 1. We observe two dissolution patterns at intermediate water depths of the Caribbean. The “deep” intermediate water pattern (1200–1894 m) resembles deep Caribbean carbonate records with dissolution during interglacial periods and preservation during glacial intervals. The “shallow” intermediate water record (<1200 m) resembles at times the Bahamas record and at other times the deep Caribbean record. These CDI records suggest that different water masses occupied the intermediate depth western North Atlantic and Caribbean during much of the late Quaternary. Observed differences between these regions may be related to variations in the flow of nutrient‐rich, low CO3= Antarctic Intermediate Water through the Caribbean or to changes in the ventilation rate of equatorial thermocline and intermediate waters. These water‐mass variations have influenced Caribbean carbon chemistry from the base of the thermocline down to abyssal water depths and may have had a significant effect on North Atlantic circulation and nutrients.
Origin of ice‐rafted debris: Pleistocene paleoceanography in the western Arctic Ocean
Authors: Bischof Jens , Clark David L. , Vincent Jean‐Serge
Published: February 2004

The composition of Pleistocene ice‐rafted debris (IRD) >250 µm was analyzed quantitatively by grain counting in five sediment cores from the western central Arctic Ocean and compared with the composition of till clasts from NW Canada in order to determine the dropstone origin and to reconstruct the Pleistocene ice driftways and surface currents. The IRD composition alternates repeatedly between carbonate‐ and quartz‐dominated assemblages, along with metamorphic and igneous rocks, clastic rocks, and some chert. The highest quartz content is found on the Alpha Ridge, while carbonate percentages are highest on the Northwind Ridge (NWR) and the Chukchi Cap. The source for the carbonates is the area around Banks and Victoria Islands and parts of northern Canada. Quartz most likely originated from the central Queen Elizabeth Islands. IRD on the southeastern Alpha Ridge is dominated by mafic crystalline rocks from northern Ellesmere Island and northern Greenland. At least six major glacial intervals are identified within the last 1 million years, during which icebergs drifted toward the west in the Beaufort Sea, straight northward in the central Arctic Ocean, and northeastward on the SE Alpha Ridge.
Barium in equatorial Pacific carbonate sediment: Terrigenous, oxide, and biogenic associations
Authors: Schroeder J. O. , Murray R. W. , Leinen M. , Pflaum R. C. , Janecek T. R.
Published: February 2004

We have analyzed carbonate sediment from Ocean Drilling Program Site 850 (equatorial Pacific Ocean) in order to assess the changing effects of sedimentation style on Ba accumulation through time. Formed along the East Pacific Rise ∼12 m.y. ago at 1°–2°S and now located at 1.3°N, Site 850 has experienced many changes through its northwesterly migration, including a crossing of the equator at ∼4 Ma. We divide sedimentation into three stages (phases I, II, and III; with phase III being youngest) according to geographic position, lithostratigraphy, and bulk accumulation rate. Of greatest importance are contrasts between phase II (7.5–4.0 m.y. ago) and phase III (4.0–0 m.y. ago). Phase II includes the previously described “biogenic bloom” as well as the depositional record of elevated productivity near the equator. In phase II the accumulation of Ba shows the strongest correlation with the accumulation of CaCO3 (r² = 0.69), opal (r² = 0.44), and Corg (r² = 0.41) compared with elsewhere through the sequence, although the correlation with terrigenous accumulation is also very strong (r² = 0.63). In phase III, which records deposition in the northern hemisphere and is thus closer to terrigenous input at the Intertropical Convergence Zone (ITCZ), there are no statistically significant correlations between the accumulations rates of Ba and CaCO3, Ba and opal, and Ba and Corg. Most significantly, through phase III the Ba accumulation rate is extremely strongly tied to terrigenous accumulation (r² = 0.89), which is the strongest correlation of any in our database, and to the accumulation of Feexcess (r² = 0.74), which we use to track the Fe‐oxide component. Cross‐equator surface sediment transects have previously indicated that beneath the ITCZ there is a pronounced local maximum in Ba accumulation, and the strong tie between Ba and terrigenous accumulation and associated Fe‐oxides at Site 850 also corresponds with its tectonic migration toward the ITCZ. We conclude that the putative link between Ba accumulation and export production may be obscured by changes in particle composition even within a dominantly biogenic sedimentary regime, as well as by early diagenetic transfer between phases, and that the non‐barite elemental Ba inventory may complicate the use of elemental Ba as a quantitative proxy for barite in the bulk sediment.
Early Holocene decadal‐scale ocean variability recorded in Gulf of California laminated sediments
Authors: Pike Jennifer , Kemp Alan E. S.
Published: February 2004

Scanning electron microscope examination of Holocene laminated sediment from Guaymas Basin, Gulf of California, resolves up to five depositional events per year. Each lamina/sublamina of an early Holocene section of continuously laminated sediment was recorded from backscattered electron imagery photomosaic analysis. Diatom mat laminae, composed predominantly of Thalassiothrix longissima, are associated with early winter mixed diatom flora laminae. Mats are probably brought into the Gulf with Pacific water during the summer, concentrated at the surface or at the pycnocline, and then rapidly deposited in the early winter as water column stratification breaks down. Time series analysis of one ∼300‐year section from piston core JPC56 revealed significant periodicities in the deposition of mat laminae at ∼50 years, ∼11 years, and 22–24 years. An ∼50‐year cycle in fish populations has been related to changing North Pacific ocean/atmosphere circulation. These Pacific‐wide changes in circulation affect the currents dominant at the mouth of the Gulf. When the California Current is dominant, fewer diatom mats are imported into the Gulf, and when the north equatorial Pacific gyre is dominant, more are imported. The ∼11‐year, 22‐ to 24‐year, and ∼50‐year cycles are all suggested to be influenced by solar cycles.
Postglacial connection of the Black Sea to the Mediterranean and its relation to the timing of sapropel formation
Authors: Lane‐Serff Gregory F. , Rohling Eelco J. , Bryden Harry L. , Charnock Henry
Published: February 2004

The opening of the connection between the Mediterranean and Black Seas as sea level rose above the Bosporus sill has long been associated with the formation of the most recent, Holocene, sapropel deposit (S1) in the eastern Mediterranean, but the mechanism has remained elusive. We present a model for the opening of the Black Sea, based on hydraulics arguments, which demonstrates that increased freshwater flux out of the Black Sea began 500–1000 years after sea level reached sill depth and that the timescale for the increased freshwater flux that drained the freshwater reservoir of the Black Sea is about 2500–3500 years. We argue that the increased freshwater discharge out of the Black Sea would lead to decreased deep water formation and higher productivity in the surface waters in the eastern Mediterranean, two conditions generally associated with sapropel formation. The delay in increased freshwater flux after the opening of the Black Sea and the period of increased freshwater discharge appear to match the onset and duration of sapropel deposits in the eastern Mediterranean.
Phytoplankton bloom dynamics in coastal ecosystems: A review with some general lessons from sustained investigation of San Francisco Bay, California
Authors: Cloern James E.
Published: February 2004

Phytoplankton blooms are prominent features of biological variability in shallow coastal ecosystems such as estuaries, lagoons, bays, and tidal rivers. Long‐term observation and research in San Francisco Bay illustrates some patterns of phytoplankton spatial and temporal variability and the underlying mechanisms of this variability. Blooms are events of rapid production and accumulation of phytoplankton biomass that are usually responses to changing physical forcings originating in the coastal ocean (e.g., tides), the atmosphere (wind), or on the land surface (precipitation and river runoff). These physical forcings have different timescales of variability, so algal blooms can be short‐term episodic events, recurrent seasonal phenomena, or rare events associated with exceptional climatic or hydrologic conditions. The biogeochemical role of phytoplankton primary production is to transform and incorporate reactive inorganic elements into organic forms, and these transformations are rapid and lead to measurable geochemical change during blooms. Examples include the depletion of inorganic nutrients (N, P, Si), supersaturation of oxygen and removal of carbon dioxide, shifts in the isotopic composition of reactive elements (C, N), production of climatically active trace gases (methyl bromide, dimethylsulfide), changes in the chemical form and toxicity of trace metals (As, Cd, Ni, Zn), changes in the biochemical composition and reactivity of the suspended particulate matter, and synthesis of organic matter required for the reproduction and growth of heterotrophs, including bacteria, zooplankton, and benthic consumer animals. Some classes of phytoplankton play special roles in the cycling of elements or synthesis of specific organic molecules, but we have only rudimentary understanding of the forces that select for and promote blooms of these species. Mounting evidence suggests that the natural cycles of bloom variability are being altered on a global scale by human activities including the input of toxic contaminants and nutrients, manipulation of river flows, and translocation of species. This hypothesis will be a key component of our effort to understand global change at the land‐sea interface. Pursuit of this hypothesis will require creative approaches for distinguishing natural and anthropogenic sources of phytoplankton population variability, as well as recognition that the modes of human disturbance of coastal bloom cycles operate interactively and cannot be studied as isolated processes.
Glaciochemistry of polar ice cores: A review
Authors: Legrand Michel , Mayewski Paul
Published: February 2004

Human activities have already modified the chemical composition of the natural atmosphere even in very remote regions of the world. The study of chemical parameters stored in solid precipitation and accumulated on polar ice sheets over the last several hundred thousand years provides a unique tool for obtaining information on the composition of the preindustrial atmosphere and its natural variability over the past. This paper deals with the chemistry of polar ice focused on the soluble mineral (Na+, NH4+, K+, Ca++, Mg++, H+, F−, Cl−, NO3−, SO4−−, and H2O2) and organic (methanesulfonate (CH3SO3−), formate (HCOO−), acetate (CH3COO−), and formaldehyde (HCHO)) species and their interpretation in terms of past atmospheric composition (aerosols and water soluble gaseous species). We discuss ice core dating, the difficulties connected with trace measurements, and the significance of the ionic composition of snow. We examine temporal (from the last decades back to the last climatic cycle) and spatial (including examples from coastal as well as central areas of Greenland and Antarctica) variations in the ionic budget of the precipitation and evaluate ice core studies in terms of the chemical composition of our past atmosphere. We review (1) how Greenland and Antarctic ice cores that span the last few centuries have provided information on the impact of human activities and (2) how the chemistry of deep ice cores provides information on various past natural phenomena such as climatic variations (glacial‐interglacial changes, El Niño), volcanic eruptions, and large boreal forest fires.
Dynamics of ionospheric ions in the magnetospheric tail: Role of the large‐scale electric field
Authors: Voronov Eugene V. , Krinberg Igor A.
Published: February 2004

The motion of ionospheric ions in the magnetospheric tail is considered using a minimal set of well‐known factors, such as the tail‐like configuration of the magnetic field and the presence of the dawn‐to‐dusk magnetospheric electric field. The calculated O+ ion density and average energy are in close agreement with satellite measurements. An abrupt increase of the magnetospheric electric field leads to formation of a hot plasma cloud in the plasma sheet, which convects earthward. In contrast, if the electric field varies slowly in time, then the mean energy of ionospheric ions in the near‐Earth tail remains almost independent of activity level.
Polarization processes in rocks: 1. Complex Dielectric Permittivity method
Authors: Levitskaya Tsylya M. , Sternberg Ben K.
Published: February 2004

This is the first part of a review of research performed in the former USSR. Experimental data were used from several regions of the USSR, including Russia, Ukraine, and Georgia. Many of the publications are available in U.S. libraries. Some of them are translated into English. This part contains results from applying the Complex Dielectric Permittivity method (Dielectric Spectroscopy) for studying the electrical response of rocks in alternating fields with frequencies from 100 Hz to 100 MHz. Data on dielectric properties of sedimentary rocks of different lithology and with various porosities, salinities of saturating solution, and hydrocarbon content are reviewed here. Measurement methods, including means for avoiding or reducing the electrode polarization, are also considered. It is shown that wet rocks exhibit a Maxwell‐Wagner polarization process at frequencies 105–107 Hz, caused by charge accumulation on the pore boundaries.
Polarization processes in rocks: 2. Complex Dielectric Permittivity method
Authors: Levitskaya Tsylya M. , Sternberg Ben K.
Published: February 2004

This is the second part of a review of research performed in the former USSR. Experimental data were used from several regions of the USSR, including Russia, Ukraine, and Georgia. Many of the publications are available in U.S. libraries. Some of them are translated into English. This part considers results of studying rocks with the Complex Electrical Resistivity method, primarily in the frequency range from 0.01 Hz to 50 MHz. Results of studying polarization processes in sedimentary rocks of different lithology and porosity with different types of saturation (salt solutions of various salinity and hydrocarbons) are described. Laboratory studies of ore‐mineral containing rocks are also considered. Describing the measurement methods, we have placed primary emphasis on means for avoiding or reducing the electrode polarization effects. It is shown that wet rocks may have two polarization processes: one at low frequencies, below 1 Hz, and one at high frequencies, above 10 kHz. The mechanisms of these processes are discussed based on experimental results.
Domain decomposition for parallelization of finite difference schemes for parabolic equations in wave propagation
Authors: Marcus Sherman W. , Degani David
Published: February 2004

Tropospheric wave propagation can be described by a parabolic differential equation which may be solved using implicit finite differences. The large grid systems which can be required in such solutions make parallelization expedient. This can be accomplished by decomposing the computational domain into several subdomains over which the solution is calculated in parallel at each step in a marching procedure. A Schwarz‐type algorithm, which has become popular in computational fluid dynamics applications, is investigated for accomplishing this domain decomposition. It is found to be unconditionally unstable for the wave propagation parabolic equation when the subdomains overlap, while for nonoverlapping subdomains, its accuracy depends strongly on the method for establishing the boundary conditions on the pseudoboundaries of the subdomains.
Analysis of electromagnetic fields in inhomogeneous dielectric gratings with periodic surface relief
Authors: Yamasaki Tsuneki , Tanaka Hirotaka , Hinata Takashi , Hosono Toshio
Published: February 2004

The electromagnetic fields in inhomogeneous dielectric gratings with periodic surface relief are analyzed using the combination of Fourier series expansion method and multilayer method. In the analysis the inhomogeneous region is divided into an assembly of stratified thin layers with modulated index, which is expressed by a fraction ƒ(z)/g(z), where ƒ(z) and g(z) are simple periodic functions. This method is applicable to a wide range of inhomogeneous dielectric gratings for both scattering and guiding problems. The order of the matrix to be solved depends only on the modal truncation number, not on the number of layers. Numerical results are given for the scattering problem of inhomogeneous dielectric gratings whose surface relief profile is sinusoidal and whose interior distribution of permittivity is a sinusoidally modulated type for both transverse electric and transverse magnetic cases. The influences of the incident angle and the frequency on the transmitted power are compared between the homogeneous medium and the inhomogeneous medium.
High‐frequency asymptotic acceleration of the fast multipole method
Authors: Burkholder Robert J. , Kwon Do‐Hoon
Published: February 2004

The plane wave translation operator of the fast multipole method (FMM) is evaluated asymptotically in the high‐frequency limit. This operator relates the plane wave components of a source region to a plane wave expansion for the radiated fields over a region away from the source. The asymptotic evaluation exploits the source‐to‐receiver directivity of the operator, leading to terms analogous to the geometrical optics (GO) and diffracted fields in the uniform theory of diffraction. Most importantly, the GO term identifies a “lit” region which may be used to define a windowing function for filtering out weak plane wave translations in the “shadow” region. The reduction in plane waves lowers the operational count of the FMM from O(N3/2) to O(N4/3) without increasing the complexity of the implementation.
Frequency‐independent features of self‐similar fractal antennas
Authors: Werner D. H. , Werner P. L.
Published: February 2004

A generalized approach to the study of frequency‐independent antennas is presented which relies on the recently developed theory of fractal geometry. It is demonstrated that this fractal geometric interpretation allows for the ability to characterize a much wider class of frequency‐independent antennas. This includes radiating structures which are self‐similar in the discrete sense, the smooth sense, and even the “rough” sense. The antenna configurations in this paper are all self‐similar and have been parameterized in terms of a common similarity factor of τ. Finally, it is shown how this new theory of self‐similar fractal radiators may be employed to develop a multiband linear array design methodology for which the directive gain is a log‐periodic function of frequency.
Electromagnetic and elastodynamic point source excitation of unbounded homogeneous anisotropic media
Authors: Marklein René , Langenberg Karl J. , Kaczorowski Torsten
Published: February 2004

Plane electromagnetic as well as plane elastodynamic waves in anisotropic media exhibit a different direction of their phase and energy propagation, resulting in slowness and group velocity surfaces. Of course, the availability of plane wave solutions gives rise to a spectral plane wave decomposition of point source excitations, i.e., Green's functions. Unfortunately, the coordinate‐free closed‐form solution of dyadic (electric) Green's functions in the Rω space is only known for electromagnetic (generalized) uniaxial media. Utilizing the relation between phase and group velocities of plane waves in uniaxial media we have been able to show that the phase and amplitude of the Green's function is related to the group velocity; i.e., time domain wave fronts reproduce group velocity surfaces. This has also been verified through numerical results obtained by the three‐dimensional (3‐D) electromagnetic finite integration technique code. In elastodynamics, where similar analytical results for anisotropic media are not available, we confirm this behavior with our numerical 3‐D elastodynamic finite integration technique code. For electromagnetic uniaxial media, we present an analytic method to derive the dyadic far‐field Green's function in Rω space from Kω space directly by utilizing the duality principle between wave vectors and ray vectors without performing the 3‐D inverse Fourier transform from Kω space to Rω space analytically.
An improved formalism for electromagnetic scattering from a perfectly conducting rough surface
Authors: Milder D. Michael
Published: February 2004

The recently introduced operator expansion formalism has brought new analytical power and computational efficiency to the scalar theory of rough‐surface scattering. Extending this method to electromagnetic scattering from a perfectly conducting surface is the subject of this paper. This simplest of boundaries imposes a Dirichlet‐like condition on the electric field component locally tangent to the surface; Maxwell's equations determine the normal component through a Neumann‐like condition, which requires an inverse operator to the normal differentiator. The resulting formalism, while considerably more elaborate than the scalar version, is nevertheless orders of magnitude more efficient than existing matrix methods, so that problems of three‐dimensional scattering from surfaces of multiscale roughness can now be computed accurately.
Application of higher‐order vector basis functions to surface integral equation formulations
Authors: Aberegg Keith R. , Taguchi Atsuo , Peterson Andrew F.
Published: February 2004

Higher‐order vector basis functions are described that provide a linear normal and quadratic tangential representation of a vector quantity on rectangular or triangular cells. These functions are used to represent the surface current density on a variety of scatterers, including perfectly conducting plates, spheres, and cone‐spheres. Results are compared with lower‐order rooftop basis functions that provide a constant normal and linear tangential vector representation. Results suggest that the higher‐order basis functions provide improved accuracy for a given number of unknowns but that special functions incorporating edge singularities are needed for a robust treatment of scatterers with corners or edges.
On the theory of electromagnetic waves scattering from the sea surface at low grazing angles
Authors: Voronovich A. G.
Published: February 2004

Scattering of electromagnetic (EM) waves from the sea surface is usually considered with the help of a two‐scale (composite surface) model. It is shown theoretically that for broad wavelength range at grazing angles less than about 20°, diffraction of the incident field on large‐scale (undulating) components cannot be considered using the tangent plane (Kirchhoff) approximation. In other words, an undulating surface cannot be treated as a set of locally plane facets, and its curvature becomes an important parameter. The effects of curvature of the undulating surface are estimated for a two‐dimensional situation. It is shown that in this case, backscattering cross sections at low grazing angles are proportional to the sin2 α rather than to the sin4 α (which would be the case according to Rice's classical formulae). This corresponds to experimental results. The theoretical curve of the σHH/σVV ratio fits well the experimental data published by Lee et al. [1995] (for “slow” signals).
Shadow boundary incremental length diffraction coefficients for perfectly conducting smooth, convex surfaces
Authors: Yaghjian Arthur D. , Shore Robert A. , Woodworth Margaret B.
Published: February 2004

General expressions are obtained for incremental length diffraction coefficients (ILDCs) that can be integrated along the shadow boundaries of perfectly conducting surfaces to correct for the fields radiated by the “shadow boundary current” missing from the physical optics (PO) current approximation. As an initial step in obtaining these shadow boundary ILDCs, uniform high‐frequency approximations are derived for the scattered and PO far fields of perfectly conducting circular cylinders illuminated by normally and obliquely incident plane waves. Subtracting the approximate PO far fields from the approximate scattered far fields and separating the resulting fields into contributions from the top and bottom of the cylinder leads to a uniform high‐frequency approximation for the fields radiated by the shadow boundary current at a single shadow boundary of the cylinder. The shadow boundary ILDCs are then determined by substituting these high‐frequency approximations for the shadow boundary fields into the general formulas derived in previous papers for obtaining three‐dimensional ILDCs from the two‐dimensional far fields of cylindrical scatterers. Comparisons with the exact eigenfunction solution for plane wave scattering by a sphere show that the integration of the ILDC fields around the shadow boundary of the sphere significantly enhances the accuracy of the PO far fields, especially for large bistatic scattering angles. Finally, the shadow boundary ILDCs are modified to increase their accuracy when they are applied to surfaces with more rapidly varying radii of curvature.
Radar imaging of equatorialFregion irregularities with maximum entropy interferometry
Authors: Hysell D. L.
Published: February 2004

Radar interferometry with multiple collinear antenna baselines has been performed at Jicamarca, near Lima, Perú, and high‐resolution images of plasma irregularities in equatorial spreadFhave been constructed from the data. The images represent the brightness distribution of the radar backscatter, which is the Fourier transform of the visibility or spatial cross correlation of the scattered radar signal on the ground. The visibility data are noisy and incompletely sampled, and we have used the maximum entropy method to perform the Fourier transform, making optimal use of the data available. Using the current antenna configuration at Jicamarca, the radar imaging technique is able to resolve structures within the radar beam that are just a few kilometers wide. By animating sequences of images, we can observe the evolution of ionospheric irregularities without confusing spatial and temporal variations.
Inverse scattering theory for optical waveguides and devices: Synthesis from rational and nonrational reflection coefficients
Authors: Jordan Arthur K. , Tamil Lakshman S.
Published: February 2004

Electromagnetic inverse scattering theory is applied to the synthesis of optical waveguides and devices from specified reflection and transmission characteristics. The permittivity profiles in the inhomogeneous core regions of the devices are reconstructed from their transverse reflection coefficients. Two applications of this theory are demonstrated with examples using specified reflection coefficients: design of a single‐mode inhomogeneous optical waveguide and design of an optical logic gate. The previous inverse scattering theory which used reflection coefficients that are rational functions of the transverse wavenumber has been supplemented with an inverse scattering theory that uses nonrational reflection coefficients. This inverse scattering theory uses an iterative discretized space‐time finite‐difference method with the “leapfrog” algorithm. The initial values for the iterative method are obtained from the inverse theory that uses rational function reflection coefficients. The inverse scattering calculations have been verified by finite‐difference frequency domain direct scattering techniques.
Transient excitation of two coupled wires over an interface between two dielectric half spaces
Authors: Rubio Bretones Amelia , Tijhuis Anton G.
Published: February 2004

The transient excitation of two identical, straight, thin wire antennas above a plane interface between two homogeneous dielectric half spaces is analyzed. The two wires are located parallel to each other and to the interface, and one of them is excited by a voltage source. By applying symmetry considerations, the problem is decomposed into two single‐wire problems, for which a method of solution is available from previous work by the authors [Rubio Bretones and Tijhuis, 1995]. The problem is solved in two steps. First, the configuration of two wires in a homogeneous medium is studied. The electric field integral equation for the total current on the wires is derived directly in the time domain and subsequently solved by using the continuous‐time discretized‐space approach. This results in a linear system of equations of a fixed dimension which is solved by marching on in frequency. Subsequently, we consider the complete configuration. As in our previous work, the field reflected by the interface is treated as a secondary incident field in the integral equation for the currents on the two wires. This leads to an integral equation of a form similar to the one describing the currents on the two wires in free space. In this equation the response of a pulsed dipole source in the two‐media configuration occurs as a Green's function. The spatial Fourier inversion involved is carried out with the aid of a fixed composite Gaussian quadature rule. This again leads to a system of equations of a fixed dimension, which can be solved by marching on in frequency. Finally, some representative numerical results are presented and discussed.
Signal intensity in the geometrical optics approximation for the magnetized ionosphere
Authors: Västberg Anders , Lundborg Bengt
Published: February 2004

The paper discusses theoretical and numerical aspects of signal intensity calculations in the geometrical optics approximation for ionospheric HF wave propagation. We present an efficient method for calculating a flux tube of rays for a very general ionosphere. The method is new in the sense that it allows for the effects of electron collisions and the anisotropy produced by the Earth's magnetic field. The basic idea is to calculate the focusing of infinitesimally separated rays by integrating the differential equations for the deviated rays along a reference ray. Some of the first results obtained by this technique are presented.
Evolution in polarimetric signatures of thin saline ice under constant growth
Authors: Nghiem S. V. , Kwok R. , Yueh S. H. , Gow A. J. , Perovich D. K. , Kong J. A. , Hsu C. C.
Published: February 2004

An experiment is carried out to measure polarimetric backscatter signatures at C band together with physical characteristics of thin saline ice grown at a constant rate under quiescent conditions. The objectives are to investigate the electromagnetic scattering mechanism in saline ice, to relate the polarimetric backscatter to ice physical characteristics, and to assess the inversion of ice thickness from backscatter data. Controlled laboratory conditions are utilized to avoid complicated variations in interrelated characteristics of saline ice and the environment. The ice sheet was grown in a refrigerated facility at the U.S. Army Cold Regions Research and Engineering Laboratory. Growth conditions, thickness and growth rate, temperatures and salinities, and internal and interfacial structures of the ice sheet were monitored. Measurements indicate that the laboratory saline ice has characteristics similar to thin sea ice in the Arctic. A strong increase of 6–10 dB is observed in backscatter as the ice grows from 3 to 11.2 cm in thickness. Ice characteristics and processes suggest that the large enhancement in backscatter relates to the interconnection and increase in the size of brine inclusions during the desalination process. Polarimetric signatures calculated with a physically based sea ice model agree with backscatter data at incident angles from 20° to 35° over the thickness range of the ice growth. Furthermore, backscattering coefficients of the saline ice sheet are shown to be similar to airborne radar measurements of thin sea ice growing in a newly opened lead in the Beaufort Sea. For the inversion the large increase in backscatter indicates that the ice thickness is retrievable for thin ice grown under the conditions in this experiment. More complicated conditions should be considered in future experiments to study their effects on the retrieval of sea ice parameters.
A physically based, simple prediction method for scattering interference
Authors: Capsoni Carlo , D'Amico Michele
Published: February 2004

This paper presents a prediction method for the evaluation of bistatic intersystem interference due to rain scatter, associated with a physical model of the meteorological environment. The precipitation phenomena are modeled through a population of isolated three‐dimensional rain cells, with exponential rain rate profile inside; each cell is characterized by a specific value of size and peak rain intensity. Starting from the complete bistatic radar equation (BRE), a simplified expression is obtained that can often be used and that preserves accuracy when computing time is at premium. The limits of applicability of the simplifications above are identified and throughly discussed. The model is then successfully validated against the database of experimental measurements, collected in the framework of the COST (European Cooperation in the Fields of Scientific and Technical Research) 210 project [Commission of the European Communities, 1991]. The importance of being able to evaluate propagation quantities (e.g., transmission loss, attenuation) jointly is also stressed: Quite often the transmission loss statistic alone, in fact, is not sufficient to characterize the interference problem.
Techniques for the measurement and decomposition of the time varying narrow bandwidth transfer function of an HF sky wave transmission
Authors: Baker P. W. , Clarke R. H. , Massie A. D. , Taylor D.
Published: February 2004

A technique is presented for the measurement and decomposition into component propagation modes of the time varying narrow bandwidth (10 kHz) transfer function of an HF sky wave frequency modulated continuous wave (FMCW) transmission at 15.085 MHz. One event on a 5244‐km transmission path which exhibited nonselective fading within the 10‐kHz bandwidth is analyzed and found to exhibit severe phase distortion due to multipath. A component transfer function for an individual propagation mode is also obtained by two‐dimensional filtering of the signal in the Doppler frequency‐time delay domain, resulting in a significant reduction in phase distortion. The filtering technique can be used on FMCW transmissions with a repetitive waveform for the time varying identification and measurement of component multipath modes with unresolvable group time delay differences.
Propagation loss prediction in the urban environment with rectangular grid plan streets
Authors: Blaunstein N. , Levin M.
Published: February 2004

This paper presents a two‐dimensional propagation model for microcellular local, personal, and mobile communication for the case when both antennas, receiver and transmitter, are located below rooftop level in an urban crossing‐street situation. The model of crossing broken waveguides with randomly distributed buildings (screens) and gaps between them (slits) is considered as a model of an urban area with a rectangular grid pattern of streets. The average field intensity attenuation at the street level and coverage effects are investigated taking into account multireflection from the building walls and multidiffraction from the building edges. The theoretical predictions were verified by path loss measurements obtained by the communication group of Tadiran Telecommunication. The comparison shows good agreement with experimentally found field intensity loss at the urban street level and redistribution of field energy at the intersections of streets in an urban area with rectangular grid plan streets.
A new spatial interferometry capability using the Arecibo 430‐MHz radar
Authors: Palmer Robert D. , Larsen Miguel F. , Howell Perry B. , Cho John Y. N. , Narayanan Ram M. , Kelley Michael C.
Published: February 2004

This note describes the new spatial interferometry (SI) capability of the 430‐MHz radar system of the Arecibo Observatory (AO). Three limitations exist when the 430‐MHz radar is used for observations in the tropospheric and stratospheric regions. First, the AO radar currently has a lower height limitation of approximately 6 km because of the existing transmit/receive switch. Second, fading ground clutter limits the useful data which can be obtained and complicates the analysis. Third, the large mass of the feed system, which slows down beam steering, reduces the temporal resolution of any measurements requiring multiple beam positions. For these reasons, an SI system has been developed which will be shown to reduce these limitations. A brief description of the hardware is provided, and preliminary data are presented.
An investigation of biases in the full spectral analysis technique
Authors: Holdsworth David A.
Published: February 2004

The full spectral analysis (FSA) technique is investigated using both experimental and model‐generated data. The relationship between the formulae presented by different authors is discussed, and the similarities and differences inherent in using these formulae are evaluated. The FSA is applied both to sample spectra calculated using individual time series records and to spectra produced by the incoherent average of the sample spectra. The results reveal that the velocities obtained using sample spectra can be significantly biased in the situation where large turbulent motions exist for two particular implementations of the technique. These biases are shown to decrease as the number of sample spectra used in the incoherent average are increased, thereby producing FSA velocity estimates in excellent agreement with those produced using the full correlation analysis (FCA) technique.
A new method for the measurement of meteor speeds: The pre‐to phase technique
Authors: Cervera M. A. , Elford W. G. , Steel D. I.
Published: February 2004

In this paper we describe a new technique for measuring the speeds of individual meteors based upon the variation of the phase received in specular reflections from the meteor train prior to the meteoroid reaching the point of closestapproach to the observation site. The technique is characterized by high precision, high yield, and is less subject to selection effects than the techniques used heretofore. About 75% of observed meteors are able to have a speed determined, compared with only 10% previously. It has application to the measurement of the velocities of meteoroid streams and the distribution of speeds of sporadic meteors. The latter is vital to the modeling of the response of HF/VHF radars to meteor backscatter [Elford, 1964; Thomas et al., 1988], as well as having astronomical implications. Results of observations with our narrow‐beam VHF radar are presented. Data collection carried out during the epoch of the θ Ophiuchid meteor shower, whose radiant passes through the response function of the VHF radar, renders speeds which are in accord with the previously known speed for that shower. The speed distribution of sporadic meteors is determined and compared with the distributions obtained in previous radar and optical programs
Growth stratal records of instantaneous and progressive limb rotation in the Precordillera thrust belt and Bermejo basin, Argentina
Authors: Zapata Tomás R. , Allmendinger Richard W.
Published: February 2004

Analysis of synorogenic deposits preserved near the thrust front zone of the Precordillera fold and thrust belt and in the Bermejo foreland basin in central Argentina documents the evolution of deformation during the last 5 Myr as well as the thrust system kinematics. Seismic lines across the area display examples of progressive and instantaneous limb rotations. The easternmost thrust plate of the Central Precordillera, the Niquivil thrust, experienced episodic motion in two main stages: a first thrust movement as a fault‐propagation fold and a second movement as a high‐angle anticlinal breakthrough fault after a period of quiescence. Growth strata deposited in the La Pareja intermontane basin and the Las Salinas and Bermejo anticline recorded continuous growth of Eastern Precordilleran structures beginning at ∼2.7 Ma, with uplift rates of ∼0.3 mm/yr for the Niquivil anticline, 1.08 mm/yr for the Las Salinas anticline, and between ∼0.6 and 0.38 mm/yr during the last ∼2 Myr for the Bermejo anticline. Once the Eastern Precordillera began to grow, the propagation of the Niquivil thrust stopped, restricting the deformation to the young Vallecito out‐of sequence thrust. The complex geometry of growth strata deposited on the back limb of the Las Salinas anticline can be explained by using a model of a two‐step fault propagation fold with constant layer thickness. The Bermejo anticline of the Eastern Precordillera is formed by the simultaneous propagation of a shallow fault, responsible for the fold shape, and a deep fault that produced vertical uplift. A growth triangle that documents instantaneous forelimb rotation for a fault‐propagation fold is recorded for the first time in a published seismic line.
Mechanical twin sets in calcite as markers of recent collisional events in a fold‐and‐thrust belt: Evidence from the reefal limestones of southwestern Taiwan
Authors: Rocher M. , Lacombe O. , Angelier J. , Chen H.‐W.
Published: February 2004

Stress reconstructions were carried out based on inversion of calcite twin data from slightly deformed Quaternary reefal limestones from SW Taiwan. Results demonstrate that twinning reliably recorded the complex Quaternary and present‐day stress fields in a young mountain range undergoing collisional shortening. The stress regimes reconstructed at microscopic scale are predominantly compressional and reflect the main stages of the neotectonic evolution of the collision belt. No twinning has been observed in a sample collected in the Holocene platform of southern Taiwan, indicating that these formations did not undergo sufficient stress to initiate calcite twinning. Three different stress regimes have been recorded in the samples collected in Pleistocene reef limestones that developed in relation to active fold systems of the Western Foothills. These regimes appear to be homogeneous at the site scale and at the scale of SW Taiwan. The reconstructed ENE and NW trending compressions, only present in SW Taiwan, probably reflect local perturbations of the regional 105° trending compression. The perturbation might be due to activation of a set of folded‐faulted blocks in the studied area. A NW‐SE extension, also identified in these samples, is related to extrados stretching due to fold development. The results are consistent with independent stress data, i.e., Quaternary faulting, borehole breakouts, and focal mechanisms of earthquakes. The analysis of mechanical twins in calcite rocks constitutes a powerful tool for tectonic analyses in fold‐and‐thrust belts.
Gondwanian age dextral transpression and spatial kinematic partitioning within the Heritage Range, Ellsworth Mountains, West Antarctica
Authors: Curtis Michael L.
Published: February 2004

The Ellsworth Mountains, West Antarctica, consist of two mountain ranges; the Sentinel, and Heritage ranges. The more southerly Heritage Range is composed of a lower Paleozoic sedimentary and volcanic rock sequence, deformed during a single major deformation event in the early Mesozoic. This Gondwanian Orogeny possibly resulted from Andean‐style convergence along the southern margin of Gondwana, prior to break‐up of the super‐continent, and the subsequent translation of the Ellsworth Mountains from a position close to the Natal embayment of southern Africa to that of the present day. Rocks of the Heritage Range are intensely folded, with close to tight, upright to inclined folds, plunging gently about a horizontal axis trending NNW‐SSE. Locally, folds plunge moderately to subvertical toward the NNW, possessing asymmetries consistent with a dextral sense of shear. Cleavage is generally axial planar displaying downdip and strike‐parallel stretching lineations that are frequently associated with domains of reverse, and dextral shear striking parallel to the regional structural grain. The spatial and temporal relationship of fractures developed within these domains as a result of noncoaxial shear, in addition to the progressive incremental strain histories derived from mineral fibres in strain shadows, indicate the contemporaneous nature of these shear domains. Strain analysis of deformed tuffaceous diamictites and oncolithic limestones reveal k‐values <1 (mean 0.59) throughout the Heritage Range. The coexistence of strike‐parallel dextral, oblique and reverse‐shear domains, abrupt reorientation of progressive strain axes, steep cleavage dips, and k‐values <1 are all consistent with a dextral transpressive deformation regime, previously unrecognized in the Ellsworth Mountains. A model of highly oblique (pure‐shear dominated) transpression, associated with efficient spatial partitioning of the strike‐slip component of shear is proposed to describe the structural relationships observed.
The Bilila‐Mtakataka fault in Malaŵi: An active, 100‐km long, normal fault segment in thick seismogenic crust
Authors: Jackson James , Blenkinsop Tom
Published: February 2004

Some parts of the east African rift system have deeper earthquakes (30–40 km), a larger effective elastic thickness (∼35 km), and wider half grabens (∼50 km) than are typical in other regions of continental extension. One such region is the southern part of the western branch of the east African rift in Malawi. In this region we describe a normal fault scarp that is up to 15 m high and continuous over a distance of more than 100 km. It represents the latest increment of slip, perhaps in a single earthquake, on a fault (the Bilila‐Mtakataka fault) with a total offset of about 1000 m. It is the longest continuous normal fault segment that we know of on the continents, and it supports the suggestion that the thickness of the seismogenic layer is the fundamental control on the scale of geological structures that form within it. A consequence of this association is the possibility of very large (Mw 8.0), though infrequent, normal faulting earthquakes in east Africa.