bluuebunny/crossref_metadata_2025 huggingface dataset:
Could Early Cenozoic thermohaline circulation have warmed the poles?
Authors: Lyle Mitchell
Published: February 2004

The Paleocene and Eocene epochs provide examples of oceanographic conditions when the Earth had a stable climate much warmer than that of the modern world. Lower Cenozoic sediments thus provide a way to investigate how oceanographic processes might respond to extreme warming. Because high‐latitude sea surface temperatures were significantly warmer during the early Cenozoic while tropical sea surface temperatures were similar to or somewhat less than modern, there may have been a much higher poleward oceanic heat transport at that time [Shackleton and Boersma, 1981; Barron, 1987; Sloan et al. 1995]. Here I develop a simple ocean thermohaline model to demonstrate that poleward oceanic heat transport could strongly warm the polar regions only if the strength of vertical mixing (vertical eddy diffusion) in the oceans is sensitive to changes in oceanic density structure. If this is true, the early Cenozoic oceans, because they had significantly less density stratification, could have had a thermohaline circulation as much as 3 times as strong as that of the modern ocean despite reduced temperature gradients. The model also predicts that an optimum bottom water temperature exists for maximum poleward heat transport: continued warming of the Earth does not continue to increase oceanic heat transport to the poles. The hypothesized sensitivity of oceanic vertical mixing to the density gradient should provide a positive feedback to both the transition to ice ages and to a future transition out of them. A test of the sensitivity of vertical eddy diffusion to density gradients can be studied through map and depth transects. In the horizontal direction, high vertical diffusion would be marked by relatively strong bottom water temperature gradients away from bottom water source regions. In the vertical direction, high vertical diffusion should be indicated by very small temperature gradients.
Barium and the Late Paleocene δ13C maximum: Evidence of increased marine surface productivity
Authors: Thompson Elisabet I. , Schmitz Birger
Published: February 2004

Deep‐sea sediment Ba* (Ba/Al2O3(sample) × 15% ‐ Ba(aluminosilicate) records show increasing values synchronous with the evolution of the late Paleocene global δ13C maximum, reflecting an increase in marine surface primary production and biogenic barite formation at this time. At two oligotrophic locations, Deep Sea Drilling Project (DSDP) Sites 384 and 527 in the North and South Atlantic, respectively, Ba* increases from 160–360 ppm in the early Paleocene to 1100–3000 ppm during the δ13C maximum. At equatorial DSDP Site 577, positioned within or near the high‐productivity zone, Ba* increases from ∼15,500 ppm in the early Paleocene to ∼25,400 ppm in conjunction with late Paleocene maximum δ13C values. Linear fitted correlation plots of sediment Ba* content versus surface water δ13C in all three regions support barite originating in the euphotic zone. The early to late Paleocene relative increase in Ba* illustrates how burial rates of Corg (relative to Al2O3) accelerated by a factor of ∼1.8 and ∼6.0 in the eutrophic and oligotrophic areas, respectively. A tentative estimate, weighing our result for the entire ocean, suggests that accumulation rates of organic carbon increased by a factor of 2 during the late Paleocene δ13C maximum.
Response of a coupled ocean/energy balance model to restricted flow through the Central American Isthmus
Authors: Mikolajewicz Uwe , Crowley Thomas J.
Published: February 2004

Prior ocean modeling work suggested that an open central American isthmus would cause a collapse of the North Atlantic thermohaline circulation because of free exchange of low salinity water between the Atlantic and the Pacific. Geological data provide some support for this response, but the data also indicate that some North Atlantic Deep Water formation occurred before final closure of the isthmus. We previously postulated that this “early switch on” could reflect a more limited exchange of Atlantic waters with the Pacific. In this study we discuss model sensitivity experiments testing that hypothesis and interpret the response in terms of shifts between multiple steady states of the model. Two simulations are conducted with a version of the Hamburg large‐scale geostrophic ocean model that is coupled to an atmospheric energy balance model. Constrictions of throughflow through the central American isthmus is mimicked by locally changing the frictional drag coefficient in the ocean model. Results indicate that modest levels of throughflow can maintain some level of thermohaline circulation. These results support the conjecture in our earlier study. However, the overturning cell is about 300 m shallower than in the control run, with deep water production nearly eliminated in the Labrador Sea. These latter responses should be testable with marine data.
Modeled Glacial North Atlantic ice‐rafted debris pattern and its sensitivity to various boundary conditions
Authors: Matsumoto Katsumi
Published: February 2004

An iceberg drift and decay model is used to compute the ice‐rafted debris (IRD) flux to the glacial North Atlantic under steady iceberg discharge from the Greenland and Laurentide Ice Sheets. I investigate the sensitivity of modeled IRD distribution pattern to formulation of glacial iceberg decay, meltwater drainage of Laurentide Ice Sheet at its southern margin, and the choice of ocean current and wind fields used as model boundary conditions. Modeled IRD patterns are compared with the observed “basic glacial” mode IRD pattern [Ruddiman, 1977] and the last five Heinrich layer patterns [Grousset et al., 1993]. Model results show that the first‐order control on IRD deposition pattern is the prevailing ocean currents, and the choice of current fields is critical in determining the results. Winds can have significant effect when they exert drag in concert with ocean currents. Formulation of glacial iceberg decay directly affects its survival and the expanse of IRD distribution, but the formulation is perhaps one of the largest uncertainties in the model. Drainage of the southern margin of the Laurentide Ice Sheet through the Laurentian Channel by meltwater instead of glaciers predicts no significant IRD deposition in the open North Atlantic in contrast to observations, suggesting that glacier (i.e., iceberg) drainage through the Laurentian Channel was significant during the times of observed IRD deposition. In most experiments, the model predicts a high IRD deposition band that roughly coincides with the latitudinal band of high Heinrich layer deposition but are measurably south of the basic glacial mode high IRD deposition band. This discordance suggests either that the flow fields used as model boundary conditions are inaccurate or that much of the basic glacial mode IRD was not delivered by icebergs but perhaps sea ice.
What is a global auroral substorm?
Authors: Elphinstone R. D. , Murphree J. S. , Cogger L. L.
Published: February 2004

The departure of the aurora from quiet levels in a dynamic manner constitutes some type of auroral “breakup” event. Research into the auroral breakup predates the International Geophysical Year (1957/1958). This feature of the aurora, and the later, more global concept of the auroral substorm, has become a focus for much of the auroral research that occurs today. New instrumentation and global collaborations continue to refine our knowledge of the substorm process and how it proceeds in the ionosphere. In particular, global auroral imaging has advanced our understanding of the dynamics of the process and has given us the ability to put localized observations into a global perspective. Fundamentally new cycles of auroral activity are now understood to exist, and this has provided a means by which auroral activity can answer questions about magnetospheric substorm dynamics. Along with this wealth of observations has come a wide range of theories purporting to explain the mechanism of the onset of this phenomenon. There is, however, no single theory which stands out as clearly explaining the wide range of active auroral phenomena. A synthesis which combines these theories and allows them to each explain individual aspects of the problem appears to be required. This has led to a new way of understanding the active aurora as a set of processes or modules which occur either coupled together or independent of one another to form a particular event. This view represents a fundamental departure from the view of the substorm as a single unchanging entity. Auroral activity can rather be thought of as the earthward end of a diverse set of ionospheric and magnetospheric processes which couple together to form different cyclical patterns. A symbolic representation of this modularization is presented to simplify future schematics of large‐scale auroral dynamics.
Nonthermal escape of the atmospheres of Venus, Earth, and Mars
Authors: Shizgal Bernie D. , Arkos Gregory G.
Published: February 2004

Atmospheric loss from planetary atmospheres is an important geophysical problem with implications for planetary evolution. This is a multidisciplinary research field that requires an expertise in a wide range of subjects including statistical mechanics, fluid mechanics, plasma physics, collision theory, and surface science. This paper is a review of the current state of our understanding of atmospheric loss from the terrestrial planets. A detailed discussion is provided of the basic concepts required to understand the processes occurring in the high‐altitude portion of a planetary atmosphere referred to as the exosphere. Light atomic species with sufficient translational energy can escape from an atmosphere. The translational energy required for escape could be thermal energy and proportional to the ambient temperature or the result of some collisional processes energizing the species above thermal energies. These collisional processes, which include charge exchange and dissociative recombination between energetic ions, neutrals, and electrons, are referred to as nonthermal escape processes. We highlight the similarities and differences in the important escape mechanisms on the terrestrial planets and comment on application of these mechanisms to evolutionary theories of the terrestrial atmospheres. The emphasis in this paper is directed toward the need to consider the exosphere as collisional.
Silicate melts: Relaxation, rheology, and the glass transition
Authors: Webb Sharon
Published: February 2004

Movement of magma within the Earth depends upon the density, compressibility, thermal expansion, viscosity, diffusivity, heat capacity, and thermal conductivity of silicate melts as a function of temperature, pressure, and composition. These physical properties are controlled by the atomic coordination and bond strengths of the melt. To contribute more to our understanding of geological processes than numerous measurements of physical properties of individual magmas of local interest, the relationship between melt structure and physical properties needs to be understood. In geological settings where the viscosity of the molten component is high (108‐1014 Pa s), the melt structure requires a long time (seconds to weeks) to equilibrate in response to changes in pressure, temperature, and composition. The effects of this structural equilibration upon the viscosity, density, and compressibility of magmas and the time required for equilibration need to be included in discussions of magma ascent and emplacement and of crystal nucleation, growth, and segregation within magmas. The structural relaxation discussed here is the slowest relaxation process occurring in a silicate melt at a specific temperature and therefore represents the glass transition, which is a kinetic transition whose temperature is not a constant specific to a melt composition but depends upon the timescale on which measurements are performed. On the basis of the agreement between nuclear magnetic resonance measurements of Si‐O bond exchange timescales and measured structural relaxation timescales, the structural equilibration observed in silicate melts appears to be due to the lifetime of the Si‐O bonds in the melt. The following is a discussion of a range of different studies to determine the density, viscosity, thermal expansion, and compressibility of silicate melts, based on a central theme of the structural relaxation phenomenon and its use to calculate the physical properties of a melt, for example, using calorimetry data to calculate viscosity, volume, and thermal expansion in silicate melts at temperatures 50 K above the glass transition. This results in the determination of melt properties at conditions of direct geological interest, for example, the densities of granitic melts at 1050 K and viscosities of 109–1012 Pa s; observations of anomalously low, non‐Newtonian viscosities and compressibilities in melts at conditions of high strain rate; the viscosities of volatile‐bearing melts; and the viscosity of the melt phase in natural crystal‐ and bubble‐bearing obsidians.
Application of tree ring analysis to paleoseismology
Authors: Jacoby Gordon C.
Published: February 2004

Knowledge of a region's seismicity is one of the keys to estimating earthquake hazards. Unfortunately, historical records are generally inadequate for evaluations of seismicity. Paleoseismology addresses this problem using various techniques for dating earthquake‐disturbed materials. Trees, with widespread distribution, identifiable annual growth increments, and sensitivity to environmental change, can provide a unique tool for dating past earthquake events. Geomorphic and hydrologic changes and dynamic stress resulting from earthquakes can cause a variety of effects in individual trees and communities of trees. Tree ring analysis can (1) produce the actual year and sometimes the season for disturbance events and (2) establish synchroneity for events that may be beyond the range of absolute calendar dating. Tree ring dating, or dendrochronology, is used to establish exact dates based on patterns of annual ring variations through time. Mere counting of rings is inadequate and may lead to errors because of possible missing, micro or false rings. Trees ranging in age from 300 to 500 years grow in many places and can be used to identify previously unknown seismic disturbances or to better define events that are partially known. Longer time spans can be covered in some instances. Earthquakes may be more precisely located in space and time or have their magnitudes better estimated by analysis of tree rings. A number of studies have established the validity of tree ring application to paleoseismology but only a few studies have contributed new information to the paleoseismic record. The science should now move from the discovery phase to a wider application phase. Successful applications will add important information to the records of seismicity and the evaluation of earthquake hazards.
Ground‐based observations of subionospheric pulse pairs
Authors: Smith David A. , Holden Daniel N.
Published: February 2004

We have developed a unique multiple‐channel subband system to detect and record broadband electromagnetic transient events in carrier‐dominated radio environments. Between May and September of 1994 we used this system to detect and record over six thousand transient radio frequency events in the band from 3 to 30 MHz. Approximately 500 of the events have been characterized as paired bursts of radio noise with individual burst durations of 4 to 10 μs and separations between the bursts of 5 to 160 μs. The paired transients are typically 5 to 40 dB brighter than the background electromagnetic spectrum between carrier signals. We have termed these events subionospheric pulse pairs (SIPPs) and presently have no explanation as to their source. Our observations of SIPPs resemble observations of transionospheric pulse pairs (TIPPs) recorded by the Blackbeard instrument on the ALEXIS satellite; the source of TIPP events is also unknown. Most SIPPs do not exhibit dispersion, implying groundwave (line‐of‐sight) propagation; but seven of the pairs exhibit dispersion characteristic of ionospheric skywave propagation. For our experimental configuration the maximum line‐of‐sight observation range for radio propagation extends to 300 km for a source at an altitude of 5 km and 520 km for a source at 15 km. Refraction from the ionosphere may allow HF skywave radiation to propagate around the globe. Information about the times and locations of sources, in conjunction with regional weather maps and computer models of ionospheric propagation, suggests that some SIPP events originate in the vicinity of large‐scale thunderstorm complexes.
Transient electromagnetic field of a vertical electric dipole above an atmospheric surface duct
Authors: Abo Seliem Adel A. S. , Bishay Samira T.
Published: February 2004

The pulsed electromagnetic radiation from a vertical electric dipole above an atmospheric surface duct is investigated theoretically. The duct model used is that of Kahan and Eckart [1950] and consists of a discontinuous drop of the otherwise constant relative permittivity at the upper duct boundary. The Earth is assumed to be an ideal conductor and planar. The modified Cagniard method is used to derive closed form expressions for the Hertzian vector anywhere above the duct. From the physical point of view, Cagniard's idea is applicable as it is based on evaluating the field in a series of image sources of the primary source. The step ‐ function solution of the problem can then be determined as an infinite sum of definite integrals over finite intervals. Two cases would be distinguished on the basis of the distance between the receiving and transmitting ends and whether it is greater or lesser than the total reflection distance.
Velocity fluctuations associated with polar cap patches
Authors: MacDougall J. W. , Grant I. F. , Hamza A.
Published: February 2004

The digital ionosonde technique has been used to measure the convection in the vicinity of polar cap patches. Two types of velocity fluctuations were seen: (1) parallel fluctuations which are a slowing down and speeding up of the convection without a change in direction, and (2) perpendicular fluctuations which are perpendicular to the average convection. The parallel type of fluctuation was most often seen when the patch density ratio was large, and the patch tended to be overhead at the time of the minimum of the velocity fluctuation. The perpendicular fluctuation behavior tended to be seen when the patch density ratio was small and the patch tended to be overhead at an extremum of the velocity fluctuation. Two mechanisms that can possibly explain the behavior are discussed: (1) Convection changes are produced “internally” because of electric field fluctuations caused by the enhanced conductivity of the patch interacting with the cross polar cap current system. (2) The electric field fluctuations are imposed “externally” by fluctuations in the induced electric field from the solar wind. It was found that the internal mechanism seems to be most consistent with the properties of the parallel fluctuations, and the external mechanism seemed most consistent with the perpendicular fluctuations.
Calculation of surface impedance effects on transient antenna radiation
Authors: Booker S. M. , Lambert A. P. , Smith P. D.
Published: February 2004

A numerical time domain approach to radiation and scattering problems is introduced for surfaces of arbitrary conductivity. This approach is based upon a method of moments solution of the electric field integral equation. The problem is posed as a second‐kind integral equation in discrete time for those regions of the surface with finite conductivity; it is shown that this formulation possesses desirable stability properties. The numerical method introduced is used to determine the far field radiation of both directive and nondirective transient antennas excited by temporally compact, ultrawideband pulses when the antenna surfaces are subject to a Wu‐King surface impedance profile. Such loads are shown to suppress the “late time ringing” resulting from reflected surface currents without significantly affecting the initial radiated waveform. In the case of nondirective transient radiators, such as the bicone or bow tie antenna, the presence of a surface loading has no effect on the peak radiated field strength. In the case of a directive transient radiator, such as a transverse electromagnetic horn, the introduction of a surface impedance reduces the radiated field strength somewhat. The difference in the physical behavior of these two classes of antenna is accounted for.
Benchmarking high‐performance computing platforms in analyzing electrically large planar conducting structures via a parallel computed method of moments technique
Authors: Kaklamani Dimitra I. , Marsh Andy
Published: February 2004

The analysis of electrically large planar conducting structures is examined by employing parallel computations of an integral equation technique in conjunction with a method of moments. The integral equation, in terms of the induced conductivity currents, is solved using an entire domain Galerkin technique, with Chebyshev‐type basis fonctions. The resulting algorithm parallelization enables extension of the proposed methodology above the resonance region. Numerical results are computed for several scatterer sizes and excitations source types on varying high‐performance computing platforms: Silicon Graphics Power Challenge, Intel Paragon XP/S, CRAY C‐90, and CRAY T3D. A comparison is made among these diverse computing platforms for their applicability‐suitability in solving electromagnetic problems in the domain of electrically large planar scattering structures. Their suitability is not only judged on performance obtained but also on ease of code portation.
A model of HF sky wave radar clutter
Authors: Coleman C. J.
Published: February 2004

A theoretical relationship between Doppler spread and irregularity distribution is derived. This is combined with ray‐tracing techniques to produce a model of HF sky wave radar clutter. The model is found to reproduce many important features of real radar observations.
Efficient computation of radar scattering modulation from jet engines
Authors: Ross Daniel C. , Volakis John L. , Anastassiu Hristos T.
Published: February 2004

By making use of the overlapping symmetries between the modal inlet fields and the engine face, it is found that the radar modulation due to a set of rotating blades can be computed using only one solution for any blade position. Frequency domain solutions for the stationary engine face can immediately produce modulation data with a simple postprocessing step. The validity of the modulation scheme is demonstrated with the aid of a simplified engine model possessing a semianalytical mode‐matching solution.
Finite difference time domain tests of random media propagation theory
Authors: Nickisch L. J. , Franke P. M.
Published: February 2004

The recent extension of the finite difference time domain (FDTD) method to frequency‐dependent media [Nickisch and Franke, 1992] allows FDTD to now be applied to ionospheric propagation. The FDTD method solves the Maxwell equations directly in the time domain by temporal integration. No approximations beyond that of finite differencing are necessary, although the direct enforcement of certain approximations is possible. By doing so the method can be used to explore the effect of many standard propagation approximations. We consider certain approximations typically applied to the problem of propagation in randomly structured ionization, such as the neglect of polarization coupling and the assumption of small‐angle scattering.
Mutual coupling between slots cut in rectangular cylindrical structures: Spectral domain technique
Authors: Rengarajan Sembiam R.
Published: February 2004

This paper addresses the problem of evaluating the mutual coupling between slots cut in arbitrary cylindrical structures. The analysis of fields radiated in the exterior region of such a structure requires a numerical evaluation since there exists no analytical Green's function for the general three‐dimensional (3‐D) problem. The 3‐D problem is reduced to a number of two‐dimensional (2‐D) problems by performing Fourier transform of fields and sources with respect to the axial (z) direction. The 2‐D problems are formulated in terms of integral equations in kz (wave number in the z direction) space, and they are solved by the moment method. Mutual coupling expressions for slots cut in rectangular waveguide walls are obtained from an inverse Fourier transform of the 2‐D spectral domain solutions. Comparisons to similar analyses in the prior literature for self‐admittance of isolated slots and mutual admittance between pairs of slots are made. Efficient methods of determining the spectral domain solutions, criteria for sampling and truncation of the spectrum, and some representative results are presented.
GPS/GLONASS sensing of the neutral atmosphere: Model‐independent correction of ionospheric influences
Authors: Ladreiter H. P. , Kirchengast G.
Published: February 2004

Radio occultation sensing of the terrestrial atmosphere can be performed by using signals from Global Positioning System (GPS) or Global Navigation Satellite System (GLONASS) satellites, which are received by satellites in low Earth orbit. The signal transmitted from a GPS/GLONASS satellite, when occulted, suffers propagation effects in the ionosphere and atmosphere, including carrier and group phase shifts and ray path bending. Those propagation effects give direct information on the refractivity of the medium of occultation. In order to sense the neutral atmosphere only, we have to carefully correct for the ionospheric contribution to the propagation effects whose influences are poorly known at present time. In this paper, we provide a straightforward method for model‐independent correction of the ionospheric effects based on the correction of the ionospheric bending angle rather than correction of the optical path lengths, as done by previous studies. Using the presented correction, the accuracy of retrieved temperature profiles of the atmosphere is better than 1 K up to heights of some 55 km during typical ionospheric conditions. The formalism can easily be implemented in on‐line analyses of phase data obtained by the radio occultation technique. This promising technique may thus in fact become an extremely valuable tool for supporting climate monitoring as well as weather and climate modeling and prediction.
Finite difference time domain analysis of arbitrarily biased magnetized ferrites
Authors: Schuster J. W. , Luebbers R. J.
Published: February 2004

The finite difference time domain method is extended to include three‐dimensional propagation in a saturated magnetized ferrite with an arbitrary direction of the biasing field. The recursive convolution method is used to include the frequency dependence of the ferrite. Both total and scattered field formulations are developed. This approach requires the additional storage of at most three complex numbers per cell. Transient scattering in two and three dimensions is used to demonstrate the accuracy of this approach.
Radiation characteristics of a plano‐convex lens antenna
Authors: Hongo Kohei , Kobayashi Hirokazu
Published: February 2004

Radiation characteristics of a plano‐convex lens are studied using Maslov's method, stressing the evaluation of an electromagnetic field around a caustic region when a uniform plane wave is incident axially to the flat plate side. The field expressions which are valid near a focal region are derived in terms of finite integrals with a single variable. As a special case, an approximate expression for a weakly focusing system is derived and compared with a corresponding expression obtained using Kirchhoff‐Huygens principle. Agreement between them is fairly good. Numerical computation for field distribution around a focal region is conducted, and the three‐dimensional pictures for the results are presented to help understand the focusing behavior of the lens.
Polarization conversion by hybrid modes: Theory and applications
Authors: Mertens K. , Sennewald M. , Schmitt H. J.
Published: February 2004

In strip‐loaded slab waveguides, mode coupling between the transverse magnetic (TM11) and the transverse electric (TE21) modes can occur even if isotropic materials such as InP or GaAs are used. This coupling leads to new eigenmodes with a strong hybrid character, that is, the strength of the transverse side components is in the range of the main components. We show that the effect can be explained by a perturbation analysis investigating a raised strip waveguide, which is thought as a superposition of a buried strip waveguide and a dielectric perturbation in the region of the substrate. The described mode‐coupling effect should be taken into account when designing integrated optic components, where strip and strip‐loaded slab waveguides are commonly used. On the other hand, the hybrid modes can be used to develop new TE/TM converters. We present a new device based on InP which converts an input TE polarization into a mixture of TE and TM polarization, making it an interesting candidate for coherent receiver chips.
Electrostatic image theory for an anisotropic half‐space slightly deviating from transverse isotropy
Authors: Lindell Ismo V. , Nikoskinen Keijo I. , Flykt Mikko J.
Published: February 2004

Electrostatic image theory developed earlier for a point charge in an isotropic half‐space in front of an anisotropic half‐space with transverse isotropy is extended to an anisotropic half‐space with a slight deviation from transverse isotropy. Two alternative image solutions are found: a combination of a point charge and a planar surface charge, or a combination of a point charge and a half‐infinite line charge of quadrifilar nature. Both images are shown theoretically to give the same potential in the isotropic half‐space, which is tested by finding the potential function at the interface. As an example, image expressions for a point charge above the interface of a uniaxial anisotropic medium with its axis tilted from the vertical are found, and the effect of the tilt angle is studied numerically. The image expressions can be applied, for example, in scattering problems involving an obstacle in the presence of an anisotropic half‐space, which allows one to make use of the simple Green function corresponding to the isotropic space.
Offset coupling between Gaussian beam and the fundamental mode of weakly guiding step‐index fibers
Authors: Do‐Nhat Tam
Published: February 2004

The transmitted power coefficient of a harmonically oscillating Gaussian beam into the HE11 mode of a transverse offset weakly guiding fiber of step index profile is represented as an integral of the coupling power density over all the spectral components of the beam. This coupling of a complex power density into the fundamental mode of a fiber which takes into account the Fresnel losses is expressed in closed form by using a vectorial analysis: normal mode method, the field equivalence principle, the Lorentz reciprocity theorem and the translational addition expressions for the cylindrical functions. Numerical results of the transmitted power coefficient are presented for various normalized frequency parameter V, transverse offset R, and different launching distance d.
The electromagnetic field emitted by a pulsed current point source above the interface of a nonperfectly conducting Earth
Authors: Kooij B. J.
Published: February 2004

The electromagnetic radiation due to a pulsed current point source located above a nonperfectly conducting Earth is investigated. The Cagniard‐de Hoop technique with polar variables is used in order to derive closed‐form expressions for the electric field above or on the surface of a nonperfectly conducting Earth. Early‐ and late‐time approximations are derived which are tested with an exact method that is presented in this paper. The approximate method yields excellent results and requires far less computation time. Numerical results for the electric field above the Earth are presented for both the exact and the approximate methods.
Nonreciprocal magnetoplasma sheath waves
Authors: Lüttgen Andrea A. E. , Balmain Keith G.
Published: February 2004

We present theoretical and experimental results relevant to azimuthal magnetoplasma sheath waves in the case of the magnetic field parallel to the axis of a thick conducting cylinder. For the theory, a planar geometry is used with the ambient magnetic field parallel to the conducting plane and propagation perpendicular to the magnetic field. The results show two low‐frequency transverse magnetic (TM) modes propagating in opposite directions with different propagation constants and also different cutoff frequencies separated by the cyclotron frequency. At frequencies above the upper hybrid frequency, one unidirectional TM mode is found. No transverse electric (TE) modes have been found. The experimental setup consists of a metallic cylinder with two protruding radial monopole antennas separated by an angle of 90°, placed in an axially magnetized laboratory plasma. Measurements of the two‐port S parameters clearly show nonreciprocity caused by the propagation of different wave modes in opposite directions.
Numerical solving of a new class of canonical problems and their application
Authors: Vasil'ev E. N. , Solodukhov V. V. , Makkaveeva V. F.
Published: February 2004

The method of integral equations and representation of unknown surface currents as a sum of “uniform” and “nonuniform” components are used to solve a very large class of diffraction problems, such as dielectric, multilayered, partially coated, and other objects, including the structures with surface waves. They can be used as canonical problems to considerably extend the field of application of the geometrical and physical theories of diffraction. A simple example (diffraction by a dielectric slab of finite width) is considered in order to illustrate the implementation and validity of our hybrid approach where the possibilities of both numerical and asymptotic methods are combined. The diffraction of the second order is taken into account. It is shown by comparison with the direct numerical calculations that the contribution of the third‐ and higher‐order diffraction is too small and practically may be neglected.
Aperture synthesis polarimetry: Application to the Dominion Radio Astrophysical Observatory synthesis telescope
Authors: Smegal R. J. , Landecker T. L. , Vaneldik J. F. , Routledge D. , Dewdney P. E.
Published: February 2004

Aperture synthesis is a powerful technique for imaging the radio sky and can be used to make images in all four Stokes parameters, providing a complete measurement of the polarization state of the received radiation. In centimeter‐wavelength continuum astronomy the received signals are generally partially linearly polarized, with a negligibly small fraction of circular polarization. For this application the preferred antenna configuration receives both left‐ and right‐hand circular polarization. In this paper the effects of nonideal antenna performance are analyzed, and calibration and data correction procedures are described which allow precise measurement of the four Stokes parameters. Three levels of data correction are identified. Level 1, complex channel gain correction, is the standard calibration of amplitude and gain required in every synthesis telescope. Level 2, orthogonality correction, makes the instrument appear as a set of interferometers, identical to the level of approximation involved, which have two orthogonal, but not precisely circular, polarizations. Level 3 correction converts the telescope into an array of identical antennas with exactly circular polarization. Level 3 correction gives the most accurate polarimetry; for this level of correction the polarization characteristics of one antenna, which is used as a reference, must be determined. In the absence of a precise determination of the polarization characteristics of the reference antenna, a measurement which may be very difficult, there is no value in proceeding to Level 3 correction. If the cross‐hand contamination of the antennas is less than about 15%, then only level 1 and level 2 corrections are needed to achieve a sensitivity to polarized emission of 1% of the total intensity, an accuracy of measurement of polarized intensity of better than 10% and a measurement of the position angle of linearly polarized emission better than 5°. The implementation of polarimetry at 1420 MHz on the Dominion Radio Astrophysical Observatory synthesis telescope, Penticton, British Columbia, Canada, is described, and instrument performance to the above specifications is demonstrated. Observations of the supernova remnant DA530 are presented, demonstrating a usable field of view for polarimetry of at least 1.5°.
Long alternating codes: 2. Practical search method
Authors: Markkanen Markku , Nygrén Tuomo
Published: February 2004

This paper is the second one in a series explaining a new search method of long alternating codes for incoherent scatter radars. The first paper explains the general idea of the method in terms of a special game of dominoes. This second paper gives an alternative mathematical formalism suitable for computer search. It consists of three rules and a mathematical analysis leading to a formula which can be used in practical search. Although the rules were originally experimental, a mathematical proof of their sufficiency is also given. The method has been used to make a complete search up to a length of 1,048,576 bits. Even longer codes have been found; the longest one known at the moment contains 4,194,304 bits. For demonstration, complete tables of 8‐, 16‐, 32‐, and 64‐bit codes and examples of 128‐ and 256‐bit codes are presented.
Forecasting and updating HF channel parameters on the basis of oblique chirp sounding
Authors: Ivanov Vladimir A. , Ryabova Nataliya V. , Shumaev Vladimir V. , Uryadov Valery P.
Published: February 2004

Lowest observed frequency (LOF) and maximum observed frequency (MOF) spatial‐temporal correlation data for midlatitude long‐distance paths are presented. The possibility of improving LOF and MOF forecasting accuracy for links without diagnostic systems by means of linear regression methods and a semiempincal ionospheric model is investigated, and a comparison is made of the effectiveness of providing HF communication frequencies on the basis of long‐term forecasting and real‐time channel evaluation.
Electromagnetic field in the sea due to an omnidirectional VLF antenna
Authors: King Ronold W. P. , Harrison Charles W. , Houdzoumis Vassilios A.
Published: February 2004

The physical and electrical properties of typical VLF transmitters are described, and new formulas are given for the electromagnetic field they generate on the surface of and at any depth in the sea. These apply to the spherical earth at frequencies in the range 14 ≤ ƒ ≤ 30 kHz. They are useful for distances up to 10,000 km. Special application is made to communication with submerged submarines equipped with horizontal antennas.
Surface vertical magnetic field produced by a finite loop buried in an earth containing a thin conducting sheet
Authors: Durkin John
Published: February 2004

The effect of a thin conducting sheet located at the earth‐to‐air interface on the surface vertical magnetic field created by a buried finite loop was studied. Expected field values as a function of frequency are provided for variations in the sheet's conductivity‐thickness product. Since the results would be most beneficial for purposes of through‐the‐earth communications, such as communicating with trapped miners following a mine emergency, field values were derived for a range of frequencies, mine depths, and earth conductivity values that would be typically found in such an application.
Polarization of auroral backscatter at 50 MHz
Authors: Hussey G. C. , Koehler J. A. , Sofko G. J.
Published: February 2004

Auroral backscatter from the lower E region was studied using two 50 MHz bistatic, continuous wave radar links which shared a common polarimetric receiver. The polarization parameters were defined in terms of the polarization ellipse, which is described by the ellipticity angle χ orientation angle ψ, and polarization ratio m. Spectral analysis was applied to the intensity measurements and its corresponding polarization parameters. Observations of typical auroral spectral types 1, 2, and 3 indicated that the scattering of a linearly polarized incident wave produced an essentially linear and highly polarized scattered wave. These results imply a small scattering volume and/or a small number of discrete scatterers located close to one another, “scatterer” referring to a volume where radar waves are scattered according to weak coherent scattering theory, and also reaffirms that the scattering process is a weak coherent one. These results are typical of most observations, but not all; an otherwise typical intensity spectrum may also exhibit variable polarization parameters with an appreciable reduction in the polarization ratio and/or signals of significant ellipticity. These anomalous properties can be explained as scatter coming from a number of individual scattering volumes within the scattering region (i.e., the effective radar viewing region), which are each influenced differently by Faraday rotation to and from the scattering region.
Direct current anomaly due to a wedge in conducting ground
Authors: Nikoskinen K. I. , Eloranta E. H.
Published: February 2004

In the present paper a geoelectric dc problem involving a wedge‐shaped structure in a conducting ground medium is studied. The geometry is a generalization for the well‐known dipping layer problem in geophysics. In mathematical terms the three‐dimensional Poisson equation is solved in a geometry where a point current excitation is located at the interface of the air and the ground. The latter medium is assumed to be homogeneous except for an arbitrarily tilted wedge‐shaped structure whose edge coincides with the Earth's surface. The potential anomaly due to this wedge is solved at the surface of the ground in terms of an image line current source which is located in complex space. The image representation of the wedge anomaly provides an analytical tool to simulate the response of the ground structure for arbitrary current excitations at the surface. Furthermore, the potential due to the image and the original point source represents the Green's function of the geometry in question, which can be applied to numerical studies of more general problems involving a wedge‐shaped anomaly in the ground model.
Simultaneous and colocated observation of winds and tides by MF and meteor radars over London, Canada (43°N, 81°W), during 1994–1996
Authors: Hocking W. K. , Thayaparan T.
Published: February 2004

Simultaneous and colocated comparisons of winds and tides by MF and meteor VHF radars have been made in the 85–94 km height range from July 1994 to June 1996 over London, Ontario, Canada. Results have been obtained for every month of the year. From these comparisons it is concluded that, in general, the MF spaced antenna technique and the meteor method both provide reliable means for synoptic studies of neutral air motions in the height range 85–94 km, at timescales of greater than 12 hours, and therefore are valuable tools in middle atmospheric research. However, we do find that some parameters seem to be estimated with greater precision than others. In particular, the measurements of the zonal long‐term wind variations, and the semidiurnal tide in both the zonal and meridional directions, seem particularly robust and reliable, while there is somewhat less consistency between measurements of the zonal diurnal tide and the meridional monthly mean winds. The former problem is very likely to be an artifact of the strong diurnal variation in meteor count rates. However, we cannot claim that our meridional monthly mean wind agreement is always good, and these discrepancies deserve further investigation.
A consistency check of three radar methods for monitoring eddy diffusion and energy dissipation rates through the tropopause
Authors: Delage D. , Roca R. , Bertin F. , Delcourt J. , Crémieu A. , Massebeuf M. , Ney R. , Van Velthoven P.
Published: February 2004

The capability of Stratosphere‐Troposphere radars for estimating the energy dissipation rate ϵ and eddy diffusivity Kz is still strongly questioned because the methods proposed in the literature generally make use of assumptions not yet fully justified. In order to clarify this point, two assessment campaigns, using the high‐resolution UHF PROUST radar, have been performed. Results obtained with both “width” and “power” methods are systematically compared with a statistical approach proposed by Dewan [1979] and Woodman and Rastogi [1984], using the thickness and lifetime of the turbulent layers. The ϵ and Kz derived from these three methods in observed turbulent fields near the tropopause level are found to be reasonably similar for turbulent layers whose thickness LT is smaller than 300 m. For the thicker turbulent patches (LT>300 m), the statistical approach using turbulent thickness tends to give eddy diffusivity values greater than those obtained with width and power methods. This discrepancy could be due to the nonhomogeneity of the turbulence in these large turbulent structures.
Spectral analysis of time domain diffraction tomography
Authors: Melamed Timor , Heyman Ehud
Published: February 2004

This paper is concerned with the spectral content of short‐pulse field scattered from an inhomogeneous medium. Direct diffraction tomography type relations between the time‐dependent data and the medium are determined via two spectral formulations: a time‐dependent plane wave spectrum and a local phase‐space spectrum. The former is formulated in the Radon space, wherein the time‐dependent plane wave spectrum of the data is directly related to the Radon transform of the medium. The latter is formulated as a local Radon transform in the space‐time domain, wherein scattered pulsed beams (space‐time wave packets) are directly related to localized scattering subdomains. This result leads to the formulation of a local inverse scattering scheme, wherein pulsed beams are back propagated and focused to form local images of any desirable region.