bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase transitions and melting in Mars
Authors: Weizman A. , Prialnik D. , Podolak M.
Published: February 2004

The basic thermal evolution model, described by Weizman et al. [1996], was modified to include the effect of phase transitions and the dependence of melting temperatures on pressure. The total amount of water release through Martian history was estimated from this model using the wet solidus of peridotite. Assuming a water content of 0.1% in the basalt, we find that the total amount of water released would be sufficient to cover the planet with a layer at least 3–300 m thick.
On the origin of Upper Miocene sapropelites: A case study from the Faneromeni Section, Crete (Greece)
Authors: Nijenhuis I. A. , Schenau S. J. , Van der Weijden C. H. , Hilgen F. J. , Lourens L. J. , Zachariasse W. J.
Published: February 2004

For the first time, results of an integrated geochemical and micropalaeontological study of upper Miocene sapropelites in a land‐based section in the Mediterranean are presented. Three sapropelites and adjacent homogeneous intervals in the Cretan Faneromeni section have been investigated in detail. The chemical and foraminiferal characteristics were found to be markedly similar to those of Plio‐Pleistocene sapropelites. As the physiography and climate of the late Miocene Mediterranean were not fundamentally different from those of today, our preferred model for late Miocene sapropelite formation is similar to the one recently developed for Plio‐Pleistocene sapropelites. Increased seasonality and freshwater input during northern hemisphere summer insolation maxima triggered high export production and oxygen consumption rates, which, in combination with a concomitant decrease in deep water formation rate, resulted in the observed high organic matter content of the sapropelites.
The Monterey Event in the Mediterranean: A record from shelf sediments of Malta
Authors: Jacobs Elizabeth , Weissert Helmut , Shields Graham , Stille Peter
Published: February 2004

Oligo‐Miocene carbonate platform and shelf sediments outcropping on the Maltese Islands provide an excellent archive of the paleoceanography of the central Mediterranean. A sequence of shallow water limestones, than shelf limestones, and marls, followed again by shallow water limestones, reflects drowning of a carbonate platform, the establishment of a shelf environment and, in the late Miocene, renewed progradation and aggradation of shallow water carbonates. The sequence recording the deepening of the Maltese platform contains several phosphorite hardgrounds and phosphorite pebble beds. These phosphorites were dated with strontium isotopes. Major episodes of phosphogenesis occurred between 25 and 16 Ma, and they are coeval with those phosphorite events reported from Florida and North Carolina. A Miocene carbon isotope and oxygen isotope stratigraphy was established on planktic and benthic foraminifera and on bulk samples. A major carbon isotope excursion with an amplitude of up to +l‰ between 18 and 12.5 Ma can be correlated with the globally recognized Monterey carbon isotope excursion. This is the first record of this event both in shallow water sediments and in the Mediterranean. The carbon isotope excursion precedes an oxygen isotope excursion which also was recognized in deep‐sea records. Major episodes of phosphogenesis and platform drowning preceded the carbon isotope excursion by up to millions of years.
Early Pleistocene chronology of the Vrica Section (Calabria, Italy)
Authors: Lourens L. J. , Hilgen F. J. , Raffi I. , Vergnaud‐Grazzini C.
Published: February 2004

Two alternative age models are presented for the Pleistocene part of the Pliocene‐Pleistocene boundary stratotype section at Vrica (Calabria, southern Italy). Both age models, or chronologies, are based on the correlation of the characteristic sapropel pattern to the summer insolation time series of astronomical solution La90. The first age model is based on the assumption that the succession is continuous, whereas a global near synchroneity of the base of large‐sized Gephyrocapsa in oxygen isotope stages 48–49 is assumed in the second model. This model implies that a major hiatus with exactly the same age and duration is present in both the Vrica and Singa sections. Cross‐spectral analysis between sea surface temperature and δ18O time series and the summer insolation target yields highly significant coherencies in the precession and obliquity frequency bands of the spectrum in cases where the continuous age model is applied but results in less significant coherencies in cases where the alternative age model is applied. According to the continuous model, the position of almost all calcareous plankton datum planes relative to the isotope stages is consistent with that found in the adjacent northern Atlantic. The only marked exception is the base of large Gephyrocapsa, which is found in stage 55 in the Mediterranean, whereas it occurs in stage 48 (or top 49) in the open ocean. The alternative age model solves the large Gephyrocapsa discrepancy but leads to significant discrepancies in the position of other calcareous plankton events. On the basis of these data sets, it is not justified to conclude which age model is correct, although most arguments are in favor of the continuous age model. This model is also supported by shipboard results from Ocean Drilling Program Legs 154 and 160 sites.
Numerical modeling of Late Glacial Laurentide advance of ice across Hudson Strait: Insights into terrestrial and marine geology, mass balance, and calving flux
Authors: Pfeffer W. T. , Dyurgerov M. , Kaplan M. , Dwyer J. , Sassolas C. , Jennings A. , Raup B. , Manley W.
Published: February 2004

A time‐dependent finite element model was used to reconstruct the advance of ice from a late Glacial dome on northern Quebec/Labrador across Hudson Strait to Meta Incognita Peninsula (Baffin Island) and subsequently to the 9.9–9.6 ka 14C Gold Cove position on Hall Peninsula. Terrestrial geological and geophysical information from Quebec and Labrador was used to constrain initial and boundary conditions, and the model results are compared with terrestrial geological information from Baffin Island and considered in the context of the marine event DC‐0 and the Younger Dryas cooling. We conclude that advance across Hudson Strait from Ungava Bay to Baffin Island is possible using realistic glacier physics under a variety of reasonable boundary conditions. Production of ice flux from a dome centered on northeastern Quebec and Labrador sufficient to deliver geologically inferred ice thickness at Gold Cove (Hall Peninsula) appears to require extensive penetration of sliding south from Ungava Bay. The discharge of ice into the ocean associated with advance and retreat across Hudson Strait does not peak at a time coincident with the start of the Younger Dryas and is less than minimum values proposed to influence North Atlantic thermohaline circulation; nevertheless, a significant fraction of freshwater input to the North Atlantic may have been provided abruptly and at a critical time by this event.
Monsoon related variations in Zaire (Congo) sediment load and influence of fluvial silicate supply on marine productivity in the east equatorial Atlantic during the last 200,000 years
Authors: Schneider R. R. , Price B. , Müller P. J. , Kroon D. , Alexander I.
Published: February 2004

Two sediment cores from the Zaire Fan and the Angola Margin have been investigated for their composition of terrigenous and biogenic constituents, respectively. For the late Quaternary, kaolinite/feldspar ratios and variations of terrigenous element ratios of Zr, Ti, K, Rb, and Al reveal that the composition of Zaire River sediment load has fluctuated in tune with precessional variations of boreal summer insolation. In particular, the correspondence of high kaolinite/feldspar and Al/K ratios with low‐latitude insolation maxima strongly corroborates the assumption that west African monsoonal precipitation and chemical weathering was enhanced during periods of increased central African heating. The most striking feauture observed is that opal accumulation has been 2 to 10 times higher in Zaire Fan sediments than outside in continental margin sediments off Angola, although biogenic Ba and Corg fluctuations from both areas indicate that changes of total paleoproductivity were of the same magnitude in the Zaire River plume and off Angola. From this we infer that the contribution of biogenic opal production to total paleoproductivity has been significantly higher within the Zaire River plume than in the oceanic upwelling regime farther to the south off Angola over the last 200,000 years. The pattern of opal accumulation rates with respect to that of marine organic carbon implies that enhanced opal production off the Zaire River to a great extent was the result of additional fluvial supply of dissolved silica during humid climates characterized by more intense chemical weathering on the continent, while total paleoproductivity created by oceanic upwelling was high in periods of increased zonal trade wind intensity at precessional insolation minima and during cold, more arid glacial climate conditions. We presume that paleoproductivity off the Zaire was controlled by the following two sources of nutrients: (1) marine nutrients including nitrate and phosphate as well as the uptake of Ba on particulate Corg caused by upwelling, and (2) silica, mainly delivered by the Zaire River dissolved silicate. Hence our results underline the importance of dissolved silicate in large rivers for marine silicate cycling in the tropical to subtropical Atlantic at a millenial timescale.
Temporal‐geographical meltwater influences on the North Atlantic conveyor: Implications for the Younger Dryas
Authors: Fanning Augustus F. , Weaver Andrew J.
Published: February 2004

The temporal and geographical roles of meltwater discharge (from the Laurentide ice sheet) on North Atlantic Deep Water (NADW) production are investigated utilizing a global, realistic geometry, coupled climate model which does not require the use of flux adjustments. Model results suggest that preconditioning by meltwater discharge (to the Mississippi) prior to the Younger Dryas (YD) is capable of pushing NADW beyond the limit of its sustainability. The diversion of meltwater to the St. Lawrence then merely serves to completely inhibit NADW production. The modeled change in surface air temperature generally agrees with the global pattern and magnitude of temperature change seen in paleoclimatic reconstructions of the YD and is intimately linked to changes in NADW formation. The global thermohaline circulation provides an interhemispheric teleconnection with the Southern Oceans, while changes in the atmospheric heat transport (reacting to a global redistribution of oceanic heat transport) provide a mechanism for interbasin teleconnection. Although the primary thermodynamic and hydrological cycle feedback processes are included within the atmospheric model, in the absence of additional feedbacks an equilibrium without the presence of NADW is possible. The inclusion of the wind stress/speed feedback is found to significantly contribute to the resumption of NADW production, as suggested by previous studies. Contrary to these same studies, however, the coupled model indicates an advective spin‐up timescale is required for resumption of NADW production and hence the termination of the modeled YD‐like climate event (as opposed to a decadal‐century timescale). The reason for the discrepancy is unclear but may be associated with the use of fixed salt flux fields applied in previous studies, or the duration, strength, and geographical location of the imposed meltwater applied.
Solar cycle variation in the heliosphere
Authors: Gazis P. R.
Published: February 2004

Variations in interplanetary plasma and magnetic field parameters over the course of a solar cycle affect a broad range of physical phenomena, such as planetary magnetospheres, cosmic ray modulation, and the interaction of the solar wind with the local interstellar medium. For more than 3 decades a succession of spacecraft have provided in situ measurements of these parameters. These measurements span a range of heliocentric distances from 0.3 to 61 AU and provide an explicit picture of the three‐dimensional structure of the inner and outer heliosphere in the vicinity of the ecliptic plane. In addition, ground‐based interplanetary scintillation measurements and observations from the Ulysses spacecraft have extended our knowledge to smaller heliocentric distances and higher heliographic latitudes. The structure of the heliosphere varies dramatically over the course of a solar cycle. Much of this variation can be related to the changes in the structure and inclination of the coronal magnetic field. The coronal neutral line, which separates the Sun's magnetic hemispheres, is associated with a belt of low‐speed and high‐density solar wind. Its inclination varies over the course of a solar cycle, from a low value near solar minimum to a high value near solar maximum. When the inclination of the coronal neutral line is low, its signature is evident in observations near the solar equator. When the inclination of the coronal neutral line is high, the heliosphere at moderate heliographic latitudes is swept by a succession of high‐ and low‐speed streams and is shaped by their interactions.
An ambiguity‐function‐based method for analysis of Doppler decompressed radar signals applied to EISCAT measurements of oblique UHF‐VHF meteor echoes
Authors: Wannberg G. , Pellinen‐Wannberg A. , Westman A.
Published: February 2004

Binary phase modulation using Barker codes as the modulating sequences is often used as a pulse compression technique in incoherent scatter radar measurements of the ionospheric E region. Demodulation of the compressed signal is usually accomplished by matched filtering techniques. If the target has appreciable velocity, the resulting Doppler shift detunes the scattered signal relative to the filter, and the target response spreads over a large range interval. The paper describes the development of a new data analysis method, based on an ambiguity function description of radar total‐power measurements, which uses the characteristic Doppler decompression signatures to identify, extract, and analyze high‐velocity events from conventional Barker‐coded power profile type incoherent scatter (IS) measurements. Data sets recorded with the European Incoherent Scatter UHF (931 MHz) and VHF (224 MHz) radars during the 1990‐1991 Geminid and 1993 Perseid showers are shown to contain many events which are associated with individual meteors crossing the radar beams over a wide range of aspect angles, including a few near‐radial cases. Line‐of‐sight velocities and effective cross sections are derived for some of the best dual radar events. The cross sections are very small and increase with decreasing wavelength, something not observed before. It is suggested that this is indicative of a scattering mechanism different from the one operating at near‐normal incidence at VHF, and Rayleigh scatter from compact “balls” of plasma contained within the meteor coma is proposed as a possible candidate, in qualitative agreement with the observations. Finally, some signal‐processing related aspects of the method are discussed. It is shown that most IS radars should be able to apply it to their existing programs with very little effort and without having to compromise the performance in normal E region applications. The method could therefore become a powerful tool for studies of meteor statistics at UHF frequencies.
Occurrence patterns of F layer patches over the north magnetic pole
Authors: McEwen D. J. , Harris D. P.
Published: February 2004

An analysis of some 523 F layer patches observed over Eureka, Northwest Territories, Canada (89° corrected geomagnetic latitude) during the four winters from 1990–1991 to 1993–1994 has given some definitive results on their occurrence patterns and characteristics. They are observed on average about 25% of the time and are seen at all hours of the day but with more present in the local evening hours than the morning hours. For the 182 patches for which interplanetary magnetic field data were available, 143 of the patches occurred with Bz negative and 39 with Bz positive. In most of the latter cases, Bz was either near zero or had changed to positive only 30–120 min prior to the patch observation. The average optical emission intensity of the patches over the 4‐year period was 150 R of 630 nm and 55 R of 558 nm. The excitation is by dissociation recombination of the oxygen ion with a branching ratio O[1S]/O[1D] of 0.37. They were on average less bright during the midnight hours (by about 50 R) than through the midday, a fact perhaps due to the offset of the Eureka station from the magnetic pole. The average patch intensity has decreased from 190 R in 1990–1991 to 120 R in 1993–1994 as solar activity has declined. While patch sizes in the dawn‐dusk direction varied up to >2000 km, the average cross‐sectional dimension was 500–600 km.
Polar patches and the “tongue of ionization”
Authors: Steele D. P. , Cogger L. L.
Published: February 2004

Optical images of [O I] 630.0‐nm emission from drifting F region polar patches acquired with a sensitive all‐sky imager have been analyzed to infer the ionospheric convection velocity over a period of 9 hours on December 8, 1993. The drift vectors agree well in both direction and magnitude with simultaneous measurements from a nearby digital ionosonde. The delay between interplanetary magnetic field By changes measured at IMP 8 and changes in the convection direction at the pole is estimated. A montage of the optical emission from the patches observed over the period is presented to illustrate the range of optical patch shapes, sizes, and spacings observed.
Frequency domain interferometry on spaced antenna MF radar
Authors: Brown William O. J. , Fraser Grahame J.
Published: February 2004

Frequency domain interferometry (FDI) experiments were carried out on two medium frequency spaced antenna radars located in New Zealand and the Antarctic. These experiments were the first spaced antenna FDI experiments on MF radar and allowed simultaneous measurements of wind, angular distribution, spatial interferometry, FDI range, and scattering depth to be made. It was found that when correcting the FDI scattering depths for the scale of the radar volumes, typically large on MF radars, the adjustment should include a measure of the aspect sensitivity of the scattering. FDI provides a means of clearly identifying thin layers, and the occurrence of such layers often appeared here to be related to fluctuations in the wind. A very interesting example was a thin layer event that seemed to be closely related to a reversal of the wind apparently caused by a change in phase of the semidiurnal tide; enhanced turbulence following the event suggested breaking of the wave. Multiple frequency steps were used and, with analysis by frequency correlation functions, revealed the occasional presence of multiple scattering features in the radar volume.
A fast moment method algorithm using spectral domain wavelet concepts
Authors: Kim Hyeongdong , Ling Hao , Lee Chankil
Published: February 2004

A multiresolution wavelet algorithm is developed for the fast solution of electromagnetic scattering problems. A multiscale feature of the spectral domain Green's function is observed in the joint spectral‐spatial representation. Owing to the multiscale nature of the electrodynamic Green's function in the spectral domain, wavelet application in the spectral domain (KDWT) is more appropriate in representing the Green's function than the wavelet transform applied in the space domain (SDWT). Using the KDWT, a sparse moment impedance matrix, which is a discretized form of the Green's function, is obtained and a fast multiresolution moment method algorithm is developed in conjunction with the conjugate gradient solver. For a square cylinder the sparsity of the moment method matrix and the resulting time performance are compared with those from the conventional moment method. It is found that the KDWT algorithm leads to a matrix‐vector multiplication which scales with an order less than N2.
Ionospheric diagnostics using wave field diffraction near the caustic
Authors: Galushko Vladimir G. , Yampolski Yuri M.
Published: February 2004

Results of investigating the spatial and temporal HF field structure near the skip distance are presented. The data were obtained on midlatitude radio paths less than 1000 km in length, using broadcasting stations as sources of the probe signals. The signals reflected by the ionosphere were received and processed with a multichannel coherent receiver and the largest phased array of the decameter radio telescope UTR‐2. The strong dependence of the probe signal parameters on the ionospheric plasma conditions has allowed formulating and solving a number of model inverse problems of ionospheric diagnostics. It proves possible to determine parameters of the parabolic ionospheric layer, and the effective electron collision frequency, to evaluate horizontal gradients of the electron density and some parameters of the medium‐scale‐size ionospheric irregularities. A number of tests made both within the method and by means of comparison of the restored parameters against the data of other diagnostic techniques demonstrate the high potential and accuracy of the approach described in the paper.
Finite difference and finite volume time domain techniques in electromagnetics: A comparative study
Authors: Jensen M. A. , Rahmat‐Samii Y.
Published: February 2004

The finite difference and finite volume time domain (FDTD and FVTD) methods are powerful computational simulation techniques in electromagnetics. Although the two algorithms are derived using finite difference discretizations of Maxwell's equations, they differ in their theoretical foundations, practical implementations, and domains of applicability. This paper provides a comparison of these two techniques based upon their theoretical development, simulation accuracy, and computational requirements. Concrete simulation examples are provided to illustrate their respective domains of applicability. A hybrid FVTD/FDTD algorithm is also presented which combines the capabilities of both techniques to allow efficient modeling of a wide variety of geometrical features.
Diffraction phenomena inside thick Fresnel zone plates
Authors: Kopylov Yuri V. , Popov Alexei V. , Vinogradov Alexander V.
Published: February 2004

The parabolic wave equation method is used to describe the complex wave field inside the body of a thick zone plate used to focus X ray radiation. Two analytical approaches are applied: (1) Diffraction of a plane wave incident onto a separate interface between opaque and open zones is considered. We construct an approximate analytical solution to the classical problem of diffraction by a dielectric wedge in terms of the Fresnel integral and a new special function. (2) Coupled wave theory is used to describe collective effects of diffraction by many interfaces. The zone plate is considered as thick diffraction grating with slowly varying period. An analytical solution is found for three interacting modes. A possibility to optimize the zone plate performance is shown. Taken together, these results describe all the main features observed in the output field of realistic X ray zone plates.
Cassini model rheometry
Authors: Rucker H. O. , Macher W. , Manning R. , Ladreiter H. P.
Published: February 2004

Rheometry serves as a method for the determination of effective length vectors of short antennas by means of electrolytic tank measurements. This paper reports on the application of rheometry to the three linear monopoles mounted for the purposes of the Radio and Plasma Wave Science Experiment on the Cassini spacecraft, which will fly to planet Saturn. The voltage signals induced by incoming waves from the Saturnian radio emissions will be recorded for further evaluation. By direction‐finding techniques one will trace back from the collected data to the source regions of the received radio waves and determine the wave polarization. An accurate direction finding is only possible if the effective length vectors of the antennas, which are affected by the spacecraft body, are known to a certain degree of accuracy. It is investigated how rheometry enables the determination of the effective length vectors with the help of a scale model. After a detailed discussion of the fundamentals of rheometry, the application of rheometry to the Cassini scale model is described. The results of the measurements are graphically depicted and discussed with the requirements for direction finding taken into consideration. Finally, an overview of the inflight antenna calibration is given, which will be possible by utilizing the strong Jovian radio emissions during Cassini's Jupiter flyby.
Numerical simulation of cycle slipping in diurnal variation of phase of VLF field
Authors: Perel M. V. , Stesik O. L.
Published: February 2004

A simulation of diurnal variation of the phase of VLF field was carried out to investigate the phenomenon of cycle slipping which had been previously observed on transequatorial paths of southwest direction. We present a simulated curve of the diurnal phase variation demonstrating the slipping phenomenon. The simulated phase curve has a part where phase changes so greatly during a few minutes that it produces a cycle slipping. We give a qualitative explanation of this phenomenon in the frame of a crude adiabatic approach. We demonstrate that at just the moment of the abrupt change of the calculated phase at the receiver the main modes degenerate at some point on the path. The realignment of the distance dependencies of phase velocities and attenuation rates of the degenerating modes at the moment of degeneration is found to be a cause of the jump of the phase. We present also a quantitative description of the phase anomalies involving a consideration of the mode conversion.
Runaway electrons in the atmosphere in the presence of a magnetic field
Authors: Gurevich A. V. , Valdivia J. A. , Milikh G. M. , Papadopoulos K.
Published: February 2004

This paper generalizes the theory of the electron runaway and runaway discharge to the case of a laminar electric field at an arbitrary angle to the magnetic field and derives the relevant threshold conditions. It is shown that the conditions of the runaway process depend on the angle between the electric and magnetic fields, and the ratio of their magnitudes. In fact, the geomagnetic field hinders the development of runaway breakdown in the atmosphere. This effect has implications for runaway discharges in the atmosphere caused by low—altitude lightning. The runaway discharges manifest themselves as fluxes of γ rays, as previously observed by the detector aboard Compton Gamma Ray Observatory. The geomagnetic field plays a significant role in the runaway discharge due to thunderstorms for heights above 20 km, where the cyclotron frequency of relativistic electrons exceeds their collision frequency. This effect depends on the angle between the electric and magnetic fields. Since the static electric fields from thunderclouds are directed almost vertically, one can expect a significant difference in the properties of high‐altitude discharges occurring at equatorial and high‐latitude regions.
Application of polarimetric sounding to HF ionospheric remote sensing
Authors: Bezrodny V. G. , Ponomarenko P. V. , Yampolski Y. M.
Published: February 2004

It is shown that the scattering of electromagnetic waves by refractive index inhomogeneities in a continuous random medium is accompanied by a certain polarimetric phenomenon that is analogous to the Brewster effect of classic electrodynamics. In this paper, an observational method and results are discussed for the case of bistatic HF radar sounding of small‐scale ionospheric inhomogeneities at frequencies above MUF. The height of the scattering layer, its thickness, and characteristic plasma drift velocity in the ionosphere are estimated as a result of statistical processing.
AIM: Adaptive integral method for solving large‐scale electromagnetic scattering and radiation problems
Authors: Bleszynski E. , Bleszynski M. , Jaroszewicz T.
Published: February 2004

We describe basic elements and implementation of the adaptive integral method (AIM): a fast iterative integral‐equation solver applicable to large‐scale electromagnetic scattering and radiation problems. As compared to the conventional method of moments, the AIM solver provides (for typical geometries) significantly reduced storage and solution time already for problems involving 2,000 unknowns. This reduction is achieved through a compression of the impedance matrix, split into near‐field and far‐field components. The near‐field component is computed by using the Galerkin method employing a set of N arbitrary basis functions. The far‐field matrix elements are calculated by using the Galerkin method as well, with a set of N auxiliary basis functions. The auxiliary basis functions are constructed as superpositions of pointlike current elements located on uniformly spaced Cartesian grid nodes and are required to reproduce, with a prescribed accuracy, the far field generated by the original basis functions. Algebraically, the resulting near‐field component of the impedance matrix is sparse, while its far‐field component is a product of two sparse matrices and a three‐level Toeplitz matrix. These Toeplitz properties are exploited, by using discrete fast Fourier transforms, to carry out matrix‐vector multiplications with O(N3/2log N) and O(N log N) serial complexities for surface and volumetric scattering problems, respectively. The corresponding storage requirements are O(N3/2) and O(N). In the domain‐decomposed parallelized implementation of the solver, with the number Np of processors equal to the number of domains, the total memory required in surface problems is reduced to O(N3/2/NP1/2). The speedup factor in matrix‐vector multiplication is equal to the number of processors Np. We present a detailed analysis of the errors introduced by the use of the auxiliary basis functions in computing far‐field impedance matrix elements. We also discuss the algorithm complexity and some aspects of its implementation and applications.
Quasi‐analytical method for analysis of multisection waveguide structures with step discontinuities
Authors: Lyapin V. P. , Manuilov M. B. , Sinyavsky G. P.
Published: February 2004

In this paper an effective quasi‐analytical field theory method for the analysis ofmultisection rectangular waveguide structures with E or H plane step discontinuities is presented. A version of Galerkin's method is used to solve the corresponding diffraction problems. The weghted Gegenbauer polynomials are used as basis functions, taking into account the field asymptotic at the discontinuity edges. Another important feature of the solution is the separation and analytical transform of the matrix operator static part of the obtained linear algebraic equations set. The method proposed here guarantees rapid convergence and high accuracy of the numerical results. It has been verified by numerical convergence investigation and comparison with the results of other works. The method can be applied for the effective design of a wide range of passive waveguide components: different band‐pass, stop‐band, and low‐pass filters, waveguide phase shifters, polarizers, etc. As examples of the concrete devise design, the characteristics of the inductive and capacitive iris filters, evanescent‐mode waveguide dielectric resonator filter, and E plane stub‐loaded, fixed‐phase shifter are given in the paper.
On the existence of nonsuperdirective aperture antennas with directivity patterns within desired bounds
Authors: Hay S. G. , Poulton G. T.
Published: February 2004

The problem of creating a directivity pattern within desired upper and lower bounds using a nonsuperdirective microwave antenna of prescribed aperture is studied, and a strong necessary condition for the existence of solutions is derived. The condition is tested in cases of interest in designing satellite antennas that provide regional coverages on the Earth. For these cases, aperture fields are found which closely approach directivity limits implied by the necessary condition. Realistic designs are also obtained for shaped reflectors with simple feeds, and the results confirm the strength of the necessary condition for practical problems.
Phase compensation experiments with the paired antennas method
Authors: Asaki Yoshiharu , Saito Masao , Kawabe Ryohei , Morita Ko‐Ichiro , Sasao Tetsuo
Published: February 2004

Several kinds of techniques are under development for effective correction of a tropospheric phase fluctuation in millimeter‐ and submillimeter‐wave radio interferometry, including very long baseline interferometry (VLBI). We report here the results of phase correction experiments using a method called “paired antennas method.” In the method, pairs of closely located antennas are used for obtaining the fringe phases of both an observing source and an adjacent reference calibrator simultaneously, and the temporal variation of the obtained fringe phase of the target source is corrected by using the calibrator phase. In cases where the separation angle between the target source and the calibrator is sufficiently small, the correction will be almost perfectly made. The experiments were carried out with the Nobeyama millimeter array (NMA) with a special emphasis on examining the separation angle dependence of the degree of the phase compensation. We simultaneously observed the Japanese Communication Satellite (CS) and a quasar 3C279 passing near the satellite at 19 GHz using four antennas of the NMA in order to measure the fluctuation of the difference in their fringe phases in dependence on the angular separation. The standard deviation of the differential fringe phase estimated for 500‐s time interval is reduced from 50°–70° to 10° as the angular separation decreases from 20° or greater to a few degrees. The results indicate that the method is very useful for the correction of the tropospheric phase fluctuation in millimeter‐ and submillimeter‐wave interferometry.
Impact of receiver errors on the radiometric resolution of large two‐dimensional aperture synthesis radiometers
Authors: Torres F. , Camps A. , Bará J. , Corbella I.
Published: February 2004

The specifications of the subsystems that compound a radiometer interferometer devoted to Earth observation are of main concern because they set the viability and final performance of such an instrument. The importance of these errors is related to the exact way they are generated, since this determines if a particular calibration procedure is capable of removing them or if they remain as residual errors. This paper presents a general method to analyze system errors. This method is used to analyze in detail the amplitude and phase errors of the receivers and their impact on the radiometric resolution. Special attention has been paid to nonseparable errors, since foreseen calibration procedures can only deal with separable phase and amplitude terms. Finally, the results have been used to set the receiver requirements of the instrument called MIRAS (microwave imaging radiometer by aperture synthesis), which is currently being developed by the European Space Agency (ESA).
Ultra wide band electromagnetic scattering of a fractal profile
Authors: Rouvier S. , Borderies P. , Chênerie I.
Published: February 2004

The relationship between the fractal dimension of a perfectly conducting bidimensionnal profile and the fractal dimension of the time domain scattered field is investigated. The first part of the paper is dedicated to the profile itself; implementation of the counting box method for fractal dimension estimation is described and improved by the adjunction of an iterative process involving a correlation criterion. The second part is about the field scattered by a fractal profile which is calculated by the method of moments; polarizations, directions of incidence and observation effects are studied. Influence of spectral window and of noise is also investigated. Results show that fractal dimensions of the field and of the profile are linked by a monotonous increasing function which weakly depends on the polarizations and on the directions of incidence and observation. Moreover, the fractal dimension shows robustness to noise.
Evaluation of Doppler radar velocity estimators
Authors: Lottman B. T. , Frehlich R. G.
Published: February 2004

This paper evaluates mean velocity estimator performance for typical parameter regimes for the 404‐MHz National Oceanic and Atmospheric Administration weather radar, the 915‐MHz boundary layer profiler, and the 50‐MHz mesosphere‐stratosphere‐troposphere/stratosphere‐troposphere radar wind profiling systems. Performance of the velocity estimates is characterized by the fraction of outliers and the standard deviation of the good estimates. For low signal‐to‐noise ratio, modern estimators generally have better performance than the traditional techniques; however, modern estimators require more information to produce a velocity estimate. The boundary for useful velocity estimates can be determined from the performance curves.
Crustal structure of the Middle American Trench off Costa Rica from wide‐angle seismic data
Authors: Ye S. , Bialas J. , Flueh E. R. , Stavenhagen A. , von Huene R. , Leandro G. , Hinz K.
Published: February 2004

Off Costa Rica seismic wide‐angle experiments were made using marine seismic sources of high shot density and ocean bottom hydrophones (OBH) and land stations for recording. Three profiles perpendicular to the Middle American Trench (MAT) and one parallel to it are presented here. The coverage from closely spaced OBH, and structural constraints from the coincident near‐vertical reflection seismic survey provided detailed velocity depth distributions. Over 4.0 km/s velocities were found directly underlying the thick continental margin sediment in a margin wedge that extends from below the coastal area seaward within 10 km from the MAT axis. The interpretation of the wide‐angle data required a low velocity zone (LVZ) between the upper and lower plate, indicating its nature as subducted and underplated sediment. The velocity depth distributions support the hypothesis that the continental margin wedge off Costa Rica is most likely an offshore extension of the Nicoya complex ophiolitic rocks, which are exposed along the coast. The subduction of the sediments seems very efficient and the accretionary prism appears to be confined to the lower slope.
Comparison of gravity‐driven deformation styles and behavior associated with mobile shales and salt
Authors: Morley C. K. , Guerin G.
Published: February 2004

Structures produced by salt and mobile shales are commonly similar; however, the material behavior of the two is different. Salt mobility is a fundamental material property, shale mobility only occurs if overpressured fluids are present. Dewatering of shales will stop their mobility, while renewed burial or the onset of an internal overpressuring process (e.g., diagenetic release of water or hydrocarbon generation) may renew mobility. Consequently, structures in mobile salt will envolve continuously until salt withdrawal produces touchdowns sufficient to stop salt mobility. Shale mobility may follow a deformation sequence similar to salt, or it may display a more episodic evolution reflecting critical overpressuring events. While salt mobility is confined to specific lithological units, the same is not the case for shale. The overpressuring of shales is strongly dependent on depth, so that mobile shale zones may cut across time/bedding boundaries. The differences in mechanical behavior lead to differences in structural style although many basic aspects of gravity tectonics remain the same. (1) Prekinematic structures and synkinematic deformation occur in both salt and shale tectonics. However, much of the prekinematic deformation in mobile shale‐dominated deltas may be lost by burial and conversion of the prekinematic sequence into mobile shales. (2) Fault‐dominated depocenters occur in salt and shale tectonics. In the Niger delta, overpressured shales vary in thickness from thin decollement zones to massive chaotic zones some 4–6 km thick. Fault‐controlled basins develop over mobile shales. Maximum basin depth is approximately the thickness of the mobile shale plus the thickness of the overlying pregrowth fault strata (i.e., some 4–8 km). (3) Diapirs are common to both salt and shale tectonics. Normal faults associated with reactive diapirism are common in both. Salt has the potential to almost completely evacuate a particular volume, resulting in local touchdowns or welds. Shale may also create touchdown areas, but complete collapse is uncommon because a large volume of immobile dewatered shale is usually left behind. (4) Salt nappes can cover extensive areas in the Gulf of Mexico. Some limited shale tongues occur in the Niger delta. They form imbricate thrusts that pass down dip into gravity flows and slumped blocks of shale derived from the exposed mobile shale.
Three‐dimensional model of the late Cenozoic history of the Death Valley region, southeastern California
Authors: Serpa Laura , Pavlis Terry L.
Published: February 2004

The accumulation of a large database on the timing and kinematics of late Cenozoic deformation in the Death Valley region of southeastern California indicates a complex three‐dimensional history. On the basis of paleogeographic reconstructions we suggest the system was initiated as a localized pull‐apart between two conjugate strike‐slip faults, the Garlock and Furnace Creek faults, and evolved into a system characterized by distributed transtension related to the eastern California shear zone. Our reconstructions differ from previous models in the incorporation of significant vertical axis rotations of a number of crustal blocks to explain paleomagnetic data from the region. The model may resolve (1) a long‐standing problem of the eastern termination of the Garlock fault which is explained here as a complex system of splays that initially terminated in the pull‐apart between the Furnace Creek and Garlock systems; and (2) the complex architecture of the Black Mountains which is explained here in terms of initial extreme attenuation between the Garlock and Furnace Creek systems with overprinting by a fold and normal fault system that operated simultaneously as a result of distributed transtension. This model suggests much of the displacement field is taken up in rotations and translations, and the actual crustal thinning in our model is relatively small (50–66% of original thickness).
Tectonic evolution of the central Annapurna Range, Nepalese Himalayas
Authors: Hodges K. V. , Parrish R. R. , Searle M. P.
Published: February 2004

The metamorphic core of the Himalayan orogen, or Greater Himalayan sequence, is a northward tapering prism bound at the bottom by a N dipping family of thrust faults (the Main Central thrust system) and at the top by a N dipping family of normal faults (the South Tibetan detachment system). Research in the central Annapurna Range of Nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the Early Miocene. The Main Central thrust system is represented here by a 2‐ to 3‐km‐thick zone of high strain that developed during two or more episodes of movement. Most of its displacement was concentrated along the Chomrong thrust, a sharp, late‐metamorphic discontinuity that places middle amphibolite facies rocks of the Greater Himalayan sequence on top of lower amphibolite facies rocks of the Lesser Himalayan sequence. The earliest demonstrable movement on this thrust system occurred ∼22.5 Ma; the most recent movement may be as young as Pliocene. The oldest element of the South Tibetan detachment system in this area is the Deorali detachment, which appears to have been active at the same time as the earliest shortening structures of the Main Central thrust system. Fabrics related to the Deorali detachment are disrupted by a previously unrecognized, SW vergent, thrust structure, the Modi Khola shear zone. The effect of this structure, which is constrained to be between 22.5 and 18.5 Ma, was to shorten rock packages that had been extended previously during movement on the Deorali detachment. Transition back to a local extensional regime after 18.5 Ma was marked by development of the Machhupuchhare detachment and related splays. Geologic evidence for rapid, two‐way transitions between contraction and extension in the Annapurna Range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle; it also appears to be an important factor in mountain range development.
Simultaneous development of noncylindrical folds, frontal ramps, and transfer faults in a compressional regime: Experimental investigations of Himalayan Examples
Authors: Dubey Ashok Kumar
Published: February 2004

Experiments were performed with modeling clay models to understand the simultaneous development of folds, frontal ramps, and transfer faults/oblique ramps in layered sequences. The models were deformed by layer parallel contraction. At a late stage of deformation, the models were subjected to a maximum contraction parallel to the layering and perpendicular to the early axis of maximum contraction to study the effect of the superposed deformation. The experimental results reveal that a transfer fault joining two frontal thrust ramps acts as a shear zone and controls the orientation of fold hinge lines in the adjacent region. Hence two orientations of fold hinge lines may be observed: (1) oblique to the axis of maximum compression in the vicinity of the transfer fault and (2) normal to the axis and parallel to the frontal ramps away from the transfer fault. During the longitudinal fold propagation, the two hinge lines may coalesce to form a fold curvature opposite to the tectonic transport direction. The curvature varies along the transfer fault depending on the shear zone geometry. Additional fold hinge line curvatures may result during the superposed deformation both in the early and the superposed fold hinge lines. All these curvatures produce a systematic pattern which can be analyzed with the help of the experimental results. The fold hinge line orientation may also be used to establish an oblique ramp in field as demonstrated by examples from the Lesser Himalayas, India.