bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectral characteristics of solar near‐infrared absorption in cloudy atmospheres
Authors: Harshvardhan  , Ridgway William , Ramaswamy V. , Freidenreich S. M. , Batey Michael
Published: February 2004

Theoretical and experimentally derived estimates of the atmospheric absorption of solar energy in the presence of clouds have been reported to be at variance for quite a long time. A detailed set of near‐monochromatic computations of the reflectance, transmittance, and absorptance of a standard midlatitude atmosphere with embedded water clouds is used to identify spectral features in the solar near‐infrared that can be utilized to study this discrepancy. The results are framed in terms of the cloud radiative forcing both at the surface and at the top of the atmosphere, and it is shown that water vapor windows are the most sensitive to variations in cloud optical properties and cloud placement in the vertical. The ratio of the cloud radiative forcing at the surface to that at the top of the atmosphere, R, varies from near zero in the band centers at small wavenumbers for high clouds to ∼1 in the band centers at larger wavenumbers for low clouds and to values in excess of 2 in the water vapor windows at small wavenumbers. The possibility of using measurements from space with the future Moderate Resolution Imaging Spectroradiometer (MODIS) and simultaneous surface measurements is discussed. It is also shown that horizontal inhomogeneities in the cloud layers do not alter appreciably the estimates of the R factor, but areal mean cloud absorption is lower for an inhomogeneous cloud having the same mean liquid water as the corresponding homogeneous cloud.
Extension and mantle upwelling within the San Andreas fault zone, San Francisco Bay area, California
Authors: Jové Carlos F. , Coleman Robert G.
Published: February 2004

Upper mantle and lower crustal xenoliths in Tertiary alkali olivine basalts located along the trace of the Calaveras Fault system near Coyote Lake reservoir, northern California, provide an opportunity to study physical conditions of crystallization, composition, and rheology of magmatic mantle rocks beneath the San Andreas fault system (SAF). The episode of mafic alkaline volcanism in the Calaveras Fault at the area of Coyote Lake reservoir can be related to stepping‐fault interaction. The right‐stepping en echelon fault geometry along with right‐lateral motion of the SAF and the spatial and temporal distribution of the Coyote Lake volcanics suggest that fault activity was synchronous to the volcanic event. Upper mantle xenoliths are Cr‐diopside spinel lherzolites with minor dunites both having porphyroclastic textures. Lower crustal xenoliths are two‐pyroxene metagabbros and andesine megacrysts. Two‐pyroxene and olivine‐spinel (sp‐lherzolites only) geothermometric estimates for both spinel lherzolites and two pyroxene metagabbros range between 885° and 1077° C and 915° and 1000° C, respectively. Pressures are bracketed between 12 and 16.8 kbars and 9 and 10.5 kbars for spinel lherzolites and two‐pyroxene metagabbros, respectively. These thermobarometric data agree with mantle temperatures projected along the elevated surface heat flow geotherms estimated for the SAF system and also with lithologies inferred from geophysical studies in this area. Furthermore, these thermobarometric estimates along with xenolith chemistry and the presence of alkaline volcanism within the fault suggest that this segment of the Calaveras Fault may have experienced near‐fault normal extension during the Pliocene producing mantle upwelling and decompressional melting at relatively shallow depths.
Comparison of two approaches to quantify anthropogenic CO2 in the ocean: Results from the northern Indian Ocean
Authors: Coatanoan C. , Goyet C. , Gruber N. , Sabine C. L. , Warner M.
Published: February 2004

This study compares two recent estimates of anthropogenic CO2 in the northern Indian Ocean along the World Ocean Circulation Experiment cruise II [Goyet et al., 1999; Sabine et al., 1999]. These two studies employed two different approaches to separate the anthropogenic CO2 signal from the large natural background variability. Sabine et al. [1999] used the ΔC* approach first described by Gruber et al. [1996], whereas Goyet et al. [1999] used an optimum multiparameter mixing analysis referred to as the MIX approach. Both approaches make use of similar assumptions in order to remove variations due to remineralization of organic matter and the dissolution of calcium carbonates (biological pumps). However, the two approaches use very different hypotheses in order to account for variations due to physical processes including mixing and the CO2 solubility pump. Consequently, substantial differences exist in the upper thermocline approximately between 200 and 600 m. Anthropogenic CO2 concentrations estimated using the ΔC* approach average 12 ± 4 μmol kg−1 higher in this depth range than concentrations estimated using the MIX approach. Below ∼800 m, the MIX approach estimates slightly higher anthropogenic CO2 concentrations and a deeper vertical penetration. Despite this compensatory effect, water column inventories estimated in the 0–3000 m depth range by the ΔC* approach are generally ∼20% higher than those estimated by the MIX approach, with this difference being statistically significant beyond the 0.001 level. We examine possible causes for these differences and identify a number of critical additional measurements that will make it possible to discriminate better between the two approaches.
The Indian Ocean 13C Suess Effect
Authors: Sonnerup Rolf E. , Quay Paul D. , McNichol Ann P.
Published: February 2004

The δ13C of dissolved inorganic carbon (DIC) decrease (the 13C Suess effect) in the Indian Ocean was calculated using a multiparameter linear regression comparison of the 1978 Geochemical Ocean Sections Study and 1995 World Ocean Circulation Experiment δ13C of DIC, hydrographic, and nutrient data. The surface ocean δ13C decrease rate along 80° and 115°E ranged from approximately −0.1‰ decade−1 at 55°S to a maximum of approximately −0.18‰ decade−1 at ∼ 35°S and decreased northward to around −0.13‰ decade−1 at the equator. Using a global extrapolation based on ocean model results [Bacastow et al., 1996] and previous δ13C changes calculated from 1970 to 1993 station reoccupations in the Pacific Ocean and based on preformed δ13C versus chlorofluorocarbon age trends in the Indian, Pacific, and Atlantic Oceans [Sonnerup et al., 1999], an ocean‐wide average surface ocean δ13C rate of change of −0.15 to −0.17‰ decade−1 was estimated. The average depth‐integrated δ13C change rate between 1978 and 1995 along 80° and 115°E was −6.9 ± 0.5‰ m yr−1 from 58°S to 5°N.
Sea‐air CO2 fluxes and carbon transport: A comparison of three ocean general circulation models
Authors: Sarmiento J. L. , Monfray P. , Maier‐Reimer E. , Aumont O. , Murnane R. J. , Orr J. C.
Published: February 2004

Many estimates of the atmospheric carbon budget suggest that most of the sink for CO2 produced by fossil fuel burning and cement production must be in the Northern Hemisphere. Keeling et al. [1989] hypothesized that this asymmetry could be explained instead by a northward preindustrial transport of ∼1 Pg C y−1 in the atmosphere balanced by an equal and opposite southward transport in the ocean. We explore this hypothesis by examining the processes that determine the magnitude of the preindustrial interhemispheric flux of carbon in three ocean carbon models. This study is part of the first stage of the Ocean Carbon Model Intercomparison Project organized by International Geosphere Biosphere Programme Global Analysis, Interpretation, and Modelling Task Force. We find that the combination of interhemispheric heat transport (with its associated carbon transport), a finite gas exchange, and the biological pump, yield a carbon flux of only −0.12 to +0.04 Pg C y−1across the equator (positive to the north). An important reason for the low carbon transport is the decoupling of the carbon flux from the interhemispheric heat transport due to the long sea‐air equilibration time for surface CO2. A possible additional influence on the interhemispheric exchange is oceanic transport of carbon from rivers.
A global geochemical database structure for rocks
Authors: Lehnert K. , Su Y. , Langmuir C. H. , Sarbas B. , Nohl U.
Published: February 2004

This technical brief describes a geochemical and petrological database structure based on the relational model that has broad applicability to chemical analyses of geological materials. Notable features of the database structure are its comprehensiveness and flexibility. The structure consists of 34 interrelated tables, which can accommodate any type of analytical values for all different materials of rock samples (volcanic glasses, minerals, inclusions, etc.) and for samples from any tectonic setting. A broad spectrum of supplementary information (metadata) is included that describes the quality of the analytical data and sample characteristics, such as petrography, geographical location, and sampling process, and that can be used to evaluate, filter, and sort the chemical data. All data in the database are linked to their original reference. The database structure can be implemented in any relational database management system (RDBMS). It is currently applied in two different rock database projects (RidgePetDB and GEOROC).
Transport and distribution of primary and secondary nonmethane volatile organic compounds in east Asia under continental outflow conditions
Authors: Phadnis Mahesh J. , Carmichael Gregory R.
Published: February 2004

A regional scale three‐dimensional atmospheric chemistry model is used to study the transport and chemistry of nonmethane volatile organic compounds (NMVOCs) in east Asia. A springtime episode during the month of May 1987 is investigated. The emission estimates of ethane, propane, and ethene from our calculations in east Asia are ∼1.6, ∼1.3, and ∼5.2 Tg C/yr, respectively. The model calculated hydrocarbon mixing ratios are compared with observations from a network of stations in China and Japan and also with the Pacific Exploratory Mission ‐ West B data. The model predictions generally agree favorably with observations over China and Japan, where hydrocarbon mixing ratios are between 1 and 5 ppb. The long‐lived ≥C4 alkanes are better modeled than the highly reactive short‐lived ≥C3 alkenes having mixing ratios <0.5 ppb, which are generally underpredicted by the model. The differences between these model predicted and observed values are attributed to incomplete information about the intensity of the NMVOC emissions as well as their distribution. Positive correlations are obtained for the predicted hydrocarbons against CO and SO2, an indication of similar anthropogenic emission sources. Secondary organic compounds are also analyzed to assess the extent of the oxidation of primary hydrocarbons in the troposphere. It is observed that over continental China the contributions of the oxidation products of isoprene are >80% of the pool of oxygenates. The reverse is true at marine locations, where oxidation products of long‐lived alkanes dominate. Sensitivity analysis to increase in the NOx and NMVOC emissions suggest that for the simulation conditions of the model, ozone production over most of east Asia is NOx sensitive. A 20–30% increase in ozone is predicted over China for doubling of NOx emissions.
The effect of atmospheric waves on aerosol nucleation and size distribution
Authors: Nilsson E. Douglas , Pirjola Liisa , Kulmala Markku
Published: February 2004

Atmospheric waves can enhance the aerosol particle nucleation rate up to 5 orders of magnitude because of the strongly nonlinear effect of temporal fluctuations in temperature and vapor pressures on the nucleation rate. The wave amplitude is the most important wave characteristic controlling the mean nucleation rate and net particle number concentration. A simple parameterization of the enhancement in nucleation rate can be given at certain mean conditions. The enhancement is from 1 to 5 orders of magnitude if the temperature amplitude is 5 K. Since nucleation causes a wave periodicity in the aerosol number only for particles below 3nm diameter, which is typically the lower cut for particle counters, it will be difficult to directly observe the effect of wave or eddy variability on the number of near‐nanometer size particles. In larger particles the waves will cause an increase in the mean number concentration. It is also difficult to observe a difference between the influence of nucleation on the number concentration and variations simply caused by vertical gradients in number concentration or fluxes of these particles.
Stereo topography of the south polar region of Mars: Volatile inventory and Mars Polar Lander landing site
Authors: Schenk Paul M. , Moore Jeffrey M.
Published: February 2004

Viking stereo images and topographic maps reveal that the south polar layered deposits of Mars are topographically complex and morphologically distinct from the north polar layered deposits. The dominant feature is a 500‐km‐wide topographic dome that rises 3 km above the surrounding plains. This dome underlies the residual ice cap but is at least 50% larger in area. Erosional scarps and terraces indicate that this dome was once more extensive and has undergone erosional retreat. Adjacent to the dome, layered deposits form a vast plateau 1–1.5 km high extending ∼1000 km beyond and to one side of the residual south polar cap. This plateau is relatively flat at kilometer scales, although it is cut in places by troughs and depressions, which have locally steep scarps up to 2 km high and sloping up to roughly 10°. Contiguously flat kilometer‐scale regions the size of the Mars Polar Lander (MPL) landing ellipse are present. These are in the form of plateaus 100–300 km wide and 1–2 km high. One of the largest of these plateaus has been proposed as a landing site for the Mars Polar Lander (MPL). The volume associated with the south polar layered deposits may be comparable to those of the layered deposits at the north pole. Although this doubles the current probable inventory of surface ice on Mars, it still falls far short of accounting for the inferred volume of water on Mars in the past.
Thorium abundances on the lunar surface
Authors: Lawrence D. J. , Feldman W. C. , Barraclough B. L. , Binder A. B. , Elphic R. C. , Maurice S. , Miller M. C. , Prettyman T. H.
Published: February 2004

Measurements of absolute thorium abundances on the lunar surface are presented using both the high‐ and low‐altitude data taken with the Lunar Prospector Gamma‐Ray Spectrometer. An analysis of the uncertainties shows that the measured uncertainties are <0.5 μg/g and are close to the theoretical limit of Poisson statistics. Currently, the overall systematic uncertainties are likely dominated by variations in background counts. The relative systematic uncertainties are thought to be no larger than 30% for the high‐altitude data and 15% for the low‐altitude data. A comparison of high‐ and low‐altitude data show that most regions having thorium abundances >7 μg/g are likely small area regions ≤(150 km)2. Using lunar topographic data, we have shown that the thorium abundances in the lunar high‐lands and portions of South Pole‐Aitken (SPA) Basin are larger for lower elevations. We have also studied a number of regions with anomalously high thorium abundances such as the northwestern region of SPA Basin, the crater Arago in western Tranquillitatis, and the Compton/Belkovich region in the northeastern highlands. The Compton/Belkovich region appears to be enriched with evolved rocks such as alkali anorthosite and currently represents the only such extended region on the Moon that has been identified. In contrast, Tycho crater has very low thorium abundances which suggests that KREEP was not assimilated at depth in this portion of the Moon.
A reassessment of the emplacement and erosional potential of turbulent, low‐viscosity lavas on the Moon
Authors: Williams David A. , Fagents Sarah A. , Greeley Ronald
Published: February 2004

We have reevaluated the role of thermal erosion by low‐viscosity lunar lavas as a mechanism for the formation of the lunar sinuous rules. We have adapted the model of Williams et al. [1998] and used the compositions of an Apollo 12 basalt and a terrestrial komatiitic basalt to investigate the compositional and environmental effects on the flow behavior of low‐viscosity lavas on the Moon and the Earth. Our model predicts that lunar lavas could have erupted as turbulent flows that were capable of flowing hundreds of kilometers on a sufficiently flat, unobstructed substrate. These results are consistent with previous studies. Modeling of lavas over a substrate of the same composition shows that thermal erosion rates would have been low (∼10 cm d−1). As a result, long‐duration eruptions (approximately months to years) would have been required to incise deep (tens to hundreds of meters) channels. Partial melting and mechanical removal of the substrate, a mechanism suggested by Hulme [1973] to enhance erosion, only slightly increases thermal erosion rates. Other factors, such as higher flow rates or lava superheating, could have produced deep rules by thermal erosion during shorter‐duration eruptions. A superheated lunar lava not only would have had a higher erosion rate (∼40 cm d−1) but also would have remained uncrusted for tens of kilometers, which is consistent with the open channel morphology of most sinuous rules. For lunar lavas with large volatile (i.e., vesicle) contents, the presence of vesicles would have tended to increase viscosity at low strain rates, resulting in shorter turbulent flow distances, lower thermal erosion rates, and thus shallower erosion channel depths for given eruption durations.
U/Th‐dated sclerosponges from the Indonesian Seaway record subsurface adjustments to west Pacific Winds
Authors: Moore Michael D. , Charles Christopher D. , Rubenstone James L. , Fairbanks Richard G.
Published: February 2004

Stable isotope records from sclerosponges collected at 10–20 m depth in the Indonesian Seaway and Solomon Islands are particularly well suited for reconstructing century‐scale trends in ambient temperature variability and the oceanic uptake of fossil fuel carbon. Basal U/Th dates demonstrate that the sclerosponges analyzed are 85–100 years old. Isotopic records from the Indonesian specimens suggest a strong subsurface cooling over the past 20 years that is not manifested in either surface instrumental or shallower coral proxy records. However, analysis of observed subsurface temperatures in Indonesia, observed winds in the west Pacific, and simulated subsurface temperatures from a steady state general circulation model hindcast forced by observed winds combine to suggest that thermocline adjustments could account for at least part of the recent cooling inferred from the Indonesian sclerosponges. If so, the sclerosponge data suggests that, on average, the west Pacific thermocline has shoaled significantly over at least the past 2 decades.
Surface ocean density gradients during the Last Glacial Maximum
Authors: Billups K. , Schrag D. P.
Published: February 2004

We have compiled published planktonic foraminiferal oxygen isotope values from the tropical to subtropical Atlantic and Pacific for the late Holocene and Last Glacial Maximum (LGM). We interpret the spatial distribution of δ18O values in terms of meridional density gradients in the LGM surface ocean and discuss implications on tropical ocean thermocline structure. In both oceans, glacial δ18O values display a pattern of interhemispheric asymmetry evidenced by increasingly higher glacial δ18O values toward northern latitudes. The δ18O‐derived density estimates suggest that the northern subtropical surface ocean density was higher during the LGM than today, which has significant implications on LGM thermocline ventilation. In the Atlantic, steeper density gradients in the northern subtropics during the LGM are consistent with a more compressed subtropical gyre and thermocline water formation farther south than today. In the Pacific the steeper density gradients suggest a significant reduction in freshwater fluxes to the northern tropical and subtropical regions, which raises the possibility of a more significant component of Northern Hemisphere source waters to the tropical Pacific thermocline. These predictions could be tested with numerical ocean models that assess the structure of the tropical thermocline in the glacial ocean.
Pliocene development of the east‐west hydrographic gradient in the equatorial Pacific
Authors: Chaisson W. P. , Ravelo A. C.
Published: February 2004

Foraminifer count and multispecies isotopic data from Ocean Drilling Program (ODP) Sites 806 (0°N, 165°E) and 847 (0°N, 95°W) are presented for the last 6.4 m.y. Faunal evidence shows a decline of taxa of Miocene origin between 4.5 and 4.0 Ma at both sites. Prior to this decline, Globoturborotalita species and Neogloboquadrina acostaensis dominated Sites 806 and 847 assemblages, respectively. Globoturborotalitids were not replaced by other thermocline dwellers after 4.2 Ma, which suggests a deepening of the thermocline and thickening of the warm pool at Site 806. At Site 847 in the eastern “cold tongue,” N. acostaensis was gradually replaced by N. dutertrei. After 4.2 Ma, there was also a large increase (from 0.7 to 1.3‰) in the difference between the δ18O values of Globigerinoides sacculifer, a surface dweller, and Neogloboquadrina dutertrei, a seasonal thermocline dweller at Site 847; these data, in conjunction with faunal evidence, suggest that the temperature gradient in the upper photic zone increased significantly, largely because of subsurface cooling. The events in planktonic foraminifer evolution and ecology suggest that between 4.5 and 4.0 Ma a significant step was made toward development of the modern east‐west gradient in surface hydrography. The timing of these events suggests that they were related to the closing of the Central American Seaway and subsequent changes in meridional temperature gradients and/or changes in air‐sea interactions that modified tropical winds.
Climatic responses to tropical sea surface temperature changes on a “greenhouse” Earth
Authors: Huber Matthew , Sloan L. Cirbus
Published: February 2004

The uncertainty associated with tropical sea surface temperatures (SSTs) during past “greenhouse” climates may have important and unaccounted for effects. We explore early Paleogene climatic sensitivity to changes in tropical‐subtropical SSTs with a general circulation model. We demonstrate that tropical SST changes have local and far‐field climatic effects, underscoring their importance in understanding greenhouse climates. The responses of winds, upwelling, and surface water balance to tropical SST changes are substantial. Our results indicate that current tropical SST reconstructions may have a significant cool bias despite corrections and that the existence of hot (>30°C) tropical SSTs may be realistic for greenhouse climate intervals, including the Eocene.
Intensification of the Northeast Pacific oxygen minimum zone during the Bölling‐Alleröd Warm Period
Authors: Zheng Yan , van Geen Alexander , Anderson Robert F. , Gardner James V. , Dean Walter E.
Published: February 2004

Although climate records from several locations around the world show nearly synchronous and abrupt changes, the nature of the inferred teleconnection is still poorly understood. On the basis of preserved laminations and molybdenum enrichments in open margin sediments we demonstrate that the oxygen content of northeast Pacific waters at 800 m depth during the Bölling‐Alleröd warm period (15–13 kyr) was greatly reduced. Existing oxygen isotopic records of benthic and planktonic foraminifera suggest that this was probably due to suppressed ventilation at higher latitudes of the North Pacific. Comparison with ventilation records for the North Atlantic indicates an antiphased pattern of convection relative to the North Pacific over the past 22 kyr, perhaps due to variations in water vapor transport across Central America.
Eocene oceanic responses to orbital forcing on precessional time scales
Authors: Sloan L. Cirbus , Huber Matthew
Published: February 2004

The goal of our study was to gain an estimate of the variability of ocean‐related climate processes driven by insolation forcing over a realistic precessional cycle in an Eocene greenhouse world. Between endmembers of a precessional cycle mean annual sea surface temperatures (SSTs) vary by up to 5°C at high northern latitudes, with minimal tropical SST response. Extratropical regions of the Pacific, Atlantic, and Tethys Oceans show up to a two‐fold variation in upwelling strength, while oceanic regions adjacent to northwestern Africa, India, and South America exhibit little oceanic upweliing variability. The response of ocean surface moisture balance to the forcing is greatest in the tropics, varying by as much as 60%. Continental runoff varies by up to a factor of two in some regions. These results may be useful in identifying locations with maximum likelihood of future recovery of orbital cyclicity in deep‐sea sediments.
The role of the ionosphere in aurora and space weather
Authors: Newell Patrick T. , Greenwald Raymond A. , Ruohoniemi J. Michael
Published: February 2004

Recent research strongly suggests that the ionosphere plays a crucial role in the dynamics of space weather. Although the ionosphere is by volume only a small fraction of the magnetosphere, it serves as a variably conducting boundary, modulating the global electrodynamic circuit in crucial ways. A striking example is the behavior of intense aurora, which have recently been discovered to occur only when the background ionospheric conductivity is low. It is now clear that auroral acceleration occurs at the interface between the ionosphere and the magnetosphere and is controlled by magnetospheric‐ionospheric coupling, with the solar cycle variations arising from a surprising source: variations in solar EUV flux. The discovery of diverging electric fields with their possibly corresponding black aurora provides a new symmetry to magnetosphere‐ionosphere coupling processes. The far‐reaching scope of the ionosphere in space weather problems is illustrated here in several ways. Ionospheric convection is suggested to be a major player in space weather, by creating global coherence in the magnetosphere on timescales not otherwise practical. Even a problem seemingly as far removed as possible from the ionosphere, namely, that of charge neutrality in polar rain (superthermal solar wind electron) entry into the distant magnetotail, is shown to be coupled to the problem of polar wind outflow from the ionosphere.
Long cells exposed to a transverse 50‐ to 60‐Hz electric field
Authors: King Ronold W. P.
Published: February 2004

The penetration of 50‐ to 60‐Hz electric fields into the membrane and interior of a long nerve or muscle cell is investigated analytically when the incident electric field is perpendicular to the length of the cell. Explicit formulas are derived for the induced electric fields in the interior and in the membrane. It is shown that these are comparable to those in a spherical cell with the same radius; that is, the induced transverse electric field in the interior of the long cell is very small. The induced voltage across the membrane is also determined. These results supplement earlier ones which showed that when the incident electric field is parallel to the length of the cell, the electric field induced in the axon and membrane is the same as the incident field and that there is no voltage induced across the membrane. The combination of the induced axial and transverse electric fields provides a complete picture of the induced electric field in both the membrane and the interior of a long cell in an arbitrarily incident electric field.
Transverse magnetic scattering from a composite semielliptic cylinder
Authors: Byun W. J. , Myung N. H.
Published: February 2004

An analytic series solution is developed in this paper for the electromagnetic field scattered by a confocal semielliptic cylinder consisting of dielectric and conductor for a transverse magnetic polarized line source illumination. The scattered field is represented in terms of Mathieu functions with unknown coefficients. By applying appropriate boundary conditions the unknown coefficients are obtained. Some numerical results are shown for mutual interactions between dielectric and conductor with the different eccentricities and permittivities.
Analysis of foliage effects on mobile propagation in dense urban environments
Authors: Bronshtein Alexander , Mazar Reuven , Lu I‐Tai
Published: February 2004

Attempts to reduce the interference level and to increase the spectral efficiency of cellular radio communication systems operating in dense urban and suburban areas lead to the microcellular approach with a consequent requirement to lower antenna heights. In large metropolitan areas having high buildings this requirement causes a situation where the transmitting and receiving antennas are both located below the rooftops, and the city street acts as a type of a waveguiding channel for the propagating signal. In this work, the city street is modeled as a random multislit waveguide with randomly distributed regions of foliage parallel to the building boundaries. The statistical propagation characteristics are expressed in terms of multiple ray‐fields approaching the observer. Algorithms for predicting the path‐loss along the waveguide and for computing the transverse field structure are presented.
Application of perturbation techniques to the problem of low‐frequency electromagnetic wave scattering from an air‐ground interface
Authors: Vazouras C. N. , Yarovoy A. G. , Moyssidis M. A. , de Jongh R. V. , Fikioris J. G. , Ligthart L. P.
Published: February 2004

The two‐dimensional problem of horizontally polarized wave scattering from an airground interface is considered. The diffraction problem is formulated by means of the extinction theorem, leading to a system of two simultaneous surface integral equations. Three perturbation techniques, namely the small‐perturbation method, the phase perturbation method, and the small‐slope approximation, have been applied to this system. Iterative relations for the terms of arbitrary order are obtained for each perturbation series. For the sinusoidal surface a benchmark solution, which remains stable near the points of Wood's anomalies, is developed. It is used to establish the area of validity of each perturbation technique by means of numerical comparison.
A note on corrections often used in two‐scale surface‐scattering models
Authors: Craeye Christophe , Guissard Albert , Sobieski Piotr
Published: February 2004

Two‐scale models have been used for nearly two decades to estimate the scattering properties of rough surfaces described by their spectrum. Nevertheless, the solutions proposed in the literature present numerous differences. Through a rigorous treatment of the large‐scale slopes we explain why some popular correcting factors actually should be omitted. Besides its simplicity, the resulting model is not restricted to situations where the large‐scale process has small slopes. The proof is given for the general case of bistatic scattering from lossy dielectric surfaces.
Plane wave scattering by edges in unidirectionally conducting screens
Authors: Nepa Paolo , Manara Giuliano , Armogida Andreina
Published: February 2004

According to its original definition a unidirectionally electrically conducting (UEC) screen is a penetrable anisotropic surface which is perfectly conducting in one direction and perfectly insulating in the orthogonal direction. The scattering of an arbitrarily polarized plane wave obliquely incident on the edge of either a UEC half plane or a planar junction between a UEC screen and a perfectly conducting half plane is analyzed in this paper. The direction of infinite conductivity of the UEC screen is arbitrarily oriented with respect to the edge. Exact closed form expressions for the scattered field are derived, which contain the standard transition function of the uniform geometrical theory of diffraction and simple ratios of trigonometric functions. Specific attention is devoted to the phenomenon of surface wave excitation at the diffracting edge of the structures under investigation. The features of this wave guided by the anisotropic screen are discussed.
Tilted atmospheric layer and gravity wave studies by simultaneous DBS/SDI observations with the MU radar during the passage of a trough
Authors: Kawano Noriyuki , Luce Hubert , Yamamoto Mamoru , Fukao Shoichiro
Published: February 2004

In the present paper, simultaneous radar observations in Doppler beam swinging (DBS) and spatial domain interferometry (SDI) modes with the middle and upper atmosphere (MU) radar are shown for the first time. High aspect sensitive echo layers were found at a 6.0 to 8.6 km altitude range owing to observations in DBS mode. From SDI analysis, the angle of arrival of the backscattered signals was also deduced. It was found that the layers were tilted from the horizontal plane along the zonal axis with a tilt angle of approximately 0.7°. Because it is known that tilted aspect sensitive layers introduce a bias in the radial velocity measurements, we corrected the vertical velocities directly measured by the vertical beam in DBS mode, by taking account of the tilt angle of the layers estimated by the SDI technique. On the other hand, from the analysis of horizontal wind profiles, we observed a quasi‐monochromatic gravity wave related to the tilted layers. It was found that the tilt angle of the phase of wave front is well associated with the inclination of the echo layers if the corrected vertical velocity is used. Finally, the Richardson number estimated at an altitude range where the aspect sensitive echo layers occur presents large positive peaks at the position of the aspect sensitive echo layers and then seems to be too high for turbulent instabilities. Consequently, the observed echo layers may be stable layers at the most stable phase of the gravity wave.
Analysis of Perseid meteor head echo data collected using the Advanced Research Projects Agency Long‐Range Tracking and Instrumentation Radar (ALTAIR)
Authors: Close S. , Hunt S. M. , Minardi M. J. , McKeen F. M.
Published: February 2004

The 1998 Perseid meteor shower was observed using the Advanced Research Projects Agency (ARPA) Long‐Range Tracking and Instrumentation Radar (ALTAIR) in order to study potential meteoroid impact on orbiting spacecraft. ALTAIR is a dual‐frequency radar that operates at VHF and UHF, and its high sensitivity and precise calibration make it uniquely suited for detecting meteor head echoes. ALTAIR is dually polarized and records left‐circularly and right‐circularly polarized signal returns, which allow the determination of polarization ratios. ALTAIR uses a multihorn feed and interferometry to measure target angle of arrival. This paper contains analysis on Perseid data that were collected at VHF (158 MHz). Meteor head echo statistics are presented, including mean altitude, radial velocity, radar cross section (RCS), and polarization ratio. ALTAIR's VHF detection rate was approximately 1 head echo per second. An in‐depth analysis on select head echoes to estimate meteor decelerations and densities has also been included.
A multiblock generalized forward‐backward method
Authors: Pino M. R. , Obelleiro F. , Rodríguez J. L. , Burkholder R. J.
Published: February 2004

In a previous work, the generalized forward‐backward (GFB) method has been proposed to obtain the scattering from targets on rough ocean‐like surfaces. In this paper a generalization of the GFB method is presented which allows larger targets and multiple targets to be handled efficiently. The solution is based on a multiblock resolution of each target which is divided into a set of small subregions (blocks). The solution is obtained via a standard method of moments matrix factorization for the small blocks, combined with a conventional forward‐backward iterative procedure to account both for the interactions between blocks and for the regions without obstacles. A relaxation parameter is also introduced to improve convergence. The proposed method provides a general formulation completely independent of the number, the size and the position of the obstacles over the sea surface and allows a significant reduction in the computational and storage costs.
A three‐dimensional characteristic mode solution of two perforated parallel planes separating different dielectric media
Authors: Kabalan Karim Y. , El‐Hajj Ali , Rayes Asaad
Published: February 2004

A three‐dimensional characteristic mode solution is developed for electromagnetic scattering by rectangular apertures in two parallel perfectly conducting planes separating three different media. The formulation is performed when the system is excited by an incident wave whose magnetic field is tangential to the conducting planes. The characteristic modes, obtained from the solution, are used to determine the fields and the radiated power with respect to the plane separations and aperture transverse location.
Errors in direction finding of VLF waves caused by a sloping ionosphere
Authors: Sao Kazuo
Published: February 2004

Lightning discharges emit very low frequency (VLF) radio waves commonly called “atmospherics.” In the past, lightning source locations were determined by the triangulation method from recordings at three widely separated stations in Japan. The three direction‐of‐arrival (DOA) lines, however, sometimes formed large triangles resulting in large uncertainties in source location. To approach this problem, the variational treatment is used here in which the sloping ionosphere plays an important role. By obtaining the time required to propagate from the source to the receiver via the Earth‐ionosphere waveguide, it is found that the direction of propagation of a VLF wave can be bent along a parabolic curve. Using this concept of parabolic propagation, the locations of lightning discharges are compared with those determined by the triangular method.
Worldwide monitoring of VLF‐LF propagation and atmospheric noise
Authors: Tomko A. A. , Hepner T.
Published: February 2004

A joint effort is underway between The Johns Hopkins University Applied Physics Laboratory and the Space and Naval Warfare (SPAWAR) Systems Center, San Diego, to deploy monitoring equipment capable of characterizing worldwide VLF‐LF radio wave propagation and atmospheric noise levels. The monitoring equipment consists of a PC‐based spectrum analyzer and orthogonal ferrite core magnetic loop antennas. The analyzer performs continuous measurements of the radio spectrum from 12 to 62 kHz and records time histories of VLF‐LF signals (equivalent vertical electric field strength), noise amplitude probability distribution, noise impulsiveness, and average noise field strength. Data are downloaded via the Internet to a central database server. The Internet connection also provides for system reconfiguration and clock synchronization. Data collected by the monitoring network will be used to improve communication coverage forecasts and to analyze transient and long‐term propagation effects. This paper provides an overview of the monitoring network and samples of data collected by it.
Generalization of ionospheric tomography on diverse data sources: Reconstruction of the three‐dimensional ionosphere from simultaneous vertical ionograms, backscatter ionograms, and total electron content data
Authors: Fridman S. V. , Nickisch L. J.
Published: February 2004

An ionospheric reconstruction method is developed that is capable of simultaneously utilizing information from all possible sources of ionospheric measurement. We describe the method and demonstrate its performance on ionospheric data collected by an over‐the‐horizon radar (vertical and backscatter ionograms) and total electron content data collected by receivers operated in the Caribbean by the Applied Research Laboratories of the University of Texas at Austin. For each data set the method produces a smooth three‐dimensional ionospheric model that is consistent with all the measurements. The method is based on the Newton‐Kontorovich method for nonlinear operator equations and Tikhonov's regularization technique for ill‐posed problems. We demonstrate here the ability of our technique to reconstruct ionospheric spatial variations from global‐scale to medium‐scale irregularities or traveling ionospheric disturbances. Overall results of this study demonstrate that the technique suggested makes it possible to unite data from various ionospheric instruments to provide monitoring of the ionosphere over a large geographical area in a wide range of spatial scales.
Architecture of transpressional thrust faulting in the San Bernardino Mountains, southern California, from deformation of a deeply weathered surface
Authors: Spotila James A. , Sieh Kerry
Published: February 2004

To investigate the architecture of transpressional deformation and its long‐term relationship to plate motion in southern California, we have studied the deformation pattern and structural geometry of orogeny within the San Andreas fault system. The San Bernardino Mountains have formed recently at the hub of several active structures that intersect the San Andreas fault east of Los Angeles. This mountain range consists of a group of crystalline blocks that have risen in association with transpressive plate motion along both high‐ and low‐angle faults of a complex structural array. We have used a deeply weathered erosion surface as a structural datum to constrain the pattern of vertical deformation across fault blocks in and adjacent to this mountain range. By subtracting the hanging wall and footwall positions of this preuplift horizon we have determined vertical displacement along two major thrust faults. We conclude that one fault, the North Frontal thrust, has played a more significant role in raising the large fault blocks and can explain the uplift of all but a few crustal slivers. On the basis of the pattern of displacement associated with this thrust fault we have also inferred fault zone geometry beneath the range. Rather than simply steepening into a high‐angle fault zone or flattening into a decollement, the thrust fault may have a complex, curviplanar geometry. The pattern of rock uplift also enables us to calculate the total motion accommodated by this orogeny. We estimate that >6 km of convergence (5% of the total plate motion in the last 2 Myr) has occurred. This horizontal shortening is associated spatially with the 15‐km‐wide restraining bend in the San Andreas fault zone near San Gorgonio Pass. The entire range may thus have risen because of a small geometric complexity in the San Andreas fault rather than the obliquity of far‐field plate motion.
Paleothermal structure of the Point San Luis slab of central California: Effects of Late Cretaceous underplating, out‐of‐sequence thrusting, and late Cenozoic dextral offset
Authors: Underwood Michael B. , Laughland Matthew M.
Published: February 2004

Late Cretaceous shale and sandstone turbidites of the Point San Luis “slab” are isoclinally folded, locally sheared, and faulted, but their severity of stratal disruption is relatively mild when compared to adjacent polymictic mélange of the Franciscan Complex. We tested the interpretation of a trench‐slope basin origin for these strata by documenting their paleothermal structure, including contacts between turbidites and mélange. Values of mean random vitrinite reflectance (Rm) from turbidites are 0.9–1.7% estimates of maximum paleotemperature are 135°–200°C. Mélange matrix samples yieldRmvalues of 1.1–2.5%, with an average of 1.5%, and peak temperatures between 160° and 240°C. The turbidite‐over‐mélange contact is locally “cooler over warmer” and was folded after peak heating. The relatively high paleotemperatures cast doubt on a shallow slope basin model (i.e., 1–2 km burial depth). We suggest, instead, that thermal maturation of the Point San Luis slab occurred much deeper in an accretionary prism (10–15 km), where offscraped trench wedge deposits were faulted against underplated mélange. The paleothermal structure was offset and tilted after peak heating by two out‐of‐sequence faults. Late Oligocene to Pliocene strata rest unconformably above the Franciscan, and there is a significant gap in thermal maturity across this unconformity, with no evidence to show that Franciscan rocks were reset thermally following the main episode of uplift and erosion. Three‐dimensional orientations of isoreflectance surfaces on opposite sides of the San Gregorio‐San Simeon‐Hosgri fault system also can be used to test interpretations of strike‐slip neotectonics. The failure to match these geometries among suspected piercing points at Point San Luis, Cambria, and Point Sur favors a suggestion that differential, post thermal peak, dextral offset of Franciscan basement has not exceeded 10–15 km.
Upper plate contraction north of the migrating Mendocino triple junction, northern California: Implications for partitioning of strain
Authors: McCrory Patricia A.
Published: February 2004

Geologic measurement of permanent contraction across the Cascadia subduction margin constrains one component of the tectonic deformation along the convergent plate boundary, the component critical for the seismic hazard assessment of crustal faults. A comprehensive survey of active faults in onshore subduction margin rocks at the southern end of the Cascadia subduction zone indicates that these thrust faults accommodate ∼10 mm/yr of convergence oriented 020°–045°. Seismotectonic models of subduction zones typically assign this upper plate strain to the estimate of aseismic slip on the megathrust. Geodetic models include this permanent crustal strain within estimates of elastic strain accumulation on the megathrust. Both types of models underestimate the seismic hazard associated with crustal faults. Subtracting the observed contraction from the plate convergence rate (40–50 mm/yr; directed 040°–055°) leaves 30–40 mm/yr of convergence to be partitioned between slip on the megathrust, contraction within the southern Juan de Fuca plate, and crustal contraction outside the subduction complex rocks. This simple estimate of slip partitioning neglects the discrepancy between the plate convergence and contraction directions in the vicinity of the Mendocino triple junction.

The San Andreas and Cascadia limbs of the Mendocino triple junction are not collinear. The eastern edge of the broad San Andreas boundary is ∼85 km east of the Cascadia subduction boundary, and across this zone the Pacific plate converges directly with the North America plate. The skewed orientation of crustal structures just north of the leading edge of the Pacific plate suggests that they are deforming in a hybrid stress field resulting from both Juan de Fuca‐North America motion and Pacific‐North America motion. The composite convergence direction (50 mm/yr; directed 023°) is consistent with the compressive stress axis (020°) inferred from focal mechanisms of crustal earthquakes in the Humboldt region. Deformation in such a hybrid stress field implies that the crustal faults are being loaded from two major tectonic sources. The slip on crustal faults north of the Mendocino triple junction may consume 4–5 mm/yr of Pacific‐Humboldt convergence. The remaining 17–18 mm/yr of convergence may be consumed as distributed shortening expressed in the high rates of uplift in the Cape Mendocino region or as northward translation of the continental margin, north of the triple junction.