bluuebunny/crossref_metadata_2025 huggingface dataset:
Late Pleistocene bryozoan reef mounds of the Great Australian Bight: Isotope stratigraphy and benthic foraminiferal record
Authors: Holbourn Ann , Kuhnt Wolfgang , James Noel
Published: October 2002

Cores from Sites 1129, 1131, and 1132 (Ocean Drilling Program (ODP) Leg 182) on the uppermost slope at the edge of the continental shelf in the Great Australian Bight reveal the existence of upper Pleistocene bryozoan reef mounds, previously only detected on seismic lines. Benthic foraminiferal oxygen isotope data for the last 450,000 years indicate that bryozoan reef mounds predominantly accumulated during periods of lower sea level and colder climate since stage 8 at Sites 1129 and 1132 and since stage 4 at the deeper Site 1131. During glacials and interstadials (stages 2–8) the combination of lowered sea level, increased upwelling, and absence of the Leeuwin Current probably led to an enhanced carbon flux at the seafloor that favored prolific bryozoan growth and mound formation at Site 1132. At Site 1129, higher temperatures and downwelling appear to have inhibited the full development of bryozoan mounds during stages 2–4. During that time, favorable hydrographic conditions for the growth of bryozoan mounds shifted downslope from Site 1129 to Site 1131. Superimposed on these glacial‐interglacial fluctuations is a distinct long‐term paleoceanographic change. Prior to stage 8, benthic foraminiferal assemblages indicate low carbon flux to the seafloor, and bryozoan mounds, although present closer inshore, did not accumulate significantly at Sites 1129 and 1132, even during glacials. Our results show that the interplay of sea level change (eustatic and local, linked to platform progradation), glacial‐interglacial carbon flux fluctuations (linked to local hydrographic variations), and possibly long‐term climatic change strongly influenced the evolution of the Great Australian Bight carbonate margin during the late Pleistocene.
Carbon cycling and burial in the glacially influenced polar North Atlantic
Authors: Taylor J. , Tranter M. , Munhoven G.
Published: October 2002

We have collated published records of carbon storage (wt % calcium carbonate and organic carbon) in polar North Atlantic sediments in order to assess the role that the glacial history of Greenland and Fennoscandia may have had on carbon cycling in this oceanographically important region. The proportion of carbonate in sediment varies between 0 and ∼50%, while that of organic carbon varies between 0 and ∼2.0%. The spatial variation of the concentration and accumulation of both constituents is markedly different. Bulk accumulation shows a strong relationship with depth, distance offshore, and the location of major glacial outlets on neighboring landmasses. Therefore, ice sheet dynamics and erosion influence carbon (especially organic carbon) storage strongly during the late Weichselian (27‐12 14C ka) via their impact on sedimentation rates and constituents. In contrast, water mass characteristics are important in determining the pattern of carbon storage during the Holocene. Carbonate fluxes to the polar North Atlantic sediment column fall by ∼50% during glacials to ∼1.1 × 1013 kg kyr−1, but organic carbon storage is maintained at or greater than interglacial levels (∼4.6 × 1011 kg kyr−1). This represents a 100% change in the ratio of preserved inorganic to organic carbon. When combined with reduced deep water ventilation, respiration of this relatively greater organic carbon flux in both the water and sediment columns provides a good explanation for the observed periodic enhanced dissolution of carbonate in polar North Atlantic late Weichselian sections, perhaps enhancing CO2 storage in deep waters.
Modeling the effect of freshwater pulses on the early Holocene climate: The influence of high‐frequency climate variability
Authors: Renssen H. , Goosse H. , Fichefet T.
Published: October 2002

The effect of freshwater pulses on the early Holocene climate is investigated with a global coupled atmosphere‐sea ice‐ocean model. In the model an early Holocene equilibrium climate state is perturbed by releasing a fixed amount of freshwater (4.67 × 1014 m3) into the Labrador Sea at three different constant rates: 1.5 Sv (1 Sv = 1 × 106 m3 s−1) in 10 years, 0.75 Sv in 20 years, and 0.3 Sv in 50 years. For each rate, five ensemble experiments have been performed, varying in initial conditions. The freshwater pulses produce a weakening of the thermohaline circulation. The perturbed state is in agreement with proxy evidence for the 8.2 ka event. Two types of recovery of the thermohaline circulation occurred, differing in time‐scale: (1) ≤200 years and (2) >200 years. In the experiments with 10 year and 20 year pulses, both types of recovery were observed. This suggests that the model response is unpredictable in the range of parameters studied here. It is hypothesized that the unpredictability is associated with annual‐to‐decadal climate variability. Our results demonstrate that several types of recovery may exist with the same kind of perturbation. The interpretation of events observed in proxy data may be thus more complex than realized until now since the magnitude and duration of climatic events caused by freshwater pulses is likely to depend strongly on nonlinear dynamics inside the coupled atmosphere–sea ice–ocean system.
Evidence of surface water oligotrophy during the Paleocene‐Eocene thermal maximum: Nannofossil assemblage data from Ocean Drilling Program Site 690, Maud Rise, Weddell Sea
Authors: Bralower Timothy J.
Published: October 2002

Nannoplankton assemblages at Ocean Drilling Program Site 690 (Maud Rise, Weddell Sea) experienced an abrupt and dramatic transformation at the onset of the Paleocene‐Eocene Thermal Maximum (PETM) at ∼55 m.y. The major assemblage shift suggests a change from colder, more productive surface waters to warmer, more oligotrophic conditions. Significant restructuring of assemblages during the later part of the PETM indicates that nannoplankton communities were not stable and that surface water conditions changed, although they remained warm and oligotrophic. Combined with benthic foraminiferal assemblage data, nannoplankton assemblage results suggest increased sequestration of nutrients in shelf environments and starvation of the open ocean. Although the PETM was a short‐lived event, it appears to have had long‐term effects on nannoplankton, leading to the extinction of Fasciculithus, a dominant Paleocene genus.
El Niño's tropical climate and teleconnections as a blueprint for pre‐Ice Age climates
Authors: Molnar Peter , Cane Mark A.
Published: October 2002

At ∼2.7 million years ago the warm equable climates of early and “middle” Pliocene time (used here to mean from ∼5 to ∼2.7 Ma) were replaced by recurring ice ages. Most attempts to explain the change appeal either to changes in CO2 in the atmosphere or reduced heat transport by the Atlantic Ocean. The sources of the strongest teleconnections in the current climate, however, lie in the tropics, and such connections occur by transport of heat and moisture by the atmosphere. The most prominent of these teleconnections link aberrations in sea surface temperatures in the equatorial Pacific, El Niño–Southern Oscillation (ENSO) variations, with warm and dry or cool and wet anomalies in extratropical climates. We show that in most cases early and middle Pliocene climate both in equatorial regions and in the extratropics differ from present‐day climates with the same spatial pattern as that associated with ENSO. For instance, not only was Canada warmer during early Pliocene time than at present, as it is during El Niño, but the region surrounding the Gulf of Mexico appears to have been cooler and a bit wetter, as it commonly is during El Niño. A virtually permanent El Niño‐like state appears to have characterized pre‐Ice Age climates, suggesting that transport of heat by the atmosphere was the principal mechanism that maintained extratropical warmth. Accordingly, cooling and the growth of recurring ice sheets in the Northern Hemisphere resulted from the development of a strong Walker circulation and a weakening of the Hadley circulation.
Intrusion of a branch of the Oyashio Current into the Japan Sea during the Holocene
Authors: Takei Tomohiro , Minoura Koji , Tsukawaki Shinji , Nakamura Toshio
Published: October 2002

The Holocene history of the Japan Sea was reconstructed using foraminiferal assemblages and stable isotope records from two piston cores recovered from west of the Tsugaru Strait, with the overall goal to resolve a controversy regarding flow in and out of the Japan Sea during the Holocene. In the northern core the planktonic foraminifer N. pachyderma changed from left‐coiling to right‐coiling at 8.3 ka. However, in the southern core the left‐coiling form remained dominant until 4.8 ka. Some indices of the stable isotopic records also changed at 8.3 and 4.8 ka. These results suggest that the Tsushima Current started to inflow into the Japan Sea at 8.3 ka, but a branch of the Oyashio Current continued to inflow until 4.8 ka. We attribute the coexistence of the two currents between 8.3 and 4.8 ka to the opposite direction flows between the shallow and the deep portions of the Tsugaru Strait driven by baroclinic components.
Cenozoic evolution of Asian climate and sources of Pacific seawater Pb and Nd derived from eolian dust of sediment core LL44‐GPC3
Authors: Pettke Thomas , Halliday Alex N. , Rea David K.
Published: October 2002

The large‐diameter piston core LL44‐GPC3 from the central North Pacific Ocean records continuous sedimentation of eolian dust since the Late Cretaceous. Two intervals resolved by Nd and Pb isotopic data relate to dust coming from America (prior to ∼40 Ma) and dust coming from Asia (since ∼40 Ma). The Intertropical Convergence Zone (ITCZ) separates these depositional regimes today and may have been at a paleolatitude of ∼23°N prior to 40 Ma. Such a northerly location of the ITCZ is consistent with sluggish atmospheric circulation and warm climate for the Northern Hemisphere of the early to middle Eocene. Since ∼40 Ma, correlations between Nd (−7.55 > εNd(t) > −10.81) and Pb (18.625 < 206/4Pb < 18.879; 15.624 < 207/4Pb < 15.666; 38.611 < 208/4Pb < 38.960; 0.8294 < 207/6Pb < 0.8389; 2.0539 < 208/6Pb < 2.0743) isotopes reflect the progressive drying of central Asia triggered by the westward retreat of the paleo‐Tethys. Comparisons between the changes with time in the isotopically well‐defined dust flux and Nd and Pb isotopic compositions of Pacific deep water allow one to draw two major conclusions: (1) dust‐bound Nd became a resolvable contribution to Pacific seawater only after the one order of magnitude increase in dust flux starting at ∼3.5 Ma. Therefore eolian Nd was unimportant for Pacific seawater Nd prior to 3.5 Ma. (2) The lack of a response of Pacific deep water Pb to this huge flux increase suggests that dust‐bound Pb has never been important. Instead, mobile Pb associated with island arc volcanic exhalatives probably consists of a significant contribution to Pacific deep water Pb and possibly to seawater elsewhere far away from landmasses.
Could changing ocean circulation have destabilized methane hydrate at the Paleocene/Eocene boundary?
Authors: Bice Karen L. , Marotzke Jochem
Published: October 2002

During the Paleocene‐Eocene Thermal Maximum (PETM, ∼55 Ma), marine and terrestrial carbon isotope values exhibit a negative shift of at least 2.5‰, indicative of massive destabilization of marine methane hydrates, releasing ∼2000 gigatons of methane carbon. The cause of the hydrate destabilization is unknown but has been speculated to be warming due to a change from high‐latitude to low‐latitude deepwater formation. Here we present results from a numerical ocean model indicating that a sudden switch of deepwater formation from southern to northern high latitudes caused middepth and deep‐ocean warming of 3°–5°C. The switch is caused by a slow increase in the intensity of the atmospheric hydrologic cycle, as expected under increasing temperatures and consistent with PETM sedimentary evidence. Deepened subtropical subduction prior to the thermohaline circulation switch causes warming of 1°–4°C in limited areas at thermocline through upper intermediate depths, which could destabilize methane hydrates gradually and at progressively greater depths. Warming accompanying a south‐to‐north switch in deepwater formation would produce sufficient warming to destabilize seafloor gas hydrates over most of the world ocean to a water depth of at least 1900 m.
High‐ and low‐latitude climate control on the position of the southern Peru‐Chile Current during the Holocene
Authors: Lamy Frank , Rühlemann Carsten , Hebbeln Dierk , Wefer Gerold
Published: October 2002

We reconstructed changes of temperature, salinity, and productivity within the southern Peru‐Chile Current during the last 8000 years from a high‐resolution sediment core recovered at 41°S using alkenones, isotope ratios of planktic foraminifera, biogenic opal, and organic carbon. Paleotemperatures and paleosalinities reached maximum values at ∼5500 years ago and thereafter declined to modern values, whereas paleoproductivity continuously increased throughout the last 8000 years. We ascribe these long‐term Holocene trends primarily to latitudinal shifts of the Antarctic Circumpolar Current (ACC). The concurrence with shifts in the position of the Southern Westerlies points to a common response of atmospheric and oceanographic circulation patterns off southern Chile. Millennial‐ to centennial‐scale fluctuations of paleotemperatures and paleosalinities, on the other hand, lag displacements in the position of the Southern Westerlies but reveal a significant correlation to short‐term temperature changes in Antarctica, indicating a high‐latitude control of the ACC at these timescales.
Lightness timescale for terrestrial sediments in the past 500,000 years
Authors: Han Zhiyong , Liu Dunyi , Hovan Steven A.
Published: October 2002

The Nihewan Beds at Yangyuan reflect apparently continuous, high‐resolution deposition of fluvial‐lacustrine sediment in the climatically important loess deposit regions of north China. The Hutouliang section forms the upper portion of Nihewan Beds deposited during the Brunhes magnetic polarity chron. Lightness values measured on successive samples display similar variability to the aeolian flux record of oceanic sediment core V21‐146 located downwind from this area in the North Pacific. The oxygen isotope timescale of core V21‐146 was transferred to the lightness curve to provide a preliminary timescale allowing more detailed age control in the Hutouliang section. This timescale will provide useful assistance in dating boundary ages of the loess‐soil sequence in China and interpreting regional climatic data.
Latest Eocene‐earliest Miocene Sr isotopic reference section, Site 522, eastern South Atlantic
Authors: Reilly Timothy J. , Miller Kenneth G. , Feigenson Mark D.
Published: October 2002

We present a revised calibration of Sr isotopes to the geomagnetic polarity timescale (GPTS) using closely spaced (∼0.15 m.y. resolution) samples from the classic uppermost Eocene through lowermost Miocene section at Site 522, eastern South Atlantic. The Sr isotopic data are fit with two linear segments with a sharp change in slope at circa 27.5 Ma from 0.000038/m.y. (27.5 to 34.4 Ma) to 0.000051/m.y. (23.8 to 27.5 Ma). Regression analysis indicates that stratigraphic resolution ranges from ±1 m.y. (for one analysis) to ±0.6 m.y. (for three analyses) for the younger interval and ±1.2 m.y. (for one analysis) to ±0.7 m.y. (for three analyses) for the older interval, representing an increase in resolution from previous studies of ±1–2 m.y. The paleoceanographic significance of this change in slope is unclear. It occurs during an interval of intermittent Antarctic glaciation, between the Oi2a and Oi2b glaciations. The subsequent interval from circa 27 to 24 Ma appears to be an interval of minimal glaciation. Thus this observation does not support previous suggestions that increases in rates of Sr isotopic change are directly associated with the frequency of Antarctic glaciations. Rather, the increase in slope may be related to increased weathering associated with the “mid‐Oligocene” glaciation.
Accurate estimation of sea surface temperatures using dissolution‐corrected calibrations for Mg/Ca paleothermometry
Authors: Rosenthal Yair , Lohmann George P.
Published: October 2002

Paired δ18O and Mg/Ca measurements on the same foraminiferal shells offer the ability to independently estimate sea surface temperature (SST) changes and assess their temporal relationship to the growth and decay of continental ice sheets. The accuracy of this method is confounded, however, by the absence of a quantitative method to correct Mg/Ca records for alteration by dissolution. Here we describe dissolution‐corrected calibrations for Mg/Ca‐paleothermometry in which the preexponent constant is a function of size‐normalized shell weight: (1) for G. ruber (212–300 μm) (Mg/Ca)ruber = (0.025 wt + 0.11) e0.095T and (b) for G. sacculifer (355–425 μm) (Mg/Ca)sacc = (0.0032 wt + 0.181) e0.095T. The new calibrations improve the accuracy of SST estimates and are globally applicable. With this correction, eastern equatorial Atlantic SST during the Last Glacial Maximum is estimated to be 2.9° ± 0.4°C colder than today.
Application of polar cap absorption events to the calibration of riometer systems
Authors: Hargreaves J. K. , Detrick D. L.
Published: October 2002

The spatial uniformity of polar cap absorption events has been exploited to test the performance of riometer systems. In an imaging riometer the use of a fixed conversion factor for obtaining the equivalent zenithal absorption from that measured with an oblique beam is verified for most of the beams, and revised factors are suggested. The readings from the corner beams are found to be unreliable, however. Factors for converting the readings from a wide‐beam antenna to true zenithal values are derived. The significance of antenna sidelobes is pointed out, and the upper limits to the absorption measurement due to the temperature of the mesosphere and to uncertainty of riometer calibration at the lowest signal levels are demonstrated.
Latitudinal variations of scintillation activity and zonal plasma drifts in South America
Authors: Kil Hyosub , Kintner Paul M. , de Paula Eurico R. , Kantor I. J.
Published: October 2002

Latitudinal variations of scintillation activity and zonal plasma drifts were investigated simultaneously at three locations in Brazil during 23 November to 26 December 1999 using Global Positioning System measurements. The scintillation morphology at 1.575 GHz showed large latitudinal differences in scintillation activity. At the magnetic equator the occurrence probability was very low without showing strong scintillation (S4 > 0.5) during the solar maximum period, whereas strong scintillation was observed during most of the days at the equatorial ionization anomaly (EIA). The scintillation activity was mostly limited to 1900–2400 LT at the magnetic equator and to 2000–0200 LT at the EIA. The scintillation onset time delay of about an hour at the EIA compared to that at the magnetic equator illustrates the development of ionospheric irregularities at the magnetic equator and then their expansion to higher latitude by drifting upward. The zonal velocities of the ionospheric irregularities were inferred using the cross‐correlation technique, and the eastward velocity of all observations decreased with local time. However, the zonal velocity magnitude also decreased in proceeding from the magnetic equator to the EIA, which indicates negative vertical shear of the eastward plasma drift velocity at nighttime in the equatorial ionosphere.
Spacecraft radio occultations using multiple Doppler readouts
Authors: Tinto Massimo
Published: October 2002

We present an innovative technique that eliminates the phase noise of the onboard frequency reference as the main limiting factor on uplink radio occultations, while allowing full utilization of the expected improvement in SNR over downlink experiments. This technique relies on conducting simultaneous uplink and downlink occultation measurements to synthesize a phase reference of quality determined by the stability of the atomic clock at the ground station, eliminating the frequency fluctuations of the onboard clock as the limiting factor. Atomic frequency standards typically have frequency stability that is about 2–3 orders of magnitude better than space‐qualified state‐of‐the‐art clocks. The quantum leap improvement in reference phase stability and, simultaneously, in measurement SNR opens the door to fundamentally new science capabilities for radio occultation observations of planetary rings and atmospheres. In particular, the improvement is expected to enhance sensitivity to tenuous target regions, to increase the dynamic range within opaque regions, and to improve the spatial resolution of mapping large‐ and small‐scale target structures.
A generalized integral equation formulation for mixed dielectric‐PEC scatterers
Authors: Herschlein Alexander , von Hagen Jürgen , Wiesbeck Werner
Published: October 2002

The known integral representation of electromagnetic fields is reviewed. Based on the review, a new formulation allows us to model any general case. The formulation of PEC, dielectric, dielectric with metallic surfaces, and dielectric scatterers with dielectric or PEC inclusions is derived. Finally, a new operator notation allows a concise and accurate representation of the integral equation formulations. The electric field integral equation and the magnetic field integral equation are contained in this formulation.
Validation of single‐station lightning location technique
Authors: Brundell James B. , Rodger Craig J. , Dowden Richard L.
Published: October 2002

Lightning discharges are powerful impulsive sources of electromagnetic energy over a wide bandwidth allowing passive methods to determine lightning location through the energy released by the lightning discharge. While multistation lightning location techniques provide high location accuracy, in some instances it is impossible to deploy a multistation network, and for this reason, techniques have been developed to allow single‐station lightning location. We consider the validation of the “Kharkov” single‐station lightning location method proposed by Rafalsky et al. [1995a], through comparing the locations of ELF/VLF observed sferics determined by the Kharkov technique with the positions of coincident lightning flashes recorded by a commercial lightning detection network. Making use of 85 sferics observed at Robertson Army Barracks, near Darwin, Australia, in the period from 20 November 1997 until 7 January 1998, we find that that ∼68% of Kharkov method bearing estimates are accurate to within 4.1° and that ∼68% of the Kharkov method range estimates are accurate to within 73.2 km.
Noise in wireless systems produced by solar radio bursts
Authors: Bala B. , Lanzerotti L. J. , Gary D. E. , Thomson D. J.
Published: October 2002

We have carried out an investigation of 40 years of solar radio burst data in a wide range of frequencies that have been reported by observing stations around the world during 1960–1999. The data were compiled by the National Geophysical Data Center of the National Oceanic and Atmospheric Administration. This period covers three full and two partial solar cycles. We find that the number of bursts per day with amplitudes >103 solar flux units (SFU) falls as an approximate power law with increasing flux level for the frequency bands investigated(1–10 and 10–20 GHz). Also, the number of events with peak flux density >103 SFU varies, as expected, with the solar cycle in the bands investigated (1–2, 2–4, and 4–10 GHz). We discuss the rate of occurrence of events (>103 SFU) in the context of the noise levels in typical wireless communications systems. We find that statistically, depending upon wireless system parameters, several solar events per year are likely to occur that could cause severe interference in a given cell site during solar maximum periods.
Systematic behavior of semiempirical global ionospheric models in quiet geomagnetic conditions
Authors: Meza A. M. , Díaz A. R. , Brunini C. A. , Van Zele M. A.
Published: October 2002

Semiempirical ionospheric models are widely used in many applications. In this work we analyze the global representation of the vertical total electron content (VTEC) derived from the La Plata single‐layer GPS ionospheric model (LPIM), the international reference ionosphere (IRI) model, and the Bent model. To perform this analysis, we compare the output from these models with VTEC determinations from the TOPEX/Poseidon (T/P) satellite. The analysis of the differences is performed over a large number of selected quiet geomagnetic days during the year 1997. From these comparisons we can conclude that LPIM gives the best global VTEC representation, even without modeling the ionospheric equatorial anomaly. The mean global difference between the VTEC determined by T/P and computed by LPIM is in the range of 2 and 3 TECU (1 TECU = 1016el/m2). IRI and Bent models tend to show larger discrepancies with T/P during the summer solstices in both Northern and Southern Hemispheres.
Nested multigrid vector and scalar potential finite element method for three‐dimensional time‐harmonic electromagnetic analysis
Authors: Zhu Yu , Cangellaris Andreas C.
Published: October 2002

A new finite element methodology is presented for fast and robust numerical simulation of three‐dimensional electromagnetic wave phenomena. The new methodology combines nested multigrid techniques with the ungauged vector and scalar potential formulation of the finite element method. The finite element modeling is performed on nested meshes over the computational domain of interest. The iterative solution of the finite element matrix on the finest mesh is performed using the conjugate gradient method, while the nested multigrid vector and scalar potential algorithm acts as the preconditioner for the iterative solver. Numerical experiments from the application of the new methodology to three‐dimensional electromagnetic scattering are used to demonstrate its superior numerical convergence and efficient memory usage.
Calibration error of L‐band sky‐looking ground‐based radiometers
Authors: Delahaye Jean‐Yves , Golé Peter , Waldteufel Philippe
Published: October 2002

A method is presented for estimating the calibration error affecting L‐band ground‐based radiometers, using the sky as a cold source. In a first step, the optimum conditions to perform this calibration are limited by removing sky areas where large radiation contributors (Sun and Moon) are present. In the region thus selected, an accurate computation of the brightness temperature measured by the antenna is performed using available sky background temperature survey charts (radio continuum and hydrogen HI line) and integrating the sky temperature over the directional gain pattern of a representative radiometer L‐band antenna. Contributions from rear lobe are not considered. After adding the atmospheric contribution, maps of the noise temperature and its error are produced for the region of interest. The best calibration orientation for a sky‐looking radiometer at medium northern latitude would be 0° in azimuth (northward) and an elevation equal to the radiometer's latitude. It is found that the computed total sky noise contribution is 6.6 K, with 24 hour variations of ±0.2 K and a maximum bias of ±0.6 K. The results are valid for the whole year, assuming low to moderate solar activity and no rain.
On the solution of Markov's parabolic equation for the second‐order spaced frequency and position coherence function
Authors: Bitjukov A. A. , Gherm V. E. , Zernov N. N.
Published: October 2002

An analytic technique has been developed to construct the asymptotic representation of the spaced position and frequency coherence function in Markov's diffusive approximation. The technique employs the formalism of the quasi‐classic complex paths in an extended complex‐valued coordinate space. It allows the construction of the coherency for arbitrary realistic models of the structure function of the fluctuations of the refractive index of the medium of propagation. The technique has been employed to obtain explicit analytic asymptotic solutions for some realistic models of the structure function. For the quadratic structure function the method produces the known rigorous solution in an automatic fashion.
Near‐horizontal line‐of‐sight millimeter‐wave propagation measurements for the determination of outer length scales and anisotropy of turbulent refractive index fluctuations in the lower troposphere
Authors: Lüdi Andreas , Magun Andreas
Published: October 2002

Variances of the normalized power, the angle of arrival, and temporal spectra were obtained from millimeter‐wave propagation experiments. For their interpretation the existing theory for monochromatic spherical wave propagation through an anisotropic weakly turbulent medium had been expanded such that anisotropy of the refractive index fluctuations transverse to the propagation path is taken into account. The comparison of experimental and theoretical results provides path‐averaged estimates for the outer scale L0 in vertical and horizontal directions within the first few hundred meters above ground. Thus a determination of the atmospheric anisotropy is feasible, and it is found to be related to the atmospheric stability. The values obtained for L0 in vertical and horizontal directions are compatible with those presented in the literature. We found that the size and the anisotropy of L0 depend significantly on atmospheric conditions and vary accordingly widely in time.
Transmitter‐receiver‐transmitter configurations of ground‐penetrating radar
Authors: Gürel Levent , Oğuz Uğur
Published: October 2002

Three‐dimensional ground‐penetrating radar (GPR) geometries are simulated using the finite difference time domain (FDTD) method. The GPR is modeled with a receiver and two transmitters with arbitrary polarizations in order to cancel the direct signals emitted by the two transmitters at the receiver. This GPR configuration is used to simulate scenarios involving single or multiple targets with arbitrary sizes. The buried objects are modeled as cylindrical disks. Perfectly matched layer absorbing boundary conditions are used to terminate the layered FDTD computational domain.
Transionospheric pulse pairs originating in maritime, continental, and coastal thunderstorms: Pulse energy ratios
Authors: Tierney Heidi E. , Jacobson Abram R. , Roussel‐Dupré Robert , Beasley William H.
Published: October 2002

We examine the ratio of energy in the second pulse to energy in the first pulse for 2467 transionospheric pulse pairs (TIPPs). The TIPPs examined here have been attributed to thunderstorms occurring over maritime, continental, and coastal regions near the continental United States. The mean values of the energy ratios are found to be 0.39 ± 0.27 for continental TIPPs and 0.94 ± 0.62 for maritime TIPPs. The energy ratio values for the coastal TIPPs exhibit a bimodal distribution representative of both the continental and the maritime populations. Previous observational evidence has shown that the second pulse of a TIPP is the surface‐reflected signal from the same source as the first pulse. We compare the observed pulse energy ratios to the reflection coefficients of soil and seawater given by the Fresnel equations. The average values of the reflection coefficients for sources with vertical plane and horizontal plane polarization are consistent with the data.
On‐orbit direction finding of lightning radio frequency emissions recorded by the FORTE satellite
Authors: Jacobson Abram R. , Shao Xuan‐Min
Published: October 2002

The recording of radio frequency signals from space potentially provides a means for global, near‐real‐time remote sensing of vigorous convective storms and a possible early warning system for convection‐associated severe weather. In general, radio frequency signals arriving at a satellite with modest antenna gain do not directly reveal the ground location of those signals' source. We develop here a means of inferring the source location using repeated signal recordings from the same source storm, with the successive recordings taken along a significant segment of the satellite pass in view of the storm. The method is based on the ratio of received power on each of a pair of crossed dipole antennas. This method has a positional accuracy of 100–500 km. Moreover, the method has an intrinsic right‐left (with respect to the subsatellite track) location ambiguity. A promising use of this technique in future applications will be as an aid in assigning lightning RF emission sources to meteorological features from other global remote‐sensing products, for example satellite infrared imagery of clouds.
Two‐scale treatment of low‐grazing‐angle scattering from spilling breaker water waves
Authors: West James C. , Ja Shiou‐Jyh
Published: October 2002

A two‐scale scattering analysis is applied to two series of profiles that represent the measured time evolution of the crests of spilling breaker waves that were mechanically generated and measured in a wave tank. The backscattering at microwave frequencies and low‐grazing‐angle (LGA) illumination were considered. A deterministic implementation of the perturbation theory that is the foundation of any two‐scale scattering model (TSM) is used that allows an instantaneous comparison of TSM cross sections with reference “exact” cross sections found using a numerical moment‐method‐based electromagnetic approach. TSM proved unable to model the scattering from steep features that appear on the crest both before and after the onset of breaking, even in cases where the horizontal‐to‐vertical polarization ratios are as low as −10 dB. This shows that the “fast” scattering (with Doppler speeds greater than or equal to the velocity of the breaker) associated with these features is not due to Bragg scattering in the traditional sense in these cases despite the low polarization ratios. TSM is more accurate after the steep features subside and the turbulent “scar” roughness is primarily on the front face of the breaker. However, accuracy is lost as the wave propagates and the scar roughness moves to the crest top and back face of the wave. It appears that a scattering coefficient based on the spectral content of the roughness at the Bragg‐resonant condition can provide only a rough estimate of the actual LGA scattering coefficient even after the major steep features have disappeared.
A new incoherent scatter technique in the EISCAT Svalbard Radar
Authors: Lehtinen Markku , Markkanen Jussi , Väänänen Antero , Huuskonen Asko , Damtie Baylie , Nygrén Tuomo , Rahkola Jyrki
Published: October 2002

A special incoherent scatter technique in the European Incoherent Scatter (EISCAT) Svalbard Radar (ESR) was tested in November 1999. After the 70‐MHz IF stage the standard ESR receiver was replaced by a spectrum analyzer acting as a combination of a down‐converter, an AD converter, and a quadrature detector and giving complex digital samples at a rate of 4 MHz. The samples were fed into a PCI‐bus‐based programmable digital I/O card, which performed a four‐sample summing operation to give an effective sampling rate of 1 MHz, large enough to span all the frequency channels used in the experiment. Finally, the resulting samples were stored on hard disk. Hence the total multichannel signal was stored instead of separate lagged products for each frequency channel, which is the procedure in the standard hardware. This solution has some benefits; for example, the ground clutter can be eliminated with only a small loss in statistical accuracy, and the true phase of the transmitter waveform in phase‐coded experiments can be measured better than in the present ESR system. In this paper the technical arrangements of this setup are presented. A new incoherent scatter experiment is also described, and the first results obtained using the hardware are shown.
Coupling effects of two electric dipoles on an interface
Authors: Slob Evert , Fokkema Jacob
Published: October 2002

Numerical modeling of thin‐wire antennas is interesting to people working on antenna design and to people working on the processing and interpretation of radar data. They have different demands regarding the output of the model code. Our primary interest lies with improving image resolution through processing of ground‐penetrating radar data. For this goal the effects of two antennas placed close together on the Earth's surface are studied. To study the antenna to antenna coupling and antenna to ground coupling, a model for linear dipoles is used. The model uses a thin‐wire approximation for the transmit and receive antennas. The antennas are placed just above the air‐Earth interface. The Earth is modeled as a horizontal layer stack. Using this model, we study the coupling effects between two parallel antennas, either in each other's E‐plane or H‐plane. For ordinary antenna distances, the coupling between two parallel antennas when they are resistively loaded is on the order of −70 dB, which is not negligible for GPR applications. The influence of the lower half‐space on antenna behavior is strong but limited to a fraction of the wavelength in depth.
Enhancement or suppression of irregularities in the high‐latitude trough ionosphere using high‐power radio waves
Authors: Keskinen M. J.
Published: October 2002

The parametric coupling processes of a large‐amplitude electromagnetic pump wave with the temperature gradient drift instability in the high‐latitude trough ionosphere have been studied. The modified growth rate of the temperature gradient drift instability in the presence of a high‐frequency pump wave is computed for a range of pump amplitudes. We find that ground‐launched radio waves with O‐mode polarization can enhance or suppress temperature gradient drift instability‐driven irregularities in the high‐latitude trough ionosphere.
Asymmetry of 118 GHz oxygen line in the Earth atmosphere: Pressure shift as a new element in atmospheric lines
Authors: Krupnov A. F. , Shvetsov A. A.
Published: October 2002

Asymmetry of the atmospheric oxygen 118 GHz line due to the air pressure shift of the center frequency of this line is predicted, and estimations are given for some observation modes. The observability of this atmospheric line asymmetry by the appropriate instruments is shown. The necessity of considering the pressure shift in atmosphere propagation models such as the well‐known millimeter wave propagation model (MPM) is demonstrated. Line shift as a new source of information is discussed. At the moment, 183 GHz water and 118 GHz oxygen lines are the only known microwave lines with appreciable air pressure frequency shift that are used for atmosphere sounding. The possible directions of further studies are outlined.
Analysis of coaxially fed monopole antennas using an auxiliary sources technique
Authors: Papakanellos Panagiotis J. , Capsalis Christos N.
Published: October 2002

In this paper, the radiation from a circular cylindrical monopole fed through a perfectly conducting ground plane by a coaxial transmission line is analyzed using the method of auxiliary sources. The proposed method is applied by introducing a set of fictitious sources aligned with the monopole's axis for the direct description of the radiated electromagnetic field above the ground plane, under the assumption of a single transverse electromagnetic mode existing in the annular aperture of the line. The currents of the fictitious sources are derived by imposing the continuity condition of the tangential electric field on the physical surface of the dipole. For the occasion, the fictitious sources are chosen to be spatially overlapped sinusoidal dipoles. It is substantiated that this choice results in a noteworthy acceleration of the convergence rate of the solution in comparison with the one associated with infinitesimal dipoles, which are usually used in current‐model‐based techniques. Since the currents of the fictitious sources are determined, pertinent quantities of great interest, including the current distribution along the monopole, the input impedance/admittance, and the field intensities in the near and the far region, can be determined in a straightforward manner. Several examples concerning various geometries are established and compared with previously published data.
Calculation methods of the electric and magnetic fields at the Earth's surface produced by a line current
Authors: Pirjola Risto , Boteler David
Published: October 2002

Space weather storms involve intense and rapidly varying electric currents in the ionosphere, which create electric and magnetic fields at the Earth's surface. The electric fields drive geomagnetically induced currents (GIC) in technological networks and may have serious impacts. For assessing the hazards it is necessary to estimate GIC magnitudes, and this requires calculations of the electric and magnetic fields produced at the Earth's surface by the ionospheric currents. The surface fields are also affected by currents induced within the ground and influenced by the conductivity of the Earth. This also has to be taken into account. The calculation methods should be fast enough that they can be applied to forecasting the fields and GIC, for example, by using satellite observations of the solar wind. In this paper, we consider an infinitely long horizontal line current, which is the basic model of an auroral electrojet and simple enough to give insight into the physics and calculation techniques. The Earth is assumed to be composed of horizontal layers. We consider the exact integral expressions of the fields at the Earth's surface. The applicability of a series expansion technique (SER) and the complex image method (CIM), both of which were originally developed for other disciplines, are reviewed and summarized by giving the expressions of the electric and magnetic fields at the Earth's surface and by considering the mathematical assumptions required. Numerical calculations and comparisons with exact solutions show that SER and CIM are very accurate.
Modeling and design of T‐probe‐fed patch antennas using the finite difference time domain method
Authors: Guo Yong‐Xin , Luk Kwai‐Man , Lee Kai‐Fong
Published: October 2002

In this paper, the finite difference time domain method is employed to analyze the T‐probe‐proximity‐fed rectangular patch antennas. With a supporting foam substrate of thickness around 0.1 λ0, the patch antenna has shown a bandwidth (SWR ≤ 2) of 40% and an average gain of 7 dBi. Numerical results for the input impedance are presented and compared with the measurements. Good agreement between the computed and measured results is obtained. The effects of geometric parameters on the characteristics of the T‐probe patch antenna are extensively studied. For design purposes, the variations of the input impedance at resonance with different geometric parameters are plotted on Smith charts, and the peak values for the resonant resistance are summarized in tables.
Thermomechanical evolution of the crust during convergence and deep crustal pluton emplacement in the Western Province of Fiordland, New Zealand
Authors: Daczko Nathan R. , Klepeis Keith A. , Clarke Geoffrey L.
Published: October 2002

Fiordland, New Zealand, contains exposures of high‐pressure (P = 12–14 kbar) granulite facies rocks that form one of Earth's largest exposed lower crustal roots of an Early Cretaceous magmatic arc. These exposures allowed us to examine the mechanisms and processes that controlled crustal thickening and large vertical displacements at the deepest levels of a deforming arc system. We present structural and metamorphic data that show how the root of this arc was tectonically thickened by imbricate, granulite facies thrust zones during and after the emplacement of sheeted plutons into the middle and lower crust. The imbricate thrust zones form part of a well‐exposed, 12 km wide, two‐sided fold‐thrust belt that preferentially developed in crust that was thermally softened by magmatism. Changes in metamorphic mineral assemblages and microstructural data from the contact aureole of a composite arc batholith called the Western Fiordland Orthogneiss record thrust and pluton emplacement conditions of P ≈ 7–9 kbar (paleodepths of 25–30 km). These data show that thrust imbrication and tectonic loading at 30 km depth is a viable mechanism of large vertical displacements and crustal thickening at the deepest levels of magmatic arcs. This mechanism produced a characteristic up‐pressure metamorphic history that is similar to that observed in many other large magmatic belts worldwide.
Orthogonal folding of extensional detachments: Structure and origin of the Sierra Nevada elongated dome (Betics, SE Spain)
Authors: Martínez‐Martínez J. M. , Soto J. I. , Balanyá J. C.
Published: October 2002

A close relationship of the kinematics and timing between low‐angle extensional faulting and upright folding is established for the Miocene detachment systems in the hinterland of the Betics in southeastern Spain. Folding accompanied tectonic denudation developing elongated domes with fold axes both parallel and perpendicular to the direction of extension. The geometry, kinematics, and tectonic evolution of two major sequentially developed extensional fault systems have been characterized in several E‐W elongated mountain ranges of the central Betics on the basis of new cartographic and structural data and a comprehensive revision of other available geological and geophysical observations. The extensional systems have an average WSW direction of extension and led to exhumation of the two lower metamorphic complexes of the Betics, the Nevado‐Filabride and the Alpujarride, during the middle and upper Miocene (Serravallian to Tortonian). The extended domain contains a core complex, with distal and proximal antiformal hinges separated by around 60 km and with a fold amplitude of ∼6 km (measured parallel to the direction of extension). The total amount of extension across the core complex is about 109–116 km, corresponding to a stretching factor (β) of 3.5–3.9, estimated using the distance between fold axial surfaces and a geometrical model accomplishing footwall deformation during tectonic denudation via subvertical simple shear. The elongated domal and basinal geometry of the detachments and their respective footwalls is due to the interference between two sets of orthogonal large‐scale open folds, trending N‐S and E‐W. Longitudinal, N‐S trending folds are interpreted as isostatic folds, i.e., folds that developed in response to differential unloading of the extensional detachment footwalls inducing ductile flow in the middle crust. These folds formed in a rolling‐hinge anticline in which rotation migrates westward through the footwall as it is progressively unroofed. In contrast, E‐W trending folds, being subparallel to the direction of extension, have a contractional origin and progressively affect to the west the isostatically readjusted segments of the detachments once they are inactive. Orthogonal folding occurred since the Serravallian to lower Pliocene. Extension is still active from the rolling‐hinge anticline in Sierra Nevada to the west. The middle Miocene to Pliocene tectonic evolution of the Sierra Nevada elongated dome at the core of the Betic hinterland is another example of the coexistence of extension and contraction during continued overall convergence and mountain‐building.