bluuebunny/crossref_metadata_2025 huggingface dataset:
Validation of coral temperature calibrations
Authors: Crowley Thomas J. , Quinn Terrence M. , Hyde William T.
Published: December 2003

Geochemical analyses of coral skeletons are increasingly used to estimate past sea surface temperatures (SSTs). In this paper we suggest that the standard method of calibrating geochemical time series against a (usually short) local time series requires modification. In order to draw large‐scale inferences about climate from coral proxy data it is also necessary to (1) calibrate against larger fields such as the local gridded data sets and (2) validate results against an independent data set (e.g., early 20th century). This approach has been applied in a pilot study to a coral record from New Caledonia. Despite a high δ18O correlation (r=−0.88) with the in situ and gridded SST data sets, estimated early 20th‐century temperatures are more than 1.5°C colder than observed if the standard seasonal calibration is used. Regression against mean annual temperatures, which has a different slope relation, yields better estimates of early 20th‐century SSTs. However, testing of a Sr/Ca record from New Caledonia yields better agreement with early 20th century SSTs. Routine validation exercises for other coral sites are necessary to clarify the robustness of geochemical coral proxies as estimators of past environmental change.
Variation of atmospheric CO2 by ventilation of the ocean's deepest water
Authors: Toggweiler J. R.
Published: December 2003

A new box model for glacial‐interglacial changes in atmospheric CO2 produces lower levels of atmospheric CO2 without changes in biological production or nutrient chemistry. The model treats the boundary between middepth water and deep water as a chemical divide that separates low‐CO2 water above from high‐CO2 water below. Atmospheric CO2 is reduced 21 ppm by reduced ventilation of the deep water below the divide. A further reduction of 36 ppm is due to CaCO3 compensation in response to lower CO3= below the divide. Colder surface temperatures account for an additional 23 ppm of CO2 reduction. The new mechanism leaves the glacial atmosphere lighter in δ13C than in preindustrial time, as seen in ice cores and fossil plant material. Bottom water below the divide becomes strongly depleted in δ13C without a change in nutrient concentrations.
Early Paleogene warm climates and biosphere dynamics: Meeting in Göteborg makes progress in deciphering the dynamics of past greenhouse worlds
Authors: Sloan Lisa Cirbus , Schmitz Birger , Aubry Marie‐Pierre , Zachos James
Published: December 2003

The meeting on Early Paleogene Warm Climates and Biosphere Dynamics was held June 9‐13, 1999, in Göteborg, Sweden, and was organized and hosted by B. Schmitz. Schmitz was assisted by organizing committee members M.‐P. Aubry and J. Zachos. This meeting was the fourth such gathering in recent years and was the largest and most successful meeting yet. There were 120 scientists in attendance from 21 countries. The largest group came from the United States; other well‐represented countries were the United Kingdom, Russia, and Germany. The meeting, aimed at a multifaceted understanding of the early Paleogene, had a strong interdisciplinary character. Over 4 days, participants presented scientific research in oral and poster sessions. The meeting ended with a 1‐day field trip to the early Paleogene deposits of northern Denmark.
Red sprites, upward lightning, and VLF perturbations
Authors: Rodger Craig J.
Published: December 2003

In the last decade there has been a great deal of interest in the detection and understanding of phenomena occurring above active thunderstorms. The discovery of the optical phenomena now termed “red sprites” is discussed, along with the properties that have been experimentally determined. Areas of disagreement between experimentalists are pointed out. Other optical and electromagnetic phenomena associated with red sprites are presented. These include blue jets, transionospheric pulse pairs, and gamma ray flashes. Particular attention is given to the work on perturbations on very low frequency radio wave transmissions (“VLF sprites”), which has provided estimates of the electrical properties of sprites. Research into activity above thunderstorms will continue to lead to a greater understanding of the coupling between thunderstorms in the troposphere to the stratosphere, mesosphere, ionosphere, and beyond.
On the glaciological, meteorological, and climatological significance of Antarctic blue ice areas
Authors: Bintanja Richard
Published: December 2003

This paper presents the current state of our knowledge about Antarctic blue ice areas. Scattered widely over the Antarctic continent yet covering only approximately 1% of its surface area, blue ice areas have some peculiar characteristics that make them of particular interest for planetary geologists, glaciologists, meteorologists, and climatologists. For instance, blue ice areas are subject to net ablation mainly by sublimation, since any accumulation is easily swept off their smooth surfaces by winds. Moreover, they are significantly warmer than their surroundings, and the wind over them is generally more gusty. An important feature of blue ice is its relatively low albedo (0.56) compared with that of snow (0.80). Blue ice is probably considerably smoother than snow (in the aerodynamic sense), with roughness lengths as low as 3 × 10−6 m, which indicates that blue ice is probably the only permanent natural surface over which the air flow is mainly in the aerodynamic smooth regime. Further, since it is likely that very old ice is exposed at the surface of some blue ice areas, this ice would be of great interest for paleoclimatic studies. It has also been suggested that blue ice areas are sensitive to climate change; this would make them useful as Antarctic climate indicators because they can easily be monitored by satellite. Finally, this paper addresses several gaps in our knowledge about blue ice areas and gives suggestions for future blue ice area research.
Method for measuring the direction and wave mode of the radio echoes that are detected by a satellite radio sounder in the Earth's magnetosphere
Authors: Calvert W.
Published: December 2003

A method based on measuring the electric polarization of a wave has been devised to measure the direction and wave mode of the echoes that are detected by a satellite radio sounder in the Earth's magnetosphere. Except for directions that are almost exactly perpendicular to the magnetic field, this method is found to work at frequencies above about 20 kHz with an angular error of less than about 1°. A second method that is based on measuring the Faraday rotation of the axis of polarization has also been devised to measure the echo direction when the ordinary and extraordinary echoes that are produced by a radio sounder overlap to produce an elliptically or linearly polarized wave. These two methods will then be used to analyze the measurements of the Image radio sounder that is scheduled to be launched in the year 2000, as described in the feasibility analysis for the Image sounder that was published in 1995.
Transient interactions between tiny interfacial antennas on half‐space lossless dielectric
Authors: Lee Won‐Seok , Nam Sangwook
Published: December 2003

The interfacial electric‐field interaction subdivides into four cases: the HH, VV, HV, and VH cases. For each case, we derive the impulse‐response solution of the interfacial electric field generated by an infinitesimal impulse current source at the same interface, using plane‐time Fourier transform and a variant scheme of the Cagniard‐de‐Hoop method. We show that it is possible to obtain exact explicit‐form solutions for the HH and VV cases, whereas we have to evaluate the electric fields only by numerical calculations for the HV and VH cases since the exact solutions are yet unavailable. Various aspects of the impulse‐response solutions, such as the impulse radiation characteristics and the transient responses to time‐Gaussian inputs, are investigated as well as the offset‐differentiated field characteristics for the HV and VH cases. We also discuss the effect of the lower‐medium loss on the transient waveforms and the effect of the interface that makes the interfacial interaction quite different from the interaction in free space.
The contribution of the protonosphere to GPS total electron content: Experimental measurements
Authors: Lunt N. , Kersley L. , Bishop G. J. , Mazzella A. J.
Published: December 2003

Global Positioning System (GPS) satellites have orbital altitudes of about 20,200 km, while satellites in the Navy Ionospheric Monitoring System (NIMS) constellation are in circular orbits at heights of about 1100 km. Independent measurements of the electron content in the ionized atmosphere can be made using the radio signals from both satellite constellations. Differences between the two estimates can be related to the electron content on the GPS ray paths above 1100 km, through the tenuous plasma of the protonosphere. Results are reported from some 21 months of simultaneous observations of both GPS and NIMS transmissions at a European midlatitude station at solar minimum. It is shown that the average differences between the electron contents measured by the two systems are in broad agreement with the predictions from an earlier modeling study of the effects of the protonosphere on GPS total electron content. The expected influence of ray path / flux tube geometry and the rapid depletion and slow refilling of the protonosphere in response to geomagnetic storm activity can be seen in the averaged measurements.
Unified approach to predicting cellular characteristics in urban environments with rectangular grid‐plan streets
Authors: Blaunstein N. , Giladi R. , Levin M.
Published: December 2003

This work presents theoretical predictions obtained by Blaunstein et al. [1997], Blaunstein and Levin [1996], Walfisch and Bertoni [1988], Xia et al. [1993], Xia and Bertoni [1992], and Bertoni et al. [1994] for estimating microcell characteristics in a street environment in order to improve the effective service to individual subscribers to cellular wireless systems. In the case when the investigated region is built in a grid‐like manner with regularly distributed rows of buildings and both antennas are located under the average building height in conditions of line of sight (LOS) along the street, we use the model of a three‐dimensional (3‐D) multislit waveguide, according to Blaunstein et al. [1997] and Blaunstein and Levin [1996], to estimate the path loss at the street level. In the case of obstructive (NLOS) conditions, when both antennas are placed below and above the rooftops in an environment with strong shadowing surrounding them, we use the 2‐D model of multidiffraction from the building roofs and walls, according to Walfisch and Bertoni [1988], Xia et al. [1993], Xia and Bertoni [1992], and Bertoni et al. [1994], modified in conjunction with actual variations of building heights, the distances between them, and the actual base station antenna height variations; at street intersections we use the crossing‐waveguide model according to Blaunstein and Levin [1997]. The contributions to path loss are obtained to predict the experimentally observed coverage effects and a cell radius in LOS and NLOS conditions to predict the microcell shape for cellular map construction in urban and suburban areas with regularly distributed rows of buildings and rectangularly crossing streets.
The protonospheric contribution to GPS total electron content: Two‐station measurements
Authors: Lunt N. , Kersley L. , Bishop G. J. , Mazzella A. J. , Bailey G. J.
Published: December 2003

Results are presented from an experiment to estimate the contribution of plasma on ray paths through the protonosphere to measurements of total electron content (TEC) using Global Positioning System (GPS) signals. Simulations using the Sheffield University plasmasphere ionosphere model show that observations of GPS satellites made at two stations separated by a few degrees of latitude could involve a common ionospheric volume but very different intersection geometries of the ray paths with protonospheric flux tubes. Experimental results demonstrate that, on average, higher equivalent vertical TECs are measured on ray paths to the south than those to the north of the European midlatitude stations considered here. The observations are discussed in terms of the known asymmetries of the protonospheric flux tubes, and caution is advised in the use of thin‐shell ionospheric models for precise determination of TEC or correction for its effects on GPS systems.
Determining polarization parameters and angles of arrival of HF radio signals using three mutually orthogonal antennas
Authors: Afraimovich E. L. , Chernukhov V. V. , Kobzar V. A. , Palamartchouk K. S.
Published: December 2003

This paper presents results of an experimental verification of our earlier suggested spectral‐polarization method of measuring the polarization parameters and angles of arrival of HF electromagnetic waves by analyzing three mutually orthogonal projections of the radio signal field vector using a single receiving antenna. The initial stage of analysis involves calculating complex Doppler spectra of time variations of these projections. Thereupon, for each spectrum component, these data are used to determine polarization parameters and angle‐of‐arrival spectra. The measurements were made on a HF radio path about 100 km in length, with simultaneous monitoring of the ionospheric situation using the oblique‐incidence sounding chirp‐ionosonde. In an effort to eliminate multipath effects, in the analysis we used nighttime intervals, for which a stable one‐mode reflected radio signal was observed. It is shown that the proposed method gives mean values of the azimuth and zenith angle which differ by no more than 2°–5° from calculated values. Results of an experimental verification of the suggested technique are in agreement with published data obtained by classic methods.
The effect of the protonosphere on the estimation of GPS total electron content: Validation using model simulations
Authors: Lunt N. , Kersley L. , Bishop G. J. , Mazzella A. J. , Bailey G. J.
Published: December 2003

Simulated observations of total electron content (TEC) along ray paths from Global Positioning System (GPS) satellites have been used to validate the estimation of TEC using GPS measurements. The Sheffield University plasmasphere ionosphere model (SUPIM) has been used to create electron densities that were integrated along ray paths from actual configurations of the GPS constellation. The resultant slant electron contents were then used as inputs to validate the self‐calibration of pseudo‐range errors (SCORE) process for the determination of TEC from GPS observations. It is shown that if the plasma resides only in the ionosphere below 1100 km, then the SCORE procedure determines the TEC to a high degree of accuracy. When the contribution of the electrons in the protonosphere above 1100 km is included, the analysis results in TEC estimates that are high by some 2 TEC units (TECU) for conditions appropriate to European midlatitudes at solar minimum. However, if a restriction is placed in the analysis on use of observations equatorward of the station, then allowance can be made for the effect of the protonosphere. It is shown that with appropriate selection of the boundary for the observations, TEC can be estimated by SCORE to better than 1 TECU for the conditions of the simulation. Sample results are included from actual experimental observations using GPS to demonstrate the effect of compensation for the protonospheric plasma.
Toward resolving small‐scale structures in ionospheric convection from SuperDARN
Authors: André R. , Villain J.‐P. , Senior C. , Barthes L. , Hanuise C. , Cerisier J.‐C. , Thorolfsson A.
Published: December 2003

The combination of radial velocities measured by a pair of Super Dual Auroral Radar Network (SuperDARN) HF coherent radars gives, in their common field of view, the velocity vectors in a plane perpendicular to the magnetic field. The standard merging is based on a natural grid defined by the beam intersections, which provides a resolution varying between 90 and 180 km (depending upon the distance to the radars). This allows the description of structures with a typical scale size (L) of the order of 500 km. The present study is devoted to a merging method which takes advantage of individual radar grids to enhance the resolution (L ≈ 200 km). After a brief description of the standard merging method, we define the high‐resolution grid and discuss the potential problems which have to be overcome. The first problem concerns the localization of the scattering volume, whereas the second one deals with the independence of the velocity vectors. These two limitations have been addressed in previous studies [André et al., 1997; Barthes et al., 1998]. In the method proposed here, several velocity vectors are determined at each grid point, from which the selection is made by using the hypothesis of minimization of the divergence magnitude. The selected map is the one which minimizes the divergence. The performances are tested and compared to the standard merging algorithm through simulated double vortices. Finally, we apply this method to real data, and show, through two examples, its ability to describe small‐scale structures (L ≈ 200 km).
On the interpretation of the layered structures detected by mesosphere‐stratosphere‐troposphere radars in dual frequency domain interferometry mode
Authors: Luce H. , Crochet M. , Hanuise C. , Yamamoto M. , Fukao S.
Published: December 2003

The frequency domain interferometry (FDI) technique has been developed for probing thin layered structures of the atmosphere. The position and thickness of a single layer embedded within the scattering volume can be deduced from the complex normalized cross correlation (coherence) of received signals at two closely spaced frequencies. Applied in the vertical pointing direction, this technique identified layered structures (“FDI layers”) of 50–200 m in thickness in the lower atmosphere. These structures are 1 order of magnitude thicker than observed temperature sheets (about 10‐m thick) which are very likely responsible for the main part of the VHF radar echoes in vertical direction. In this paper, although the ambiguity of the dual FDI technique is well known, we emphasize that the FDI layers do not necessarily correspond to a single atmospheric layer; they can also be interpreted as a more complex structure of very thin atmospheric layers. A simple model, introduced as an example, shows that the FDI layer thickness can also approximately be interpreted as the vertical separation of two very thin atmospheric layers. This result can explain by itself the differences between the estimated thicknesses by balloon and FDI radar techniques. Finally, we stress that comparisons with high‐resolution in situ measurements are urgently needed for interpreting the FDI layers.
Interpretation of equatorial electrojet irregularities observed with a broad beam HF zenithal radar
Authors: Farges T. , Blanc E. , Villain J. P.
Published: December 2003

Observations of the equatorial ionosphere were performed in Africa with the radar of the Laboratoire de Détection et de Géophysique near the magnetic equator during the International Equatorial Electrojet Year (1993–1994). This high‐resolution zenithal HF radar uses a broad beam antenna, in which complex echo structures are observed. A diversity of irregularities appears in the 100–500 km radar range up to the F region. Some irregularities appear in range‐time‐intensity diagrams as a parabolic region between 140 and 180 km with a minimum at noon. This region is only observed for radar frequencies between 4 and 5 MHz. At these frequencies, type II irregularities arising from vertical and oblique directions in the 100–140 km range dominate the spectra, and type I irregularities are difficult to clearly identify. Ray tracing and simulations show that type I irregularities are a possible source of the observed parabolic region. Irregularities from the equatorial “150 km region” as observed with the Jicamarca radar appear at slightly higher radar ranges.
Fractals and the small perturbation scattering model
Authors: Franceschetti Giorgio , Iodice Antonio , Migliaccio Maurizio , Riccio Daniele
Published: December 2003

The two‐dimensional fractional Brownian motion (fBm) fractal model has proved to be very suitable for the description of natural surfaces. In this paper we use this model to evaluate scattering from natural rough surfaces. The small perturbation method is considered, and an expression of the backscattering coefficient is obtained. We show that use of the fBm surface model leads to scattering results that are in better agreement with a set of measured data than the ones obtained by using usual surface models. The proposed model also compares favorably with the two‐scale classical model.
Tropical precipitating systems observed with Indian MST radar
Authors: Rao T. Narayana , Rao D. Narayana , Raghavan S.
Published: December 2003

Three campaigns are conducted with the Indian mesosphere‐stratosphere‐troposphere (MST) radar, located at Gadanki (13.5°N, 79.2°E), India, to study the precipitating systems in the tropics. This study mainly deals with (1) classification of precipitating clouds and the spectral characteristics of echoes associated with these cloud systems and (2) characteristics of the radar bright band. The radar gets echoes scattered both from refractive index fluctuations and precipitation particles in moderate to heavy precipitation conditions. These echoes are separated in the spectral domain to determine the vertical air motion and the Doppler velocity of hydrometeors simultaneously. The tropical precipitating systems are classified as stratiform and convective using the reflectivity and vertical velocity distribution. The echo power, spectral width, and vertical velocities of the ambient air and hydrometeors in both the cloud systems have been compared. Aspect sensitivity of the echoes from the hydrometeors and refractive index fluctuations in both stratiform and convective atmosphere is studied. A transition stage, where the stratiform precipitation is associated with the convection, is also reported. Backscattered power from precipitation particles is used to estimate the reflectivity factor (dBZ), and these values along with spectral width and vertical velocity values are used to identify the bright band structure. The reflectivity at the bright band, up to 42 dBZ, is found to be 10–12 dB more than the average value of reflectivity below the bright band. Discussion on the factors contributing to this enhancement is also included. A clear layered structure around the 0°C isotherm in the reflectivity profile of the precipitation echo confirms the presence of the bright band. The thickness of the bright band is estimated and is correlated with the peak reflectivity at the bright band. Comparison of the average terminal velocity of hydrometeors with their average Doppler velocities below the bright band shows the presence of gentle updrafts of a few cm s−1 in stratiform precipitation. These studies are made for the first time with the Indian MST radar and also demonstrate the capability of a VHF radar in studying precipitating systems in addition to the turbulence to which these radars are highly sensitive.
High Frequency Active Auroral Research Program imaging riometer diagnostic
Authors: Rosenberg T. J. , Weatherwax A. T. , Detrick D. L. , Lutz L.
Published: December 2003

This paper describes the prototype 16‐beam, 38.6‐MHz riometer system developed by Advanced Power Technologies, Inc., and the University of Maryland for the High Frequency Active Auroral Research Program (HAARP). The prototype system is the forerunner for a full‐scale imaging riometer diagnostic instrument for characterizing the ionospheric volume perturbed by controlled radio frequency (RF) heating experiments. The prototype system, installed at the HAARP site near Gakona, Alaska, consists of a 1 × 16 antenna array phased in one dimension (beam width of ∼6.7°) and oriented approximately along the magnetic meridian. The system responds sensitively to natural variations of auroral absorption, such as those caused by magnetospheric substorms, and provides clear evidence of its capability to discern spatial structure and motion. A newly observed feature seen near dusk are intense, short‐duration absorption spikes accompanied by only weak magnetic signatures. Because the HAARP heating facility is still a work in progress, there have been only limited opportunities to conduct science investigations with it. Consistent with estimates, heater‐induced effects in the ionosphere have not been observed with the riometer at the power levels currently available. It has been noted that the proximity of the prototype system to the HAARP RF heater can result in interference to the riometer signal for some frequencies and operating modes of the heater. The full‐scale imaging capability of the proposed instrument, and a remote location, may be required to detect small‐scale modifications of the ionosphere caused by the heater.
Morphology of polar cap patch activity
Authors: Dandekar B. S. , Bullett T. W.
Published: December 2003

For the period July 1, 1989, to June 30, 1990, for high solar activity, ƒ0F2 data were recorded by the Digital Ionospheric Sounding Systems from two Qaanaaq and Sondrestromfjord; ionospheric UHF scintillation data were recorded from Thule; and airglow images were recorded from Qaanaaq. These data were used to study the morphology of polar cap patches. For this study the polar cap patch activity has been defined as excess electron density above a baseline. The megahertz scale ƒ0F2 for the strength of a patch matches well with the decibel scale of scintillation patches. On a daily basis the polar cap patch occurrence is very high (90%) for weak patches (≥3 MHz), and it decreases significantly (20%) for stronger patches (≥8 MHz). On a daily basis the occurrence of the number of patches per day shows a similar behavior, with as many as 10 weak (≥3 MHz) patches and as few as two strong (≥8 MHz) patches. On the average, patches are seen 50% of the time, whereas strong patches are seen only 5% of the time. Polar cap patches show a strong seasonal dependence, with the highest occurrence in the winter months. Average patch duration is about 55 min at the polar latitudes of Thule and Qaanaaq. The time interval between the patches is about 100 min. The patch occurrence and strength both show a diurnal behavior, with the highest occurrence around 2200–0200 UT and a minimum around 0600–1100 UT. These observations are consistent with the suggestion that the source for patches is a tongue of ionization formed by plasma convection carrying auroral/subauroral plasma into the nightside polar region. The observations support the mechanism of a chopping of a tongue of ionization for a formation of a patch close to auroral latitude, and the patch drifts across the polar cap. The patch activity seems to be weakly dependent on magnetic activity index Kp. The present study shows that the patch occurrence is 27% for By positive and 73% for By negative, and 32% for Bz positive and 68% Bz negative, supporting the present understanding that the polar cap patches predominantly occur with the southward turning of the interplanetary magnetic field.
A simulation‐based approach to radar backscatter from the sea surface
Authors: Corsini G. , Manara G. , Monorchio A.
Published: December 2003

A numerical procedure for simulating electromagnetic scattering from the sea surface is presented. The simulation scheme is based on a physical model of the sea surface; the surface is generated starting from its two‐dimensional directional spectrum. The scattered electromagnetic field is evaluated by resorting to a two‐scale electromagnetic model. Physical parameters can be taken into account in simulations, as for instance, the local wind velocity direction and strength. The procedure is applied to the case of real aperture radar systems operating at low and intermediate incidence angles. In order to demonstrate the effectiveness of the simulation technique proposed, numerical results are presented and compared with experimental data available in the literature.
Longitude structure of ionospheric total electron content at low latitudes measured by the TOPEX/Poseidon satellite
Authors: Vladimer J. A. , Jastrzebski P. , Lee M. C. , Doherty P. H. , Decker D. T. , Anderson D. N.
Published: December 2003

The longitude structure of ionospheric total electron content (TEC) at low latitudes has been evaluated using the NASA/Centre Nationale d'Etudes Spatiales TOPEX/Poseidon satellite. The TEC data set is given by the ionospheric range correction, which is computed from TOPEX dual‐frequency altimeter measurements. The satellite's orbit allows analysis of vertically measured TEC values at approximately 30° intervals of longitude across the world at local time differences of only 6–12 min. Patterns of longitudinal dependence of the equatorial anomaly were observed during the equinoxes, summers, and winters of 1993, 1994, and 1995. TOPEX observations reveal occurrence of relative maximum anomaly TEC values in the Indian/Asian longitude sector. This dominance in TEC is seen most consistently in the Asian Southern Hemisphere. Also, a relative decrease in anomaly TEC values is evident in the western American region, which is observed primarily during equinox and winter. This configuration of the equatorial anomaly TEC is observed on a day‐to‐day basis at particular periods of local time. Global theoretical ionospheric model results are presented in an attempt to reproduce the distinctive longitude structure. Variability in E × B vertical drift velocity within specific longitude sectors is shown to be a primary factor in the longitude dependence of equatorial anomaly TEC.
Modeling of subionospheric VLF signal perturbations associated with earthquakes
Authors: Rodger Craig J. , Clilverd Mark A. , Thomson Neil R.
Published: December 2003

It has been reported that propagation of very low frequency (VLF) waves in the Earthionosphere waveguide might provide an indication of imminent earthquakes [Hayakawa et al., 1996; Molchanov et al., 1998]. Narrow‐band data from Inubo, Japan, suggested that transmissions from Omega Japan, 1000 km away, might be influenced by pre‐earthquake processes. The terminator time (TT) was defined as the time where a minimum occurred in the received phase (or amplitude) during sunrise and sunset. A few days before an earthquake the TT was observed to deviate significantly from the monthly averages, producing a longer “VLF day.” The TT effect has been explained through some rather simple modeling by a 1–2 km drop in the VLF reflection height at the lower ionospheric boundary. In this study we apply more realistic propagation models to show that the changes in VLF reflection height associated with earthquakes would have to be considerably larger (∼4–11 km) than those suggested previously in order to produce the reported effect. If the reported TT changes were caused by alterations in the VLF reflection height associated in some manner with an imminent earthquake, these effects would be commensurate with the effects of a solar flare. However, this would lead to changes in received amplitude (or phase) that would be significant at all times, and not just during the day/night transition. Hence it is not at all clear that a simple height‐lowering explanation for the TT effect is correct.
Arithmetic average, geometric average, and ranking: Application to incoherent scatter radar data processing
Authors: Zhou Q. H. , Zhou Q. N. , Mathews J. D.
Published: December 2003

We examine the statistical characteristics of three commonly used estimators, arithmetic average, geometric average, and ranking as applied to incoherent scatter radar observations in the presence of interference. While the arithmetic average is effective in reducing the statistical error, it is very sensitive to “outlier” contamination, such as meteor returns. The ranking method is robust in removing outliers but is not effective in reducing the statistical variance. On the other hand, triple ranking and the geometric average are almost as effective as the arithmetic average in reducing the statistical error. If the data contain only outlier contamination, the geometric average is a better choice than either simple arithmetic average or the single ranking method. In dealing with complex interference from radars and various communications systems, one can use the triple ranking method or a combination of the arithmetic average and the single ranking method. In addition, the processes of ranking and geometric average, like those of the arithmetic average, also converge to a Gaussian function when the number of samples is large. We also show that the central limit theorem can be used to obtain interesting approximations.
Theoretical study of the effect of different types of tropical rainfall on microwave and millimeter‐wave propagation
Authors: Ajewole M. O. , Kolawole L. B. , Ajayi G. O.
Published: December 2003

In this paper, the effects of tropical rainfall on the propagation of microwave and millimeter‐wave signals on terrestrial paths have been investigated. Quantitative evaluation of spheroidal raindrop scattering parameters such as the scattering amplitude functions, total and scattering cross sections, and so on, were carried out for linear polarization (horizontal and vertical), using the least squares fitting method over the frequency range 1–100 GHz. Incident radio signals are assumed plane and arrive normally on the raindrop axis. For the purpose of application to slant paths, two other incident angles, 50° and 70°, were considered. The computed forward scattering amplitude functions were employed to compute the complex effective propagation constant of the radio signals in four tropical rainfall types: drizzle, widespread, shower, and thunderstorm. The corresponding specific attenuation and specific phase shift were computed. Specific attenuation and phase shift due to the rain types for circularly polarized radio signals were also derived. The parameters of the power law relationship between attenuation, phase shift, and rain rate were computed for each rain type and compared with existing temperate and tropical models. The problem of rain‐induced depolarization was also investigated by computing the cross polarization discrimination for the four rainfall types over the frequency range 1‐50 GHz, which is of immediate importance to system designers.
Nonmonochromatic radiation transfer in a plane slab of magnetized plasma with random irregularities
Authors: Bronin A. G. , Zabotin N. A. , Kovalenko E. S.
Published: December 2003

The radiation energy balance equation describing propagation of nonmonochromatic radiation (pulses of waves and modulated radiation) in a plane slab of magnetized plasma with random irregularities in the invariant ray coordinates has been obtained. Its solution in the small angle (in the invariant ray variables) scattering approximation has been considered.
Fast and accurate frequency‐sweep calculations using asymptotic waveform evaluation and the combined‐field integral equation
Authors: Jiao Dan , Zhu Xian‐Yang , Jin Jian‐Ming
Published: December 2003

The method of asymptotic waveform evaluation (AWE) is applied to the combined‐field integral equation (CFIE) to achieve fast and accurate frequency‐sweep calculations of electromagnetic scattering and radiation by three‐dimensional conducting and dielectric objects. The employment of the CFIE eliminates the interior resonance problem suffered by both the electric‐field integral equation and the magnetic‐field integral equation. It is shown that the use of AWE can speed up the calculation by more than an order of magnitude. It is also shown that when combined with the complex frequency hopping technique, the AWE method can produce an accurate solution within a prespecified frequency band. Numerical examples are presented to demonstrate the performance of the proposed method.
Active slivering of oceanic crust along the Molucca ridge (Indonesia‐Philippine): Implication for ophiolite incorporation in a subduction wedge?
Authors: Bader Anne Gaëlle , Pubellier Manuel , Rangin Claude , Deplus Christine , Louat Rémy
Published: December 2003

A recent marine geophysical survey in the northern Molucca Sea revealed the structure to be that of a classical active convergent margin. We observe from west to east a volcanic arc (Sangihe), a forearc basin resting on an outer ridge (the Molucca ridge), which serves as a buttress for an accretionary wedge, and a composite downgoing plate (Snellius Ridge and Philippine Sea Basin). Gravity modeling indicates a strong negative anomaly above the wedge, which cannot be explained with reasonable density values. Modeling imposes a basement deepening and a rupture of the 700‐km‐long subducting lithosphere. This process individualized the lithospheric slab from the Snellius Ridge, which in turn was separated recently from the south Philippine Basin by the incipient Philippine Trench. This induces a deformation of the forearc region with backthrusting of the outer ridge and forearc basin, visible on bathymetry and seismic data. We extrapolate the tectonic emplacement of such oceanic blocks to the Oligocene times in order to explain the origin of the Pujada Miangas outer ridge as a sliver previously incorporated to the margin, and we discuss the possibility of this deformation process being fabric for terrane accretion.
The Auriga Nunataks shear zone: Mesozoic transfer faulting and arc deformation in northwest Palmer Land, Antarctica
Authors: Vaughan Alan P. M. , Millar Ian L. , Thistlewood Laurence
Published: December 2003

The Auriga Nunataks shear zone places new tectonic and temporal constraints on the Mesozoic evolution of West Antarctica. The shear zone is a long‐lived, arc‐orthogonal, ductile transfer fault that preserves a history of regional Mesozoic compressional basement deformation and extensional arc pluton emplacement in the Antarctic Peninsula magmatic arc. It forms an east‐west trending positive flower structure 2.4 km wide, exposed along 5 km of strike. Marble, graphite‐bearing pyroxene‐granulite, granodiorite‐diorite, amphibolite, and gabbro are deformed to mylonite and marble‐hosted tectonic breccia. The Mesozoic history of the shear zone is interpreted as follows: emplacement of granodiorite‐diorite at circa 206 Ma during Early Jurassic dextral transtension and metamorphism that peaked at circa 188 Ma (D1), brecciation of marble and mylonitization of gneiss by Late Jurassic to Early Cretaceous sinistral shear during the peninsula‐wide Palmer Land orogeny (D2), Early Cretaceous dextral transtension with emplacement of gabbro and garnet leucogranite between 140 and 135 Ma (D3), and mid‐Cretaceous, ocean‐vergent thrusting and sinistral transpression between 125 Ma and 80 Ma, with a peak at circa 110 Ma that folded, mylonitized, and brecciated preexisting plutonic and metamorphic rocks (D4); this is responsible for the current geometry of the shear zone. The Auriga Nunataks shear zone transferred motion between arc‐parallel compressional and extensional structural elements and by hosting plutons appears to have acted like a leaky transform fault during episodes of regional extension.
Subsidence history, gravity anomalies, and flexure of the northeast Australian margin in Papua New Guinea
Authors: Haddad D. , Watts A. B.
Published: December 2003

Gravity and well data have been used to model the foreland basin flanking the Papua New Guinea orogenic belt. Backstripping of sediments from well data reveals a former passive margin that has probably experienced at least two major orogenic loading events. The first of these took place at or near the Miocene—Oligocene boundary and was associated with widespread carbonate deposition across the existing shelf and evolving basin. The second event is most likely related to the start of the main phase of uplift in the fold‐and‐thrust belt near the beginning of the Pliocene. Sedimentation in the Papuan Basin altered from a system dominated by carbonate accumulation to one of rapid deposition of terrigenous clastic sediments shed from the emerging orogen. For the purposes of gravity and flexure modeling, the base of the foreland sequence was defined as a horizontal surface that existed at the time of the initial collision. A total of seven gravity, topography, and stratigraphie profiles were constructed and used to model the flexure of the lithosphere supporting the onshore Papuan Basin and New Guinea orogen. The best fit model was a semiinfinite elastic plate in which the plate break was located approximately midway across the orogenic belt. The elastic plate thickness Te was found to be high in the western regions of Papua New Guinea (Te of 70–80 km) with little variation across the first four profiles, but it weakened considerably to the east where Te varied rapidly from 60 km to as little as 8 km. This sudden weakening of the lithosphere occurred as a narrow zone at the foot of the mountain belt. The weakening could be related to a low‐strength zone within the former margin. Alternatively, it reflects the western extent of late Tertiary volcanism that was produced by the subduction of the Solomon Sea Plate beneath Australia.
Patterns of tectonic stress in Sicily from borehole breakout observations and finite element modeling
Authors: Ragg Steffen , Grasso Mario , Müller Birgit
Published: December 2003

The orientation of in situ tectonic stress was deduced from borehole breakout analysis of 22 wells from onshore Sicily. The results allow us to distinguish the stress field of different geological units: (1) A nearly NNW (148°) orientation is detected in the Hyblean Plateau. (2) A NNE SHmax direction characterizes the Gela area. To the north, within the thrust belt (around Mount Judica), the SHmax direction swings to NE. In the northeastern segment of the foredeep, the Catania Plain, the direction of SHmax is roughly parallel to the NE trending grabens that mark the northern margin of the Hyblean Plateau. (3) The southwestern segment of the foredeep has no preferential SHmax orientation and may act as a transition zone with isotropic horizontal stresses. (4) In western Sicily a SHmax orientation from N–S to NW–SE is observed. This fits well the kinematics of the SE migrating Egadi and Adventure thrust belts and the direction of shortening inferred from the modern seismicity of the area. A three‐dimensional finite element modeling, including the most important tectonic features, was performed. Modeling results of the stress field indicate that the NNW trending SHmax is locally influenced by the variation of crustal thickness and local zones of weakness. In addition, the Pantelleria Rift causes a rotation of the regional NW–SE stress orientation to NNE along the northern rim of the rift system, including large on‐shore areas in the adjacent central south Sicily.
Proterozoic and Cenozoic subduction complexes: A comparison of geometric features
Authors: van der Velden Arie J. , Cook Frederick A.
Published: December 2003

Mantle reflections along the Lithoprobe Slave Northern Cordillera Lithospheric Evolution (SNORCLE) deep seismic reflection transect in northwestern Canada are interpreted as a relict subduction zone. Geometric features of the data in the vicinity of the interpreted subduction zone are compared with those of the Vancouver Island profile, which images a modern convergent margin. Comparisons are made at both a regional scale, as in the case of the location of the magmatic arc with respect to the subduction zone, as well as a local scale, such as the geometry of structures within the accretionary wedge. The comparison provides a test for the paleosubduction hypothesis and aids the interpretation of the deep reflection profile where it is difficult to trace individual features to outcrop. The similarity of geometry and scales of the structures gives support to the interpretation that the lithospheric structures in the Wopmay orogen are the products of accretionary processes at ∼1.88–1.84 Ga and that these accretionary processes are similar to those observed today.
Thermochronologic constraints on deformation and cooling history of high‐ and ultrahigh‐pressure rocks in the Qinling‐Dabie orogen, eastern China
Authors: Webb Laura E. , Hacker Bradley R. , Ratschbacher Lothar , McWilliams Michael O. , Dong Shuwen
Published: December 2003

The Hong'an block is the best place to study the exhumation of high‐pressure (HP) and ultrahigh‐pressure (UHP) metamorphic rocks in the Qinling‐Dabie orogen of eastern China because it lacks the extensive Cretaceous tectonic and thermal overprint observed in the Dabie Shan. We measured timing of deformation and rate of cooling of the HP‐UHP rocks by 40Ar/39Ar analyses of synkinematic minerals from tectonites of key structural zones in the Hong'an and Tongbai Shan. Normal‐sense shear along the north dipping Huwan detachment at the northern edge of the Hong'an block occurred between 237 and 231 Ma; this detachment facilitated the bulk of the exhumation of the HP‐UHP rocks. Our new 40Ar/39Ar ages, combined with U/Pb zircon and Sm/Nd ages of 245–240 Ma, suggest that exhumation of UHP rocks from mantle depths occurred at rates of 5–25 mm/yr from ∼245 to 230 Ma. The mountain range is a warped extensional footwall, within which white mica cooled from 225 to 205 Ma. Locally, younger extension is recorded by white mica recrystallization at 198–194 Ma, after which the entire block had cooled to below 300°C. Early Cretaceous 40Ar/39Ar ages from the Tongbai shear zone indicate that dextral shear along the southwest boundary of the orogen was contemporaneous with normal to sinistral‐oblique slip along the Xiaotian‐Mozitan fault along the northern boundary. Coeval dextral and sinistral shear zones along the northern and southwestern margins of the Hong'an and Dabie Shan would have caused eastward lateral extrusion of these two blocks, perhaps driven by collision of the Lhasa block with Eurasia.
Structural styles and regional tectonic setting of the “Gela Nappe” and frontal part of the Maghrebian thrust belt in Sicily
Authors: Lickorish W. Henry , Grasso Mario , Butler Robert W. H. , Argnani Andrea , Maniscalco Rosanna
Published: December 2003

The Gela Nappe of south central Sicily provides an example of a curved segment of an orogenic front that can be examined both onshore and offshore for deformational style and amount of shortening. Synorogenic sediments allow the deformation to be dated. Two distinct structural styles are observed in the Gela Nappe: The central salient part of the nappe (Caltanissetta basin) consists of a single thrust sheet containing a train of continuously tightening folds and the reentrant margins of the nappe (Sciacca and Monte Judica) consist of a stack of several thrust sheets. These different structural styles correspond to the pretectonic Mesozoic stratigraphy of the foreland plate. Carbonate platforms exist on the Adventure bank and Hyblean Plateau ahead of Sciacca and Monte Judica, respectively, while the Caltanissetta basin region appears to have accumulated basinal clay facies. Where the resistant carbonate stratigraphy provides a buttress to the propagation of the thrust front, deformation is taken up by imbrication on‐steep ramps through the carbonates generating a relatively thick orogenic wedge. In the basinal setting, where no strong rheology exists, the low angle of friction on the clay detachment levels requires the growing thrust wedge to be much thinner with a very low foreland dip. Hence the thrust front propagates much farther forward into the basin than it does in the adjacent platformal areas, producing a nonlinear thrust front. In the basinal region, accretion of foreland material to the nappe by imbrication was only prominent during the Messinian when subaerial exposure prevented low‐friction transport of the nappe across the highest levels of the stratigraphy. A steady thickening of the nappe by internal folding suggests an increase in friction along the basal detachment, possibly due to progressive compaction of the clays.