bluuebunny/crossref_metadata_2025 huggingface dataset:
Zooming in on Heinrich layers
Authors: Grousset Francis E. , Cortijo Elsa , Huon Sylvain , Hervé Laurence , Richter Thomas , Burdloff Didier , Duprat Josette , Weber Olivier
Published: February 2004

Theories explaining the origin of the abrupt, massive discharges of ice‐rafted detritus (IRD) into the glacial North Atlantic (the Heinrich layers (HLs)) generally point to the Laurentide ice sheet as the sole source of these events, until it was found that the IRDs also originated from Icelandic and European ice sheets [Bond and Lotti, 1995; Snoeckx et al., 1999; Grousset et al., 2000]. This apparent contradiction must be reconciled as it raises fundamental questions about the mechanism(s) of HL origin. We have analyzed two ∼12 cm thick HLs in an ultrahigh‐resolution mode (1–2 century intervals) in a mid‐Atlantic ridge piston core. The δ18O record (N. pachyderma left coiling) reveals strong excursions induced by the melting of the icebergs; these excursions are associated with a strong decrease in the amount of planktic foraminafersand with a 3°C cooling of the surface waters. Counts of coarse detrital grains reveal that IRD are deposited according to a typical sequence (1) volcanic glass, (2) quartz and feldspars, (3) detrital carbonate, that implies a chronology in the melting of the differentpan‐Atlantic ice sheets. Sr and Nd isotopic composition confirm that in both Heinrich layers H1 and H2, “precursor” IRD came from first Europe/Iceland, followed then by Laurentide‐derived IRD. An internal cyclicity can be identified: during H1 and H2, about four to six major, abrupt discharges occurred roughly on a century timescale. The δ13C and δ15N records reveal that dominant inputs of continent‐derived organic matter are associated with IRD within the HLs, hiding the plankton productivity signal.
Comparison of statistical and artificial neural network techniques for estimating past sea surface temperatures from planktonic foraminifer census data
Authors: Malmgren Björn A. , Kucera Michal , Nyberg Johan , Waelbroeck Claire
Published: February 2004

We present the first detailed and rigorous comparison of six different computational techniques used to reconstruct sea surface temperatures (SST) from planktonic foraminifer census data. These include the Imbrie‐Kipp transfer functions (IKTF), the modern analog technique (MAT), the modern analog technique with similarity index (SIMMAX), the revised analog method (RAM), and, for the first time, a set of back propagation artificial neural networks (ANN) trained on a large faunal data set, including a modification where geographical information was added among the input variables (ANND). By training the techniques on an identical database, we were able to explore the differences in SST reconstructions resulting solely from the use of different mathematical methods. The comparison indicates that while the IKTF technique consistently shows the worst performance, ANN and RAM perform slightly better than MAT and that the inclusion of the geographical information into the training database (SIMMAX and ANND) further improves the accuracy of modern SST estimates. However, when applied to an independent validation data set and an additional fossil data set, the results did not conform to this ranking. The largest differences in the reconstructed SST values occurred between groups of techniques with different approaches to SST reconstruction; that is, ANN and ANND produced SST reconstructions significantly different from those produced by RAM, SIMMAX, and MAT. The application of the various techniques to the validation data set, which allowed comparison of SST reconstructions with instrumental records, suggests that artificial neural networks might provide better paleo‐SST estimates than the other techniques.
Shifts in the position of the north equatorial current and rapid productivity changes in the western tropical Atlantic during the last glacial
Authors: Vink Annemiek , Rühlemann Carsten , Zonneveld Karin A. F. , Mulitza Stefan , Hüls Matthias , Willems Helmut
Published: February 2004

High‐resolution, well‐dated calcareous dinoflagellate cyst and organic carbon records from a 58 kyr sediment core (M35003‐4) located southeast of the island of Grenada show that rapid and pronounced changes in cyst association and accumulation and organic carbon deposition occurred, controlled by (1) a significant southward shift in the position of the North Equatorial Current during the last glacial period and the Younger Dryas cold interval and (2) rapid changes in local productivity in marine isotopic stage 3 that are associated with variations in Orinoco River nutrient discharge and coastal upwelling strength. Prominent cyst accumulation peaks representing extremely oligotrophic and stratified thermocline conditions mimic the Greenland ice core and northern Atlantic Dansgaard/Oeschger stadials and Heinrich events. We provide new evidence for a coupled tropical/high‐latitude Atlantic climate system during the last glacial period and suggest that changes in the zonality of the low‐latitude winds may play an important role in modulating rapid interhemispheric climate variability.
Uncorking the bottle: What triggered the Paleocene/Eocene thermal maximum methane release?
Authors: Katz Miriam E. , Cramer Benjamin S. , Mountain Gregory S. , Katz Samuel , Miller Kenneth G.
Published: February 2004

The Paleocene/Eocene thermal maximum (PETM) was a time of rapid global warming in both marine and continental realms that has been attributed to a massive methane (CH4) release from marine gas hydrate reservoirs. Previously proposed mechanisms for this methane release rely on a change in deepwater source region(s) to increase water temperatures rapidly enough to trigger the massive thermal dissociation of gas hydrate reservoirs beneath the seafloor. To establish constraints on thermal dissociation, we model heat flow through the sediment column and show the effect of the temperature change on the gas hydrate stability zone through time. In addition, we provide seismic evidence tied to borehole data for methane release along portions of the U.S. continental slope; the release sites are proximal to a buried Mesozoic reef front. Our model results, release site locations, published isotopic records, and ocean circulation models neither confirm nor refute thermal dissociation as the trigger for the PETM methane release. In the absence of definitive evidence to confirm thermal dissociation, we investigate an alternative hypothesis in which continental slope failure resulted in a catastrophic methane release. Seismic and isotopic evidence indicates that Antarctic source deepwater circulation and seafloor erosion caused slope retreat along the western margins of the North Atlantic in the late Paleocene. Continued erosion or seismic activity along the oversteepened continental margin may have allowed methane to escape from gas reservoirs trapped between the frozen hydrate‐bearing sediments and the underlying buried Mesozoic reef front, precipitating the Paleocene/Eocene boundary methane release. An important implication of this scenario is that the methane release caused (rather than resulted from) the transient temperature increase of the PETM. Neither thermal dissociation nor mechanical disruption of sediments can be identified unequivocally as the triggering mechanism for methane release with existing data. Further documentation with high‐resolution benthic foraminiferal isotopic records and with seismic profiles tied to borehole data is needed to clarify whether erosion, thermal dissociation, or a combination of these two was the triggering mechanism for the PETM methane release.
Pier‐based observations of polarized sea surface emission
Authors: Frasier S. J. , Wang F. , Carswell J. R. , Knapp E. J. , Swift C. T.
Published: February 2004

A Ka band polarimetric radiometer operating at 37 GHz was deployed from the seaward end of a small pier colocated with an X band Doppler scatterometer used to measure the instantaneous wave field. Brightness temperature time series from a small spot on the surface for four linear polarizations were measured: TV, TH, TP, and TM, corresponding to vertical, horizontal, +45°, and −45°, respectively. Expected behavior of the mean values of TV and TH was observed as a function of incidence and azimuth angles. Varying sensitivities to the wave field in TP and TM were observed as a function of azimuth angle with respect to the upwind direction. To the left of upwind, wave‐induced modulations of TP exceeded those of TM, while the opposite behavior was observed to the right of upwind. Since U is the mean value of the difference of these signals, it is relatively easy to interpret the sine variation of U with wind‐relative azimuth angle. Finally, modulations of brightness temperatures by the wave field were found to be consistent with the “smoothed” slope distribution provided by the radar.
Physical processes responsible for the nonlinearities of probe characteristics and their implications in space plasma measurements
Authors: Chugunov Y. V. , Mareev E. A.
Published: February 2004

Recent studies emphasize the significance of antenna‐plasma coupling consideration when analyzing the results of space plasma measurements, obtained with the Langmuir probe technique. We investigate the current‐voltage (I−V) and charge‐voltage (Q−V) curves characterizing antenna‐plasma coupling and calculate probe impedance and harmonic magnitudes under different physical conditions. The effects of absorption or reflection of charged particles by the antenna surface are taken into account. Impedance dependence on the bias probe voltage V0 and the RF signal amplitude V1 is studied. We give simple analytical formulas and numerical results for either a big or a small (with respect to the Debye length) spheric probe in isotopic plasma. I−V curve nonlinearities due to electron current exponential growth and saturation and photoelectron current contribution are considered. The main attention is paid to Q−V curve calculation: a problem which is poorly studied up to now. It is shown, in particular, that the electric capacitance of the absorbing spheric probe at low probe‐plasma voltage equals C = 1.25a(a/λD)2/3 (where a and λD are the probe radius and Debye length, respectively) instead of the commonly used expression C = a(a/λD). Q − V curve nonlinearity due to the complicated space charge distribution around the absorbing probe (which could be described as an effective increase of the probe sheath thickness at high voltage) is investigated. It is shown that the role of particles trapped by the probe near field can be substantial for a small‐probe capacitance estimation.
VHF and HF sea echo Doppler spectrum for a finite illuminated area
Authors: Hisaki Yukiharu , Tokuda Masayuki
Published: February 2004

The prediction of the sea echo Doppler spectrum of VHF and HF radio waves is important for the ocean wave remote sensing by VHF and HF radars. The authors extended the theory of the sea echo Doppler spectrum. In previous theories it is assumed that the scale of the illuminated area is much larger than the dominant wavelength. However, the effect of the finite illuminated area may not be negligible. The authors considered the effect to predict second‐order sea echo Doppler spectrum and extended previous theories. Furthermore, the authors developed the numerical technique to compute radar cross sections of the extended theory. As a result, the agreement between measured and predicted Doppler spectra is improved.
Diffraction of an obliquely incident plane wave by a two‐face impedance half plane: Wiener‐Hopf approach
Authors: Lüneburg Ernst , Serbest A. Hamit
Published: February 2004

The problem of diffraction of plane electromagnetic waves at an imperfectly conducting half plane with different impedances on upper and lower faces for oblique (skew) incidence either leads to a Wiener‐Hopf equation with a 4×4 Fourier symbol matrix for the tangential field components or to two formally decoupled Wiener‐Hopf equations with 2×2 symbol matrices of the Daniele‐Khrapkov form for the electric and magnetic field components perpendicular to the diffracting edge. The higher‐order edge singularity of the normal field components leads to undetermined constants in the classical Wiener‐Hopf solution that are used to eliminate “unphysical” leaky wave poles that appear in the final solution by the residue calculus technique. The interrelation between both formulations involves an analytic family of polynomial transformation matrices. Consideration of the range restriction of this mapping is shown to be equivalent to the pole elimination procedure.
The difference between Doppler velocity and real wind velocity in single scattering from refractive index fluctuations
Authors: Tatarskii V. I. , Muschinski A.
Published: February 2004

Scattering of electromagnetic waves by turbulence is considered. It is shown that Doppler velocity (the ratio of Doppler shift and Bragg wave number) includes two terms: The first is the mean radial velocity in the scattering volume, and the second is what we call the “correlation velocity,” caused by a correlation between the fluctuations of velocity and those of refractive index. The correlation velocity is proportional to the imaginary part of the Bragg wave vector component of the three‐dimensional spatial cross spectrum of refractive index and radial wind velocity. In the electromagnetic case in the atmosphere the correlation velocity provides information about the combination of humidity and heat fluxes. In the acoustic case it provides information about the flux of sensible heat in the atmosphere or about some combination of salinity and heat fluxes in water. In situ measurements of the temperature, humidity, and wind fluctuations in the atmosphere are analyzed to estimate the magnitude of the correlation velocity. It appears that the correlation velocity, which cannot be directly measured with existing radar wind profilers, could be separated from the total Doppler shift by means of bistatic radars. A potential application is the profiling of fluxes using bistatic systems.
Total solar eclipse effects on VLF signals: Observations and modeling
Authors: Clilverd Mark A. , Rodger Craig J. , Thomson Neil R. , Lichtenberger János , Steinbach Péter , Cannon Paul , Angling Matthew J.
Published: February 2004

During the total solar eclipse observed in Europe on August 11, 1999, measurements were made of the amplitude and phase of four VLF transmitters in the frequency range 16–24 kHz. Five receiver sites were set up, and significant variations in phase and amplitude are reported for 17 paths, more than any previously during an eclipse. Distances from transmitter to receiver ranged from 90 to 14,510 km, although the majority were <2000 km. Typically, positive amplitude changes were observed throughout the whole eclipse period on path lengths <2000 km, while negative amplitude changes were observed on paths > 10,000 km. Negative phase changes were observed on most paths, independent of path length. Although there was significant variation from path to path, the typical changes observed were ∼3 dB and ∼50°. The changes observed were modeled using the Long Wave Propagation Capability waveguide code. Maximum eclipse effects occurred when the Wait inverse scale height parameter β was 0.5 km−1 and the effective ionospheric height parameter H′ was 79 km, compared with β=0.43 km−1 and H′=71 km for normal daytime conditions. The resulting changes in modeled amplitude and phase show good agreement with the majority of the observations. The modeling undertaken provides an interpretation of why previous estimates of height change during eclipses have shown such a range of values. A D region gas‐chemistry model was compared with electron concentration estimates inferred from the observations made during the solar eclipse. Quiet‐day H′ and β parameters were used to define the initial ionospheric profile. The gas‐chemistry model was then driven only by eclipse‐related solar radiation levels. The calculated electron concentration values at 77 km altitude throughout the period of the solar eclipse show good agreement with the values determined from observations at all times, which suggests that a linear variation in electron production rate with solar ionizing radiation is reasonable. At times of minimum electron concentration the chemical model predicts that the D region profile would be parameterized by the same β and H′ as the LWPC model values, and rocket profiles, during totality and can be considered a validation of the chemical processes defined within the model.
On generation of an assembly of images in ionospheric tomography
Authors: Andreeva E. S. , Franke S. J. , Yeh K. C. , Kunitsyn V. E. , Nesterov I. A.
Published: February 2004

Radio tomography experiments have demonstrated the promising potential of applying tomographic methods in imaging various ionospheric structures. In actual implementation of image reconstructions one is faced with many choices, which include the following: whether to use the total phase, relative phase, or Doppler as the projection data, how to approximate the projection operator, what inversion algorithm to employ, and the choice of how to include the ancillary data and constraints on the constructed image. Each choice results in an image compatible with the given or measured projection data, yet each choice results in an image different from that of the others, with its own attendant artifacts and distortions. Collectively, the images produced by all the possible choices comprise an assembly of images. In this simulation study of one ionospheric model, 113 members of such an assembly are generated. All images look similar in gross features with a root‐mean‐square deviation not more than 29% from the mean. As expected, the largest deviation occurs near the region of highest gradients. By averaging all of the images in the assembly we show that the mean image is superior because of its smallest root‐mean‐square deviation from the true image. This conclusion, drawn on the simulation study of one model, may in fact have a general applicability, and we discuss why this may be so.
Simple ray tracing of Galileo‐observed hectometric attenuation features
Authors: Higgins Charles A. , Thieman James R. , Fung Shing F. , Green James L. , Candey Robert M.
Published: February 2004

Observations of persistent structural features within Jovian hectometric (HOM) radio emission have been made with the Galileo spacecraft. Two well‐defined sinusoidal‐shaped “band” features of reduced emission intensity and occurrence probability exist at all Jovian longitudes and nearly cover the entire spectrum of HOM radio emission from ∼500 kHz to 3000 kHz. These two sinusoidal lanes have a bandwidth of 200–400 kHz and are 180° out of phase with one another, suggesting that they are a result of HOM radio emission propagation processes from opposite hemispheres. These features become more apparent when presented as intensity or occurrence probability spectrograms added together over multiple Jovian rotations. Enhancements in the HOM intensity and occurrence are seen along the edges of one of the observed sinusoidal lane features which may indicate caustic surfaces due to refraction along the propagation path. We present some simple ray tracing analyses to show that refraction from density enhancements in the Io torus flux tube may explain some of the observations. Using this simple method, we approximate the density enhancements in the Io flux tube to be 100 cm−3.
Cosmic background at long wavelengths
Authors: Tokarev Y. V.
Published: February 2004

The interpretation of the averaged spectrum of the cosmic background at long (meter, decameter, and hectometer) wavelengths with a cloudy model of the diffuse ionized gas of the galaxy is presented. It is concluded that there is strong spatial variation of synchrotron emissivity in the interstellar medium. It was found that spectrum of extragalactic background radiation outside the galactic absorption disk does not have an evident low‐frequency depression at least down to 1 MHz.
Simulation of induction‐logging response using conjugate gradient method with nonuniform fast Fourier and fast Hankel transforms
Authors: Zhang Zhong Qing , Liu Qing Huo
Published: February 2004

Previously, the conjugate gradient method combined with fast Fourier and fast Hankel transform (CG‐FFHT) was developed to solve an integral equation for borehole induction measurements in axisymmetric media. In the CG‐FFHT method, the regular fast Fourier transform algorithm uses equal‐spaced sample points, while the fast Hankel transform algorithm requires the sample points to distribute uniformly on a logarithmic scale. The uniform grid limits the utility of the CG‐FFHT method since it is not the most efficient way to discretize a problem, especially when there are fine structures in the geometry. These limitations are removed in this work by the use of the newly developed nonuniform fast Fourier transform (NUFFT) and nonuniform fast Hankel transform (NUFHT) algorithms. The combination of the CG procedures and NUFFT and NUFHT leads to the CG‐NUFFHT method. It retains the computational efficiency of the CG‐FFHT method but has the flexibility of a nonuniform grid. Excellent agreement is shown between the CG‐NUFFHT results and those from the numerical mode‐matching method.
Direct measurements on imaging riometer antenna array beam directivities
Authors: Wilson A. , Nel J. J. , Mathews M. J. , Stoker P. H.
Published: February 2004

Spatial structures in enhanced ionization of the ionosphere are observed by absorption of cosmic radio waves. These structures are resolved by using theoretically derived imaging riometer antenna array directivities. These directivities are calculated from beam phasing of 64 crossed dipole elements of the 38.2‐MHz antenna array at SANAE IV, Antarctica. In order to ensure that these derived directivities are representative of the actual viewing directions of the 64‐beams, a radio transmitter was flown by helicopter across the antenna array. In this paper variations in the receiver signal strengths, recorded when flying across beam‐viewing directions, are compared with the spatial and angular‐dependent profiles of expected receiver output responses, derived theoretically from the directivities of the antenna array. A Global Positioning System (GPS) device on board the helicopter was used for positional recording. The derived and recorded profiles did coincide occasionally, but at other instances relative displacements and differences in magnitude of responses were observed. These displacements and differences could be attributed to degradation in position fixes imposed deliberately by selective availability on the GPS system. Excellent coincidence for a number of beam crossings proved that the viewing directions are accurate in all the beam directions, since the multi‐dimensional Butler matrix produces 64 simultaneous beams.
Search for light ion outflow by incoherent scatter radar
Authors: Løvhaug U. P. , Hagfors T. , van Eyken A. P.
Published: February 2004

Regular observation of the polar wind was one of several measurements considered feasible when the European Incoherent Scatter (EISCAT) auroral zone incoherent scatter facility was first conceived. Whereas most of the other measurements proposed with EISCAT have been successful, direct observations of the polar wind using the standard incoherent scatter radar analysis schemes have proven to be elusive. This paper presents the first direct measurement of the H+ velocity by a ground‐based instrument situated at high latitudes, where thermal outflow of light ions occurs. We have avoided the difficulties associated with standard analysis by optimizing the detection method to emphasize only those features of the spectrum sensitive to the drift of the O+ and H+ ions. We have tested the analysis procedure on EISCAT VHF observed spectra. The data analyzed show unmistakable evidence of differences in drift velocities of the two major ion populations. The results indicate considerable variation in the drift velocity of the light ion component. The light ion velocity shows an overall increase with altitude consistent with H+ outflows predicted by thermal polar wind models. The observed H+ velocities and the prospects for using the procedure on a routine basis are discussed.
A regional GPS receiver network for monitoring equatorial scintillation and total electron content
Authors: Thomas R. M. , Cervera M. A. , Eftaxiadis K. , Manurung S. L. , Saroso S. , Effendy  , Ramli A. G. , Hassan W. Salwa , Rahman H. , Dalimin M. N. , Groves K. M. , Wang Y.
Published: February 2004

A network of Global Positioning System (GPS) single‐frequency (L1) receivers has been installed in Australia and Southeast Asia for the purpose of monitoring equatorial ionospheric scintillation during the current peak in solar cycle activity. Dual‐frequency receivers to measure total electron content are also operating at some stations and will be described in a later paper. With respect to equatorial scintillation our long‐term intention is to characterize its statistical properties and its effect on GPS link performance within the Asian‐Australian longitude zone (approximately from 100° to 150° E) as a function of time of day, season, sunspot number, and magnetic latitude, for comparison with available models such as WBMOD and in order to quantify the potential for navigational degradation, especially during times of elevated solar activity. In this paper, the scintillation receiver network is described, and preliminary results are presented from the first two years of operation (1998 and 1999). Our initial results include measurements of diurnal and seasonal variations in S4 and an analysis of the performance of multiple satellite links which indicates that just prior to solar maximum, GPS can approach marginal performance because of scintillation.
Use of GPS for estimation of bending angles of radio waves at low elevations
Authors: Sokolovskiy Sergey V. , Rocken Christian , Lowry Anthony R.
Published: February 2004

The paper compares three methods of calculating the bending angles of radio waves propagated from space to a ground‐based receiver: (1) from refractivity climatology corrected for refractivity at the receiving antenna, (2) from radiosonde refractivity profiles, and (3) from the refractivity at the antenna and the measured Doppler frequency shift of the GPS signals. The methods are tested with the use of radiosonde and GPS observations collocated in space and in time. We analyzed seven cases during October‐November 1999 where GPS satellites were observed to below 0.5° elevation from Point Loma, California, and which coincided closely in time with radiosonde launches from the nearby Miramar station. In all cases the bending angles calculated from Doppler and from radiosondes agree fairly well at all elevations, but in a number of cases both differ significantly at low elevations from the bending angles calculated from climatology corrected for the refractivity at the antenna. Thus GPS has the potential of being used for the correction of radar observations at low elevations instead of (or complementary to) radiosondes. The differences between the bending angles calculated from climatology corrected for the refractivity at the antenna and those calculated from the Doppler frequency shift indicate anomalies in the refractivity profile in the lower troposphere and can thus be used as an indicator of ducting conditions.
High‐order window functions and fast algorithms for calculating dyadic electromagnetic Green's functions in multilayered media
Authors: Yu Tiejun , Cai Wei
Published: February 2004

A type of bell‐shaped compact high‐order window function ψ( x, y ) is studied, and closed‐form and approximation formulas are derived for its Hankel transforms. Applications of these results are presented as fast algorithms for calculating dyadic Green's functions for electromagnetic scattering in multilayered media. Numerical results in electromagnetics are provided to demonstrate the accuracy and efficiency of the algorithms.
Cylindrical vector wave function representation of Green's dyadics for uniaxial bianisotropic media
Authors: Li Le‐Wei , Lim Nam‐Hoe , Leong Mook‐Seng , Yeo Tat‐Soon , Kong Jin Au
Published: February 2004

This paper presents a novel and rigorous eigenfunction expansion of electric‐type dyadic Green's function for an unbounded uniaxial bianisotropic medium in terms of the cylindrical vector wave functions. The Green's dyadic is obtained on the basis of the well‐known Ohm‐Rayleigh method together with some newly developed vector and tensor identities formed by the differential, curl, and dot product of the constitutive dyadics and the cylindrical vector wave functions. The above identities greatly simplify the process of finding the vector expansion coefficients of the dyadic Green's function of the uniaxial bianisotropic media. The dyadic Green's function derived is expressed in terms of the contribution from the irrotational solenoidal types of vector wave functions, with the λ integrals removed using the residue theorem.
Guided electromagnetic waves observed on a conducting ionospheric tether
Authors: James H. G. , Balmain K. G.
Published: February 2004

On the up leg of its flight through the auroral nightside ionosphere to an apogee of 824 km, the tethered double payload Observations of Electric Field Distributions in the Ionospheric Plasma: A Unique Strategy (OEDIPUS) C was the site of experiments on wire‐guided electromagnetic (EM) waves. Waves were transmitted from the upper subpayload to a receiver on the lower subpayload along a conducting wire aligned within a few degrees of the Earth's magnetic field. Such EM waves were observed at almost all frequencies in the range 0.1–8.0 MHz. There was a deep stop band between the cyclotron and upper hybrid resonance frequencies where the cold plasma theory predicts a propagation cutoff, and there were shallower attenuation bands at frequencies where hot‐plasma electrostatic waves may affect the guided EM modes. Resonances of the wire‐guided waves with the tether length were observed throughout the entire tethered portion of the flight. The resonances appear as a set of fringes when all the data are presented in a frequency‐versus‐time summary. The fringe shapes in this summary have been compared with the predictions of an early theory, which give generally good agreement. The exceptions are frequencies close to the stop band, where cold‐plasma dispersion effects are expected to be greatest. Another theory based on a different derivation of the dispersion relation includes a vacuum sheath gap outside the conductor. The absolute fringe intensities and positions predicted agree moderately well with the observations.
Transverse electric scattering from a gap on a circular waveguide
Authors: Park Jong K. , Eom Hyo J.
Published: February 2004

Electromagnetic transverse electric wave scattering from a circumferential gap on a conducting circular waveguide is theoretically investigated. The Fourier transform/series technique is used to constitute a set of simultaneous equations for the discrete modal coefficients. Residue calculus is applied to obtain a fast‐convergent series solution that is amenable to the numerical computations. Extensive calculations are performed to examine the effects of a gap on the radiation and scattering.
A numerical approach for the evaluation of the nonlinear effects on the attenuation constant in high‐temperature superconducting transmission lines
Authors: Caorsi S. , Donelli M. , Massa A. , Pastorino M.
Published: February 2004

Superconducting materials exhibit an experimentally verified nonlinear dependence with respect to the magnetic field. In this paper, this nonlinearity is taken into account in the evaluation of the attenuation constant in propagating structures of practical usage. Quadratic and cubic nonlinearities are considered, and an iterative numerical procedure is applied to calculate the attenuation constant. The nonlinearity in the penetration depth is also considered. In section 3, some typical structures are investigated. In particular, parallel‐plane transmission lines filled by dielectric materials, microstrip lines, and striplines are considered. Comparisons with exsisting results show that this nonlinear behavior cause significant changes in the attenuation parameters.
A reappraisal of the Sibson‐Scholz fault zone model: The nature of the frictional to viscous (“brittle‐ductile”) transition along a long‐lived, crustal‐scale fault, Outer Hebrides, Scotland
Authors: Imber J. , Holdsworth R. E. , Butler C. A. , Strachan R. A.
Published: February 2004

The widely cited Sibson‐Scholz conceptual fault zone model suggests that seismically active, upper crustal brittle faults pass downward across a predominantly thermally controlled transition at 10–15 km depth into ductile shear zones in which deformation occurs by aseimic viscous creep. The crustal‐scale Outer Hebrides Fault Zone (OHFZ) in NW Scotland has been described as the type example of such a continental fault zone. It cuts Precambrian basement gneisses and is deeply exhumed, allowing direct study of the deformation products and processes that occur across a wide range of crustal depths. A number of fault rock assemblages are recognized to have formed during a long‐lived displacement history lasting in excess of 1000 Myr. During Caledonian movements that are recognized along much of the 190 km onshore fault trace, brittle, cataclasite‐bearing faults in the west of the OHFZ are unequivocally overprinted to the east by a younger fabric related to a network of ductile shear zones. Field observations and regional geochronological data demonstrate that there is no evidence for reheating of the fault zone due to thrust‐related crustal thickening or shear heating. Microstructural observations show that the onset of viscous deformation was related to a major influx of hydrous fluids. This led to retrogression, with the widespread development of new finegrained phyllosilicate‐bearing fault rocks (“phyllonites”), and the onset of fluid‐assisted, grain size‐sensitive diffusional creep in the most highly deformed and altered parts of the fault zone. Phyllonitic fault rocks also occur in older, more deeply exhumed parts of the fault zone, implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature (depth) range within crustal‐scale faults. Our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal‐scale faults containing interconnected networks of phyllosilicate‐bearing fault rocks will be characterized by long‐term relative weakness and shallow (∼5 km) frictional‐viscous transition zones. Similar processes acting at depth may provide an explanation for the apparent weakness of presently active structures such as the San Andreas Fault.
Metamorphic soles from the Albanian ophiolites: Petrology, 40Ar/39Ar geochronology, and geodynamic evolution
Authors: Dimo‐Lahitte A. , Monié P. , Vergély P.
Published: February 2004

In the eastern Mediterranean region the Albanian ophiolites are remnants of one branch of the Tethyan oceanic basin obducted on carbonate margins during the Mesozoic. Pressure, temperature, time, and structural data in the metamorphic soles of these ophiolites place new constraints on the closure of this basin and the emplacement history of oceanic rocks. The petrological and structural work documents an “apparently inverted metamorphic gradient” with granulite facies at the top, decreasing downward through amphibolite, then greenschist facies until unmetamorphosed rocks. Thermobarometry indicates that the granulite facies formed at peak temperatures of 800°–860°C for pressures close to 1 Gpa. These conditions point to the subduction of the ophiolites beneath young oceanic lithosphere to a depth of 30–40 km. A large set of 40Ar/39Ar data has been obtained on the metamorphic soles and some ophiolite intrusives using single grains and bulk mineral populations. Ages range from 160 to 174 Ma, corresponding to the middle Jurassic (Bajocian‐Bathonian). Within the same sole the convergence of results using single grains and mineral populations of amphibole and muscovite and the lack of internal age gradient within muscovite are interpreted to result from fast cooling of metamorphic soles. The main result documented by this study is a systematic younging of metamorphic sole ages from south to north, with a difference of 14 Ma along the 140 km length of the belt. The age of igneous rocks, such as plagiogranites and mafic dikes in the ophiolites, is equivalent to that of metamorphic soles, which indicates that the ophiolite was still hot and young at the time of metamorphic sole formation. The emplacement of the Albanian ophiolites is the consequence of a complex tectonic evolution: continental rifting, slow mid‐ocean spreading followed by west dipping intraoceanic subduction initiated at the ridge axis, symmetric intraoceanic thrusting with opposite vergence, and at last, emplacement onto the continental margin. The geochronological and stratigraphic data suggest that this evolution lasted only a few million years. Moreover, they indicate that this evolution stopped earlier to the south with the closure of a narrow basin than to the north, where the basin was wider due to faster oceanic spreading.
Development of volcanic passive margins: Two‐dimensional laboratory models
Authors: Callot Jean‐Paul , Grigné Cécile , Geoffroy Laurent , Brun Jean‐Pierre
Published: February 2004

Volcanic margins are inferred to develop during lithosphere extension above mantle plumes. Continental breakup is characterized in such case by (1) thick seaward dipping lava sequences, (2) plutonic complexes associated with dyke swarms parallel to the coast, and (3) zones of high seismic velocity in the lower crust likely attributable to magma underplating. Comparison with classical nonvolcanic passive margins shows that a striking but nonsystematic structural character of volcanic margins is the narrowness of the domain of crustal attenuation (down to 50 km). The existence of a soft magma body at depth may considerably affect the mechanical behavior of the lithosphere during continental breakup. Here we present a series of scaled experiments designed to study the mechanical effects on lithospheric extension of rheological heterogeneities caused by magma emplacement at various levels. Four‐layer models were constructed with sand and silicone putties in order to represent the brittle and ductile layers of both crust and lithospheric mantle. The underplated magma bodies were simulated by low‐viscosity silicone putty with variable geometry and location. The experimental results are compared to interpreted refraction seismic profiles across volcanic margins in the North Atlantic. A narrow zone of necking is obtained only when the high‐strength layer of the sub‐Moho mantle is interrupted by a heterogeneity of low viscosity representing an underplated magma body. As in cross sections of volcanic margins, models show weak deformation in the brittle upper crust. Sequential addition of sand in the rifted area during extension results in the development of layers dipping toward the rift center. This geometrical pattern is directly comparable to the seaward dipping reflectors sequences. However, the normal faults which developed within the seaward dipping wedge have vergence opposite to the one currently observed in volcanic margins. This suggests a possible component of mantle‐generated stresses during extension.
Nonaqueous‐phase‐liquid dissolution in variable‐aperture fractures: Development of a depth‐averaged computational model with comparison to a physical experiment
Authors: Detwiler Russell L. , Rajaram Harihar , Glass Robert J.
Published: February 2004

Dissolution of nonaqueous‐phase liquids (NAPLs) from variable‐aperture fractures couples fluid flow, transport of the dissolved NAPL, interphase mass transfer, and the corresponding NAPL‐water‐interface movement. Each of these fundamental processes is controlled by fracture‐aperture variability and entrapped‐NAPL geometry. We develop a depth‐averaged computational model of dissolution that incorporates the fundamental processes that control dissolution at spatial resolutions that include all scales of variability within the flow field. Thus this model does not require empirical descriptions of local mass transfer rates. Furthermore, the depth‐averaged approach allows us to simulate dissolution at scales that are larger than the scale of the largest entrapped NAPL blobs. We compare simulation results with an experiment in which we dissolved residual entrapped trichloroethylene (TCE) from a 15.4×30.3 cm, analog, variable‐aperture fracture. We measured both fracture aperture and the TCE distribution within the fracture at high spatial resolution using light transmission techniques. Digital images acquired over the duration of the experiment recorded the evolution of the TCE distribution within the fracture and are directly compared with the results of a computational simulation. The evolution with time of the distribution of the entrapped TCE and the total TCE saturation are both predicted well by the dissolution model. These results suggest that detailed parametric studies, employing the depth‐averaged dissolution model, can be used to develop a comprehensive understanding of NAPL dissolution in terms of parameters characterizing aperture variability, phase structure, and hydrodynamic conditions.
Effective dispersivities for a two‐dimensional periodic fracture network by a continuous time random walk analysis of single‐intersection simulations
Authors: Grubert D.
Published: February 2004

Fracture networks are of major importance for transport problems in hydrology. Their inclusion in models requires assumptions about both the single fracture and the network structure. This paper focuses on the macroscopic effect of microscopic mixing conditions in intersections within a periodic fracture network. Mixing is characterized by the transition probabilities for particles to move from one fracture to another or to stay in the same fracture. A periodic network is used, which excludes stochastic effects and also single‐fracture dispersion. The periodic network can be solved analytically and is applicable to a variety of network structures. We find that for this type of network, effective dispersivities in the sense of asymptotic values for long times and large scales do usually exist (i.e., the asymptotic dispersion is Gaussian). For the particular case of a quadratic network, particle transition probabilities at an intersection have been obtained by microscopic lattice Boltzmann simulations of flow and transport in addition to the assumptions of complete mixing and stream routing. The resulting dispersivities show that complete mixing is not a low Péclet number limit in the presence of regions of immobile water volumes. Dispersivities depend strongly on the direction between flow and network and on the mixing model; dispersivities are large for high Péclet numbers and flow parallel to the network. Especially for these cases, we also find a scale dependence up to some orders of magnitude, which has serious implications for the interpretation of simulations and measurements. Since these results also appear in stochastic network simulations, they are therefore not specific to the periodic structure of the fracture network.
Modeling flow in phreatic and epiphreatic Karst conduits in the Hölloch Cave (Muotatal, Switzerland)
Authors: Jeannin Pierre‐Yves
Published: February 2004

The Hölloch cave is a site where the hydrodynamic behavior of a karst conduit network can be observed with a high degree of precision. Observed heads, discharge rates, conduit sizes, and conduit lengths have been compiled into a simple hydrodynamic model in order to check their consistency. It was possible to calibrate and satisfactorily fit the observed data. Model results show the following: (1) Flow models which are able to simulate turbulent flow in variably saturated conduit networks can adequately model conduit flow–dominated karst systems. (2) Karst systems may be strongly nonlinear, especially because of the presence of epiphreatic conduits. (3) Under certain circumstances, storage in the epiphreatic conduits and in the fissured limestone matrix can be neglected. (4) The typical effective hydraulic conductivity of karst conduits ranges between 1 and 10 m s−1, and the Louis formula is adequate to calculate head losses in those conduits. (5) Indirect measurements of flow velocity using scallop size indicate values of ∼30–40% of the maximal annual discharge, and velocity derived from pebble size indicates values of ∼150% of the maximal annual discharge.
Hydrologic‐Response simulations for the R‐5 catchment with a comprehensive physics‐based model
Authors: VanderKwaak Joel E. , Loague Keith
Published: February 2004

For approximately 20 years, there has been a concerted effort, by several different research groups, to simulate observed rainfall‐runoff events from the well‐known R‐5 catchment, located near Chickasha, Oklahoma. These prior simulation efforts, with relatively simple models of Horton‐type overland flow, have not been entirely successful, as the streamflow generation process for the R‐5 catchment, as now recognized, may not be totally dominated by the Horton mechanism. In the effort reported here, a new fully coupled comprehensive physics‐based hydrologic‐response model, the Integrated Hydrology Model (InHM), is tested for two R‐5 rainfall‐runoff events. The InHM simulations in this study clearly show, in a hypothesis‐testing mode, that both the Horton and Dunne overland flow mechanisms can be important streamflow generation processes for R‐5 events. The InHM simulations reported here also suggest that accurate accounting of soil water storage can be as important as exhaustive characterization of spatial variations in near‐surface permeability.
Release and transport of colloidal particles in natural porous media: 2. Experimental results and effects of ligands
Authors: Grolimund Daniel , Barmettler Kurt , Borkovec Michal
Published: February 2004

We present an extensive experimental data set of particle release from natural porous media saturated with monovalent cations. The generation process of mobile colloidal particles is studied by means of leaching of saturated laboratory columns packed with a noncalcareous soil material with various monovalent electrolytes and by analyzing the colloids in the effluent over typically 1000 pore volumes. The concentration of released particles cannot be modeled with simple first‐order kinetics but can be rationalized in terms of a distribution of release rate coefficients k. The experimentally observed effluent concentration often decays with time as a power law c ∝ t−(α+l) for long times, suggesting a distribution of release rate coefficients p(k) ∝ kα−1 for small k. The observed values of exponent α range between 0.01 and 0.8. The composition of the pore water is found to have a profound influence on the particle release characteristics. With decreasing salt concentration the rate for particle release increases. Anionic organic and inorganic ligands have a major effect on the release process. For the ligands studied, the amount of released particles decreases in the sequence malonate, chloride, phtalate, and azide.