bluuebunny/crossref_metadata_2025 huggingface dataset:
A review of critical ionization velocity
Authors: Lai Shu T.
Published: February 2004

This paper reviews the critical ionization velocity (CIV) phenomenon, with inclusion of recent research. CIV, suggested by Alfvén in 1954 as part of a larger cosmological theory accounting for the formation of the solar system, is controversial in that laboratory and space experiments to confirm its validity have yielded conflicting results. Theoretical analysis has suggested that the driving mechanism for CIV is some form of beam‐plasma instability, such as the lower hybrid instability, which leads to rapid energization of ambient electrons so that they gain enough energy to ionize the beam of neutral atoms by electron impact. The newly created beam ions energize the instability, thus fostering a cyclic process that may lead to an avalanche ionization. Because the implications of this process, if correct, are widespread, it has become important to establish a theoretical framework for its presence and its occurrence. This framework includes a variety of microscopic chemical and physicochemical processes, such as line excitation, formation of metastable states by electron impact, ionization of metastable states by electron impact, and ion‐electron recombination in the case of molecular ions. These reactions may occur in CIV experiments both in the laboratory and in space. Numerical computer models have been able to not only simulate CIV but also reveal details in nonlinear plasma evolution together with electron impact ionization of the neutral particles. Theories, laboratory experiments, and computer simulations have all shown CIV as feasible and reasonably understood, although all CIV experiments in space have yielded negative results with perhaps three exceptions. In the CIV experiments in space, not only the ionization yields were low, but also non‐CIV processes such as charge exchange, associative ionization, and stripping ionization may have occurred and may be easily mistaken as CIV. We also discuss the conditions under which the laboratory and the space experiments are carried out and highlight the differences, some of which may explain the different results.
Polar mesosphere summer echo studies at 51.5 MHz at Resolute Bay, Canada: Comparison with Poker Flat results
Authors: Huaman Mercedes M. , Kelley Michael C. , Hocking Wayne K. , Woodman Ronald F.
Published: February 2004

We present a study of seasonal and diurnal variations of polar mesosphere summer echoes (PMSE), along with temperature and wind data collected during summer 1998 using the 51.5 MHz VHF radar located in Resolute Bay, Canada (75°N, 95°W). The latitudinal dependence of PMSE occurrence and strength are examined by comparing Resolute Bay observations with earlier Poker Flat (65°N, 147°W) observations. Temperatures measurements at Resolute Bay using the radar in the meteor mode are compared with satellite measurements at similar latitudes of Resolute Bay and Poker Flat. The temperature measurements are in good agreement, and, as expected, PMSE is strongly controlled by the seasonal temperature variations. Differences found in the daily composite and monthly average winds, and in the monthly evolution of the amplitudes and phases of the diurnal and semidiurnal tides at Resolute Bay and Poker Flat, are discussed in order to establish the role that they play in the occurrence of PMSE. In general, we find that the diurnal occurrence of PMSE at Resolute Bay also follows the diurnal variation of temperature. The echoes tend to occur more frequently and with greatest intensity when the temperatures are the coldest, in a period centered around local noon. We show that the diurnal wind components at Resolute Bay appear to correlate with the occurrence of PMSE, as also seen at Poker Flat. PMSE at Resolute Bay are not as strong or as frequent as reported at Poker Flat, differing with what we had expected since strongest and more frequent echoes have been observed in the European sector approaching the pole. New observations are needed to establish a possible longitudinal dependence of PMSE occurrence given that the present American sector data are compared with data taken over 10 years ago.
Resolute Bay VHF radar: A multipurpose tool for studies of tropospheric motions, middle atmosphere dynamics, meteor physics, and ionospheric physics
Authors: Hocking W. K. , Kelley M. , Rogers R. , Brown W. O. J. , Moorcroft D. , St. Maurice J.‐P.
Published: February 2004

A VHF radar has been established at a site near Resolute Bay in Nunavut, Canada (75°N, 95°W), which has the capability to make a variety of measurements relating to the atmospheric and ionospheric environment in the polar regions. The site is very close to the north geomagnetic pole, and therefore the radar is well situated to make some unique measurements. The system is a multipurpose instrument with good remote control capabilities. It can be used as a wind profiler radar to study the lower troposphere, as a mesospheric radar to study polar mesosphere summer echoes (PMSE) in summer, as a meteor radar to determine winds in the altitude region of 80–100 km, and as an ionospheric radar to study 3 m scale irregularities in the E and F regions. The radar has some unique design features, partly dictated by the rough terrain in which it is sited. In this paper, the radar system is described, including description of some unusual approaches to deal with special conditions at the site, and then some key early results are presented. Important findings include error determinations for tropospheric wind measurements, detection of PMSE, correlations between PMSE and atmospheric temperatures at 86 km altitude, measurements of mean winds and tidal characteristics over a full year, and detection of various normal modes of oscillation in the 80–100 km region, especially in nonsummer months. Some of these features will be discussed here, but more detailed discussions will be left to related papers in this issue.
Ionospheric E region remote sensing with ELF radio atmospherics
Authors: Cummer Steven A. , Inan Umran S.
Published: February 2004

Radio remote sensing of the ionospheric E region can be difficult, particularly in the nighttime E region valley which lies between two regions of higher electron density. We show that extremely low frequency (ELF) electromagnetic waves launched from below penetrate this region because of their low attenuation and can be reflected from both the D region and the top of the E region valley. This double reflection, also caused by sporadic E layers, produces a measurable effect on the subionospheric propagation of ELF waves, which we demonstrate with numerical Earth‐ionosphere waveguide simulations. This sensitivity opens the possibility of remotely sensing the E region and detecting sporadic E layers with broadband ELF propagation measurements using lightning discharges as the source. From nighttime observations of ELF lightning radiation over the same propagation path on different days, we extract E region electron density profiles that show significant variability not only from day to day but over the course of a single night. This technique is inherently a path‐integrated measurement, enabling tomographic large‐scale ionospheric remote sensing with multiple sources and receivers.
Radio wave propagation over a nonconstant immittance plane
Authors: Janaswamy Ramakrishna
Published: February 2004

The problem of electromagnetic wave propagation over a horizontal, nonconstant immittance plane, whose immittance value is a function of incident grazing angle, is considered. An equivalent specification of the immittance surface is in terms of its angle‐dependent reflection coefficient. Expressions are provided for the field on a vertical line given the field on a previous vertical line. The vertical line field is initialized at the plane containing the source where its aperture current distribution is specified. Both two‐dimensional and three‐dimensional fields are considered, and the expressions are valid for either polarization. The form of expressions is particularly suited for implementing with the Fourier split‐step algorithm of the parabolic wave equation. Extension to inhomogeneous atmosphere to account for mild atmospheric inhomogeneities is presented. Several examples are considered where the immittance arises from small‐scale and large‐scale surface roughnesses. A numerical procedure is described wherein incomplete or approximate reflection coefficient data are made to conform to the assumptions made in the development of the expressions. This is demonstrated for a surface reflection coefficient which is governed by the Miller‐Brown‐Vegh roughness reduction factor. Numerical results are presented for propagation under ducting conditions over a rough surface for frequencies from HF through microwave.
Wideband model of HF atmospheric radio noise
Authors: Lemmon John J.
Published: February 2004

A model of the waveform generated by high‐frequency atmospheric radio noise is presented. Cumulative probability distributions of the noise envelope are derived and shown to be in good agreement with a large database collected from a wide range of noise environments. The model includes correlations in the waveforms that simulate the burst structure of measured atmospheric noise. The bandwidth dependence of the voltage deviation parameter, which parameterizes the impulsiveness of the noise, shows behavior that is qualitatively similar to a limited amount of measured data.
Penetration of an electromagnetic wave above a reflection point layer
Authors: Krasnov V. M.
Published: February 2004

A generalized solution of the problem of electromagnetic wave propagation in a linear dielectric layer indicates that variation of a phase of an incident wave varies the height locations of the nodes and antinodes of the standing wave under the reflection point. The depth of electromagnetic wave penetration above the reflection point additionally depends on the initial phase of the incident wave. Thus there is an opportunity to control wave penetration above a reflection point.
Microwave Radiometer for Cloud Carthography: A 22‐channel ground‐based microwave radiometer for atmospheric research
Authors: Crewell Susanne , Czekala Harald , Löhnert Ulrich , Simmer Clemens , Rose Thomas , Zimmermann Ralf , Zimmermann Rüdiger
Published: February 2004

A 22‐channel Microwave Radiometer for Cloud Carthography (MICCY) for the profiling of tropospheric temperature, humidity, and cloud liquid water has been developed. The radiometer has 10 channels along the high‐frequency wing of the 22.235 GHz water vapor line, 10 channels along the low‐frequency side of the 60 GHz oxygen complex, and 2 channels at 90 GHz. Two features make the radiometer a unique tool for the observation of small‐scale structures related to cloud processes: (1) Atmospheric brightness temperatures are measured simultaneously at all channels with an integration time of 1 s. (2) A Cassegrain system including a 90 cm off‐axis parabolic mirror leads to a spatial resolution better than 1° full width at half maximum for all frequencies. The necessity of these features for cloud observations is demonstrated. Algorithms for the retrieval of integrated water vapor, integrated cloud liquid water, and the profiles of temperature and humidity were developed on the basis of artificial neural networks. Measurement examples, including comparisons with radiosondes, demonstrate the systems capabilities for high‐resolution atmospheric monitoring.
An algorithm for inferring the two‐dimensional structure of the nighttime ionosphere from radiative recombination measurements
Authors: Dymond K. F. , Thomas R. J.
Published: February 2004

We present a technique for retrieving the two‐dimensional structure of the ionosphere using the measured variations of ultraviolet emissions produced by radiative recombination at 911 and 1356 Å as measured by satellite‐based limb scanning instruments. The algorithm uses an iterative scheme based on discrete inverse theory to determine the best fit to the data. We present the results of a simulation that demonstrates the algorithm's performance for an inversion covering the nighttime half of the orbit. The algorithm was tested using “data” generated with the international reference ionosphere (IRI‐90; Bilitza [1990]). The algorithm accurately retrieved the nighttime F region electron density.
Overview of the 1998 Caribbean Ionospheric Campaign
Authors: Thonnard S. E. , McCoy R. P.
Published: February 2004

The Office of Naval Research and the Naval Research Laboratory have coordinated the first of a series of scientific campaigns to gather ionospheric measurements to characterize the dynamics of space weather. The campaign participants include several universities, government agencies, and private corporations. The focus of the campaign is to gather high‐quality, spatial and temporal coincident data to study midlatitude ionospheric specification, compare various ionospheric measurement techniques, and lay the groundwork for validating new ionospheric monitoring instruments and models. The location of the campaign was chosen to further investigate midlatitude ionospheric depletions previously observed above Puerto Rico. In addition to direct comparisons an ionospheric specification model currently under construction will assimilate the data set collected during these campaigns. This paper presents an overview of the first campaign that occurred in the Caribbean, centered about Puerto Rico, in June 1998. The paper also includes a brief description of the variety of monitoring techniques used and the preliminary results of the comparison.
Heliospheric current sheet effect on propagation of type II interplanetary radio bursts from coronal mass ejections
Authors: Ivanov Kim G. , Romashets Eugene P.
Published: February 2004

In this paper, we present our hypothesis that a disturbed heliospheric current sheet and heliospheric plasma sheet can exert a very strong influence on propagation of type II radio waves through interplanetary space. We make a conjecture that these layers become more dense and the critical frequency becomes higher, so radio emission induced in the bow shock region in front of the coronal mass ejection is reflected or deflected more strongly. The idea is illustrated by two examples of type II radio bursts: in January 1997 and in May 1997. In the first case, the effect of the heliospheric current sheet and heliospheric plasma sheet was minimal because they were some 30° below the ecliptic plane and did not play an important role in the propagation of radio waves to the Earth, while during the second event the layers were located in the way of radio waves, and attenuated them strongly.
Wave scattering from lossy dielectric random rough surfaces using the physics‐based two‐grid method in conjunction with the multilevel fast multipole method
Authors: Li Qin , Tsang Leung
Published: February 2004

To study wave scattering from random lossy dielectric rough surfaces with large permittivities using the method of moments, a dense grid is needed for accurate results. A dense grid requires more CPU and memory. The physics‐based two‐grid (PBTG) method can reduce both CPU and memory requirements. In this paper, the PBTG is used in conjunction with the multilevel fast multipole method (FMM) to solve wave scattering from one‐dimensional random lossy dielectric rough surfaces. The proposed algorithm has the computational complexities of O(Ndg) for near‐field interactions and O(Ncg) for nonnear‐field interactions, where Ndg and Ncg are the number of sampling points on the dense and coarse grids, respectively. Using the proposed algorithm, wave scattering from Gaussian and non‐Gaussian rough surfaces is investigated and illustrated. Special emphasis is put on checking the accuracy of the algorithm and energy conservation.
Ionospheric irregularity diagnostics from the phase structure functions of MF/HF radio echoes
Authors: Zabotin N. A. , Wright J. W.
Published: February 2004

We present a new approach to investigating ionospheric irregularities, using the temporal structure function of totally reflected radio echo phase variations. Modern digital ionosondes (e.g., the dynasonde) measure the echo phase with very high resolution and precision, at closely spaced antennas, frequencies, and times. A “stringing” procedure gives continuous and unambiguous phase variation data for time intervals of any desired length. Quasi‐periods of tens of seconds up through several minutes are caused by large‐scale movements of the ionospheric plasma, while shorter‐period phase variations result from the interaction of the sounding signal with small‐scale irregularities. The relevant irregularity spatial domain extends from decameter radio wavelengths to the first Fresnel scale, a few kilometers. We obtain a theoretical relation between structure functions of the temporal phase variations and spatial irregularities with a simple model of frozen horizontal drift. The relation permits solutions of both the direct and inverse problems. Although long‐period phase measurements are practicable and essential to exploring larger irregularity scales, they require observing modes dedicated to multiple fixed‐frequency time series, and this undesirably limits the number of altitudes that can be monitored simultaneously. An alternative “rudimentary structure function” is obtainable from standard dynasonde “B‐mode” ionograms; it offers good altitude and time resolution for irregularity studies while permitting other established diagnostics (electron density profiles, vector velocities, critical frequencies, etc.) with the same data. We show some example analyses by these methods as applied to auroral and magnetic‐equatorial dynasonde observations. We find irregularity amplitudes in the range 0.001 < ΔN/N < 0.1 (for a nominal scale of 1 km) and spectral indices in the range 2 < v < 4, with evidence of diurnal variation in both quantities at both locations.
Rapid numerical simulation of axisymmetric single‐well induction data using the extended Born approximation
Authors: Torres‐Verdín Carlos , Habashy Tarek M.
Published: February 2004

We derive an extended Born approximation for electromagnetic scattering due to rock formations that exhibit axial symmetry around a well bore. An alternative formulation is also developed to improve the accuracy of the approximation when scattering takes place in close prroximity with the source. The resulting equations are solved via a numerical procedure that is as efficient as the first‐order Born approximation but substantially more accurate. This conclusion follows from studies carried out on a variety of source‐receiver configurations and frequencies typical of single‐well induction measurements.
High‐frequency bistatic cross sections of the ocean surface
Authors: Gill E. W. , Walsh J.
Published: February 2004

An analysis leading to the first and second‐order bistatic cross sections of the ocean surface in the context of high‐frequency ground wave radar operation is presented. Initially, a pulsed dipole source is introduced into the previously derived electric field expressions for the bistatic reception of vertically polarized radiation scattered from rough surfaces that do not vary with time. To make application to the ocean, a time‐varying surface is introduced via a three‐dimensional Fourier series with two spatial variables and one temporal variable. The surface randomness is accounted for by allowing the Fourier coefficients to be zero‐mean Gaussian random variables. Fourier transformation of the autocorrelations of the resulting fields gives the appropriate power spectral densities. The latter are used in the bistatic radar range equation to produce the cross sections. The features of the bistatic case are seen to reduce to the well‐known monostatic results when the appropriate geometry is introduced. Illustrative comparisons of monostatic and bistatic reception are presented.
On the numerical calculation of the magnetic current frill model
Authors: Hui Hon Tat , Yung Edward K. N. , Chan Kam Yuen
Published: February 2004

The numerical calculation of the singular electric field from a magnetic current frill is reexamined with respect to two types of coaxially fed wire antennas. It is shown that the moment method with the Galerkin matching procedure allows direct and efficient numerical integrations to yield accurate results on the input impedance without any special singularity extraction methods. This demonstrates a simple but efficient and accurate method for the calculation of the input impedance of wire antennas by using the magnetic current frill excitation model. Numerical results on the fast convergence of the input impedance are presented.
Multilevel fast multipole algorithm for general dielectric targets in the presence of a lossy half‐space
Authors: He Jiangqi , Sullivan Anders , Carin Lawrence
Published: February 2004

The multilevel fast multipole algorithm (MLFMA) is extended to the problem of an arbitrarily shaped dielectric target in the presence of a lossy, dispersive half‐space. The near MLFMA terms are treated rigorously, via a complex‐image‐technique‐based evaluation of the Sommerfeld integrals inherent to the half‐space Green's function. The Green's function components for the far MLFMA terms are evaluated approximately, but accurately, via an asymptotic analysis. In this paper, we detail the scattering formulation and perform a comparison of MLFMA‐generated results with those from other, simpler (and less general) models.
Wideband transhorizon channel characterization
Authors: Ndzi David , Austin John , Vilar Enric
Published: February 2004

The paper describes a 31.25 MHz bandwidth wideband channel sounder used to characterize a transhorizon path over the English Channel (La Manche) at 11.64 GHz. The measurements were done continuously for a period of 8 months (June 5, 1996, to January 5, 1997), capturing 515 Mb of data each day. A nonlinear regression technique, singular value decomposition prony (SVD‐P), was used to estimate the channel impulse response from the measured transfer function. The received signal levels obtained together with their dynamic signatures were used to categorize the mechanisms into three broad groups: ducting, strong enhancements, and troposcatter. The statistical analysis carried out gave strong correlation between signal levels, delay spread, coherence bandwidth, and Doppler spread. The results also help to highlight the limitation of traditional channel characterization parameters such as delay spread. The investigation has also revealed the potential and propagation conditions of an over the sea transhorizon path as a high‐data‐rate communication channel. The research has qualified and quantified the interference potential in spectrum reuse and the frequency decorrelation probabilities to combat it.
Effect of background fields on three‐dimensional finite element analysis of induction logging
Authors: Badea Eugene A. , Everett Mark E. , Shen Liang C. , Weiss Chester J.
Published: February 2004

Three‐dimensional controlled‐source electromagnetic induction algorithms are generally formulated in terms of secondary fields or potentials. The selection of the primary solution is somewhat arbitrary but can greatly influence the efficiency of finite element codes. A simple induction logging problem demonstrates this statement. Responses are computed using a three‐dimensional finite element method for two different selections of the primary potential, one that includes the borehole effect and one that does not. The accuracy of the numerical solution, for a given mesh size, increases dramatically when the borehole effect is included in the primary solution. This is because the sharp gradients in the electromagnetic field near the transmitter due to the presence of the borehole are coded as an analytic source term and do not have to be captured by the numerical approximation. Whenever possible, finite element analysts should select a convenient primary solution that closely matches the anticipated total solution in the immediate vicinity of the transmitter.
Atmospheric radar imaging using multiple‐receiver and multiple‐frequency techniques
Authors: Yu Tian‐You , Palmer Robert D.
Published: February 2004

Atmospheric radar imaging techniques have shown promise in revealing the fine‐scale structure of the atmosphere within the resolution volume of the radar. Enhanced resolution can be obtained in both angle and range by using spaced receivers and shifted frequencies, respectively. The distinct techniques have been termed coherent radar imaging (CRI) for angular resolution enhancement and range imaging (RIM) for radial resolution improvement. Because of the mathematical similarities between CRI and RIM it is possible to derive a generalization of both techniques. In this work, the three‐dimensional (3‐D) imaging technique, which uses multiple receivers and multiple frequencies simultaneously, is developed for the first time. Three‐dimensional imaging has the advantage of mitigating the limitations of beam width as well as pulse width of a conventional radar to simultaneously improve both angular and range resolution. It is shown that CRI and RIM are special cases of 3‐D imaging. The mathematical problem is formulated as an inverse problem with solutions provided by the Fourier, Capon, and maximum entropy (MaxEnt) methods. These three 3‐D imaging methods are verified and statistically tested through numerical simulations.
Pretectonic and posttectonic emplacements of the granitoids in the south central Okchon belt, South Korea: Implications for the timing of strike‐slip shearing and thrusting
Authors: Ree Jin‐Han , Kwon Sang‐Hoon , Park Youngdo , Kwon Sung‐Tack , Park Sung‐Hyun
Published: February 2004

Structural analyses of three granitic plutons in the south central Okchon belt of the Korean peninsula reveal that the Baegnok and Cheongsan plutons are pretectonic with respect to right lateral strike‐slip ductile shearing along the Cheongsan shear zone and later top to the ESE thrusting, whereas the Boeun pluton is posttectonic with respect to these two deformation events. U‐Pb sphene age data from the three plutons indicate that the intrusion ages of the Baegnok, Cheongsan, and Boeun plutons are 222.7 ± 2.1 Ma, 216.9 ± 2.2 Ma, and 171.7 ± 1.4 Ma, respectively. Microstructural evidence preserved in deformed rocks from the NE striking Cheongsan shear zone and adjacent, NNE striking thrusts suggests that the right lateral ductile shearing event along the shear zone occurred before the ESE directed thrusting. Since these thrusts crosscut the late Triassic ‐ Early Jurassic Taedong Group, we estimate that the thrusts developed at ∼180 Ma during the Daebo tectonic event, just prior to the intrusion of the Boeun pluton. We also constrain the age of the Cheongsan shear zone at Middle ‐ Late Triassic time, corresponding to the Songrim orogeny, so that the shear zone is not related to deformation along the Middle Jurassic Honam shear zone (∼176 Ma). The Honam shear zone is not a simple tectonic boundary between the Okchon belt and the Yongnam massif but an anastomosing ductile shear zone not only affecting the Okchon belt and the Yongnam massif but also crosscutting the boundary between them at a low angle. We suggest that the right lateral Cheongsan shear zone is a major boundary between the Okchon zone and the Yongnam massif with Paleozoic cover rocks. The Kyonggi massif and the Okchon zone (South China block) were juxtaposed against the Yongnam massif and Taebaeksan basin (North China block) along the Cheongsan shear zone during a late stage of the Middle Triassic Songrim orogeny, probably related to the late Paleozoic ‐ early Mesozoic collision between the North and South China blocks.
Structure and metamorphism of the granitic basement around Antananarivo: A key to the Pan‐African history of central Madagascar and its Gondwana connections
Authors: Nédélec Anne , Ralison Bruno , Bouchez Jean‐Luc , Grégoire Vincent
Published: February 2004

The Precambrian basement of Madagascar acquired a polyphase imprint during the Pan‐African orogeny. In northern central Madagascar, emplacement of stratoid alkaline granites at midcrustal depth (4–5 kbars) led to formation of a layered crust in a postcollisional extensional regime at 630 Ma (D1). Subsequently, the structures of the stratoid granites were rotated by the sinistral and transpressive E‐W Antananarivo flexure (or virgation) zone (D2). East of Antananarivo the structures of the D1 layered crust and the D2 virgation are crosscut by the steeply dipping N‐S foliations of the Angavo belt. Lineations gently plunging to the north attest that the Angavo belt is a major strike‐slip shear zone that formed under low‐pressure granulitic conditions (3 kbars, 790°C). The nearby porphyritic Carion granite was emplaced at the end of this period of N‐S shearing (D3), which can thus be no younger than 530 Ma. Late‐Pan‐African (580–550 Ma) strike‐slip motion along broadly N‐S shear zones has been recognized elsewhere in Madagascar and in its Gondwana connections. Continuation of the Angavo belt as one of the high strain belts of the Arabian‐Nubian Shield is discussed in the general framework of Gondwana assembly.
The structure of the Africa‐Anatolia plate boundary in the eastern Mediterranean
Authors: Vidal Neus , Alvarez‐Marrón Joaquina , Klaeschen Dirk
Published: February 2004

New marine deep seismic reflection data from south of Cyprus to the Syrian coast provide images of the upper crustal structure of the Cyprean Arc indicating that the deformation is partitioned along strike‐slip fault systems distributed over a wide zone, rather than forming a sharp plate boundary between African and Anatolian plates. Three major submarine strike‐slip fault systems, tens of kilometers in length, are mapped, which follow bathymetric features and merge together toward the east. These structures exhibit the three‐dimensional characteristics typical of strike‐slip deformation zones throughout the seven pre‐stack depth‐migrated sections, including several sets of positive flower structures forming bathymetric ridges, and the intervening contemporaneous subbasins. Beneath the Plio‐Quaternary sediments (500 m thick) that are blanketing the whole area and that reflect only the main surface traces of the fault systems, the subbasins are of varied dimensions and have rapid lateral changes in the thickness of the sedimentary fill. They include two major unconformities that have been correlated throughout the data marked by the M and K reflections. The M reflection is well imaged above varied thickness of Messinian evaporites, and the K reflection commonly appears at the base of syntectonic Tertiary age sediments. Within the eastern Cyprean Arc the K reflection corresponds to the basement‐cover contact, indicating that the strike‐slip tectonic scenario may have been active since the uppermost Cretaceous or lowermost Paleogene times to present. The active deformation front of the Alpine belt in the eastern Mediterranean corresponds to a strike‐slip fault system that forms a 110° arc and coincides at the southern slope of the Hecateaus Rise, continuing along the Latakia Ridge to the Syrian coast. The mapped structures fit within a general kinematic framework of left‐lateral transcurrent deformation that transfers slip from the subduction zone southwest of Cyprus into the Dead Sea transform system in the east. This change in the mode of deformation at the Africa‐Anatolia plate boundary occurs toward the junction between African, Anatolian, and Arabian plates.
Reflection seismic evidence for Caledonian deformed sediments above Sveconorwegian basement in the southwestern Baltic Sea
Authors: Lassen Alexander , Thybo Hans , Berthelsen Asger
Published: February 2004

Two complexes of orogenic structures are interpreted to the south of the Baltic Shield from seismic reflection sections in the southern Baltic Sea. Dipping reflections from the pre‐Mesozoic basement are interpreted as (1) an upper, Caledonian (450–440 Ma) complex of SSW dipping low‐angle thrusts that formed in relation to Avalonia's accretion to Baltica and (2) a lower, Sveconorwegian (1150–900 Ma) complex of west to SW dipping ductile thrusts and shear zones in the crystalline crust of Baltica. The two complexes are separated by a regional seismic marker, the O horizon, from the basal part of Baltica's lower Palaeozoic cover and, in places, also by reflections from an underlying Neoproterozoic clastic rift sequence. The complex tectonic structures were resolved from a network of commercial seismic reflection profiles to 4 s two‐way travel time. The results imply that (1) the late Proterozoic crust of southwestern Baltica extends southward, with little lateral offset, across the Sorgenfrei‐Tornquist Zone, below the southwestern Baltic Sea and the North German Basin: (2) in the study area a distinction should be made between the northernmost occurrence of Caledonian deformation structures and the Caledonian suture proper: (3) Caledonian deformation did not involve the crystalline basement in the study area, but was accommodated by the main decollement, near the top of the O horizon, and by thrusts in the overlying lower Palaeozoic sequence; and (4) with such data it is possible to add more detail and partly revise previous interpretations of the Neoproterozoic and Caledonian tectonic evolution of the southwestern part of Baltica.
A semianalytical solution for steady infiltration in unsaturated fractured rock
Authors: Ho Clifford K.
Published: February 2004

A semianalytical solution is developed for an equivalent continuum model of one‐dimensional steady infiltration in unsaturated fractured rock. The differential form of the mass conservation equation is integrated to yield an analytical expression relating elevation to a function of capillary pressure and relative permeability of the fracture and rock matrix. Constitutive relationships for unsaturated flow in this analysis are taken from van Genuchten [1980] and Mualem [1976], but alternative relations can also be implemented in the integral solution. Expressions are presented for the liquid saturations and pore velocities in the fracture, matrix, and effective continuum materials as a function of capillary pressure and elevation. Results of the analytical solution are applied to examples of infiltration in fractured rock consisting of both homogeneous and composite (layered) domains. The analytical results are also compared to numerical simulations to demonstrate the use of the analytical solution as a benchmarking tool to address computational issues such as grid refinement.
Quantifying the thermal dynamics of a permafrost site near Ny‐Ålesund, Svalbard
Authors: Roth Kurt , Boike Julia
Published: February 2004

The dynamics of permafrost soils is manifest in the soil temperature which can be measured with high accuracy and high temporal resolution. Using continuous data over a period of 778 days from a mineral hummock field at the Bayelva site, Svalbard, we deduce and quantify the processes which constitute the dynamics. In particular, conductive heat flux, generation of heat from phase transitions, and migration of water vapor are analyzed. Support for the interpretation of the data comes from high‐resolution time domain reflectometry measurements of the liquid water content.
Sudden drawdown and drainage of a horizontal aquifer
Authors: Parlange J.‐Y. , Stagnitti F. , Heilig A. , Szilagyi J. , Parlange M. B. , Steenhuis T. S. , Hogarth W. L. , Barry D. A. , Li L.
Published: February 2004

Drainage of a saturated horizontal aquifer following a sudden drawdown is reanalyzed using the Boussinesq equation. The effect of the finite length of the aquifer is considered in detail. An analytical approximation based on a superposition principle yields a very good estimate of the outflow when compared to accurate numerical solutions. An illustration of the new analytical approach to analyze basin‐scale field data is used to demonstrate possible field applications of the new solution.
Coupling stochastic models of different timescales
Authors: Koutsoyiannis Demetris
Published: February 2004

A methodology is proposed for coupling stochastic models of hydrologic processes applying to different timescales so that time series generated by the different models be consistent. Given two multivariate time series, generated by two separate (unrelated) stochastic models of the same hydrologic process, each applying to a different timescale, a transformation is developed (referred to as a coupling transformation) that appropriately modifies the time series of the lower‐level (finer) timescale so that this series becomes consistent with the time series of the higher‐level (coarser) timescale without affecting the second‐order stochastic structure of the former and also establishes appropriate correlations between the two time series. The coupling transformation is based on a developed generalized mathematical proposition, which ensures preservation of marginal and joint second‐order statistics and of linear relationships between lower‐ and higher‐level processes. Several specific forms of the coupling transformation are studied, from the simplest single variate to the full multivariate. In addition, techniques for evaluating parameters of the coupling transformation based on second‐order moments of the lower‐level process are studied. Furthermore, two methods are proposed to enable preservation of the skewness of the processes in addition to that of second‐order statistics. The overall methodology can be applied to problems involving disaggregation of annual to seasonal and seasonal to subseasonal timescales, as well as problems involving finer timescales (e.g., daily to hourly), with the only requirement that a specific stochastic model is available for each involved timescale. The performance of the methodology is demonstrated by means of a detailed numerical example.
Solute transport in heterogeneous formations of bimodal conductivity distribution: 1. Theory
Authors: Dagan G. , Lessoff S. C.
Published: February 2004

Transport of a conservative solute takes place in a formation made up from a matrix of conductivity K0 and porosity ϑ0 and inclusions of properties K, ϑ. For given inclusions shape, the system is characterized by the two parameters κ = K/K0 and the inclusions volume fraction n. In the past, approximate solutions of the flow and transport problems were obtained under the limit of low variability, i.e., κ − 1 ≪ 1, and arbitrary n [Rubin, 1995]. The present study aims at solving the problem under the opposite limit of a dilute system, i.e., n ≪ 1 and arbitrary κ. We are particularly interested in elongated inclusions (high length/thickness ratio) of high‐permeability contrast to the matrix. Such configurations are related to applications in which lenses or cracks are present in a medium of highly different conductivity (Figure 1). The basic procedure was developed by Eames and Bush [1999] for cylindrical or spherical inclusions, with no porosity contrast. They compute the macrodispersion coefficient, for advective transport past a large number of inclusions located at random. It is based on the solution for the distortion of a material surface of marked particles, moving past an individual inclusion in an unbounded domain and with uniform flow at infinity. In the present study we extend the approach to inclusions of arbitrary porosity and elliptical shape, characterized by the parameter e, the ratio between the small and large axes, with emphasis on e ≪ 1. We present the analytical solution of the flow problem and the procedure, requiring two quadratures, to calculate the macrodispersivity. Analytical solutions are obtained for two particular limits: κ ≪1 and κ ≃ 1. The latter is compared with the limit n ≪ 1 of the solution of Rubin [1995]. The theoretical results are applied to a few cases of hydrological interest [Lessoff and Dagan, this issue].
Pore‐scale modeling extension of constitutive relationships in the range of residual saturations
Authors: Held Rudolf J. , Celia Michael A.
Published: February 2004

A pore network model is used to describe constitutive relationships between saturation, capillary pressure, relative permeabilities, and interfacial areas over the full range of saturations. We focus on residual nonwetting‐phase saturations, that is, the saturation range between main drainage and primary drainage. Interphase mass transfer and dynamic miscible transport are modeled to generate the range of saturations over which the extended constitutive relationships are calculated. To accommodate all saturations in a consistent way, we define capillary pressure as the areal average of local capillary pressures associated with each fluid‐fluid interface. The extended constitutive relationships provide input for continuum‐scale models covering the entire saturation range and indicate that special care is needed in the general formulation of continuum‐scale equations.
Derivation of effective hydraulic parameters of a Karst Aquifer from discharge hydrograph analysis
Authors: Baedke S. J. , Krothe N. C.
Published: February 2004

In well‐developed karst terrains, three or more distinct portions of the karst continuum can be identified from hydrographs of springs issuing from the karst aquifer. Hydrographs from two karst springs within the same drainage basin at the Crane Naval Surface Warfare Center, Indiana, have been analyzed, and ratios of transmissivity and specific yield (T/Sy) have been established for the conduit and diffuse flow systems. These ratios have been compared with values of T derived from aquifer tests, so that independent values of Sy can be calculated for the diffuse system. Similarly, if the value of Sy is assumed to be 1.0 for a pure conduit, then independent values of T can be calculated for this end‐member of the karst continuum. The values of T and Sy derived from this study are similar to values obtained from a dye trace of the conduit‐dominated flow system and of values derived from aquifer tests of the diffuse flow system. Values of T for the conduit system of these springs may need to be established at a local scale, while the values for the diffuse flow system may be applicable at a regional scale. A hydrograph separation using isotopic data suggests that the intermediate‐flow system represents a mix of water from the conduit and diffuse flow systems. If this portion of the hydrograph is a truly mixing phenomena, ratios of T/Sy cannot be determined from the hydrograph analysis presented herein. However, if instead, the intermediate‐flow system represents water released from a third reservoir (such as small fractures), ratios of T/Sy can be established for the intermediate‐flow system.
A water balance approach to assessing the hydrologic buffering potential of an alluvial fan
Authors: Herron Natasha , Wilson Cathy
Published: February 2004

The hydrologic connectivity of a catchment determines the efficiency with which runoff is moved through the catchment and hence opportunities for runoff storage. Potential storages include valley floors and alluvial fans. Monitoring results from a small alluvial fan indicate that alluvial fans can be significant hydrologic buffers. Between 20% and 100% of surface runoff delivered to the study fan from a 26‐ha catchment was slowed and/or stored during the monitored events. Interevent differences in buffering response are attributed to antecedent moisture conditions. In one event, when catchment conditions were very wet, only 20% of incoming runoff was absorbed by the fan. Under dry conditions, smaller proportions of total rainfall were delivered to the fan head as surface runoff, of which >60% was absorbed by the fan. Because sediment storages can modify surface runoff delivery, because of their distinctive morphologic and sedimentologic properties, it is suggested that their distribution through a catchment will influence catchment hydrologic connectivity hence runoff and sediment delivery through catchments.
Pore‐scale modeling and upscaling of nonaqueous phase liquid mass transfer
Authors: Held Rudolf J. , Celia Michael A.
Published: February 2004

A pore‐scale model is developed to simulate mass transfer between two fluid phases in a porous medium. The approach uses a network description of the pore space and builds on explicit tracking of the fluid‐fluid interfaces in the pore network. Mass transfer is computed as local mass fluxes across each interface, and transport equations are solved in the pore network by a characteristic method. The concept of stagnant‐layer diffusion is used to describe the interface mass transfer, where calculated local concentrations control the rates of mass transfer. The model results predict dissolution fronts developed in column experiments of porous media initially at residual nonaqueous phase saturation. The definition of macroscopic mass transfer coefficients is investigated, and comparisons are made for rigorously upscaled quantities.
Modeling transport of dissolved silica in a forested headwater catchment: Implications for defining the hydrochemical response of observed flow pathways
Authors: Scanlon Todd M. , Raffensperger Jeff P. , Hornberger George M.
Published: February 2004

Groundwater, subsurface stormflow, and overland flow components of discharge, derived from a hydrological model that was applied to a forested headwater catchment in north central Virginia, were used with measured stream water and lysimeter concentrations of dissolved silica to investigate the hydrochemical behavior of the catchment. Concentrations in base flow, taken to be a reflection of groundwater, vary with discharge, an observation in conflict with the typical assumption of constant concentration used in end‐member mixing analyses. This observed flow dependence was modeled by considering the concentration in groundwater to be related to the saturation deficit in this zone. A positive correlation between the average groundwater saturation deficit and base flow dissolved silica concentrations is consistent with batch experiments and petrographic analysis of regolith core samples, which both indicate an increase in silica available for dissolution with depth in the groundwater zone. In the absence of subsurface storm flow zone sampling during rainfall events a constant concentration was assumed for this zone. Concentration‐discharge (C‐Q) paths in the stream were used to evaluate the modeled stream silica concentrations. An inconsistency in the direction of the modeled C‐Q rotations suggests that the storm flow zone dissolved silica concentration may also vary with time, because of the “flushing” of high‐concentration, preevent soil water on the rising limb of the storm hydrograph. For this catchment in Virginia the assumption of a constant concentration for subsurface storm flow, as well as for base flow, appears to be invalid.
Macroturbulent structure of open‐channel flow over gravel beds
Authors: Shvidchenko Audrey B. , Pender Gareth
Published: February 2004

The turbulent structure of open‐channel flow over a mobile gravel bed was investigated in an 8 m long, 0.3 m wide, and 0.3 m deep tilting flume. A flow visualization technique was used and complemented by measurements of flow velocity fluctuations near the bed. The experiments reveal that turbulent flow consists of a sequence of large‐scale eddies with a vertical size close to the flow depth, an average length equal to four to five depths, and a width of about two depths. The downstream motion of these eddies causes quasiperiodic fluctuations of the local flow velocity components. The development of longitudinal troughs and ridges on the mobile bed and preferential transport of bed particles along troughs are related to the effect of the eddies. The experimental results indicate that the depth‐scale eddies are an important turbulence mechanism contributing to sediment transport.