bluuebunny/crossref_metadata_2025 huggingface dataset:
Determining flow field singularities from drifter trajectories
Authors: Halide Halmar , Sanderson Brian G.
Published: February 2008

We use the method developed by Kirwan et al. and Halide to obtain the differential kinematic properties (DKP) and the position and velocity of a flow field singularity from a single drifter trajectory. This method, hereinafter called the OK method, has been successfully tested using artificial trajectories that were generated from flow fields containing one singularity. The OK method requires fourth‐order time derivatives of data. A new regression method that calculates DKP from a cluster of drifters is presented. This new regression method does not artificially set the flow center to the cluster centroid. It has been successfully tested by application to the trajectories generated from flow fields containing one singularity. Depending upon the spatial distribution of drifters relative to the singularity, the new regression method can be much less sensitive to noise than the OK method. Trajectories generated by flow fields with both saddle and vortex points were analyzed. Both methods worked when trajectories were close to the singularity, but not otherwise. Trajectories measured on the Scotian shelf were not amenable to either method of analysis, presumably because the trajectories did not result from only one singularity.
Seasonal variability of bio‐optical and physical properties in the Sargasso Sea
Authors: Dickey T. , Granata T. , Marra J. , Langdon C. , Wiggert J. , Chai‐Jochner Z. , Hamilton M. , Vazquez J. , Stramska M. , Bidigare R. , Siegel D.
Published: February 2008

The seasonal variability of bio‐optical and physical properties within the upper ocean at a site in the Sargasso Sea (34°N, 70°W) has been observed using multivariable moored systems (MVMS) during a 9‐month period (March through November 1987). In addition, complementary meteorological data, sea surface height (Geosat) and sea surface temperature maps, and expendable bathythermograph (XBT) and shipboard profile data (physical and bio‐optical) have been utilized for interpretation. The observations during March are characteristic of late wintertime conditions of a deep isothermal layer (∼18–19°C), but with intervening periods of warming due to the advection of warm outbreak waters associated with Gulf Stream meanders. The mixed layer depth shoals from greater than 160 m to about 25 m in late March (spring transition). Phytoplankton blooms follow the mixed layer shoaling. A succession of phytoplankton populations occurs during this transitional interval. Mesoscale variability associated with cold core rings and warm outbreak waters associated with the Gulf Stream are evident at various times. The mixed layer remains near 25 m for the summer and deepens in mid‐September. A relatively intense subsurface maximum in chlorophyll develops at ∼75 m following the spring transition. The maximum persists, but weakens in mid‐summer. The present study clearly indicates that important processes associated with and contributing to the seasonal cycle occur on short time and space scales and that integrated data sets obtained from moorings, ships, and satellites can be used to effectively study bio‐optical and physical phenomena on time scales from minutes to seasons.
Global validation of the wave model WAM over a one‐year period using Geosat wave height data
Authors: Romeiser Roland
Published: February 2008

The high quality of wave fields simulated by the third‐generation wave model WAM has already been demonstrated in various validation studies using in situ measurements as well as data from satellites as reference. However, owing to limitations of the reference data sets, the previous studies concentrated on relatively small regions or short time periods only, for which adequate measurements were available. In this paper the first global verification of the WAM model over a full 1‐year period is presented. The significant wave heights hindcast for 1988 by the WAM model as implemented at the European Centre for Medium Range Weather Forecasts are compared with measurements obtained by the Geosat radar altimeter. The wave heights from WAM and Geosat show good agreement in general. However, significant regional and seasonal differences are found. The underestimation of WAM wave heights in the southern hemisphere, which was already known from a validation study for the Seasat period, shows significant seasonal variations. The hindcast wave heights are underestimated by about 20% in large parts of the southern hemisphere and the tropical region during May‐;September. For the rest of the time, the agreement with Geosat data is fairly good. Together with the fact that also the rms variability of wave heights in the tropical region is clearly underestimated by WAM, this can possibly be attributed to simplifications like the neglect of atmospheric stratification effects when converting wind speeds to the wind stress fields driving WAM. Furthermore, the intercomparison indicates that low wave heights below ∼ 1.5 m are generally overestimated by WAM. As it is planned to use altimeter wave heights for updating wave models in future data assimilation systems, it is quite important to have efficient quality control criteria for these data. We show that some additional Geosat parameters, e.g., the off‐nadir angle of the altimeter, can be useful quality parameters. The difference between the Geosat and WAM wave heights shows a clear dependence on the additional parameters in some cases, which must be related to quality problems of the Geosat data. Some new criteria for the rejection of incorrect Geosat data points are obtained.
The vertical turbulence structure of the coastal marine atmospheric boundary layer
Authors: Tjernström Michael , Smedman Ann‐Sofi
Published: February 2008

The vertical turbulence structure in the marine atmosphere along a shoreline has been investigated using data from tower and aircraft measurements performed along the Baltic coast in the southeast of Sweden. Two properties make the Baltic Sea particularly interesting. It is surrounded by land in all directions within moderate advection distances, and it features a significant annual lag in sea surface temperature as compared with inland surface temperature. The present data were collected mostly during spring or early summer, when the water is cool, i.e., with a stably or neutrally stratified marine boundary layer usually capped by an inversion. Substantial daytime heating over the land area results in a considerable horizontal thermal contrast. Measurements were made on a small island, on a tower with a good sea fetch, and with an airborne instrument package. The profile data from the aircraft is from 25 slant soundings performed in connection to low level boundary layer flights. The results from the profiles are extracted through filtering techniques on individual time (space) series (individual profiles), applying different normalization and finally averaging over all or over groups of profiles. The land‐based data are from a low tower situated on the shoreline of a small island with a wide sector of unobstructed sea fetch. Several factors are found that add to the apparent complexity of the coastal marine environment: the state of the sea appears to have a major impact on the turbulence structure of the surface layer, jet‐shaped wind speed profiles were very common at the top of the boundary layer (in about 50% of the cases) and distinct layers with increased turbulence were frequently found well above the boundary layer (in about 80% of the cases). The present paper will concentrate on a description of the experiment, the analysis methods, and a general description of the boundary layer turbulence structure over the Baltic Sea.
Methane and carbon monoxide production, oxidation, and turnover times in the Caribbean Sea as influenced by the Orinoco River
Authors: Jones Ronald D. , Amador José A.
Published: February 2008

The surface distribution of CH4, photoproduction capacity of seawater for CO, and CH4 and CO microbial oxidation rates and turnover times were determined for the surface waters of the southeastern Caribbean Sea and Gulf of Paria as affected by the Orinoco River. Measurements were made during the spring (low river flow) and the fall (high river flow) in order to determine the influence of Orinoco River discharge on these parameters. Methane concentrations were generally lower in the fall than in the spring. Low CH4 oxidation rates and similar surface distributions were observed during both seasons. Methane oxidation in the river plume was significantly higher in the fall. Potential CO photoproduction and CO oxidation rates were generally higher during the fall. Possible effects of the Orinoco River on potential CO photoproduction capacity were observed as far as Puerto Rico. Turnover times for CH4 were in the order of years, whereas CO turnover times were in the order of hours. The Orinoco River has a significant impact on the consumption of CO and CH4 and the photoproduction of CO in the southeastern Caribbean Sea, presumably resulting from inputs of riverine carbon and nutrients.
Reactive nitrogen and its correlation with ozone in the lower stratosphere and upper troposphere
Authors: Murphy D. M. , Fahey D. W. , Proffitt M. H. , Liu S. C. , Chan K. R. , Eubank C. S. , Kawa S. R. , Kelly K. K.
Published: February 2008

Reactive nitrogen (NOy) and O3 were measured simultaneously from a NASA ER‐2 aircraft during 1987 through 1989. These high resolution measurements cover a broad range of latitudes in the upper troposphere and lower stratosphere. The data show a striking positive correlation between NOy and O3 in the lower stratosphere at all scales sampled. The ratio NOy/O3 is nearly independent of altitude from the tropopause to above 20 km, with ratios in the stratosphere of 0.0015–0.002 in the tropics and 0.0025–0.004 elsewhere. The ratio is much more constant than either individual species, thus providing an excellent reference point for comparing limited data sets with models. Two‐dimensional models reproduce general features of the vertical profile of NOy/O3 but not the gradient in the lower stratosphere between tropics and mid‐latitudes. NOy and O3 are better correlated in the lower stratosphere than in the upper troposphere. The magnitude and variability of both NOy mixing ratios and NOy/O3 ratios indicate a source of NOy in the upper troposphere. The most plausible source in the tropics is lightning production of NOx. Condensation of NOy onto aerosol particles is often possible in the tropical upper troposphere and may play a role in determining the vertical distribution of NOy. In the mid‐latitude upper troposphere the data suggest long‐range transport of NOy. NOy mixing ratios in the tropical upper troposphere were usually between 150 and 600 pptv, enough so that upward transport can affect the NOy abundance in the tropical lower stratosphere.
Algorithms and sensitivity analyses for Stratospheric Aerosol and Gas Experiment II water vapor retrieval
Authors: Chu W. P. , Chiou E. W. , Larsen J. C. , Thomason L. W. , Rind D. , Buglia J. J. , Oltmans S. , McCormick M. P. , McMaster L. M.
Published: February 2008

This paper provides a detailed description of the current operational inversion algorithm for the retrieval of water vapor vertical profiles from the Stratospheric Aerosol and Gas Experiment II (SAGE II) occultation data at the 0.94‐μm wavelength channel. This algorithm is different from the algorithm used for the retrieval of the other species such as aerosol, ozone, and nitrogen dioxide because of the nonlinear relationship between the concentration versus the broad band absorption characteristics of water vapor. Included in the discussion of the retrieval algorithm are problems related to the accuracy of the computational scheme, accuracy of the removal of other interfering species, and the expected uncertainty of the retrieved profile. A comparative analysis on the computational schemes used for the calculation of the water vapor transmission at the 0.94‐μm wavelength region is presented. Analyses are also presented on the sensitivity of the retrievals to interferences from the other species which contribute to the total signature as observed at the 0.94‐μm wavelength channel on SAGE II instrument. Error analyses of the SAGE II water vapor retrieval will be shown, indicating that good quality water vapor data are being produced by the SAGE II measurements.
Relationship between total ozone amounts and stratospheric temperature at Syowa, Antarctica
Authors: Chubachi Shigeru
Published: February 2008

Using statistical methods, the relationship has been studied between total ozone and 100‐mbar temperatures at Syowa Station, Antarctica (69°S, 40°E), based on data obtained in 1961–1981 and 1982–1988, the time of ozone depletion in Antarctica. Results indicate a strong, positive correlation between total ozone and 100‐mbar stratospheric temperatures during September‐March for all years, but lower ozone values at 100‐mbar stratospheric temperatures colder than about −60°C during the 1982–1988 period. Ozone destruction by heterogeneous photochemical processes is the main cause of ozone depletion over Syowa during the 1980's, with a lesser contribution from a change in air dynamics (heat, ozone, and momentum transport to Antarctica during the austral spring) that increased polar vortex stability, thereby promoting photochemical ozone depression within the vortex.
Modeling of inductive charge separation in rainshafts with variable vertical electric fields
Authors: Canosa E. Freire , List Roland , Stewart R. E.
Published: February 2008

The inductive dissipation of uniform vertical electric fields in warm clouds is assessed with a parallel‐capacitor model and with raindrops in 20 size classes having diameters between 0.1 and 4 mm. The collision/breakup parameterization of Low and List [1982 a, b] is used together with the inductive charging proposed by Latham and Mason [1962] and confirmed by Canosa and List [this issue]. The selection of collision, breakup, coalescence, and bouncing events is made on the basis of probabilistic considerations. The results are: (1) The original field decays rapidly and sinusoidally at rainfall rates > 2.4 mm h−1. This dissipative effect has not been recognized in earlier warm‐cloud electrification models. It is a consequence of drop interaction experiments that show standard breakup configurations with the large fragments below the separating smaller fragments (disjection) (2) For disjection, electric fields and raindrop charges oscillate sinusoidally in damped fashion, with the period decreasing with increasing rainrate (3) The results for an initial field of −10 kV m−1 can be linearly scaled to any other initial field. (4) In light rain, inductive charging may produce buildup of vertical fields, but this is not sufficient to cause significant electrification. Threshold limits are given for this process to occur.
Measurements of high number densities of ice crystals in the tops of tropical cumulonimbus
Authors: Knollenberg R. G. , Kelly K. , Wilson J. C.
Published: February 2008

Imaging and light scattering instruments were used during the January/February 1987 STEP Tropical Experiment at Darwin, Australia, to measure ice crystal size distributions in the tops of tropical cumulonimbus anvils associated with tropical cyclones and related cloud systems. Two light scattering instruments covered particles from 0.1‐μm to 78‐μm diameter. Particles larger than 50‐μm diameter were imaged with a two‐dimensional Grey optical array imaging probe. The measurements were made at altitudes ranging from 13 to 18 km at temperatures ranging from −60° to −90°C. Additional measurements made in continental cumulonimbus anvils in the western United States offer a comparative data set. The tropical anvil penetrations revealed surprisingly high concentrations of ice crystals. Number densities were typically greater than 10 cm−3 with up to 100 cm−3 if one includes all particles larger than 0.1 μm and can approach condensation nuclei in total concentration. In order to explain the high number densities, ice crystal nucleation at altitude is proposed with the freezing of fairly concentrated solution droplets in equilibrium at low relative humidities. Any dilute liquid phase is hypothesized to be transitory with a vanishingly short lifetime and limited to cloud levels nearer −40°C. Homogeneous nucleation of ice involving H2SO4 nuclei is attractive in explaining the high number densities of small ice crystals observed near cloud top at temperatures below −60°C. The tropical size distributions were converted to mass using a spherical equivalent size, while the continental anvil data were treated as crystalline plates. Comparisons of the ice water contents integrated from the mass distributions with total water contents measured with NOAA Lyman‐alpha instruments require bulk densities equivalent to solid ice for best agreement. Correlation between the two data sets for a number of flight passes was quite good and was further improved by subtraction of water vapor density values ranging between ice and water saturation. Ice water contents up to 0.07 g m−3 were observed in the tropical anvils with over 0.1 g m−3 in continental anvils. The size distributions in tropical anvils generally reveal mass modes at sizes of 20–40 μm. With rare exceptions, particles larger than 100 μm were not observed near the cloud tops. In continental cumulonimbus anvils, much larger plate crystals approaching 1 mm in size account for the majority of the ice water. Most of the ice crystal mass lofted to anvil altitudes falls to lower levels prior to evaporating. The anvils can experience strong radiational heating as well as cooling depending upon lower cloud cover, particle size distribution, and time of day.
Miniature vortices produced by electrical corona
Authors: Jonsson H. H. , Vonnegut B.
Published: February 2008

Laboratory experiment shows how a small whirlwind can be created under the influence of an electric field. This demonstrates one mechanism whereby a portion of the electrical energy in a thundercloud might be transformed into kinetic energy within atmospheric vortices.
New laboratory measurements on ammonia's inversion spectrum, with implications for planetary atmospheres
Authors: Spilker Thomas R.
Published: February 2008

Microwave spectral measurements have been performed on pure room‐temperature gaseous ammonia at frequencies from 1.75 to 18 GHz (1.7–17 cm), at 50‐, 100‐, and 300‐torr pressures. These measurements are part of a laboratory program to measure the microwave absorption spectrum of ammonia, under conditions applicable to giant planet atmospheres, now in progress at the Jet Propulsion Laboratory. The pure ammonia data reported here agree well with previous data by Bleaney and Loubser (1950) at 100 and 300 torrs, and with predictions of the absorptivity formalism published by Berge and Gulkis. Success with pure ammonia but failure with mixtures of ammonia in hydrogen and helium (Spilker, 1990) indicates that the Berge and Gulkis formalism does not correctly handle foreign‐gas effects on ammonia inversion lines. This may require modifying conclusions of radio astronomical and radio occultation studies that used this formalism. Notably, a suggested depletion of ammonia and superabundance of hydrogen sulfide may have been exaggerated as a result of inaccuracies in the Berge and Gulkis formalism.
SO2‐rock interaction on Io: Reaction under highly oxidizing conditions
Authors: Johnson M. L. , Burnett D. S.
Published: February 2008

Laboratory simulations have been carried out to test the possibility that interactions of SO2 and silicates can produce Na‐S compounds which can account for the observed surface enrichment of Na relative to Si on Io. A wide variety of silicate compositions were heated under oxidizing conditions with a SO2/O2 = 1 gas phase at a mid‐level crustal temperature for Io (1123K). All experiments produced sulfate deposits on the silicate surfaces. The nature of the sulfate changed systematically with the silicate Ca/Na ratio, with mixtures of CaSO4 and Na‐rich sulfate formed by basaltic compositions having higher Ca/Na but only alkali‐rich sulfates formed for more granitic (low Ca/Na) compositions. For crystalline albite and an albite‐orthoclase eutectic glass composition, K and Al‐rich sulfates were formed. Assuming that burial of SO2 to the temperatures of our experiments is plausible on Io and that somewhat less oxidizing conditions do not qualitatively change the results, SO2‐rock interactions producing Na‐rich sulfates could account for the Io surface enrichment in Na. Observations on minor elements, such as K or Ca, in the atomic cloud or in magnetospheric ion spectra could be used to identify sulfates as Io surface phases and, conceivably, even define Ionian crustal magma types. Many analyses show significant S excesses which are best interpreted as due to the presence of bisulfate (HSO4.) components, and NaHSO4 deserves consideration as an Io surface mineral. The possibility of elemental S on Io can be regarded as a totally separate issue from the problem of the surface Na enrichment.
Impact‐induced tensional failure in rock
Authors: Ahrens Thomas J. , Rubin Allan M.
Published: February 2008

Planar impact experiments were employed to induce dynamic tensile failure in Bedford limestone. Rock discs were impacted with aluminum and polymethyl methacralate flyer plates at velocities of 10 to 25 m/s. This resulted in tensile stresses in the range of ∼11 to 160 MPa. Tensile stress durations of 0.5 and 1.3 μs induced microcrack growth which in many experiments were insufficient to cause complete spalling of the samples. Ultrasonic P and S wave velocities of recovered targets were compared to the velocities prior to impact. Velocity reduction, and by inference microcrack production, occurred in samples subjected to stresses above 35 MPa in the 1.3‐μs PMMA experiments and 60 MPa in the 0.5‐μs aluminum experiments. Apparent fracture toughnesses of 2.4 and 2.5 MPa m1/2 are computed for the 1.3‐ and 0.5‐μs experiments. These are a factor of ∼2 to 6 greater than quasi‐static determinations. Three‐dimensional impact experiments were conducted on 20 cm‐sized blocks of Bedford limestone and San Marcos gabbro. Compressional wave velocity deficits up to 50–60% were observed in the vicinity of the crater. These damage levels correspond to O'Connell and Budiansky damage parameters of 0.4 as compared to the unshocked rock. The damage decreases as ∼r−1.5 from the crater indicating a dependence on the magnitude and duration of the tensile pulse. Using the observed variation in damage with tensile stress from the one‐dimensional experiments, and estimates of the variation of peak dynamic tensile stress and tensile stress duration with distance from an impact on an elastic half‐space, the observed dependence of damage with radius in the three‐dimensional experiments are theoretically predicted and compare favorably to experimental data.
Pleistocene δ13C Variability of North Atlantic Intermediate Water
Authors: deMenocal Peter B. , Oppo Delia W. , Fairbanks Richard G. , Prell Warren L.
Published: February 2008

Variations in the stable isotopic composition of benthic foraminifera from Deep‐Sea Drilling Project (DSDP) site 502B in the Caribbean Sea are used to reconstruct Atlantic intermediate water circulation variability over the last 1.2 m.y. Comparison of this record with other North Atlantic benthic isotope records indicates that Atlantic intermediate water circulation was relatively enhanced during glacial maxima when North Atlantic deep water (NADW) production was reduced. However, a simple, compensatory relationship between intermediate and deepwater circulation is not apparent. Geochemical models have shown that such changes in ocean circulation can affect atmospheric CO2 levels by changing vertical nutrient and alkalinity profiles. The Δδ13C difference between Caribbean site 502B and deep equatorial Pacific site 677 is highly coherent and in phase with ice volume. Like the δ18O record, there is an increase in amplitude (40%) and a large increase in 100 kyr power after 0.7 Ma. The 1.2‰ Δδ13C amplitude scales to 70 ppm V in atmospheric CO2 using Boyle's (1986) box model result. The implied increase in CO2 amplitude after 0.7 Ma may suggest a positive feedback role in effecting the higher‐amplitude climatic fluctuations which characterize the last 0.7 m.y.
The influence of high‐latitude surface forcing on the global thermohaline circulation
Authors: Stocker Thomas F. , Wright Daniel G. , Broecker Wallace S.
Published: February 2008

The basin‐averaged, latitude‐depth ocean model of Wright and Stocker (1992) is used to simulate the deep circulation of the world ocean. Under present‐day surface forcing, sinking occurs in the North Atlantic and the southern ocean, and realistic temperature and salinity structures are obtained in the Atlantic, Pacific, and Indian oceans. “Color” tracers and radiocarbon are used to identify the composition of the deepwater masses and the associated renewal time scales. While broad agreement with observations is found in all basins, the water masses in the southern ocean are too young. The global thermohaline circulation and the composition of the deepwater masses are sensitive to the buoyancy contrast between the southern ocean and the North Atlantic. This contrast can be modified by changing relaxation values of temperature and salinity at the northern and southern high latitudes. If the model is forced with the zonal averages of the observed surface salinity, North Atlantic Deep Water is the dominant deep ocean water mass, and hardly any Antarctic Bottom Water flows into the Atlantic. Choosing instead the observed salinities of the newly formed deep water as the restoring values, the model realistically simulates the penetration of Antarctic Bottom Water into the different ocean basins. This has a global effect through reducing both strength and depth of North Atlantic Deep Water formation. If higher surface salinity values are applied in the southern ocean, a steady state is obtained whose tracer distributions and overturning are consistent with reconstructions of the deep circulation during the last glacial maximum. The two states are stable also under mixed boundary conditions and transitions are possible by smoothly varying the surface freshwater flux of one state to that of the other. These experiments suggest the importance of modified high‐latitude forcing in glacial‐to‐interglacial transitions.
Formation of auroral arcs via magnetosphere‐ionosphere coupling
Authors: Lyons L. R.
Published: February 2008

Brilliant displays of light are visible within a polar band of latitudes that surround each of the Earth's geomagnetic poles. These emissions are known as the aurora, and they are a dramatic consequence of the electromagnetic interaction between the Sun and the Earth. Energy is carried from the Sun by the plasma of the solar wind. This plasma impinges upon the magnetic field of the Earth and powers a wide range of electrodynamical phenomena that include the aurora. The aurora itself results from electrons that first are energized at very high altitudes by the solar wind‐Earth interactions and then travel down magnetic field lines until they hit the atmosphere. In this tutorial I discuss the basic physical processes that lead to the energization of these electrons and thus to the aurora.
Laboratory chamber experiments exploring the potential use of artificially ionized layers of gas as a Bragg reflector for over‐the‐horizon signals
Authors: Kuo S. P. , Zhang Y. S. , Lee M. C. , Kossey Paul , Barker Robert J.
Published: February 2008

A set of parallel plasma layers is generated by two intersecting microwave pulses in a chamber containing dry air at a pressure comparable to the upper atmosphere. The dependence of breakdown conditions on the pressure and pulse length is examined. The results are shown to be consistent with the appearance of tail erosion of microwave pulse caused by air breakdown. Bragg scattering experiments, using the plasma layers as a Bragg reflector, are then performed. Both time domain and frequency domain measurements of wave scattering are conducted. The experimental results are found to agree very well with the theory. Moreover, the time domain measurement of wave scattering provides an unambiguous way for determining the temporal evolution of electron density during the first 100 μs period. A Langmuir double probe is then used to determine the decay rate of electron density during a later time interval (1 to 1.1 ms). The propagation of high‐power microwave pulses through the air is also studied experimentally. The mechanisms responsible for two different degrees of tail erosion have been identified. The optimum amplitude of a 1.1 μs pulse for maximum energy transfer through the air has been determined.
Gravity wave parameters derived from traveling ionospheric disturbances observations in the auroral zone
Authors: Natorf L. , Schlegel K. , Wernik A. W.
Published: February 2008

Large‐scale wavelike fluctuations of ion velocity, as measured by the European incoherent scatter radar along the geomagnetic field line, have been attributed to gravity wave effects. The height‐dependent parameters of the causative gravity waves are calculated, taking into account the neutral horizontal wind and the electric field. The results are compared with the solutions of a dissipative dispersion relation. Much better agreement is achieved for the imaginary part of the vertical wave vector than for its real part. The calculated wave damping is greater than that given by theory. The possible reasons for this are discussed. It is suggested that E × B drift of the ions and vertical neutral winds, which are characteristic features of the auroral zone ionosphere, may contribute to the observed discrepancies.
Higher‐order mode theory of three‐dimensional irregular waveguides
Authors: Zernov Nikolay N.
Published: February 2008

The method of asymptotic solution of waveguide equations for higher‐order modes is generalized to the three‐dimensional case. Geometrical optics and full wave‐type solutions are constructed in the form of integral representations. In the geometrical optics case, two sorts of partial waves are used to construct the integral representation of the field: ray‐type waves and Gaussian beam‐type waves. In full wave solution the elementary waves of the integral representation are constructed by means of complex phase in Fresnel diffraction approximation for forward scattering. Asymptotic integral representations developed take into account coupling of the modes, describe the multiray effects and focusing of the higher‐order mode field. The method is valid for waveguides with either varying height or varying boundary conditions and dielectric properties.
Microwave propagation constant for a vegetation canopy at X band
Authors: Tavakoli Ahad , Sarabandi Kamal , Ulaby Fawwaz
Published: February 2008

An equivalent‐medium model is developed for vegetation media to relate the propagation constant γ, associated with propagation of the mean field through a vegetation canopy, to the geometrical and dielectric parameters of the canopy constituents at high frequencies. The model is intended for media containing vertical dielectric cylinders, representing the stalks, and randomly oriented, arbitrary shaped thin dielectric disks, representing the leaves. The formulation accounts for absorption and scattering losses by both stalks and leaves. A resistive sheet model in conjunction with the physical optics approximation is used to model scattering by the canopy leaves, which is valid when the leaf dimensions are larger than a wavelength. The model is found to be in good agreement with experimental results at 10.2 GHz. The experimental component of the study included measurements of the attenuation loss for horizontally polarized and vertically polarized waves transmitted through a fully grown corn canopy. The measurements were made at incidence angles of 20°, 40°, 60°, and 90° relative to normal incidence. The proposed model is suitable for cornlike canopies, provided the leaves are larger than λ in size.
Plasma sheath and presheath waves: Theory and experiment
Authors: Morin Gilbert A. , Balmain Keith G.
Published: February 2008

A theoretical and experimental study of characteristic waves propagating along a wire immersed in a weakly ionized, isotropic plasma is described. The theoretical model uses the hydrodynamic equations with a sheath profile composed of uniform plasma density steps. Boundary conditions at each step boundary and at the wire surface complete the model. The sheath wave characteristic mode is studied, and also a new family of characteristic waves named “presheath waves” is discovered and investigated. A novel experimental setup is described, one that facilitates the observation of these waves by means of impedance measurements. The measured sheath wave complex propagation constant agrees very well with theory for frequencies below approximately half the plasma frequency. It is shown that the “series resonance” found on small probes is directly related to the cutoff frequency of sheath wave propagation. A family of presheath waves is observed experimentally for the first time through a set of resonances that occur close to the frequencies predicted, above the sheath wave cutoff frequency.
Analysis of diffraction by material discontinuities in thin material‐coated planar surfaces based on Maliuzhinets method
Authors: Ly H. C. , Rojas R. G.
Published: February 2008

The electromagnetic diffraction of a plane wave (transverse electric case) by a two‐part material coated perfectly conducting ground plane and a material‐coated perfectly conducting half plane is studied using the Maliuzhinets method. Each of the coatings is assumed to be electrically thin so that it can be replaced in the analysis by a generalized impedance boundary condition of 0(t), where t is the corresponding coating thickness. To solve each of the boundary value problems completely, one needs to impose an additional constraint which is determined here by matching an approximate quasi‐static solution with the corresponding Maliuzhinets solution in the common region of overlap. This requires the asymptotic evaluation of the Maliuzhinets functions in various regions of the spectral domain. Furthermore, several constants that appear in the analysis are exactly related to the field and its derivatives evaluated at the edge or junction of the body under study. The backscattered and bistatic echo widths of a partially coated perfectly conducting half plane are computed with the solutions developed here (including multiple interaction) and compared with an independent moment method solution. The agreement between the two solutions is excellent.
Scattering of radio waves by high atomic Rydberg states
Authors: Smid Thomas S.
Published: February 2008

The scattering coefficient of highly excited atomic levels formed by recombination from a plasma and energetically broadened by plasma field fluctuations is determined. For this purpose, the population of the levels is calculated from a detailed balance equation which is solved consistently together with the corresponding equation for the free electron spectrum. Numerical results are obtained for parameters appropriate for ionospheric conditions. The solution proves to be in no way related to LTE situations, therewith invalidating usual treatments of the problem, which are not consistently based on a detailed balance approach. The theoretical result for the equilibrium coefficient for resonant scattering of radio waves by Rydberg atoms is consistent with experimental data obtained from the ionosphere. This suggests that the considered mechanism is exclusively responsible for the scattering of radio waves in particular and electromagnetic radiation in general.
Geology of the Hazelton Volcanic Belt in British Columbia: Implications for the Early to Middle Jurassic evolution of Stikinia
Authors: Marsden Henry , Thorkelson Derek J.
Published: February 2008

The Hazelton Group is an Early to Middle Jurassic volcanic and sedimentary succession that was deposited on the Stikine Terrane (Stikinia) of the Canadian Cordillera. The group is distributed across the entire width of Stikinia, a distance of about 300 km. Prior to post‐Hazelton contractional deformation, the width across which volcanism had occurred was at least 450 km. A review of chronologic and stratigraphic information corroborates an earlier suggestion that the Hazelton Group formed as a pair of coeval, partly subaerial volcanic chains separated by a subsiding, mainly sedimentary marine basin (Hazelton Trough). The trough developed in response to extension of Stikinia during the latest Triassic and/or earliest Jurassic, and remained the locus of moderate extension during deposition of the Hazelton Group. Volcanic rocks of the group are diverse, comprising subaerially and subaqueously deposited lavas and volcaniclastic rocks of mafic to felsic composition. Calc‐alkaline to tholeiitic characteristics, and depletion of high‐field‐strength elements relative to alkali and alkaline earth elements indicate a subduction‐related origin. The degree of alkalinity (medium‐ to high‐K) is fairly consistent throughout the group. The original width across which volcanism occurred is two to five times greater than widths of modern volcanic arcs. The anomalous width cannot be accounted for by time transgression of a single volcanic arc, because the two volcanic chains were coeval. Extremely low‐angle subduction of a single oceanic plate is also rejected as an explanation because alkalinity does not differ significantly between the two volcanic chains. Genesis of the Hazelton Group above a single subduction zone is therefore considered unlikely. We propose a model in which a pair of oceanic plates were subducted beneath Stikinia from opposite sides, in a manner analagous to the tectonic regime of the Philippine archipelago. Subduction angles of the two downgoing plates are postulated to be 30° to a depth of 150 km, a configuration consistent with a postulated forearc width of about 175 km on each side. Stikinia is thereby considered to have been an 800‐km‐wide microplate on which two island arcs developed. The Hazelton Trough was a back‐arc region common to both volcanic arcs. During deposition of the Hazelton Group, Stikinia was not part of the Intermontane Superterrane, an amalgam composed of more inboard terranes. Rather, it belonged to an oceanic microplate that accreted to the inboard terranes in late Early Jurassic to early Middle Jurassic time, during which Hazelton volcanism waned and ended.
Progressive evolution from uplift to orogen‐parallel transport in a late‐acadian, upper amphibolite‐ to granulite‐facies shear zone, south‐central Massachusetts
Authors: Peterson Virginia L. , Robinson Peter
Published: February 2008

Fabrics within a 1‐km‐wide shear zone along the eastern margin of the Bronson Hill anticlinorium in central Massachusetts provide important information on the regional tectonic development during the Devonian Acadian orogeny. Two distinct mineral lineations observed within the zone are each defined by alignment of coarse sillimanite needles and elongated quartz and feldspar grains and aggregates. Both lineations are associated with shearing and development of mylonitic fabrics. The character of the two lineations and related fabrics differs mainly in orientation and in the relationship of each lineation to the local orientation of quartz c axis fabrics. The interpreted earlier lineation has a steep west plunge and the later lineation has a shallow plunge and north‐south to southwest trend, parallel to the regional trend of the orogen. Extensive dynamic recrystallization of feldspar, the possible activity of prism slip in quartz, and the mineral assemblages in apparent equilibrium during deformation suggest that both lineations probably formed under upper amphibolite facies conditions, just after the thermal peak of Acadian metamorphism. A progressive change between the two deformation fabrics is suggested by evidence of overlap between the two, by their striking similarity in style and metamorphic conditions of formation, and by the similarity in orientation of their bulk strain axes, with a change only in the transport direction between them, kinematic indicators associated with each phase of lineation development are consistent throughout the shear zone. The earlier phase has a west‐side‐up sense of shear and the later phase indicates west‐side‐north movement. These phases are correlated with the regional backfold and dome stages of Acadian deformation. The kinematics and relationships between them suggest a revised tectonic model for the later part of the Acadian orogeny and for development of orogen‐parallel lineations in this region. In this model, uplift and thickening of basement gneisses in the Bronson Hill anticlinorium during the backfold stage was impeded by gravitational instability in a thermally weakened crust. This led to a change from subvertical to subhorizontal extension. East‐west Acadian shortening was still in progress, constraining this extension to a north‐south direction, parallel to the trend of the orogen.
Permian paleomagnetism of the northern Tien Shan and its tectonic implications
Authors: Audibert Mark , Bazhenov Mikhail L.
Published: February 2008

As a part of structural and paleomagnetic investigations of the late Paleozoic and younger tectonism in the Tien Shan, Permian sedimentary and volcanic rocks were sampled at four localities within the Northern Tien Shan west of the Issyk Kyl Lake. After complete thermal demagnetization, prefolding characteristic components were isolated for four localities, one of which also yielded a synfolding magnetization. When compared with the European reference data, the inclinations of these Permian rocks appear to be close to the reference ones. Mean declination for the northernmost locality also agrees with the European data, while the results for the southern parts of the area point to counterclockwise rotations. Comparison with data from an independent study (Thomas et al., 1992) shows that the rotations pre‐date Alpine deformation but probably postdate the initial stage of the folding which affected the Permian rocks. Analysis of paleomagnetic data from different parts of Kazakhstan together with our results from the Northern Tien Shan has led to the conclusion that these blocks had already been welded with the European plate in Permian time.
Progressive overprinting of normal fault systems and their role in Tertiary exhumation of the East Humboldt‐Wood Hills Metamorphic Complex northeast Nevada
Authors: Mueller Karl J. , Snoke Arthur W.
Published: February 2008

Low‐ and high‐angle, normal‐sense faults present along the northern margin of the East Humboldt‐Wood Hills metamorphic complex record a protracted history of episodic extensional unroofing. The earliest extension associated with the exhumation of the metamorphic complex occurred in the mid‐Eocene (about 50 Ma) during slip along the west‐rooted Wells Peak fault. Subsequent extension developed during movement along the east‐rooted Black Mountain fault system in the late Eocene (about 35 Ma). Continued exhumation of the high‐grade metamorphic rocks, occurred during the late Oligocene to early Miocene (about 29–23 Ma) along a west‐rooted, normal‐sense, plastic‐to‐brittle shear zone exposed along the length of the East Humboldt Range and much of the Ruby Mountains. The mylonitic shear zone, Wells Peak fault, and Black Mountain fault system were all overprinted by the west‐rooted, low‐ to high‐angle Mary's River fault system which was active from mid‐Miocene to Holocene. Broadly coeval extension in the Windermere Hills was also accommodated by the development of high‐angle, north and east dipping normal faults between approximately 35–12 Ma and 12–10 Ma.
Structure of the Irian Jaya Mobile BElt, Irian Jaya, Indonesia
Authors: Nash C. R. , Artmont G. , Gillan M. L. , Lennie D. , O'Connor G. , Parris K. R.
Published: February 2008

Image interpretation and field investigation over the Central Ranges of Irian Jaya has provided a structural overview of the inaccessible and largely unmapped Irian Jaya Mobile Belt (IJMB) in the region bounded by longitudes 136°30′E and 141°00′E. Throughout much of the southern (para‐autochthonous) part of the belt, structural facing directions are visible on imagery through selective illumination of hogbacks and cuestas formed on thick‐bedded Mesozoic siliciclastics and extensive Late Cretaceous‐Neogene platform carbonate sequences. North of the regional Derewo Fault Zone, the Derewo Metamorphics are regarded in part as correlatives of the “metamorphosed Om Beds” in western Papua New Guinea and display a regional structural grain parallel to the orogen. The limited extent of spectrally anomalous features associated with ultramafic rocks indicate that ophiolites are confined to the southern part of the “Irian Jaya Ophiolite Belt”, which may be largely composed of metamorphic rocks. Interpreted macroscopic and megascopic structures suggest the presence of seven discrete structural domains in the IJMB. From north to south these are (1) North Coast Basin region, consisting of Paleogene‐Neogene volcanics and sediments overlain by a Pliocene‐Pleistocene successor basin sequence; (2) an allochthonous terrane composed of ophiolites and high‐grade metamorphic rocks; (3) the Derewo metamorphic assemblage, displaying polyphase deformation; (4) a complex marginal zone within Mesozoic‐Paleogene miogeoclinal sediments which includes steep duplex structures and remnant klippen; (5) a 40 to 50‐km‐wide partly inverted synclinorium composed of miogeoclinal sediments; (6) a regional south vergent overturned anticlinorium formed by incompetent Paleozoic sediments; and (7) a foreland thrust domain involving both Mesozoic‐Cenozoic miogeoclinal cover and a deformed Neogene foreland molasse basin sequence. The regional disposition of interpreted structures in the Irian Jaya Mobile Belt and reference to published mapping in western Irian Jaya and western Papua New Guinea strongly indicates polyphase deformation. Late Oligocene‐Miocene docking of metamorphics, island arc assemblages and ophiolites produced tectonically stacked E‐W trending structures thrust on to the north margin of the Australian continent with its miogeoclinal cover. Late Miocene‐Pliocene collision with the Melanesian Arc brought the accreted Australian margin into contact with the westward moving Pacific Plate and instituted a regime of oblique transpressional convergence. Resulting structures include E‐W sinistral wrenching and NW thrusts along lateral E‐W ramps.