bluuebunny/crossref_metadata_2025 huggingface dataset:
Etale homotopy and sums-of-squares formulas
Authors: DUGGER DANIEL , ISAKSEN DANIEL C.
Published: April 2008

This paper uses a relative ofBP-cohomology to prove a theorem in characteristicpalgebra. Specifically, we obtain some new necessary conditions for the existence of sums-of-squares formulas over fields of characteristicp>2. These conditions were previously known in characteristic zero by results of Davis. Our proof uses a generalized étale cohomology theory called étaleBP2.
Man, Woman, Artist? Rethinking the Muse in Vienna 1900
Authors: Brandow-Faller Megan
Published: April 2008

Veni, CreatorSpiritus! From the triumphant chorus of Gustav Mahler's Eighth Symphony to the muse-choir adorning Gustav Klimt's Beethoven Frieze, the Vienna Moderns celebrated the inspirational powers of the feminine. Muses, or the spiritual forces guiding acts of artistic and literary creation, have long been associated with the creative output of the Vienna Moderns, the groups and individuals responsible for the development of modernism in the arts and literature in Vienna from 1890 to 1910. These inspiring women—commonly personified as femmes fatales, femmes fragiles, and femmes savantes—played a crucial role in bringing the art, architecture, and writing of Viennese Modernism to fruition.
The Husbandman: Tomáš Masaryk's Leader Cult in Interwar Czechoslovakia
Authors: Orzoff Andrea
Published: April 2008

For almost a century now, Tomáš Masaryk has been one of the most famous Czechs in world history. He was nominated at least twice for a Nobel Prize and lionized by noteworthies across the globe. However, that esteem paled when compared to the adoration expressed for him at home. During and after his lifetime, Masaryk was presented as the embodiment of moral rectitude, cosmopolitan erudition, and democratic egalitarianism. By the end of the 1930s, Masaryk's association with the state he had helped found was impressively total. But it was not accidental. Many different groups and factors collaborated to position Masaryk as the only man capable of superseding Czechoslovakia's ethnic and political divisions, a devoted democrat to lead Europe's self-styled ideal new democracy.
Screening Transcendence: Austria's Emigrantenfilm and the Construction of an Austrofascist Identity in Singende Jugend
Authors: von Dassanowsky Robert
Published: April 2008

Political developments between 1933 and 1934 placed Austrian cinema under more governmental control than at any time since World War I. In 1934 the new chancellor, Engelbert Dollfuss, attempted to counter a looming civil war and the growing power of the Austrian National Socialists by disbanding the embattled parliament and instituting a nonparty clerico-authoritarian corporate state, often referred to as Austrofascist. Although Dollfuss's Fatherland Front was intended to be a national unity movement above party politics, it was, in fact, led by the conservative, Catholic-oriented Christian Social Party. Subsequent laws, which outlawed all political parties, may have temporarily silenced the National Socialists, but they also alienated a substantial portion of Austria's electorate that had supported the Social Democrats.
Transporter-mediated GABA responses in horizontal and bipolar cells of zebrafish retina
Authors: NELSON RALPH , BENDER ANNA M. , CONNAUGHTON VICTORIA P.
Published: April 2008

GABA-mediated interactions between horizontal cells (HCs) and bipolar cells (BCs) transform signals within the image-processing circuitry of distal retina. To further understand this process, we have studied the GABA-driven membrane responses from isolated retinal neurons. Papain-dissociated retinal cells from adult zebrafish were exposed to GABAergic ligands while transmembrane potentials were monitored with a fluorescent voltage-sensitive dye (oxonol, DiBaC4(5)). In HCs hyperpolarizing, ionotropic GABA responses were almost never seen, nor were responses to baclofen or glycine. A GABA-induced depolarization followed by after hyperpolarization (dep/AHP) occurred in 38% of HCs. The median fluorescence increase (depcomponent) was 0.17 log units, about 22 mV. HC dep/AHP was not blocked by bicuculline or picrotoxin. Muscimol rarely evoked dep/AHP responses. In BCs picrotoxin sensitive, hyperpolarizing, ionotropic GABA and muscimol responses occurred in most cells. A picrotoxin insensitive dep/AHP response was seen in about 5% of BCs. The median fluorescence increase (depcomponent) was 0.18 log units, about 23 mV. Some BCs expressed both muscimol-induced hyperpolarizations and GABA-induced dep/AHP responses. For all cells, the pooled Hill fit to mediandepamplitudes, in response to treatments with a GABA concentration series, gave an apparentkof 0.61 μM and annof 1.1. The dep/AHP responses of all cells required both extracellular Na+and Cl−, as dep/AHP was blocked reversibly by Li+substituted for Na+and irreversibly by isethionate substituted for Cl−. All cells with dep/AHP responses in zebrafish have the membrane physiology of neurons expressing GABA transporters. These cells likely accumulate GABA, a characteristic of GABAergic neurons. We suggest Na+drives GABA into these cells, depolarizing the plasma membrane and triggering Na+, K+-dependent ATPase. The ATPase activity generates AHP. In addition to a GABA clearance function, these large-amplitude transporter responses may provide an outer plexiform layer GABA sensor mechanism.
Electrophysiological evidence of GABAA and GABAC receptors on zebrafish retinal bipolar cells
Authors: CONNAUGHTON VICTORIA P. , NELSON RALPH , BENDER ANNA M.
Published: April 2008

To refine inhibitory circuitry models for ON and OFF pathways in zebrafish retina, GABAergic properties of zebrafish bipolar cells were studied with two techniques: whole cell patch responses to GABA puffs in retinal slice, and voltage probe responses in isolated cells. Retinal slices documented predominantly axon terminal responses; isolated cells revealed mainly soma-dendritic responses. In the slice, GABA elicited a conductance increase, GABA responses were more robust at axon terminals than dendrites, and Erev varied with [Cl−]in. Axon terminals of ON- and OFF-type cells were similarly sensitive to GABA (30–40 pA peak current); axotomized cells were unresponsive. Bicuculline-sensitive, picrotoxin-sensitive, and picrotoxin-insensitive components were identified. Muscimol was as effective as GABA; baclofen was ineffective. Isolated bipolar cells were either intact or axotomized. Even in cells without an axon, GABA or muscimol (but not baclofen) hyperpolarized dendritic and somatic regions, suggesting significant distal expression. Median fluorescence change for GABA was −0.22 log units (∼ −16 mV); median half-amplitude dose was 0.4 μM. Reduced [Cl−]out blocked GABA responses. GABA hyperpolarized isolated ON-bipolar cells; OFF-cells were either unresponsive or depolarized. Hyperpolarizing GABA responses in isolated cells were bicuculline and TPMPA insensitive, but blocked or partially blocked by picrotoxin or zinc. In summary, axon terminals contain bicuculline-sensitive GABAA receptors and both picrotoxin-sensitive and insensitive GABAC receptors. Dendritic processes express zinc- and picrotoxin-sensitive GABAC receptors.
Reply to Philipona and O'Regan
Authors: JOHNSON KENT , WRIGHT WAYNE
Published: April 2008

This paper responds to Philipona and O'Regan (2006), which attempts to account for certain color phenomena by appeal to singularities in the space of “accessible information” in the light striking the retina. Three points are discussed. First, it is unclear what the empirical significance/import is of the mathematical analysis of the data regarding the accessible information in the light. Second, the singularity index employed in the study is both mathematically and empirically faulty. Third, the connection drawn between their findings and some data from the World Color Survey is lacking in quantitative analysis in places where it is needed. The difficulties raised prevent Philipona & O'Regan's conclusions from being accepted.
Long-term cellular and regional specificity of the photoreceptor toxin, iodoacetic acid (IAA), in the rabbit retina
Authors: LIANG LI , KATAGIRI YOSHIAKI , FRANCO LUISA M. , YAMAUCHI YASUYUKI , ENZMANN VOLKER , KAPLAN HENRY J. , SANDELL JULIE H.
Published: April 2008

This study investigated the anatomical consequences of a photoreceptor toxin, iodoacetic acid (IAA), in the rabbit retina. Retinae were examined 2 weeks, 1, 3, and 6 months after systemic IAA injection. The retinae were processed using standard histological methods to assess the gross morphology and topographical distribution of damage, and by immunohistochemistry to examine specific cell populations in the retina. Degeneration was restricted to the photoreceptors and was most common in the ventral retina and visual streak. In damaged regions, the outer nuclear layer was reduced in thickness or eliminated entirely, with a concomitant loss of immunoreactivity for rhodopsin. However, the magnitude of the effect varied between animals with the same IAA dose and survival time, suggesting individual differences in the bioavailability of the toxin. In all eyes, the inner retina remained intact, as judged by the thickness of the inner nuclear layer, and by the pattern of immunoreactivity for protein kinase C-α (rod bipolar cells) and calbindin D-28 (horizontal cells). Müller cell stalks became immunoreactive for glial fibrillary acidic protein (GFAP) even in IAA-treated retinae that had no signs of cell loss, indicating a response of the retina to the toxin. However, no marked hypertrophy or proliferation of Müller cells was observed with either GFAP or vimentin immunohistochemistry. Thus the selective, long lasting damage to the photoreceptors produced by this toxin did not lead to a reorganization of the surviving cells, at least with survival as long as 6 months, in contrast to the remodeling of the inner retina that is observed in inherited retinal degenerations such as retinitis pigmentosa and retinal injuries such as retinal detachment.
Retinal ganglion cell density of the black rhinoceros (Diceros bicornis): Calculating visual resolution
Authors: PETTIGREW JOHN D. , MANGER PAUL R.
Published: April 2008

A single right retina from a black rhinoceros was whole mounted, stained and analyzed to determine the visual resolution of the rhinoceros, an animal with reputedly poor eyesight. A range of small (15-μm diameter) to large (100-μm diameter) ganglion cell types was seen across the retina. We observed two regions of high density of retinal ganglion cells at either end of a long, but thin, horizontal streak. The temporal specialization, which receives light from the anterior visual field, exhibited a ganglion cell density of approximately 2000/mm2, while the nasal specialization exhibited a density of approximately 1500/mm2. The retina exhibited a ganglion cell density bias toward the upper half, especially so, the upper temporal quadrant, indicating that the rhinoceros would be processing visual information from the visual field below the anterior horizon for the most part. Our calculations indicate that the rhinoceros has a visual resolution of 6 cycles/degree. While this resolution is one-tenth that of humans (60 cycles/deg) and less than that of the domestic cat (9 cycles/deg), it is comparable to that of the rabbit (6 cycles/deg), and exceeds that seen in a variety of other mammals including seals, dolphins, microbats, and rats. Thus, the reputation of the rhinoceros as a myopic, weakly visual animal is not supported by our observations of the retina. We calculate that the black rhinoceros could readily distinguish a 30 cm wide human at a distance of around 200 m given the appropriate visual background.
Reply to Johnson and Wright
Authors: Philipona David , O'Regan J. Kevin
Published: April 2008

Johnson and Wright (hereafter J&W) claim, in their brief communication entitled “Reply to Philipona and O'Regan” that there are various difficulties in the reasoning in Philipona and O'Regan's (2006) paper entitled “Color naming, unique hues and hue cancellation predicted from singularities in reflection properties,” which was published in this journal in 2006. These difficulties supposedly prevent our paper's conclusions from being accepted.
Responses to direction and transparent motion stimuli in area FST of the macaque
Authors: ROSENBERG ARI , WALLISCH PASCAL , BRADLEY DAVID C.
Published: April 2008

Motion transparency occurs when multiple object velocities are present within a local region of retinotopic space. Transparent signals can carry information useful in the segmentation of moving objects and in the extraction of three-dimensional structure from relative motion cues. However, the physiological substrate underlying the detection of motion transparency is poorly understood. Direction tuned neurons in area MT are suppressed by transparent stimuli, suggesting that other motion sensitive areas may be needed to represent this signal robustly. Recent neuroimaging evidence implicated two such areas in the macaque superior temporal sulcus. We studied one of these, FST, with electrophysiological methods and found that a large fraction of the neurons responded well to two opposite directions of motion and to transparent stimuli containing those same directions. A linear combination of MT-like responses qualitatively reproduces this behavior and predicts that FST neurons can be tuned for transparent motion containing specific direction and depth components. We suggest that FST plays a role in motion segmentation based on transparent signals.
Strong motion compatible source geometry
Authors: Raghu Kanth S. T. G. , Iyengar R. N.
Published: April 2008

Strong motion array records are analyzed in this paper to identify and map the source zone of four past earthquakes. The source is represented as a sequence of double couples evolving as ramp functions, triggering at different instants, distributed in a region yet to be mapped. The known surface level ground motion time histories are treated as responses to the unknown double couples on the fault surface. The location, orientation, magnitude, and risetime of the double couples are found by minimizing the mean square error between analytical solution and instrumental data. Numerical results are presented for Chi‐Chi, Imperial Valley, San Fernando, and Uttarakashi earthquakes. Results obtained are in good agreement with field investigations and those obtained from conventional finite fault source inversions.
Rise dynamics and relative ash distribution in vulcanian eruption plumes at Santiaguito Volcano, Guatemala, revealed using an ultraviolet imaging camera
Authors: Yamamoto Hiroko , Watson I. Matthew , Phillips Jeremy C. , Bluth Gregg J.
Published: April 2008

Santiaguito Volcano, Guatemala, regularly produces small vulcanian eruption plumes which rise to heights of up to 2 km. A combined study using a novel UV camera coupled with classical analysis of the fluid dynamics of finite‐volume buoyant releases (thermals) has been used to develop a detection algorithm for ground‐based volcanic ash monitoring. Analysis of plume rise dynamics shows that vulcanian eruption plumes at Santiaguito behave as axisymmetric buoyant thermals, and this behaviour is consistent with eruption of volcanic gas and particle mixtures whose initial momentum is dissipated by flow through a porous capping layer. The UV imager has been adapted to detect relative ash burdens through the use of a single filter, centred on 307 nm, an edge detection and background fitting procedure and normalising to a simple theoretical model. The method provides the capability to observe and measure the internal structure of the plume, and processes occurring during plume rise, including concentration of ash over time into the thermal ‘head’, increased ash at the plume edges during early formation of the thermal and dilution at the top of the plume head as entrainment occurs.
Nonlinear mirror waves in non‐Maxwellian space plasmas
Authors: Pokhotelov O. A. , Sagdeev R. Z. , Balikhin M. A. , Onishchenko O. G. , Fedun V. N.
Published: April 2008

A theory of finite‐amplitude mirror type waves in non‐Maxwellian space plasmas is developed. The collisionless kinetic theory in a guiding center approximation, modified for accounting of the finite ion Larmor radius effects, is used as the starting point. The model equation governing the nonlinear dynamics of mirror waves near instability threshold is derived. In the linear approximation it describes the classical mirror instability that is valid for a wide class of the velocity distribution functions. In the nonlinear regime the mirror waves form solitary structures that have the shape of magnetic holes. The formation of such structures and their nonlinear dynamics has been analyzed both analytically and numerically. It is suggested that the main nonlinear mechanism responsible for mirror instability saturation is associated with modification (flattening) of the shape of the background ion distribution function in the region of small parallel particle velocities. The width of this region is of the order of the particle trapping zone in the mirror hole. Near the mirror instability threshold the saturation arises before its width reaches the ion thermal velocity. The nonlinear mode coupling effects in this approximation are smaller and unable to take control over evolution of the space profile of saturated mirror waves or lead to their magnetic collapse. This results in the appearance of quasi‐stable solitary mirror structures having the form of deep magnetic depressions. A phenomenological description of this process is formulated. The relevance of the theoretical results to recent satellite observations is stressed.
The teleseismic signature of fossil subduction: Northwestern Canada
Authors: Mercier J.‐P. , Bostock M. G. , Audet P. , Gaherty J. B. , Garnero E. J. , Revenaugh J.
Published: April 2008

Between June 2003 and September 2005, 20 broadband, three‐component seismometers were deployed along the MacKenzie‐Liard Highway in Canada's Northwest Territories as part of the joint Lithoprobe‐IRIS Canada Northwest Experiment (CANOE). These stations traverse a paleo‐Proterozoic suture and subduction zone that has been previously documented to mantle depths using seismic reflection profiling. Teleseismic receiver functions computed from ∼250 earthquakes clearly reveal the response of the ancient subduction zone. On the radial component, the suture is evident as a direct conversion from the Moho, the depth of which increases from ∼30 km to ∼50 km over a horizontal distance of ∼70 km before its signature disappears. The structure is still better defined on the transverse component where the Moho appears as the upper boundary of a 10 km thick layer of anisotropy that can be traced from 30 km to at least 90 km depth. The seismic response of this layer is characterized by a frequency dependence that can be modeled by upper and lower boundaries that are discontinuous in material properties and their gradients, respectively. Anisotropy can be characterized by a ±5% variation in shear velocity and hexagonal symmetry with a fast axis that plunges at an oblique angle to the subduction plane. The identification of this structure provides an unambiguous connection between fossil subduction and fine‐scale, anisotropic mantle layering. Previous documentation of similar layering below the adjacent Slave province and from a range of Precambrian terranes across the globe provides strong support for the thesis that early cratonic blocks were stabilized through processes of shallow subduction.
Formation and export of dense shelf water from the Adélie Depression, East Antarctica
Authors: Williams G. D. , Bindoff N. L. , Marsland S. J. , Rintoul S. R.
Published: April 2008

Coastal polynyas in the Adélie Depression are an important source of Antarctic Bottom Water to the Australian‐Antarctic Basin. We present time series (April 1998 to May 1999 and August 1999 to February 2000) of data from temperature‐salinity sensors, in both the Adélie Depression and the known outflow region of the Adélie Sill, to describe the annual cycle of shelf water densities. From April through September, salinification beneath the polynya produces dense shelf waters. During September–October, shelf water densities in the depression peak at 27.94 kg m−3, and the cooling and freshening signature of Ice Shelf Water is observed north of Buchanan Bay. In November–December, shelf water densities decrease as intrusions of warm and relatively fresh modified Circumpolar Deep Water enter east of the Adélie Sill. From January–March the surface layer is conditioned by the cooling of the atmosphere, which overturns the upper water column. At the Adélie Sill, observed daily mean currents were approximately 10 cm s−1 with intense instantaneous currents greater than 50 cm s−1 at the sill depth. Using an idealized outflow region with a rectangular cross‐sectional area (6 × 106 m2), we present the first estimates of shelf water export by potential density class. Assuming shelf water with a minimum density of 27.88 kg m−3 has sufficient negative buoyancy for downslope mixing and a fourfold volume increase (1:3 mixing ratio) from entrainment, the dense shelf water export of 0.1–0.5 Sv results in an annual average production of bottom water in this region of between 0.4 and 2.0 Sv. The wide range in bottom water estimate results from data limitations, and a narrowing of this range requires further mooring observations.
Use of SF6 to estimate anthropogenic CO2 in the upper ocean
Authors: Tanhua Toste , Waugh Darryn W. , Wallace Douglas W. R.
Published: April 2008

The highest concentrations of anthropogenic carbon (Cant) are found in the upper layers of the world ocean. However, this is where seasonal variability of inorganic carbon and related parameters due to thermal and biological effects complicates use of back‐calculation approaches for Cant. Tracer based approaches to Cant estimation are unaffected by biological variability and have found wide application. However, slow‐down, even reversal, of the atmospheric growth of chlorofluorocarbons (CFCs) restricts use of these tracers for Cant estimation for waters ventilated since the mid 1990s. Here we apply SF6, a tracer that continues to increase in the atmosphere, as a basis for the Cant estimation, using samples collected in the midlatitude North Atlantic in 2004. Cant estimates derived from water mass transit time distributions (TTDs) calculated with SF6 are compared to those based on CFC‐12. For recently ventilated waters (pCFC‐12 > ∼450 ppt), the uncertainty of SF6 based estimates of Cant is ∼6 μmol kg−1 less than that of CFC‐12 based estimates. CFC‐12 based estimates remain more reliable for older (deeper) water masses, as a result of the longer input history and more readily detectable concentrations of CFC‐12. Historical data suggest that the near‐surface saturation of CFC‐12 has increased over time, in inverse proportion to its atmospheric growth rate. Use of a time‐dependent saturation of CFC‐12 in TTD calculations appears to provide more reliable estimation of Cant.
Factors controlling the fluvial export of large woody debris, and its contribution to organic carbon budgets at watershed scales
Authors: Seo Jung Il , Nakamura Futoshi , Nakano Daisuke , Ichiyanagi Hidetaka , Chun Kun Woo
Published: April 2008

The fluvial export of large woody debris (LWD) was monitored in 131 reservoirs throughout Japan. Published data on the fluvial export of dissolved and particulate organic carbon were used to estimate the contributions of LWD in carbon budgets. Of all variables tested, watershed area was most important in explaining LWD carbon (LWDC) export, followed by annual precipitation. LWDC export per unit area was relatively high in small watersheds, highest in intermediate‐sized watersheds, and decreased in large watersheds. In small watersheds, a large proportion of LWD retained on narrow valley floors may fragment or decay and eventually be exported in forms other than LWD. In intermediate‐sized watersheds, LWD supplied from upstream and recruited by bank erosion is consistently transported downstream. In large watersheds, LWD recruitment is limited and LWD transported from upstream is stored on large floodplains. These differences in LWD recruitment, retention and transport in watersheds of different sizes lead to the proportion of LWDC in organic carbon exports to be maximum in intermediate‐sized watersheds and decline rapidly in large watersheds.
Derivation of travel time of limestone cave drip water using tritium/helium 3 dating method
Authors: Yamada Makoto , Ohsawa Shinji , Matsuoka Hiroshige , Watanabe Yumiko , Brahmantyo Budi , Maryunani Khoiril Anwar , Tagami Takahiro , Kitaoka Koichi , Takemura Keiji , Yoden Shigeo
Published: April 2008

A method of deriving the travel time of drip water based on the tritium/helium 3 (3H/3He) dating method was demonstrated in limestone caves in Indonesia. The examined drip waters from two caves, which were taken using different methods, included 2.15 (± 0.84) TU and 1.35 (± 0.77) TU of tritiogenic 3He. From the derived tritiogenic 3He contents and the measured tritium concentrations of the drip water samples (2.02 ± 0.27 TU and 1.19 ± 0.40 TU), the 3H/3He ages (travel times) of the drip waters from the two caves were estimated as 12.9 ± 3.8 years and 13.5 ± 6.3 years. The latter age is probably overestimated because re‐equilibration of He between drip water and cave air is expected to occur during sampling. For reliable derivation of 3H/3He age, it is advisable to take a drip water sample that is kept isolated from the cave air.
Determination of radar coverage diagrams in complex environments using closed form ray tracing
Authors: Abdulla Shakir , Raouf Samir , Al‐anbari Abdulamir , Kadri Ashraf
Published: April 2008

In the present work, radio ray tracing equations are derived for linear refractive index profiles. The solution is obtained in closed form without approximations. Wave front reconstruction is proposed to infer wave parameters for a large number of points from few ray traces. A computer program to evaluate radar coverage diagrams was developed utilizing a closed form solution in a complex troposphere consisting of several linear refractive index profile layers in conjunction with complex topography. The computer program is expected to be highly superior in speed compared to similar programs due to the use of wave front reconstruction and closed form ray tracing.
Application of a generalized Leibniz rule for calculating electromagnetic fields within continuous source regions
Authors: Silberstein Marian
Published: April 2008

In deriving the electric and magnetic fields in a continuous source region by differentiating the vector potential, Yaghjian (1985) explains that the central obstacle is the dependence of the integration limits on the differentiation variable. Since it is not mathematically rigorous to assume the curl and integral signs are interchangeable, he uses an integration variable substitution to circumvent this problematic dependence. Here, we present an alternative derivation, which evaluates the curl of the vector potential volume integral directly, retaining the dependence of the limits of integration on the differentiation variable. It involves deriving a three‐dimensional version of Leibniz' rule for differentiating an integral with variable limits of integration, and using the generalized rule to find the Maxwellian and cavity fields in the source region.
Computation of transient electromagnetic waves in inhomogeneous media
Authors: Moghaddam M. , Chew W. C. , Anderson B. , Yannakakis E. , Liu Q. H.
Published: April 2008

A brief summary of the methods of solving transient electromagnetic wave problems in inhomogeneous media will be given. The two distinct general techniques, which are the inverse Fourier transformation of time‐harmonic solutions and the direct time‐domain formulation, will be illustrated by way of two examples. In the first example, an efficient numerical mode‐matching method to obtain the response of an electromagnetic source in a two‐dimensional cylindrical inhomogeneity is described. Using this method, the solution is first obtained in the frequency domain. The time‐domain solution is then found by inverse Fourier transforming the frequency‐domain solution. In the second method, a finite‐difference scheme is used to find the transient response of a point source in a two‐dimensional inhomogeneity. Two different methods are proposed to treat the source region singularity. Transmitting boundary conditions are applied on the walls of the finite difference grid so that a finite‐sized box can be used to model an infinite region.
Iterative methods for solving integral equations
Authors: Kleinman R. E. , van den Berg P. M.
Published: April 2008

A number of iterative algorithms to solve integral equations arising in field problems are discussed. We describe the essential features of the Neumann Series, overrelaxation methods, Krylov subspace methods, and the conjugate gradient technique. Proofs of convergence of the conjugate gradient method are directly available when the underlying integral operator is self‐adjoint, and in this case the method is equivalent to the Krylov method. However, for non‐self‐adjoint operators the conjugate gradient method requires an implicit symmetrization which results in poorer convergence than that obtained using the Krylov method. Some convergence results are also available for overrelaxation methods for both self‐adjoint and non‐self‐adjoint operators. Relations between all of the methods will be described and numerical performance will be contrasted using a uniform square error criterion. All the methods are treated in the continuous operator form which is especially useful in using the physical setting to arrive at effective preconditioners.
Prediction of compact antenna range performance by radiative coupling
Authors: McNair P. A. C. , Olver A. D.
Published: April 2008

The performance of compact antenna ranges is analyzed in terms of the radiative coupling between two near‐field antennas. Both a geometric optics model and a more exact model based on an angular spectrum of plane waves are used. A two dimensional model shows that an anomaly exists when the test antenna sees the edges of the range reflector. The relationship between test antenna size and range reflector size is investigated and the influence of the shape of the range reflector field studied.
Adiabatically tapered directional coupling between dielectric waveguides
Authors: Leminger Ottokar , Zesch Gerhard
Published: April 2008

Conventional dielectric dual‐channel directional couplers used in integrated optics are subject to severe tolerance restrictions on the waveguide parameters, in order to guarantee nearly complete power transfer. These restrictions are relaxed considerably by tapering suitably both waveguides. Such adiabatically tapered couplers can be designed easily along the beam propagation method combined with the effective index method. Computer simulations confirm their advantage.
Asymptotic description of ultrashort electromagnetic pulse propagation in a linear, causally dispersive medium
Authors: Oughstun Kurt Edmund , Laurens Judith Elizabeth Keil
Published: April 2008

The asymptotic description of the propagation of an input pulse‐modulated harmonic signal of fixed angular frequency ωc and initial pulse width T into the half‐space z > 0 that is occupied by a single resonance Lorentz medium with resonance frequency ω0 and damping constant δ is described. Particular attention is given to both the input rectangle‐modulated and Gaussian‐modulated fields. The dynamical evolution of the pulse with increasing propagation distance z > 0 is described for both long and very short initial pulse widths T when the signal frequency is either below, near, or above the medium resonance frequency. For the input rectangular pulse envelope the asymptotic theory clearly shows that the resultant pulse distortion in the dispersive medium is due primarily to the Sommerfeld and Brillouin precursor fields that are excited by the leading and trailing edges of the input pulse. It is shown that the pulse distortion becomes severe when the propagation distance z is such that the precursor fields that are associated with the trailing edge of the pulse interfere with the precursor fields associated with the leading edge. Such is not the case, however, for the input Gaussian pulse envelope. For this pulse shape the asymptotic theory shows that the propagated field can be expressed solely in terms of a generalized Sommerfeld and Brillouin precursor field. If the applied signal frequency ωc is either below or near the medium resonance frequency, the propagated field is found to be dominated by a generalized Brillouin precursor field structure, while if ωc is well above the medium resonance frequency, the propagated field is dominated by a generalized Sommerfeld precursor field structure. The pulse distortion is shown to be due solely to the manner in which the initial Gaussian pulse‐envelope spectrum modifies the precursor field amplitude. Finally, the frequency dependence of the pulse velocity in the single resonance Lorentz medium is compared for these two different pulse types.
A highly accurate feed model for microstrip, dual patch microstrip and microstrip backfire antennas
Authors: Vandenbosch G. , Van de Capelle A.
Published: April 2008

A new, more accurate model for the coaxial feed of microstrip, dual patch microstrip and microstrip backfire antennas is presented. In combination with the use of edge expansion functions in the method of moments this leads to a highly accurate analysis of these three types of antennas in a homogeneous medium. Results obtained using the model are compared with experimental results for few structures.
Ray formulation of waves guided by circular cylindrically stratified dielectrics
Authors: Felsen L. B. , Naishadham K.
Published: April 2008

Curved dielectric layers, either by themselves or as claddings on perfectly conducting substrates, are useful elements for guiding and confining electromagnetic energy. The prototype configuration, explored here, has circular cylindrical symmetry. The fields excited by an arbitrarily located and oriented dipole source are the basic building blocks, from which other source configurations can be synthesized. By recourse to the general theory of spectral representations, various alternative formulations of the solution can be constructed, and, via asymptotic techniques at high frequencies, examined concerning their utility and physical interpretation. In this paper, special emphasis is placed on source and observer locations on the outermost shell boundary; this is the relevant Green's function for application to stripline or printed circuits that are deposited there. A good parametrization of the relevant wave phenomena is achieved by treating the radial ρ portion of the problem in terms of the radial discrete mode spectrum, while retaining a continuum for the azimuthal ф and longitudinal z portions, with the ф domain extended to infinity. By asymptotic reduction the radial mode fields guided inside or on the layers follow helical (ф, z) trajectories originating at the source; as a result of dispersion, these waves have anisotropic speeds depending on the departure angle. Therefore in an equivalent two‐dimensional rectilinear (ф, z) space the local modes can be modeled as ray fields that propagate away from the source in a homogeneous anisotropic medium. The theory is summarized briefly. For the special case of a single layer on a perfectly conducting substrate, excited by an axial electric current element, modal dispersion surfaces (azimuthal versus longitudinal wave number) shown for various layer thicknesses reveal different degrees of anisotropy. Full calculation of the spectral integral, both numerically (for reference solution) and asymptotically, is in progress.
Planar microwave imaging camera for biomedical applications: Critical and prospective analysis of reconstruction algorithms
Authors: Bolomey J. Ch. , Pichot Ch. , Garboriaud G.
Published: April 2008

This paper is dealing with the biomedical applications of active microwave imaging. As a result of extensive preliminary evaluations conducted by means of a 2.45‐GHz planar camera, it has been proven that active microwave imaging is able to provide a very sensitive means of investigation in such applications. While technological problems have been conveniently solved, the main problem still remains to determine the most efficient way to process the measured data in order to get the best quality of the reconstructed images. Until now, diffraction tomography algorithms based on a spectral approach have been used. The main limitations of this approach and some attempts to reduce them are analyzed. A more prospective discussion allows to identify the most promising reconstruction techniques. Among them, iterative space‐domain formulations appear to provide a suitable base for achieving quantitative imaging thanks to a convenient use of the available a priori information.
A transverse aperture‐integral equation solution for edge diffraction by multiple layers of homogeneous material
Authors: Pearson L. W. , Whitaker R. A.
Published: April 2008

The transverse aperture‐integral equation method provides a means of computation for diffraction coefficients at blunt edges of a broad class of stratified layers, including sheet‐anisotropy models for conducting composites. In this paper we concentrate on the application of the method when the material profile comprises layers of homogeneous, potentially lossy material. The method proceeds from defining an artificial aperture perpendicular to a semi‐infinite, planar, stratified region and passing through the terminal edge of the region. An integral equation is formulated over this infinite‐extent aperture, and the solution to the integral equation represents the influence of the edge. The kernel in the integral equation is a weighted sum of the Green's functions for the respective half spaces lying on either side of the aperture plane. The vector wave equation is separable in each of these half spaces, resulting in Green's functions that are expressible analytically. The Green's function for the stratified half space is stated in terms of a Sommerfeld‐type integral.
High‐frequency electromagnetic scattering by open‐ended waveguide cavities
Authors: Pathak P. H. , Burkholder R. J.
Published: April 2008

Some approaches for analyzing the high‐frequency electromagnetic scattering by open‐ended waveguide cavities with a planar interior termination are discussed. The contribution to scattering arising from the open end is found using the geometrical theory of diffraction based equivalent current method, while the interior cavity scattering contribution can be treated by several different approaches, for example, hybrid modal, ray, or beam methods; other contributions are ignored for the reasons outlined in the paper. Numerical results based on these methods are presented.
Electromagnetic wave scattering by a two layered piecewise homogeneous confocal elliptic cylinder
Authors: Sebak A. , Shafai L. , Ragheb H. A.
Published: April 2008

The scattering properties of a homogeneous elliptic cylinder with a confocal dielectric or magnetic coating are investigated analytically. The method of separation of variables is used to determine, exactly, the field distributions in each region for both transverse electric and transverse magnetic excitations. The technique can be easily extended to handle any number of layers. The behavior of the scattered field in the far zone is illustrated with numerical results for different core and coating material types, axial ratios, and electrical sizes.
Diffusion approximation in the theory of weak localization of radiation in a discrete random medium
Authors: Barabanenkov Yu. N. , Ozrin V. D.
Published: April 2008

Coherent backscattering of scalar waves from a semi‐infinite discrete nonabsorbing random medium of nonisotropic scatterers is considered. The dependence of the line shape of the backscattering peak of the scattering nonisotropy is studied in the frameworks of the diffusion approximation. Various versions of this approximation are applied in which, in particular, the gradiental terms are taken into account, and various forms of boundary conditions are used. It is shown that the peak line shape is almost independent of the form of boundary conditions only in the vicinity of the backscattering direction. The validity of the diffusion approximation is discussed.
A new method of calculating the transmission and reflection coefficients and fields in a magnetized plasma layer
Authors: Zhang D. Y.
Published: April 2008

It is shown that the calculation of the transmission and reflection coefficients and the fields of electromagnetic waves in a magnetized plasma layer can be solved as a two‐point boundary value problem. Test results are presented to demonstrate the applicability of this method.
Natural frequencies and eigencurrents of the elliptic disk
Authors: Björkberg Jonas
Published: April 2008

The natural frequencies of an object determine the late time response of a transient excitation and govern the time harmonic scattering in the resonance region. In this paper the natural frequencies and the corresponding eigencurrents of a perfectly conducting elliptic disk are treated using the null‐field approach. The elliptic disk is obtained as the zero thickness limit of a general ellipsoid. It is found that the dominant frequencies, i.e., the frequencies that stay in the finite complex frequency plane when the disk approaches the “wire limit,” line up in two series. The first series, closest to the real axis, can be identified as the resonances of a thin wire antenna, corresponding to quasi‐stationary current waves on the disk. The second series has more involved nonstationary current patterns. The theoretical results are illustrated with numerical calculations.
The far sidelobes and noise temperature of a small paraboloidal antenna used for radio astronomy
Authors: Anderson M. D. , Landecker T. L. , Routledge D. , Vaneldik J. F.
Published: April 2008

Measurements have been made of the radiation pattern of a symmetrical, prime‐focus paraboloidal antenna which is used as a radio telescope at 1420 MHz. A transmitter was placed on a nearby hilltop, and the test antenna, used as a receiver, was driven through the range of directions permitted by its mounting; about 55% of the radiation pattern was accessible to measurement. The main beam, near sidelobes, and spillover lobes have been measured, and the conical sidelobes generated by scattering from the feed support struts are clearly seen. The effective temperature of the ground at 1420 MHz has been determined by radiometry. By convolving the radiation pattern with the ground temperature distribution the antenna temperature has been calculated and the contribution from various parts of the radiation pattern estimated. Of a measured total antenna temperature of 26.8 K with the antenna pointing at the zenith, the following contributions can be accounted for: cosmic microwave background (2.7 K), galactic emission (1.0 K), atmospheric emission (2.0 K), direct spillover from the ground into the feed (8.0 K), leakage through the reflector mesh (5.9 K), and diffraction around the reflector rim (0.6 K). It is concluded that ground radiation scattered from the feed support struts into the aperture is a significant contributor to antenna noise, and 5.8 ± 2.5 K has been attributed to this cause. Of the total noise from the ground, 1.1 K is contributed by hills surrounding the site; most of this enters the antenna through the sidelobes generated by the feed support struts. On a completely flat site, strut scattering would contribute about 4.7 K.
Millimeter‐wave radar scattering from snow 1. Radiative transfer model
Authors: Kuga Yasuo , Ulaby Fawwaz T. , Haddock Thomas F. , DeRoo Roger D.
Published: April 2008

Millimeter‐wave (MMW) remote sensing of ground snow has attracted considerable interest in recent years. Because the size of the snow ice particle is comparable to the wavelength in the millimeter‐wave region, we can no longer use a simple Rayleigh phase function or the small particle approximation usually used at microwave frequencies for calculating the extinction coefficient. In this paper we present a model for MMW scattering from snow using the vector radiative transfer theory and a Mie phase function. Assuming snow to consist of randomly distributed spherical particles embedded in a mixture of air and water, the vector radiative transfer theory is solved using the discrete ordinate method. The values of the extinction coefficient used in the calculations are based on a combination of experimental data and calculations using the quasi‐crystalline approximation. The backscattering coefficient is calculated for different liquid water contents at 35, 95, and 140 GHz. We show that the backscattering coefficient is sensitive to liquid water content at all three frequencies, with 35 GHz being the most sensitive. Except for normal incidence, the effect of snow surface roughness is negligibly small for dry snow, and it is somewhat significant for wet snow at 35 GHz, but not at the higher frequencies.
Hydrodynamic radio‐frequency model of an ion sheath near a conductor in a plasma
Authors: Morin Gilbert A. , Balmain Keith G.
Published: April 2008

A warm‐plasma, hydrodynamic, radio‐frequency model of the ion sheath region adjacent to a conductor in a plasma is developed. The sheath has a stepped density profile consisting of several contiguous regions of uniform plasma density. A consistent set of boundary conditions developed from hydrodynamic theory is imposed on the fields at the density steps, and the absorptive boundary condition is used at the conductor‐plasma interface. When applied to calculate the radio‐frequency admittance of a spherical probe, the single‐step version of this theoretical model (1) agrees closely with the admittance calculated using hydrodynamic theory and a continuous density profile and (2) agrees at low frequencies with the admittance calculated using kinetic theory. As a consequence, the single‐step or multistep sheath models of this type should be useful in theoretical analyses of a wide range of radio‐frequency interactions involving a conductor in contact with a plasma.
Dissolved organic carbon in modeling oceanic new production
Authors: Bacastow R. , Maier‐Reimer E.
Published: April 2008

The flux of organic carbon associated with new production has been modeled by advection of dissolved organic carbon in addition to falling particulate organic carbon, in a carbon cycle model that is based on an oceanic general circulation model. Model predictions of chemical species involved in the carbon cycle are compared with observations. Relative to a model in which new production is carried only by falling particulate organic carbon, there is significantly better agreement between predicted and observed oceanic phosphate and oxygen concentrations if a large part of the new production flux is carried by dissolved organic carbon.