bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic model and force control of the redundantly actuated parallel manipulator of a 5-DOF hybrid machine tool
Authors: Wu Jun , Wang Jinsong , Wang Liping , Li Tiemin
Published: April 2008

This paper deals with the dynamic model and force control of the redundantly actuated parallel manipulator of a 5-DOF hybrid machine tool. The inverse dynamic model is derived by using the Newton–Euler method. The driving force is optimized by the least-square method. Based on the kinematic and dynamic models, the redundant chain is controlled by force mode and other chains by position mode. The redundantly actuated parallel manipulator is incorporated into a 5-DOF hybrid machine tool which also includes a worktable with a translational DOF and a rotational DOF. The experiments wherein the machine moves along a straight-line trajectory and a circular trajectory show that the machine has a good contouring performance.
Despair unto Death? Attempted Suicide in Early 1930s Vienna
Authors: Bowman William
Published: April 2008

From the late eighteenth century until the present, suicide has become a fundamental issue for professional Europeans' understanding of themselves and especially of the supposed health of their societies. From the second half of the eighteenth century onward, medical doctors, moral statisticians, sociologists, psychologists, and other professionally trained and university-educated experts produced increasing numbers of studies on suicide. Prior to 1750, suicide had been a popular theme for commentary from moralists, philosophers, and, above all, theologians and clerics. Of course, these groups of commentators have continued to write on the topic through the modern period, but the emergence of doctors, sociologists, and psychologists, among others, in the debates surrounding suicide has been a hallmark of the modern era. Claiming to offer complete or partial answers to the issue and causes of suicide, these professional groups increasingly asserted their own pivotal roles in analyzing the problems of European societies. Suicide was supposedly a modern problem that required expert commentary and intervention.
Survival and remodeling of melanopsin cells during retinal dystrophy
Authors: VUGLER ANTHONY A. , SEMO MA'AYAN , JOSEPH ANNA , JEFFERY GLEN
Published: April 2008

The melanopsin positive, intrinsically photosensitive retinal ganglion cells (ipRGCs) of the inner retina have been shown to send wide-ranging projections throughout the brain. To investigate the response of this important cell type during retinal dystrophy, we use the Royal College of Surgeons (RCS) dystrophic rat, a major model of retinal degeneration. We find that ipRGCs exhibit a distinctive molecular profile that remains unaltered during early stages of outer retinal pathology (15 weeks of age). In particular, these cells express βIII tubulin, α-acetylated tubulin, and microtubule-associated proteins (MAPs), while remaining negative for other RGC markers such as neurofilaments, calretinin, and parvalbumin. By 14 months of age, melanopsin positive fibers invade ectopic locations in the dystrophic retina and ipRGC axons/dendrites become distorted (a process that may involve vascular remodeling). The morphological abnormalities in melanopsin processes are associated with elevated immunoreactivity for MAP1b and a reduction in α-acetylated tubulin. Quantification of ipRGCs in whole mounts reveals reduced melanopsin cell number with increasing age. Focusing on the retinal periphery, we find a significant decline in melanopsin cell density contrasted by a stability of melanopsin positive processes. In addition to these findings, we describe for the first time, a distinct plexus of melanopsin processes in the far peripheral retina, a structure that is coincident with a short wavelength opsin cone-enriched rim. We conclude that some ipRGCs are lost in RCS dystrophic rats as the disease progresses and that this loss may involve vascular remodeling. However, a significant number of melanopsin positive cells survive into advanced stages of retinal degeneration and show indications of remodeling in response to pathology. Our findings underline the importance of early intervention in human retinal disease in order to preserve integrity of the inner retinal photoreceptive network.
Foveal evoked magneto-encephalography features related to the parvocellular pathway
Authors: YANG CHIA-YEN , HSIEH JEN-CHUEN , CHANG YIN
Published: April 2008

The aim of this study was to use non-invasive magneto-encephalographic techniques, together with visual stimulus paradigms that can psychophysically separate the M- and P-pathways, to examine the physiological relations of the pathways at the fovea with (1) the magneto-encephalography components M70 and M100 (in latency and amplitude), and (2) the cortical oscillatory activities (alpha, beta, and gamma), respectively. The checkerboard stimuli accompanied with different spatial frequencies (SFs) (0.5 or 4 cycles per degree) were presented (within 2° of the retinal center) to six healthy subjects by using steady-pedestal and pulse paradigms, which could activate distinct populations of M- and P-neurons. SF analyzed brain responses in each paradigm. The results show a consistent trend in M70 and M100 with increased latencies and amplitudes in response to the high SF. Mean while, the beta to gamma activities are apparently enhanced by the stimulus of high SF, especially under pulse paradigm (p = 0.03). In this study, we suggest that M70 can be a good clue to characterize the P-pathway. Moreover, in the frequency analysis, the beta oscillations may serve for more detailed visual information, while the gamma oscillations seem to reflect the signal processing in the P-pathway and with sensitivity to the fovea.
Effects of unoprostone on phosphorylated extracellular signal-regulated kinase expression in endothelin-1-induced retinal and optic nerve damage
Authors: MUNEMASA YASUNARI , KITAOKA YASUSHI , HAYASHI YASUHIRO , TAKEDA HIROYUKI , FUJINO HIROMI , OHTANI-KANEKO RITSUKO , HIRATA KAZUAKI , UENO SATOKI
Published: April 2008

Endothelin-1 (ET-1), a potent vasoconstrictor peptide, has been implicated in the development of normal- and high-tension glaucoma. We investigated the effects of unoprostone on extracellular signal-regulated kinase (ERK) in ET-1-induced retinal ganglion cell (RGC) death and optic nerve injury. Our morphometric study showed that intravitreal injection of ET-1 led to cell loss in the RGC layer (RGCL) in 28 days. Western blot analysis showed decreased neurofilament (NF) protein in the optic nerve 28 days after ET-1 injection. In this in vivo model, increased phosphorylated ERK (p-ERK) was observed in the retina on 1 day and subsequently in the optic nerve from 7 days after ET-1 injection. Simultaneous injection of M1, as a metabolite of unoprostone, showed further increased p-ERK levels compared with ET-1 injection alone. Our morphometric study of flat-mount preparations stained with cresyl violet or retrograde labeling with a neuro-tracer and Western blot analysis of NF showed that inhibition of ERK phosphorylation led to acceleration of ET-1-induced RGC death and optic nerve damage. In addition, M1 significantly attenuated both RGC loss and the decrease in NF protein induced by ET-1. The protective effects of M1 were significantly inhibited by U0126, an ERK inhibitor. These results suggest that unoprostone has neuroprotective effects against ET-1-induced neuronal injury through ERK phosphorylation.
Improvement in the prediction of solar wind conditions using near‐real time solar magnetic field updates
Authors: Arge C. N. , Pizzo V. J.
Published: April 2008

The Wang‐Sheeley model is an empirical model that can predict the background solar wind speed and interplanetary magnetic field (IMF) polarity. We make a number of modifications to the basic technique that greatly improve the performance and reliability of the model. First, we establish a continuous empirical function that relates magnetic expansion factor to solar wind velocity at the source surface. Second, we propagate the wind from the source surface to the Earth using the assumption of radial streams and a simple scheme to account for their interactions. Third, we develop and apply a method for identifying and removing problematic magnetograms from the Wilcox Solar Observatory (WSO). Fourth, we correct WSO line‐of‐sight magnetograms for polar field strength modulation effects that result from the annual variation in the solar b angle. Fifth, we explore a number of techniques to optimize construction of daily updated synoptic maps from the WSO magnetograms. We report on a comprehensive statistical analysis comparing Wang‐Sheeley model predictions with the WIND satellite data set during a 3‐year period centered about the May 1996 solar minimum. The predicted and observed solar wind speeds have a statistically significant correlation (∼0.4) and an average fractional deviation of 0.15. When a single (6‐month) period with large data gaps is excluded from the comparison, the solar wind speed is correctly predicted to within 10–15%. The IMF polarity is correctly predicted ∼75% of the time. The solar wind prediction technique presented here has direct applications to space weather research and forecasting.
Non‐equilibrium between dust and gas temperatures in the Mars atmosphere
Authors: Goldenson Naomi , Desch Steven , Christensen Philip
Published: April 2008

A radiative‐conductive balance calculation is presented to predict dust temperatures in the Mars atmosphere and to evaluate the assumption that dust and gas in the atmosphere are in thermal equilibrium. Mie theory is used to calculate the absorption and scattering properties of dust, important for the heating of dust by sunlight; scattering is neglected in the treatment of infrared absorption and emission by grains. Besides radiating in the infrared, dust grains cool via collisions with gas molecules, at a rate proportional to the gas density. We find that dust grains are in thermal equilibrium with the atmospheric gas near the surface. At a predictable pressure, which depends on grain size, the dust and gas temperatures decouple, as radiative absorption and emission increasingly dominate over exchange of thermal energy due to collisions with gas molecules. This non‐equilibrium between dust and gas will affect calculations of heating in the atmosphere.
Relativistic turning acceleration of radiation belt electrons by whistler mode chorus
Authors: Furuya Naoki , Omura Yoshiharu , Summers Danny
Published: April 2008

We perform test particle simulations assuming whistler mode chorus wave packets that are generated at the geomagnetic equator propagate away from the equator in both poleward directions. While electrons in the energy range 10–100 keV are primarily responsible for the generation of chorus waves through pitch angle diffusion into the loss cone, it has been found that a fraction of the higher‐energy electrons from a few hundred keV to a few MeV are effectively accelerated by chorus due to a special nonlinear trapping process called relativistic turning acceleration (RTA). This mechanism has been recently demonstrated for a coherent whistler mode wave packet with a constant amplitude and constant frequency. In the present study we confirm that the RTA process takes place for a wave packet with variable frequency such as that occurring in a rising tone of chorus emissions. We study the efficiency of the RTA process for different particle energies. A Green's function method is used to describe the evolution of the particle energy distribution function. The RTA process due to chorus emissions creates a high‐energy tail in the electron energy distribution function. The shape of the high‐energy tail is determined by the distribution function of the seed electrons in the lower‐energy range. RTA can accelerate electrons in a much shorter timescale than that estimated by quasi‐linear diffusion theory, e.g., it typically takes tens of minutes to hours for ∼100 keV seed electrons to be accelerated to energies of a few MeV by RTA.
Ground‐based validation of EOS‐Aura OMI NO2 vertical column data in the midlatitude mountain ranges of Tien Shan (Kyrgyzstan) and Alps (France)
Authors: Ionov D. V. , Timofeyev Y. M. , Sinyakov V. P. , Semenov V. K. , Goutail F. , Pommereau J.‐P. , Bucsela E. J. , Celarier E. A. , Kroon M.
Published: April 2008

Ground‐based UV‐visible instruments for NO2 vertical column measurements have been operating at Issyk‐Kul station, in Kyrgyzstan, and Observatoire de Haute‐Provence (OHP), in France, since 1983 and 1992, respectively. These measurements have already been used for validation of ERS‐2 Global Ozone Monitoring Experiment (GOME) and Envisat Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) NO2 column data. Building upon the successful missions of GOME and SCIAMACHY, the Ozone Monitoring Experiment (OMI) was launched by NASA onboard the EOS Aura satellite in July 2004. Here we present the results of recent comparisons between OMI NO2 operational data (standard product) and correlative ground‐based twilight measurements in midlatitudes, at Issyk‐Kul and OHP, in 2004–2006. The stratospheric NO2 columns, observed by OMI and our ground‐based instruments, have been corrected for NO2 diurnal change and normalized to local noon values using a photochemical box model. According to our comparison, OMI stratospheric NO2 columns underestimate ground‐based measurements by (0.3 ± 0.3) × 1015 molecules/cm2 and (0.7 ± 0.6) × 1015 molecules/cm2 at Issyk‐Kul and OHP, respectively. The effect of tropospheric pollution on the NO2 measurements in both regions of observations has been identified and discussed.
Megatides in the Arctic Ocean under glacial conditions
Authors: Griffiths Stephen D. , Peltier W. R.
Published: April 2008

Over the history of the Earth, changes in ocean depth and coastal configuration have led to considerable variations in the pattern and amplitude of ocean tides. Here we perform global simulations of ocean tides for the Last Glacial Maximum, using new data sets for both ocean depth and density stratification. We show how the configuration of the Arctic Ocean, which was almost entirely enclosed by continents at that time, leads to the near‐resonant excitation of large semi‐diurnal tides. Under certain conditions, this previously unidentified Arctic tide is massively amplified in the Canadian Archipelago. Such tides may have played a role in destabilizing the coastal margins of North American ice sheets, with implications for rapid changes in the Earth's climate and ocean circulation.
Radiocarbon decay and oxygen utilization in the Deep Atlantic Ocean
Authors: Broecker Wallace S. , Blanton Sean , Smethie William M. , Ostlund Gote
Published: April 2008

A parameter based on the sum of the concentrations of PO4 and O2 (divided by the Redfield coefficient‐ΔO2/δPO4) is used to separate the contributions of the northern and southern components to deep waters in the Atlantic. This separation allows the amount of radiocarbon lost by radiodecay and the amount of oxygen lost to respiration during residence in the deep Atlantic to be calculated. Maps of these quantities reveal strong west to east gradients and weak north to south gradients consistent with ventilation along the western boundary from both ends of the ocean coupled with mixing outward from the boundary. The O2 and 14C deficiences are highly correlated, suggesting an O2 utilization rate of 12 μm/kg per century. The apparent mean isolation time of water in the deep Atlantic is about 200 years.
The Canadian geophysical long baseline interferometer
Authors: Yen J. L. , Leone P. , Watson G. A. , Zao J. K. , Popelar J. , Petrachenko W. T. , Feil G. , Cannon W. H. , Mathieu P. , Newby P. , Tan H. , Wietfeldt R. D. , Galt J. A.
Published: April 2008

The Canadian geophysical long baseline interferometer, a new very long baseline interferometer system, has been developed and put into operation. The system tracks source delay and fringe rotation during observation using a wave front clock. After the low‐noise receiver the signals from radio sources are channelized and down converted to baseband, each baseband channel is one bit digitized at a rate of 12 mbar/s and then recorded on a single video cassette recorder for processing. The total bandwidth of the system can be easily increased by replication of baseband channels. During processing the tapes are played back in synchronism and the recovered astronomy data are processed in a simple correlator to obtain the source visibility. The use of the wave front clock allows bursts of wideband astronomy data to be sampled at a high rate at each station and then recorded at a lower rate for processing. Because bursts recorded at different stations at the same wave front clock epoch are emitted by the source at the same time, they are correlated. The fringe visibility obtained from processing the bursts would provide high‐delay resolution geodetic measurements commensurate with the wide bandwidth of the bursts.
A simple near‐field envelope formula for square aperture antennas
Authors: Hansen R. C.
Published: April 2008

A closed form near‐field power density envelope is derived from the geometrical theory of diffraction formulation. The envelopes exist on lines parallel to the aperture in principal planes. These results, and those from an equivalent edge current integral, are compared with numerically integrated results.
Electromagnetic resonant scattering by multiple objects
Authors: Zheng Wenxin , Shao Hanguang
Published: April 2008

Electromagnetic scattering by multiple objects in the resonance region is investigated numerically with particular emphasis on direct application to object characterization. The influence of coupling between objects on scattering properties is demonstrated. The average scattering cross section is suggested as a tool to extract useful information about an unknown object adjacent to other known objects from total scattering data measured from multiple scatterers.
Transmission characteristics of modes in dielectric slab waveguides with finite‐length, uniformly bent sections
Authors: Morita N. , Yamashita I.
Published: April 2008

A new, rigorous theory is presented for analyzing radiation and transmission of electromagnetic waves in uniformly curved sections of slab waveguides. General fields are expressed, in this theory, in terms of a spectral integral of eigenmodes with continuous eigenvalues. The theory is applied to the investigation of mode transmission characteristics of a circularly bent, finite‐length dielectric slab waveguide, sandwiched between the two straight sections. It is demonstrated that the analysis method is also applicable, though approximately, to the study of radiation and transmission of electromagnetic fields in a uniform bend of a three‐dimensional dielectric waveguide.
Computations of electromagnetic fields by the multiple multipole method (generalized multipole technique)
Authors: Hafner C. , Kuster N.
Published: April 2008

In this paper a numerical method for computations of electromagnetic fields is presented, which has been developed by the authors and their colleagues since 1978 under the name of the multiple multipole (MMP) method. In the meantime several groups are working independently on similar methods (with sometimes different approaches and different names). The majority of them agreed to use the name generalized multipole technique (GMT) in the future (Ludwig, 1989). The main advantages are as follows: (1) Only the boundaries of the domains have to be discretized (even in the case of lossy bodies). (2) No integrals have to be solved numerically which reduces the computation time and simplifies the codes. (3) Plotting errors on the boundaries and fields allows an excellent control of the results. (4) The fields may be computed accurately in the near‐field region and even on boundaries for closed and open problems. (5) Efficient programs including graphic input and output routines (also animated pictures) are available even for personal computers.
Finite‐element modeling of three‐dimensional electromagnetic fields in inhomogeneous media
Authors: Mur Gerrit
Published: April 2008

An efficient and accurate finite‐element method is presented for computing three‐dimensional time‐harmonic electromagnetic fields in strongly inhomogeneous, (an)isotropic media such as are encountered in the human body, subsurface structures of the Earth, and many engineering applications. For this purpose, two types of elements are used. In the domain of computation the corresponding program decides locally what type of element has to be used for obtaining the user‐specified accuracy of modeling the field. In this way, optimum results are obtained both as regards computational efficiency and as regards desired accuracy. A method is introduced for imposing the divergence condition in a weighted sense. Numerical results are presented that demonstrate our approach to be efficient as well as accurate.
A new approach to the synthesis of reflector antennas
Authors: Kildal Per‐Simon
Published: April 2008

This paper presents a new ray method to synthesize shaped offset multireflector antennas which involves solving linear equations. In the other existing techniques, differential equations have to be solved. The advantage of the technique is that the solution is easier to control, so that, for example, low cross‐polarization can be obtained and an exact solution can be found. The method is applied to synthesize a dual‐reflector feed for the spherical reflector of the radio telescope in Arecibo.
The Lorentz reciprocity in random media—The derivation from Maxwell's equations
Authors: Tateiba M.
Published: April 2008

The Lorentz reciprocity in random media is mathematically derived from Maxwell's equations on the assumption that the media are isotropic and that the material parameters ε and μ are uniformly bounded random functions and exist in a finite region. It should be emphasized that the assumption is sufficiently satisfied in physics and that other conditions for ε and μ are not assumed. The Lorentz reciprocity is valid in the sense that the reciprocal relation holds for any order moment of electromagnetic fields. The result and the derivation process presented here are useful for analyzing the wave propagation and scattering in random media.
The induced polarization influence in transient geoelectromagnetics
Authors: Wait James R.
Published: April 2008

In inductive methods of electromagnetic prospecting the transient magnetic field response, for a step function current in the source loop, is measured. It has been observed that the waveform of the received signal may change sign if the medium is polarizable in the sense that the complex conductivity is a function of frequency. We present a simple spherical shell model which incorporates such IP (induced polarization) in the formulation of the time dependent secondary field. It is shown that the electromagnetic and the induced polarization effects are additive if the target is weakly polarizable. In general, however, the EM and the IP influences are intertwined in a nonsimple manner. We also approach the problem using a general functional formulation which leads to a relatively simple procedure to deduce the form of the negative slow tail of the waveform which is the portion resulting from frequency dispersion. We conclude that such dispersive effects may have a profound influence even when the medium is weakly polarizable.
Diffraction by a generalized impedance half plane
Authors: Senior Thomas B. A.
Published: April 2008

When boundary conditions of higher order than the first are applied to an edged structure such as a wedge or half plane, difficulties occur associated with the uniqueness and reciprocity of the solution. These are illustrated in the case of a plane wave incident on a half plane subject to arbitrary second order boundary conditions different on the two faces. Using an extension of the standard Maliuzhinets method, it is shown how reciprocity can be explicitly enforced, and a solution is then obtained involving an arbitrary constant. The constant is related to the surface value of the field at the edge, and this additional information is necessary for a unique solution.
Evaluation of excess inductance of microstrip discontinuities
Authors: Djordjević Antonije R. , Sarkar Tapan K. , Marićević Zoran
Published: April 2008

Excess inductance of various microstrip discontinuities is evaluated numerically, but solving an integral equation for the surface‐current distribution. The equation is based on the boundary conditions for the electric field. The unknown current density is expressed in terms of an auxiliary scalar function. This function is evaluated by the method of moments, and it is approximated by linear, two‐dimensional functions over a set of triangles. The excess inductance is evaluated from the resulting current distribution.
Analysis of plane periodic surfaces appearing in phased arrays and scatterers using grounded dielectric‐slab Green function and iterative methods
Authors: Cuevas J. G.
Published: April 2008

A numerical scheme is presented to analyze plane periodic surfaces used in phased arrays and scattering applications. Use is made of the grounded dielectric‐slab Green function in formulating the electric field integral equation that is solved by iterative methods. Results comparing several iterative methods are presented as well as other results for reflection coefficients, active resistance, etc.
Analysis of electromagnetic scattering from doubly periodic arrays of penetrable bodies using a patch‐dipole current model
Authors: Boag Amir , Leviatan Yehuda , Boag Alona
Published: April 2008

A novel solution is presented for the problem of three‐dimensional electromagnetic scattering of a time‐harmonic plane wave from a doubly periodic infinite array of disjoint finite‐size penetrable bodies. A set of fictitious doubly periodic and properly modulated patches of magnetic current of cross polarization is used to simulate the scattered field. A set of fictitious elemental lectric dipoles of cross polarization is used to simulate the field inside the penetrable bodies. The complex amplitudes of the two sets of fictitious sources are adjusted to render the tangential components of the electric and magnetic fields continuous at a selected set of points on the surface of any of the scatterers. The suggested solution procedure is simple to implement and is applicable to doubly periodic arrays composed of homogeneous bodies of smooth, but otherwise arbitrary, shape. The accuracy of the method has been demonstrated by means of several accuracy checks. It has also been shown that in the limiting case of widely spaced spherical scatterers the numerical solution agrees well with an approximate analytic solution.
Averaging method for analyzing waveguides with anisotropic filling
Authors: Tretyakov Sergei A. , Cherepanov Andrei S. , Oksanen Markku I.
Published: April 2008

The idea of locally quasi‐static averaging fields inside a slab is applied to thin anisotropic layers. This approach leads to approximate relations between tangential fields on opposite sides of the slab. Then, by considering tangential electric and magnetic fields as vector voltages and currents, an analogous vector circuit is introduced for anisotropic slabs. A comparison between exact and approximate results is given, and some applications are discussed.
Superdirective antenna with passive resonant reflector
Authors: Veremy Vladimer V. , Shestopalov Victor P.
Published: April 2008

The effect of the finite number of two‐dimensional resonant scatterers ‐ longitudinally slotted circular cylinder on forming the far‐field radiation of line magnetic current is investigated. Location and orientation of unclosed cylinders providing the maximum directivity is determined numerically. The possibility of superdirective radiation of an antenna having a reflector made of resonant scatterers with cross‐section dimensions smaller than a wavelength is shown.
Spectral properties of open disordered systems and propagation of waves in random media
Authors: Freylikher V. D. , Gredeskul S. A.
Published: April 2008

A discussion of statistical properties of wave fields in randomly layered media is presented. It is shown that such media, because of quasi‐stationary states existing therein, represents an efficient fluctuation waveguide.
A determination of the orientation of Polar Mesosphere Summer Echo layers using EISCAT as a dual beam radar
Authors: van Eyken A. P. , Hall C. , Williams P. J. S.
Published: April 2008

An extensive international campaign to observe Polar Mesospheric Summer Echoes was conducted with the European Incoherent Scatter radars in July and August 1988. During this period, two short experiments used the flexibility of the VHF system to transmit two simultaneous beams which could be used to determine the horizontal variation of the scattering irregularities. On the first occasion the echoes appear to originate from noncoplanar points near 84 km altitude. On the second occasion, only the temporal behaviour could be studied for a short interval.
Reflection and transmission for planar‐layered anisotropic structures
Authors: Audone Bruno , Uslenghi Piergiorgio L. E.
Published: April 2008

A structure is considered that consists of any number of planar layers of different thicknesses and materials, each represented by permittivity and permeability tensors with complex elements. In addition, anisotropic jump‐immittance sheets are located at the interfaces between layers. When a plane wave of arbitrary polarization is obliquely incident upon the structure, the reflection and, if appropriate, transmission coefficients are determined via a chain‐matrix algorithm. The related computer code is tested on a variety of structures.
Structure of ionospheric irregularities from amplitude and phase scintillation observations
Authors: Bhattacharyya A. , Rastogi R. G.
Published: April 2008

The mutual coherence function Γ2, or the second moment of the complex amplitude of a radio wave which traverses through equatorial F region irregularities, is computed from amplitude and phase scintillation data. Theoretically, the equation satisfied by the coherence function has an analytic solution over the whole range of scintillation strength. This solution is directlly related to the structure function for the phase fluctuations produced by the irregularities. Hence the shape of the correlation function for variations in the total electron content along the signal path can be derived from the computed values of Γ2. With a suitable power‐law model for the irregularities, an “intermediate break scale,” 10, as well as the rms density fluctuation are deduced from a comparison of computed values for short time lags with those expected from theory. During a postsunset scintillation event, 10 is found to increase with local time. In the context of the generalized Rayleigh‐Taylor instability, which is the likely source of the irregularities, this increase may be attributed to a decline in the effective electric field prevailing in the region of the irregularities.
Fast and accurate computation of fields near caustics and cusps
Authors: Stamnes J. J. , Gelius L.‐J.
Published: April 2008

For a 2.5‐dimensional geometry involving a point source in a two‐dimensional medium we present fast and accurate procedures for computing the field near caustics and cusps. These procedures combine dynamic ray tracing with uniform asymptotic methods.
Numerical simulation of backscatter from linear and nonlinear ocean surface realizations
Authors: Rino Charles L. , Crystal Thomas L. , Koide Alan K. , Ngo Hoc D. , Guthart Harold
Published: April 2008

In this paper, numerical simulations of the scattering from time‐dependent realizations of one‐dimensional ocean surface waves are described. A new technique is used that allows efficient generation of ocean surface realizations that preserve the dominant nonlinear hydrodynamic characteristics. Thus unique scattering effects of real ocean surface waves can be explored. Until very recently, numerical simulations of rough‐surface scattering were used mainly to test and/or improve theoretical models that predict the average bistatic scatter cross section. We carry the simulations further by generating Doppler spectra from dynamically evolving surface realizations. Doppler spectra of signals scattered from the ocean surface are affected by both hydrodynamic nonlinearities and higher‐order scatter terms. The simulated Doppler spectra from nonlinear surface realizations reproduce the measured characteristics of ocean and wave‐tank data for low and high wind conditions. We also show that the results are essentially reproduced by the second‐order Kirchhoff approximation.
Nonlinear beam waves in inhomogeneous media
Authors: Bass F. G. , Konotop V. V. , Pritula G. M. , Vekslerchik V. E.
Published: April 2008

Nonlinear potential space charge waves, caused by particle beam propagation in statistically inhomogeneous resistive media, are considered. Variations of wave parameters caused by the fluctuation are obtained. Complete analogy with the corresponding linear case is stated. All calculations are carried out in the hydrodynamic approximation.
Transfer functions and pulse distortion for an ionospheric reflection channel with embedded random irregularities
Authors: Rand Timothy W. , Yeh K. C.
Published: April 2008

The ionospheric reflection channel is known to be dispersive, time varying, and random. The dispersive nature of the ionosphere and the propagation geometry make the transfer function frequency selective, i.e., frequency dependent; while the ionospheric motions cause the communication channel to be time dependent or fading. Frequently, the background ionosphere is permeated by random irregularities. A radio wave propagating through such an irregular ionosphere will have its phase randomized, followed by phase mixing through diffraction. As a result, the transfer function also acquires a random character. The transfer function for a random, dispersive, and time‐varying ionospheric reflection channel can be simulated numerically using the phase‐screen‐diffraction‐layer method. The detailed behavior of the calculated transfer function is examined in several different ways. First, the spatial behavior of the transfer function, both magnitude and phase, is depicted in three‐dimensional plots. This spatial behavior can also be interpreted as a temporal behavior for a moving ionosphere. A curious phase jump of 180° accompanied by a deep fade at some frequency within the passband of the signal is found to occur. To facilitate the examination of this phase jump, locus plots are made. Using real and imaginary parts of the transfer function as coordinates, the transfer function traces out a locus with increasing frequency on this complex plane. In the presence of severe selective fading the locus is shown to pass through the origin occasionally. When this happens, the magnitude of the transfer function dips down to zero with an accompanied phase jump of 180°. Such an event is caused by destructive interference among multipath components. Second, the channel behavior is examined by taking temporal moments of the received pulse. This is done for several sample pulses, each up to the moments of the fourth order, giving the arrival time, pulsewidth, skewness, and kurtosis. Each of these four quantities is then treated as a time series in the computation of the mean, standard deviation, histogram, power spectrum, and autocorrelation function. The stochastic behavior of these four quantities is thus thoroughly studied.
Beyond the Kirchhoff approximation II electromagnetic scattering
Authors: Rodríguez Ernesto
Published: April 2008

In a previous paper the momentum transfer expansion was introduced for scalar wave scattering. It was shown that, using this expansion, one could obtain wavelength dependent curvature corrections to the Kirchhoff approximation. In this paper we extend the momentum transfer perturbation expansion to electromagnetic waves. Curvature corrections to the surface current are obtained. Using these results, the specular field and the backscatter cross section are calculated.
Scattered near‐field and induced current of a beam wave by pits on optical disks using boundary element analysis
Authors: Miyazaki Yasumitsu , Manabe Katsuya
Published: April 2008

Numerical solutions of electromagnetic fields are presented for the scattering by pits and guide grooves on optical disks. The boundary element method can be applied to pit scattering of various pit forms under different conditions of the incident beam. Scattering characteristics from concave or convex pits with a section of trapezoid shapes are calculated as functions of the width or the depth (height) of pits. The maximum interference effects due to disk pits are discussed, and optimum pit shapes are found. Near‐field and induced currents are also studied for different polarizations.
A wideband model for electromagnetic interference from corona on multiconductor electric power lines
Authors: Olsen Robert G. , Wu Mingde
Published: April 2008

An analytic model has been developed for predicting the electromagnetic interference from multiconductor power lines over the frequency range from 100 kHz to 20 MHz. Results are provided for typical transmission lines. These include lateral and vertical profiles of the interference as well as radiation patterns.
Distributions of underdense meteor trail amplitudes and its application to meteor scatter communication system design
Authors: Weitzen J. A. , Bourque S. , Ostergaard J. C. , Bench P. M. , Baily A. D.
Published: April 2008

Analysis of data from recent experiments leads to the observation that distributions of underdense meteor trail peak signal amplitudes differ from classic predictions. In this paper the distribution of trail amplitudes in decibels relative to 1 W (dBw) is considered, and it is shown that Lindberg's theorem can be used to apply central limit arguments to this problem. It is illustrated that a Gaussian model for the distribution of the logarithm of the peak received signal level of underdense trails provides a better fit to data than classic approaches. Distributions of underdense meteor trail amplitudes at five frequencies are compared to a Gaussian distribution and the classic model. Implications of the Gaussian assumption on the design of communication systems are discussed.
Characterization of biconical transmitting antennas
Authors: Grimes Dale M. , Badii Vahid , Tomiyama Ken
Published: April 2008

We have obtained a general, numerical solution of an ideal biconical transmitting antenna, with arbitrary arm length and conic angle. The solution evaluates the field coefficients associated with the electric multipolar expansion terms both within the antenna region and external to it; the set must necessarily be truncated, and we truncate our set with 16 terms. From the field coefficients we obtain surface current and charge densities resulting from a normalized input current, for several antennas. We evaluate resonant frequencies versus cone angle for the first three resonances. Using a normalized input voltage, we obtain the complex power at all radii; conservation of energy, in turn, permits us to solve for the reactive energy of the field per unit radius.
Refraction effects on the Galileo probe telemetry carrier frequency
Authors: Atkinson D. H. , Spilker T. R.
Published: April 2008

As the Galileo probe Relay Radio Link (RRL) signal propagates outward through the Jovian atmosphere, the atmosphere will manifest itself in two ways. First, the geometric path length of the signal is increased, resulting in a small change of the RRL signal departure angle from the probe (transmitter). Secondly, the velocity of the signal is decreased. For a spherical, static atmosphere with a known profile of refractivity versus altitude the effects of refraction on the RRL frequency can be found using a variation of standard ray‐tracing techniques, whereby the ray departure angle is found by an iterative process. From the dispersive characteristics of a mixture of hydrogen and helium with trace amounts of methane and ammonia a simple model of the Jovian atmosphere is constructed assuming spherical symmetry and uniform mixing. The contribution to the RRL Doppler frequency arising from refraction is calculated, and its effect on the Doppler wind measurements is discussed.
Diffraction by a multilayer slab recessed in a ground plane via generalized impedance boundary conditions
Authors: Ricoy M. A. , Volakis J. L.
Published: April 2008

The diffraction problem associated with a multilayer material slab recessed in a perfectly conducting ground plane is formulated and solved via the generalized scattering matrix formulation (GSMF) in conjunction with the dual integral equation approach. The multilayer slab is replaced by a surface obeying a generalized impedance boundary condition (GIBC) to facilitate the computation of the pertinent Wiener Hopf split functions and their zeros. Both Ez and Hz polarizations are considered and a number of scattering patterns are presented, some of which are compared to exact results available for a homogeneous recessed slab.
Skew incidence on a material junction
Authors: Senior Thomas B. A.
Published: April 2008

The solution is determined for the problem of a plane wave at skew incidence on the junction of two coplanar resistive half planes. Using duality and the fact that coplanar electric and magnetic current sheets do not interact, it is then shown that for any pair of half planes subject to first‐order transition or boundary conditions, the diffracted field can be expressed in terms of the diffracted field for the resistive half planes.
Methane consumption and emission by Taiga
Authors: Whalen S. C. , Reeburgh W. S. , Kizer K. S.
Published: April 2008

Taiga or boreal forest environments are a poorly understood component of the global CH4 budget. Results from a 1‐year study of CH4 fluxes at a range of representative floodplain and upland taiga sites in the Bonanza Creek long term ecological research area show that soil consumption of atmospheric CH4 was the dominant process. Methane emission occurred only sporadically in the earliest successional stages in the floodplain system; all other floodplain and upland sites were net CH4 consumers. Our results suggest that upland and floodplain taiga soils are an atmospheric CH4 sink of up to 0.8 Tg yr−1. Point‐source bogs and fens are the only important CH4‐emitting sites in taiga.
Keeping global change honest
Authors: Broecker W. S.
Published: April 2008

In my reading and travels I frequently come across statements regarding the importance of biological cycles in the ocean to the uptake of CO2 produced by fossil fuel burning and forest cutting. While these are often prefaced with an admission that physical processes are more important, the listener or reader comes away with a feeling that somehow biology has a very important role to play. This supposed importance is being used as a defense for the funding of a wide range of biologically oriented ocean programs. I challenge this defense.
A simple inverse carbon cycle model
Authors: Wigley T. M. L.
Published: April 2008

The convolution integral form of a carbon cycle model is transformed to a simple first‐order differential equation. This allows one to derive an extremely efficient numerical algorithm for the carbon cycle model. The algorithm can be easily inverted to obtain a practical and efficient inverse carbon cycle model. The model is described and an example is given showing how modelled land‐use‐change emissions over 1765–1990 vary with assumptions made regarding the efficiency of oceanic CO2 uptake.
Seasonal patterns of methane uptake and carbon dioxide release by a temperate woodland soil
Authors: Crill Patrick M.
Published: April 2008

Direct measurements of CH4 and CO2 atmosphere/soil exchange with a drained upland inceptisol were made over a 2‐year period in a mixed hardwood forest in New Hampshire. Soil gas concentrations of CH4 and CO2 were also monitored over the same period. Soil incubation experiments were used to characterize the depth variation and the temperature response of the consumption and production reactions. CH4 was always taken up by the soils after spring thaw in April. Maximum rates of consumption were 4.8 mg CH4 m−2 d−1 in 1989 and 4.9 mg CH4 m−2 d−1 in 1990. CO2 efflux was much higher with rates over 25 g CO2 m−2 d−1 measured in July 1990. Annually, about 600 mg CH4 were consumed and approximately 2 kg CO2 emitted per square meter. There was an apparent negative correlation between soil CH4 and CO2 concentrations. Gas samples taken in the surface litter layer showed CH4 to be depleted and CO2 enhanced within 1 cm of the surface. Below the surface, CH4 decreased and CO2 increased with depth. At 15 cm, CH4 was never greater than 0.3 ppm and was usually less than 0.2 ppm. At the same depth, CO2 concentrations ranged from 1100 to over 7500 ppm. Incubation experiments indicated that CO2 was being produced throughout the top 15 cm of surface soil with a similar temperature response. Significant CH4 oxidation was measured only in a zone at the top of the mineral soil layer. Both activities could be stopped by autoclaving. CO2 flux from the ground throughout the year was driven by biological activity, mainly soil and root respiration. CH4 uptake, on the other hand, was more complicated. Biological activity controlled the establishment of soil concentration gradients, and so in spring, CH4 influx was tightly linked to rates of consumption. However, in summer and fall, diffusive supply of CH4 to its site of consumption in the soils limited flux rates.
The carbon budget of Mono Lake
Authors: Oxburgh Rachel , Broecker Wallace S. , Wanninkhof Richard H.
Published: April 2008

Radiocarbon measurements suggest that 14C‐free carbon enters from beneath Mono Lake at a rate of about 1 mol/m2/yr. An input of this magnitude should be manifested in the inorganic carbon budget of the lake and with this in mind we have devised a model to reconstruct the evolution of the partial pressure of CO2 (pCO2) over the past 150 years. This encompasses a period (1945 to present) during which major diversions of source waters via the Los Angeles aqueduct have been in effect, significantly increasing the salinity of the lake and hence its pCO2. The model has been constrained by experimental characterization of the carbonate chemistry of the lake water, by the temperature dependence of pCO2 for the lake water, and by pCO2 measurements made on the lake water in 1966, 1969,1981, and 1989. Our calculations suggest that prior to 1945 the pCO2 of Mono Lake water was about 1.3 times the atmospheric value. To produce this excess, an input of CO2 of about 3.3 mol/m2/yr is required. Volcanic activity beneath the lake is a probable source of this input.
Oceanic uptake of anthropogenic CO2: The major uncertainties
Authors: Sarmiento J. L.
Published: April 2008

The recent notes by Broecker [1991] and by Smith and Mackenzie [1991] rightfully point out that many oceanographers do not have a clear grasp of the relationship between the oceanic biological pump and anthropogenic CO2 transient. This misunderstanding has unfortunately led some scientists to make misleading claims as to the importance of the biological pump in the anthropogenic transient. It is remarkable that someone who contributed as much to our understanding of the oceanic carbon cycle as Revelle could have incorrectly stated that the biological pump takes up anthropogenic CO2 [Revelle, 199O]. However, both notes go too far when they dismiss the biological pump as being of minimal relevance to global change, with a particular indictment by Broecker of the Joint Global Ocean Flux Study (JGOFS) as having been guilty of “tarnishing the integrity of global change research” by “hitching their wagons to the greenhouse star.”