Showing papers on "Point source published in 1987"

Lagrangian measurement of fluid-particle motion in an isotropic turbulent field

Abstract: By means of an optical tracer-particle tracking method, measurements for Lagrangian characteristics of turbulence, including the mean-square value of lateral diffusion from a point source was weakened by the decay of turbulent energy, particularly so for long diffusion times.

Polarization of light scattered from the winds of early-type stars

Abstract: There is evidence from visible wavelength polarimetry that the matter in the winds of early-type stars is not distributed uniformly about the stars. In this paper equations for the observable polarization from an axisymmetric distribution of electrons are derived first from the basic point source model of Brown and McLean (1977) and then from extended atmosphere radiative transfer theory. A factor is derived which corrects the Brown and McLean model for the finite size of stars. This finite disk factor is found to reduce the magnitude of the predicted polarization to less than half of the point source prediction. The effects of having the mass outflow concentrated in a plume are considered. The effects of absorption in an unocculted plume are shown to give rise to a wavelength-dependent polarization that is described using a 'polarization color' parameter. A continuum opacity index is defined that allows for a straightforward interpretation of the polarization of stars of differing effective temperatures.

The Bakerian Lecture, 1986. Ships from Space

Abstract: Narrow V-shaped wakes extending some 20 km behind surface ships were first found on Synthetic Aperture Radar (SAR) images from SEASAT in 1978. The V-wake geometry differed strikingly from the traditional Kelvin wake geometry consisting of divergent and transverse wave components generated by a travelling pressure point. The SAR images can be accounted for in terms of Bragg scatter from relatively short waves generated by the surface vessel. An essential ingredient of this hypothesis is that the wave generation is by an intermittent rather than a steady point source. Optical images from a hand-held camera on a 1985 space shuttle mission revealed many V-like wakes behind surface ships. There is no Bragg scattering from the ocean surface at optical wavelengths, so an alternative hypothesis is called for. We can interpret the observed features in terms of sun glitter from the tilted facets of a Kelvin wake. An essential ingredient is the generation by complex sources rather than by a single point source. We regard the present study as a step towards the interpretation of many unexplained naturally occurring features at the edge of the Sun glitter.

Response of a Point Source in a Multicylindrcally Layered Medium

Abstract: An algorithm using 2 x 2 matrices to compute the response of an arbitrary point source in the vicinity of a multicylindrically ally layered medium is presented. The algorithm is physical and efficient and can be implemented easily for any number of layers, and with the source location in any of the layers. The formalism can be easily extended to related structures and the algorithm has potential applications in the areas of optics, defense, and geophysical exploration. It also corresponds to finding the Green's function to the multicylindrically layered geometry. Reflection and transmission coefficients are defined such that a simple recursive algorithm can be given to determine the fields in all the layers.

Dipole radiation in a multilayer geometry

Abstract: There are several kinds of experiments that can be done with multilayer stacks of dielectric media which require an understanding of light emission by sources within the stack for their analysis. These experiments may involve, for example, light-emitting tunnel junctions, Raman scattering in Kretschmann and other multilayered geometries, and Rayleigh scattering by small amounts of surface or interface roughness, either alone or in combination with other processes. A set of electromagnetic Green's functions for a multilayer stack of isotropic dielectric media [D. L. Mills and A. A. Maradudin, Phys. Rev. B 12, 2943 (1975)] gives the electric fields produced everywhere by a point source of current oscillating at a frequency f. These Green's functions can thus be used to solve this type of problem. In this paper we show how these Green's functions can be written in terms of 2\ifmmode\times\else\texttimes\fi{}2 transfer matrices of the type commonly used to find the fields in a dielectric stack due to an incident plane wave. With this simplification we can easily evaluate the Green's functions for a stack with an arbitrary number of layers. We further show that, when the electric fields generated by a point source within the stack are evaluated far away, they can be written directly in terms of the electric fields that would be generated at the location of the current source by plane waves incident from the direction of the observation point. We show that this follows from the Lorentz reciprocity theorem. Thus, in this case the formalism of Green's functions is not needed.

A Survey of IRAS Point Sources in Taurus for High-Velocity Molecular Gas

Abstract: A sample of young stellar objects selected from the IRAS Point Source Catalog (1985) has been surveyed for the presence of high-velocity (C-12)O J = 1-0 emission with the Five College Radio Astronomy Observatory 14-m telescope. Three new molecular outflows have been detected from this survey. The high-velocity emission from these regions is dominated by redshifted material indicative of monopolar outflow. The energy injection rate from all stellar winds within the Taurus cloud complex is compared to the energy dissipation rate due to radiative collisions of clumps. It is uncertain, based on these observations, that the stellar winds can provide the energy necessary to sustain the supersonic turbulent motions which characterize molecular clouds. From a map of (C-13)O J = 1-0 emission of the B18 cloud, the ring of enhanced line widths detected from an earlier study of (C-12)O J = 1-0 emission is confirmed. This ring may be due to the cumulative interaction of accelerated shells driven by stellar winds from several T Tauri stars associated with the cloud. 43 references.

CAL 83 - A puzzling X-ray source in the Large Magellanic Cloud

Abstract: Spectroscopic observations of the X-ray point source no. 83 in the Columbia Astrophysics Laboratory Einstein survey of the Large Magellanic Cloud have been accumulated over 4 yr. The optical spectrum shows no stellar absorption features, but only emission lines typical of an accretion disk. Radial velocity measurements reveal a small (K = 30 km/s) velocity variation modulated at 0.93 d, which appears to be the orbital period. No strong constraints can be put on the mass of the collapsed object, but its companion must be a low-mass, evolved star. Evidence for a precessing disk with a possible period of 69 days is presented. Exosat X-ray observations reveal no short-term (less than 6 hr) periodicities, although erratic, random variations were observed. The X-ray spectrum is very soft, reminiscent of some of the candidate black-hole sources. IUE ultraviolet spectra show only weak emission lines of N V and He II. The UV flux is variable with a mean effective temperature of about 19,000 K. Optical B, V photometry during November 1985 showed random variations of about 0.2 mag with a mean V = 17.3 mag.

Map of the Galactic center region in the 1. 8 MeV Al-26 gamma-ray line

Abstract: The mapping of the Galactic center region in the 1.8 MeV gamma-ray line based on observations made with the MPI Compton telescope during a balloon flight is reported. The measured gamma-ray line profile and the intensity map of the Galactic center region are presented, and the measured map is compared with maps expected from distributions of those objects that have been suggested as possible sources. The consistency of the results with a point source at the Galactic center raises questions about the validity of the generally assumed origin of the gamma-ray line in interstellar space. 26 references.

Effects of Electron Scattering on the Oscillations of an X-Ray Source

Abstract: The time variability observed at infinity due to a variable point source at the center of a spherical cloud of radius R and optical depth to electron scattering tau is analytically determined. The emissin pattern of the source and its time variability are assumed to be of the following three forms: (1) isotropic emission with intensity varying sinusoidally in time with angular frequency Omega(L), (2) emission in the form of a delta-function beam rotating with angular frequency Omega(R) about a fixed axis and with intensity constant in time, and (3) emission in the form of a delta-function beam rotating with angular frequency Omega(R) about a fixed axis and with intensity varying sinusoidally in time with angular frequency Omega(L). More complicated source emissions and variabilities are studied by superposing the above forms. The results of our calculations reveal the conditions under which quasi-periodic oscillations can be observed from X-ray sources, while periodic oscillations are completely smeared out. Furthermore, these results can be used to study the X-ray oscillations of such sources as Her X-1, Cyg X-3, and the Vela pulsar, which are believed to be embedded in scattering clouds. 35 references.

Evolution of Copper Vapor from the Cathode of a Diffuse Vacuum Arc

Abstract: The metal-vapor evolution from the cathode of a diffuse vacuum arc is described by a model that starts from the expression for the collisionless expansion into vacuum of atoms originating from an instantaneous point source. The velocity distribution is assumed to be Maxwellian at the moment the atoms are released from the source. By convolution with the vapor generation rate, which is given by an effective erosion rate and the waveform of the arc current, this expression is generalized to yield the atomic density at an arbitrary distance from a point source which emits atoms for a finite period. The result is integrated over the cathode surface which is treated as an extended homogeneous source of vapor. The copper-vapor density was calculated for a vacuum arc driven by a sinusoidal 50-Hz current half-cycle of 500 A rms, for the center of the contact gap. With a vapor temperature of 2000 K and an effective copper-vapor erosion rate of 3 ?/C, the model well describes the measured decay of the copper-vapor density from about 5 × 1017 m-3 at 300 ?s before current-zero to 5 × 1014 m-3 at 400 ?s after current-zero. Comparison with calculations based on the assumption that metal vapor is generated predominantly by molten droplets evaporating in flight indicates that before current-zero the contribution of the droplets to the vapor density is negligibly small, while after current-zero both vapor generation mechanisms produce vapor at the same order of magnitude.

Bio-optical model describing the distribution of irradiance at the sea surface resulting from a point source embedded in the ocean

Abstract: A model of the optical properties of the ocean, providing the absorption and scattering coefficients of the medium as nonlinear functions of the concentration of pigments associated with phytoplankton and their immediate detrital material, is presented. Monte Carlo computations of the attenuation coefficient of downwelling irradiance Kd for an ocean–atmosphere system illuminated by the sun at zenith, agree well with experimental data and demonstrate the validity of such a model for studying the influence of phytoplankton biomass on the propagation to the surface of light generated through bioluminescence. The radiative transfer equation for the irradiance at the sea surface resulting from illumination by a point source embedded in the water is solved by Monte Carlo techniques. The solution technique is validated through comparison with an asymptotic analytic solution for isotropic scattering. The computations show that the irradiance distribution just beneath the surface as a function of R, the distance measured along the surface from a point vertically above the source, is described by two regimes: (1) a regime in which the irradiance is governed mostly by absorption and geometry with scattering playing a negligible role—the near field; (2) a regime in which the light field at the surface is very diffuse and the irradiance decays approximately exponentially in R and is a very weak function of the source depth—the diffusion regime. The near field is of primary interest because it contains most of the power reaching the sea surface. An analytical model of the irradiance distribution just beneath the surface as a function of R, the source depth, and the pigment concentration for the near field is presented. This model is based on the observation that at most scattering events the change in the photon's direction is slight, and therefore, scattering is rather ineffective in attenuating the irradiance. An analytic solution for the irradiance from the point source, then, is first carried out ignoring scattering altogether; however, recognizing that backscattering will attenuate the irradiance, the absorption coefficient is replaced by an effective attenuation coefficient k. This effective attenuation coefficient is determined by fitting the total power just beneath the surface determined from the Monte Carlo computations to the analytical model. The resulting k is closely related to Kd, and the Monte Carlo irradiance as a function of R and source depth in the near-field regime can be approximated with high accuracy using the model. These results indicate Kd can be estimated at night by releasing a point source in the water, measuring the irradiance at the surface as it sinks, and fitting the measurements to the relationships developed here to determine k. The analytic model also enables estimation of the source depth and power from the irradiance distribution just beneath the surface.

The Meckering earthquake of 14 October 1968: A possible downward propagating rupture

Abstract: Average source parameters of the 1968 Meckering, Australia earthquake are obtained by the inversion of body waves. The objectives of the inversion are the elements of the moment tensor and the source-time history. An optimum source depth of 3 km is determined, but because of source complexity the point source assumption fails and the moment tensor obtained at that depth has a large nondouble-couple term, compensated linear vector dipole = 34 per cent. The source parameters of the major double-couple are: strike = 341°; dip = 37°; rake = 61°; and seismic moment = 8.2 × 1025 dyne-cm. The source-time function is of approximately 4 sec duration, with a long rise time and a sharp fall-off. The fault length is constrained on the surface by the observed surface break, and results from vertical displacement modeling suggest a width of approximately 10 km in the middle, assuming a dip of 37°. That restricts the entire faulted area to lie above 6 km depth. Two finite fault models for the earthquake are presented, with rupture initiating at a point (1) near the top of the fault and (2) at the bottom of the fault. Both models produce similar long-period synthetics, but based on the short-period waveforms, model 1 is favored. It is argued that such a rupture process is the most reasonable in this cold shield region.

Intensity covariance of a point source in a random medium with a Kolmogorov spectrum and an inner scale of turbulence

Abstract: The intensity covariance of a point source of waves propagating in a random medium is calculated by using the asymptotic theory (strong-scattering regime) and a Kolmogorov refractive-index spectrum with an inner-scale cutoff. The variance and small-scale correlation scale are both larger than predicted for zero inner scale. The behavior of the small-scale structure is dominated by irregularities near the receiver, whereas the behavior of the large-scale structure is dominated by irregularities near the transmitter. The theoretical predictions are compared with experimental results. Higher terms in the asymptotic theory are required for intensity covariance to be predicted adequately. Saturation of intensity fluctuations (variance of unity) is unlikely for laser propagation in typical atmospheric conditions. The calculation of intensity covariance in an extended medium cannot be simplified by substituting an equivalent thin screen in the midpoint of the propagation path.

A Horizontal Plane Source Model for Ground‐Water Transport

Abstract: A model was derived through the use of Green's functions to simulate three-dimensional contaminant transport from a horizontal plane source (HPS). This analytical model incorporates retardation and decay, and can simulate varying source emission rates. Appropriate uses of the model include simulations of contaminant transport from landfills, waste lagoons, land treatment facilities, and areas of pesticide application. Comparison between HPS and point source solutions indicates that for such simulations, the HPS model will provide more accurate results than the point source solution, particularly near the source. Representative model applications indicate the model's sensitivity to variations in retardation, decay, and temporal period of source emission.

An analytically soluble problem in fully nonlinear statistical gravitational lensing

Abstract: The amplification probability distribution p(I)dI for a point source behind a random star field which acts as the deflector exhibits a I exp-3 behavior for large amplification, as can be shown from the universality of the lens equation near critical lines. In this paper it is shown that the amplitude of the I exp-3 tail can be derived exactly for arbitrary mass distribution of the stars, surface mass density of stars and smoothly distributed matter, and large-scale shear. This is then compared with the corresponding linear result.

Time dependence of the field energy densities surrounding sources: Application to scalar mesons near point sources and to electromagnetic fields near molecules.

Abstract: The time dependence of the dressing-undressing process, i.e., the acquiring or losing by a source of a boson field intensity and hence of a field energy density in its neighborhood, is considered by examining some simple soluble models. First, the loss of the virtual field is followed in time when a point source is suddenly decoupled from a neutral scalar meson field. Second, an initially bare point source acquires a virtual meson cloud as the coupling is switched on. The third example is that of an initially bare molecule interacting with the vacuum of the electromagnetic field to acquire a virtual photon cloud. In all three cases the dressing-undressing is shown to take place within an expanding sphere of radius r = ct centered at the source. At each point in space the energy density tends, for large times, to that of the ground state of the total system. Differences in the time dependence of the dressing between the massive scalar field and the massless electromagnetic field are discussed. The results are also briefly discussed in the light of Feinberg's ideas on the nature of half-dressed states in quantum field theory.

Determination of seismic attenuation structure and source strength by inversion of seismic intensity data tohoku district, northeastern japan arc

Abstract: As the first application of the method of Hashida and Shimazaki (J. Phys. Earth, 32, 299-316, 1984), a three-dimensional seismic attenuation structure and source strengths are estimated by inversion of seismic intensity data for earthquakes that occurred in the Tohoku district, Japan. We carefully selected 1, 630 intensity data from 101 earthquakes so that the intensities are consistent with accelerations converted from Kawasumi's relation and so that a well-resolved attenuation structure is obtained. A reasonable fit of the formula proposed in our preceding paper to the actual intensity data guarantees that a systematically biased attenuation structure is not obtained. A comparison of the obtained attenuation structure with velocity structures estimated by previous studies shows that high (low) Q nearly corresponds to high (low) V. The correlation of both structures indicates that the attenuation structure estimated by the proposed method is reliable. The resultant attenuation structure shows a remarkable contrast in the attenuation coefficient and two prominent features. The first feature is low-Q zones down to a depth of 90 km, which corresponds to the distribution of volcanoes. The second is high-Q zones that correspond to the subducting Pacific slab. The high-Q slab is in contact with the high-Q zone in a depth range of 30-60 km, which lies on the east side of the volcanic front. The presence of high-stress earthquakes in this depth range, such as the 1978 Miyagi-ken-oki earthquake, is explained by a model in which the contact of the underthrusting Pacific plate with the surface high-Q zone accumulates higher stress and thus causes stronger seismic coupling. The estimated source strength, which is expressed as a point source acceleration, correlates well with earthquake magnitude. Normalized source acceleration, which is an average acceleration over a source area, is estimated. The acceleration suggests that the stress drop of an earthquake becomes higher with magnitude and with depth. The relation between JMA magnitude (MJ) and seismic intensity at a hypocentral distance of 100 km (I100) is found to be I100=1.5 MJ-6.5 for crustal events. This I100 is in agreement with the value reported by Utsu (Bull. Earthq. Res. Inst., 59, 219-233, 1984) which was determined from events excluding those which show anomalous distributions of intensity data. This agreement suggests that our method of estimating earthquake magnitude from intensity data is effective for removing the effect of structure.

Light absorption analyser

Abstract: A light absorption analyzer can work in either a reflection or a transmission mode, according to which probe head is fitted to it. Light is caused to be of a desired wavelength for the analysis by passing non-monochromatic light from a high-intensity flash tube source (which is not a point source) to an interference filter. Only parallel light emanating from the filter is focused by a concave parabolic mirror on a point where at least one fibre-optic collector is positioned. This light can then be passed directly to the probe for passage through the sample, since it is only light of a specified wavelength which will have travelled parallel from the filter and will therefore be focused at the point. Some of the light is taken off a reference detector to provide a reference signal for comparison with that derived from a test detetor fed by a return fibre-optic from the probe. A comparator is programmed to give a quantitiative readout of the absorption by the test sample.

A criterion for an energy vortex in a sound field

Abstract: Measurements with intensity meters have shown that energy vortices exist in certain sound fields. In these vortices, sound energy flows around closed paths, in the steady state. Vortices occur in some sound fields (e.g., that of a point source near a reflecting edge), but not in others (e.g., that of a plane rigid piston in a plane rigid baffle). It is shown that in a two‐dimensional or axisymmetric sound field, a necessary and sufficient condition for a vortex to exist is the presence of an isolated maximum or minimum in the stream function. Two examples are given for a vortex in (a) a duct of square cross section, and (b) the field of two monopole sources, one of which just extinguishes the other. Some relations for the interaction between two monopole sources are also given.

Point-Source Location Using Hexagonal Detector Arrays

Abstract: For staring sensors, improved performance in the location of point sources can be achieved by use of an array of hexagonal detectors instead of the usual array of square or rectangular detectors. This improvement is demonstrated by calculating the accuracy of the centroid algorithm as a function of signal-to-noise ratio and blur spot size for both types of detector arrays. The probability density function for the centroid random variable is derived and is used to perform all noise analysis. The analysis indicates that the algorithm error is reduced by as much as a factor of 3, the sensitivity to noise is reduced by 17%, the computational load is decreased by 23%, and the data storage requirement is reduced by 22%. The clutter-induced noise, as measured by the clutter equivalent target, is essentially identical for square and hexagonal detectors of the same area.

Two dimensional digital spatial filter for enhancement of point source

Abstract: A point source spatial filter determines the quality of a central point source or pixel under test to thereby detect point sources and minimize the signals from all sources other than the desired point source. The spatial filter incorporates a buffer apparatus for receiving pixel signals, memory apparatus for storing pixel data for the test pixel and surrounding pixels, comparator apparatus for comparing test pixel values against a given number of surrounding pixel values to determine if the test pixel is a local maximum or minimum value, subtraction apparatus for subtracting the value of the nearest-valued surrounding pixel from the test pixel value when the test pixel is determined to be a local maximum or minimum, and apparatus for indicating the difference between the value of the test pixel and that of the surrounding pixel of closest value. This difference value becomes the filter output, and serves to establish a value for the point source signal intensity which is then compared to a threshold to provide a detection flag.

Light source for fibre optic instruments

Abstract: A light source device for illuminating a selected receptor end of a number of fibre optic cables, having a light source which provides a concentrated point source of light, a movable selector to allow alignment of a selected aperture in which a receptor end of a fibre optic is located, with the light source, and further having an adjustable positioning means for displacing the light source and the selected aperture relative to each other, so as to allow the concentrated point source of light to be exactly focused on the receptor end located in the selected aperture.

Simultaneous determination of source wavelet and velocity profile using impulsive point-source reflections from a layered fluid*

Abstract: SUMMARY The determination of source signature is a major calibration problem in reflection seismology. This ‘deconvolution’ problem is conventionally approached by way of statistical methods, by direct measurement, or by the location of a clean reflection in an otherwise quiet part of a reflection section. We show that a quasi-impulsive, isotropic point source may be recovered simultaneously with the velocity profile from reflection data over a layered fluid, in linear (perturbation) approximation. Our approach is completely deterministic, and does not depend on the presence of an isolated reflection in a quiet part of the section, as we illustrate with a numerical example, After describing the algorithm and a numerical implementation, we give a complete mathematical treatment, which shows that our estimates of source wavelet and velocity profile are stable in a certain sense. Because of this stability property we conjecture that our approach to simultaneous estimation of source and medium parameters actually applies to a much broader class of models than that treated here.

A simple model for estimating excess attenuation of road traffic noise

Abstract: Sound propagation over ground from a point source having a typical spectrum of motor vehicle noise was investigated by a computational study using theoretical models. Employing new parameters of an average propagation height and a classified resistivity of ground surface, a practical expression to estimate A-weighted excess attenuation was obtained with some charts for various source and receiver locations. Results of field and scale model experiment show a good agreement with the values estimated from this practical model. It is applicable to predict the sound levels of road traffic noise propagating over the ground with a typical acoustic impedance.

A comparison of gamma-ray buildup factors for low-Z material and for low energies using discrete ordinates and point Monte Carlo methods

Abstract: This paper discusses the calculation of exposure and absorbed dose buildup factors for a photon point source in infinite beryllium in the low-energy range of 003 to 03 MeV, for penetration depth up to 40 mfp, using two discrete ordinates codes, PALLAS-PL, SP-Br, and ANISN Comparisons of both result to values obtained by point Monte Carlo calculations using the electron gamma shower version 4 code show reasonable agreement for two types of sources: normally incident and point isotropic sources The fitting parameters of a geometric-progression method formula are determined for the resulting buildup factor data These fitting parameters are in good agreement with the basic data within 5% over a great variation in magnitude

Sound wave propagation from a point source over a homogeneous surface and over a surface with an impedance discontinuity

Abstract: The sound field from a point source above an infinite plane with an impedance discontinuity is studied by considering the Helmholtz equation. With boundary conditions the Helmholtz equation is transformed into a singular integral equation and an exact solution formula is given. The formula is evaluated for the case when the source and the receiver are close to the boundary, many wavelengths from the discontinuity. The evaluation gives, among other things, information on the dependence of the surface wave structure on the impedances and—in the case of no surface waves—information on the farfield solution. If one of the impedances gives a surface wave in the case of a homogeneous boundary, then a surface wave appears also in the nonhomogeneous case and is modified depending on the other impedance. In the case of no surface waves, the rapid decay of the sound field in the homogeneous case does not appear in the nonhomogeneous case since the discontinuity acts as a weak source of slowly decaying waves. A numb...

The Host Galaxy of Quasar IRAS 00275-2859: an Interacting System

Abstract: Optical imaging of the recently discovered infrared quasar IRAS 00275-2859 shows that it can be decomposed into two point sources embedded in an underlying extended image. The brighter point source is the quasar; the extended image together with the fainter point source is identified as the host galaxy system. The IRAS 00275-2859 QSO 1 + host-galaxy system appear to be typical of radio-quiet QSOs. The following interpretation of the present observations is offered: IRAS 00275-2859 is an interacting system of galaxies. QSO 1 is associated with one of the galaxies and may be a direct result of the interaction. The fainter point source visible in the optical image is the nucleus of the second galaxy. The extended low-surface-brightness region is the tidal signature of the interaction. Enhanced star formation resulting from the interaction powers most of the infrared radiation. 40 references.