VTechWorks Repository :: Browsing by Author "Dennison, Brian K."

Browsing by Author "Dennison, Brian K."

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14-year program monitoring the flux densities of 33 radio sources at low frequencies
Salgado, José Francisco; Altschuler, Daniel R.; Ghosh, Tapasi; Dennison, Brian K.; Mitchell, Kenneth J.; Payne, Harry E. (IOP PUBLISHING LTD, 1999-01)
We present the results of a low-frequency flux density monitoring program of 33 extragalactic radio sources. The light curves at 318 and 430 MHz over a 14 yr period are presented. The measurements were made with the NAIC Arecibo 305 m radio telescope at approximately bimonthly intervals between 1980 January and 1989 February and at less regular intervals between 1989 October and 1993 October, for a total of 64 observing sessions. In addition, we provide a first discussion of the results, pointing out several source properties and interesting objects.
Analytic Results for Hopping Models with Excluded Volume Constraint
Toroczkai, Zoltan (Virginia Tech, 1997-09-04)
Part I: The Theory of Brownian Vacancy Driven Walk We analyze the lattice walk performed by a tagged member of an infinite 'sea' of particles filling a d-dimensional lattice, in the presence of a single vacancy. The vacancy is allowed to be occupied with probability 1/2d by any of its 2d nearest neighbors, so that it executes a Brownian walk. Particle-particle exchange is forbidden; the only interaction between them being hard core exclusion. Thus, the tagged particle, differing from the others only by its tag, moves only when it exchanges places with the hole. In this sense, it is a random walk "driven" by the Brownian vacancy. The probability distributions for its displacement and for the number of steps taken, after n-steps of the vacancy, are derived. Neither is a Gaussian! We also show that the only nontrivial dimension where the walk is recurrent is d=2. As an application, we compute the expected energy shift caused by a Brownian vacancy in a model for an extreme anisotropic binary alloy. In the last chapter we present a Monte-Carlo study and a mean-field analysis for interface erosion caused by mobile vacancies. Part II: One-Dimensional Periodic Hopping Models with Broken Translational Invariance.Case of a Mobile Directional Impurity We study a random walk on a one-dimensional periodic lattice with arbitrary hopping rates. Further, the lattice contains a single mobile, directional impurity (defect bond), across which the rate is fixed at another arbitrary value. Due to the defect, translational invariance is broken, even if all other rates are identical. The structure of Master equations lead naturally to the introduction of a new entity, associated with the walker-impurity pair which we call the quasi-walker. Analytic solution for the distributions in the steady state limit is obtained. The velocities and diffusion constants for both the random walker and impurity are given, being simply related to that of the quasi-particle through physically meaningful equations. As an application, we extend the Duke-Rubinstein reputation model of gel electrophoresis to include polymers with impurities and give the exact distribution of the steady state.
An Arcminute-Resolution Imaging Study of the H-alpha & [S II] Emission of the ISM from the Local Perseus Arm Using the Virginia Tech Spectral-Line Survey
Nelson, Keith Phillip (Virginia Tech, 2011-08-08)
The Virginia Tech Spectral-Line Survey (VTSS) is a series of 10-degree wide, arcminute-resolution images of the warm ionized interstellar medium (WIM) within the Milky Way for declinations greater than -15 degrees. The Wisconsin Hydrogen-Alpha Mapper (WHAM) studies the kinematics and distribution of this same material on an angular scale of one degree. Comparing the spatially-resolved images of the VTSS with WHAM's high spectral resolution images shows that the ISM's characteristics appear to be very similar at both degree and arcminute scales --- we see similar structures, and notice the same [S II]/H-alpha trend between those structures at both degree and arcminute scales. VTSS fields show three basic types of structures --- compact clouds with diameters greater than several degrees, those that are 1-degree or less in diameter, and extended filaments that differ from the clouds by spanning several degrees in length but having thicknesses of only a few tens of arcminutes. The latter two morphological types cannot be observed by WHAM. Additionally, VTSS data confirms that the [S II] intensity values are directly proportional to H-alpha intensities, a result that is also observed at degree resolutions. Finally, VTSS data show that [S II]/H-alpha ratios are, on average, nearly six times higher in the filaments. This would indicate that collisional excitation of singly-ionized sulfur ions is the dominant emission source within filaments. In clouds, the lower [S II]/H-alpha values observed are evidence that the H-alpha recombination line of photo-excited neutral hydrogen dominates. Because automating the VTSS was a key part of obtaining many of the images used in this project, I begin with a discussion of general observatory automation. I then address the specific processes and techniques used in automating the VTSS before discussing data collection and reduction techniques.
Atomic Clock Augmentation For Receivers Using the Global Positioning System
Kline, Paul A. (Virginia Tech, 1997-02-07)
For receivers using the Global Positioning System (GPS), it is standard procedure to treat the receiver clock bias from GPS time as an unknown. This requires four range measurements to the satellites in order to solve for three dimensional position and clock offset. If the receiver clock could be synchronized with GPS time, the extra range measurement would not be necessary. To achieve this synchronization, a stable frequency reference must be incorporated into the GPS user set. This concept is known as clock aiding or clock augmentation of GPS receivers. Clock augmentation increases the availability of the navigation function because only three GPS satellites are required. Also, it is shown that clock augmentation improves vertical accuracy by reducing the vertical dilution of precision (VDOP), which is a unitless multiplier that translates range measurement error into vertical position error. This improvement in vertical accuracy is particularly beneficial for applications involving final approach and landing of aircraft using GPS, because GPS typically provides better horizontal accuracy than vertical accuracy. The benefits of atomic clock augmentation are limited by factors that cause a loss of synchronization either between the receiver and GPS time, or between ground station and airborne receivers processing GPS data in differential mode (DGPS). Among the error sources that cause a clock offset are antenna rotation, hardware drifts due to temperature variations, and relativistic effects for GPS receivers on moving platforms. Antenna rotation and temperature effects are addressed and supported by experimental data. It is shown that two particular relativity terms thought to be missing from GPS receiver algorithms are not evident in data collected during a flight test experiment. Upon addressing the error sources, the dissertation concludes with analysis of DGPS data collected during a flight test at the Federal Aviation Administration (FAA) Tech Center in Atlantic City, during which external rubidium oscillators were used by airborne (Boeing 757-B) and ground station GPS receivers. A new method of clock modeling is introduced, and this clock model is used to demonstrate the improvement in vertical accuracy, as well as three-satellite navigation.
Automation of the Spectral-Line Imaging Camera for the Virginia Tech Spectral-Line Survey
Portock, Kenneth Patrick (Virginia Tech, 2002-12-09)
The Virginia Tech Spectral-Line Survey (VTSS) is a high sensitivity, high resolution survey of Hydrogen-$\alpha$ and Sulfur-II emission in the full northern hemisphere. The instrument used for the survey is the Spectral Line Imaging Camera (SLIC). SLIC uses a fast(f/1.2) lens attached to a cryogenically cooled, TK 512x512 CCD with 27 micron pixels. The focal length of the lens is 58mm which gives a pixel size of 1.6 arcminutes. The diameter of each field is 10degrees. A filter wheel ahead of the lens allows for imaging at different wavelengths. Automating the imaging system is desirable and advantageous due to the large scope of the survey. A variety of devices have been developed in order to implement automation of the SLIC observatory. They include an automated focusing mechanism, filter wheel, liquid nitrogen auto fill system, motorized roll-off roof, cloud monitor, and an equatorial mount. A PERL script, called SLICAR (Spectral Line Imaging Camera Automation Routine), was written to control and communicate with the various hardware and software components. The program also implements a user prepared Observing File, and makes decisions based on observing conditions.
Classical and quantum gravity with Ashtekar variables
Soo, Chopin (Virginia Tech, 1992)
This thesis is a study of classical and quantum gravity with Ashtekar variables. The Ashtekar constraints are shown to capture the essence of the constraints and constraint algebra of General Relativity in four dimensions. A classification scheme of the solution space of the Ashtekar constraints is proposed and the corresponding physics is investigated. The manifestly covariant equations of motion for the Ashtekar variables are derived. Explicit examples are discussed and new classical solutions of General Relativity are constructed by exploiting the properties of the Ashtekar variables. Non-perturbative canonical quantization of the theory is performed. The ordering of the quantum constraints as well as the formal closure of the quantum constraint algebra are explored. A detailed Becchi-Rouet-Stora-Tyutin (BRST) analysis of the theory is given. The results demonstrate explicitly that in quantum gravity, fluctuations in topology can occur and there are strong evidences of phases in the theory. There is a phase which is described by a topological quantum field theory (TQFT) of the Donaldson-Witten type and an Abelian antiinstanton phase wherein self-interactions of the gravitational fields produce symmetry breaking from SO(3) to U(1). The full theory is much richer and includes fluctuations which bring the system out of the various restricted sectors while preserving diffeomorphism invariance. Invariants of the quantum theory with are constructed through BRST descents. They provide a clear and systematic characterization of non-local observables in quantum gravity, and can yield further differential invariants of four-manifolds.
Electron-positron annihilation into photons at √s = 50 to 64 GeV
Sterner, Kevin L. (Virginia Tech, 1993)
We present a study of e⁺e⁻ collisions where only photons are visible in the final state in data taken with the AMY detector at TRISTAN. Data are presented at CM energies from 50 to 64 GeV, with a total integrated luminosity of 189.1 pb⁻¹. Differential cross sections for e⁺e⁻ → γγ, γγγ are measured and compared to 𝛰(α³) QED. A search for electron compositeness through an e⁺e⁻ γγ contact interaction is conducted, and limits are presented. A search for the pair production of unstable photinos is also presented with limits. Finally, the result of a search for anomalous γγ production is presented, based upon energy scan data taken in December, 1992.
Fermion Quantum Field Theory In Black-hole Spacetimes
Ahmad, Syed Alwi B. (Virginia Tech, 1997-04-18)
The need to construct a fermion quantum field theory in black-hole spacetimes is an acute one. The study of gravitational collapse necessitates the need of such. In this dissertation, we construct the theory of free fermions living on the static Schwarzschild black-hole and the rotating Kerr black-hole. The construction capitalises upon the fact that both black-holes are stationary axisymmetric solutions to Einstein's equation. A factorisability ansatz is developed whereby simple quantum modes can be found, for such stationary spacetimes with azimuthal symmetry. These modes are then employed for the purposes of a canonical quantisation of the corresponding fermionic theory. At the same time, we suggest that it may be impossible to extend a quantum field theory continuously across an event horizon. This split of a quantum field theory ensures the thermal character of the Hawking radiation. In our case, we compute and prove that the spectrum of neutrinos emitted from a black-hole via the Hawking process is indeed thermal. We also study fermion scattering amplitudes off the Schwarzschild black-hole.
Nonlinear evolution of Vlasov equilibria
Demeio, Lucio (Virginia Polytechnic Institute and State University, 1989)
In this work, we investigate numerically the evolution of perturbed Vlasov equilibria. according to the full nonlinear system with particular emphasis on analyzing the asymptotic states towards which the system evolves. The simulations are carried out with the numerical code that we have implemented on the Cray X-MP of the Pittsburgh Supercomputing Center and which is based on the splitting scheme algorithm. Maxwellian symmetric and one-sided bump-on-tail and two-stream type of equilibrium distributions are considered: the only distribution which seems to evolve towards a BGK equilibrium is the two-stream while the asymptotic states for the other distributions are better described by superpositions of possible BGK modes. Perturbations with wave-like dependence in space and both symmetric and non-symmetric dependence on velocity are considered. For weakly unstable modes, the problem of the discrepancy between different theoretical models about the scaling of the saturation amplitude with the growth rate is addressed for the first time with the splitting scheme algorithm. The results are in agreement with the ones obtained in the past with less accurate algorithms and do not exhibit spurious numerical effects present in those. Finally, collisions are included in the splitting scheme in the form of the Krook model and some simulations are performed whose results are in agreement with existing theoretical models.
Radiation emission and absorption in a hydrogen plasma of a laser engine
Estublier, Denis L. (Virginia Tech, 1990-05-05)
In this work, we describe all the possible radiative processes occurring in a low temperature hydrogen plasma. Some of the fundamental concepts involving ionized gases and collision phenomena are presented, and a rigorous approach is used to show that classical mechanics is quite appropriate to our study. As an application to a laser engine, we investigate the effects of the maximum temperature, the temperature gradient, the stretching of the plasma shape, the engine pressure, and the equivalent sphere radius, on the total emitted power, including absorbing mechanisms through the equation of radiative transfer. Graphs related to spectral radiative exitances are included, and a complete set of graphs of the total power, permitting interpolations with respect to the above relevant parameters, are also provided.
Religious appeals in the popular presentation of cosmology
Guarino, Michael J. (Virginia Tech, 1996-05-08)
This thesis explores the use of religious language, metaphors, and images in the popular presentation of cosmology. Specifically looking at books written by cosmologists and intended for a lay audience, I argue that these religious appeals present a particular model, or picture, of cosmology to the public. Through the religious language, metaphors, and images, the texts create scientific-religious imagery which gives cosmology and cosmologists religious functions (e.g., the metaphor of cosmologists as priests). Taking these images together, the texts present a model in which cosmology performs like a religion -a model of Cosmology-as-Religion. Thus, the religious appeals appropriate religious authority through cosmology's assumption of religious functions. However, the texts also contain arguments justifying cosmology and the need for its continued financial support. The texts link these justificatory arguments to the scientific-religious imagery, and this associates the texts' appropriation of religious authority with the justificatory arguments. As a result, the appropriated functions give cosmology additional social benefits, providing a socio-religious context to the arguments. This, in turn, increases cosmology's importance to society. In this way, the religious appeals help legitimate the justification of cosmology.
The Scattering of H-alpha Emission Associated with the Rosette Nebula in the Monoceros Region Studied Using Polarimetry
Topasna, Gregory A. (Virginia Tech, 1999-05-07)
Polarimetric CCD images of HII regions were obtained using a rotating polarizer device designed, built, and used in conjunction with the Spectral Line Imaging Camera (SLIC) at Virginia Tech's Martin Observatory in Giles County, Virginia. The SLIC uses a narrow bandpass interference filter coupled with a 58 mm camera lens and cryogenically cooled CCD camera to image diffuse, extended H-alpha emission over a 10° angular extent. A rotating polarizer device was placed in front of the H-alpha filter with images recorded at every 45° with respect to a fiducial setting. Stoke's parameters and were obtained and polarization maps of selected HII regions were created. Maps of the Monoceros supernova remnant and the Rosette Nebula (NGC 2237-9) were made in an attempt to detect polarization by selective extinction in H light. While this was not detected, polarization by scattering in a dust shell around the Rosette Nebula (NGC 2237-9) was observed. Scattered continuum light from the central star cluster NGC 2244 in the H-alpha bandpass was ruled out. Using Celnik's (1985) map of extinction across the Rosette Nebula at the H wavelength, coupled with Serkowski's empirical relationship between maximum polarization and color excess, it was shown that the maximum degree of polarization seen in the Rosette Nebula should be no more than 3% to 4%. The polarization observed in this project reaches values as high as 10%. It was found that a correlation exists between the H-alpha intensity and infrared emission by dust grains in all four IRAS waveband images in the suspected scattering region of the Rosette Nebula. A radial comparison between [SII] images and H-alpha images in the region of high polarization showed that the H-alpha intensity in that region is dominated by scattered H-alpha light from the Rosette Nebula. A single scattering model was constructed in an effort to predict the observed polarization. The model used parameters based on 21 cm observations by Kuchar and Bania (1993) of the HI shell which surrounds the HII region of the Rosette Nebula. The single scattering model can not accurately predict the degree of polarization. It was concluded that a multiple scattering model is required. A spatial comparison of the 12 m emission with the degree of polarization strongly suggested that multiple scattering is important in describing the observed radial behavior of polarization. Polarization images of regions in Cygnus were obtained. A polarization map of the North America Nebula (NGC 7000) and surroundings reveals a large amount of polarization. The map reveals that scattering of H-alpha light from the North America Nebula is the most likely cause of polarization in these images. From the analysis in this thesis, I conclude that in the northwest quadrant, at radial distances greater than 40 from the center of the Rosette Nebula, the observed H-alpha intensity is due to scattered H light from the nebula itself. This implies that, in H-alpha , the Rosette Nebula appears slightly larger than it actually is. With evidence of polarization by scattered H supported by the polarization map of the North America Nebula (NGC 7000), it is concluded that other HII regions may very well appear larger in H-alpha than they actually are. Thus, scattered H-alpha light may account for a small part of the more extended warm ionized medium as well.
A search for slow, lightly ionizing particles in cosmic rays
Solie, Daniel J. (Virginia Tech, 1991)
A surface search was made using a small-area four-layer scintillator detector. Slow lightly ionizing particles created in cosmic ray interactions in the atmosphere which arrived within 13 ms of the shower front were searched for. In a live time of 2.55 x 10⁵ s 394,608 showers of two or more muons and 21,038 showers of four or more muons were recorded. No multiple muon events were recorded in which a trailing particle was identified above the expected random background, with a velocity between 7.4 x 10⁻⁵c and 6.7 x 10⁻³c. The detector operated without a trigger and used interplanar timing information to identify potential events, and operated above a threshold of 1/400 of that of a minimum ionizing muon.
Spatiotemporal dynamics of a photorefractive phase-conjugate resonator
Korwan, Daniel R. (Virginia Tech, 1996-04-15)
The spatiotemporal dynamics of a photorefractive phase-conjugate resonator (PPCR) is studied both experimentally and analytically. The resonator is a confocal cavity bounded by a dielectric mirror and a phase-conjugate mirror in a four wave mixing geometry. The effect of the Bragg mismatch, which is caused by the misalignment of the pump fields, is experimentally shown to break the cylindrical symmetry of the system and to increase the speed of the dynamics. By studying the first non stationary state at a cavity Fresnel number of F=2.0, the effect of the transverse component of the mismatch is shown to add a transverse phase to the wavefront of the phase-conjugate field, leading to the periodic nucleation of a pair of phase defects. A model of this state is developed in terms of the competition of a few transverse patterns. The model is experimentally verified using a holographic optical correlator designed to identify the modes presumed by the model. The dynamics are also studied using a Karhunen-Loeve decomposition in which the eigenvectors of the covariance matrix are calculated. The covariance matrix is obtained from the transverse intensity fluctuations of the cavity field and the eigenvectors are interpreted as the active cavity modes of the resonator. The results of the application of this experimental method to the F=2.0 state match those obtained by the correlator. This demonstrates its validity as a useful tool for studying the system. Application of the decomposition to states at higher F reveal that aperiodic and periodic states can have very similar active mode structures. An analytical model of the PPCR is then developed using a plane wave decomposition of the cavity field and the n1aterial variables contained in Kukhtarev's equations. Numerical simulations using the model demonstrate its accuracy. In addition, the different effects of the longitudinal and transverse components of the Bragg mismatch on the dynamics and defect nucleation are revealed. The relevant assumptions involved in the development of the model are discussed in detail.
Spectral characteristics of low-frequency variability in compact extragalactic radio sources
Pantazopoulou, Maria J. (Virginia Tech, 1996)
We examine the refractive scintillation hypothesis of low frequency variability via numerical simulation in order to account for the spectral characteristics of the observed fluctuations. Plane waves from extragalactic radio sources propagating through the interstellar medium, a medium with fluctuations of the refractive index due to electron density irregularities, emerge from that medium corrugated. Since fluctuations present on scales greater than the Fresnel scale act refractively, the emerging wavefront has a curvature which produces a refractive amplification or deamplification in the flux density. We develop a numerical algorithm to characterize the phase fluctuations in the wavefront and we simulate the resulting intensity distribution in the limit of geometrical optics. We then produce light curves by taking trajectories in the simulated intensity distribution plane and we compare our results with statistical properties of existing data from a 5—year monitoring program of 32 extragalactic sources at frequencies 0.318, 0.430, 0.606, 0.880 and 1.4 GHz. We find that the refractive scintillation hypothesis is in good agreement with the data at comparable timescales and that the variability in the simulated light curves diminishes within an octave of frequency, in agreement with those of the observed light curves with the same behavior.
Spontaneous CP-Violation in Two Higgs Doublet Supersymmetric Models
Lebedev, Oleg (Virginia Tech, 1998-07-08)
An alternative approach to the problem of CP-violation is presented. It is based on the possibility of spontaneous CP-breakdown in models with two Higgs doublets. General features of the phenomenon such as stability of the vacuum and the existence of a light axion are discussed. We investigate the feasibility of spontaneously broken CP in the minimal supersymmetric models - the MSSM and NMSSM. The latter is shown to be experimentally viable. The phenomenological implications of the model such as CP-violating effects in the kaon systems and a nonzero neutron electric dipole moment are studied.
Steady State Properties of Some Driven Diffusive Systems
Mazilu, Irina (Virginia Tech, 2002-08-30)
In an attempt to reach a better understanding of the properties and critical behavior of non-equilibrium systems, we investigate the steady state properties of three simple models, variations of the prototype, the driven Ising lattice gas. Our first system studied is the bilayer model, a stack of two driven Ising lattice gases allowed to interact. We study this model using a very simple analytic approximation, the high temperature expansion. Building on existing simulation data and field theory results, our goal is to test how faithfully the series expansion can reproduce the Monte Carlo phase diagram. We find that the agreement between our calculations and the already reported simulations results is remarkably good. Next, we investigate the critical behavior of a two-dimensional Ising lattice gas driven into a non-equilibrium steady state, subject to a local modification of the dynamics, namely, having anisotropic attempt frequencies for exchanges along different spatial directions. We employ both Monte Carlo simulation techniques and a high temperature expansion approximation and find the phase diagram of the system, perform a finite-size scaling study in order to determine the universality class of the model and compare our simulation results with the phase diagram obtained using the high temperature expansion. We conclude that the bias in the jump rates does not affect the universal critical properties of the system: the modified model is in the same universality class as the driven Ising lattice gas. Our last objective concerns a different inroad into the study of non-equilibrium steady states. Instead of investigating a non-equilibrium steady state via indirect observables, such as correlation functions and order parameters, we seek to compute the steady state probability distribution directly. This is feasible only for systems with a small number of degrees of freedom. We chose to study a one-dimensional version of the so-called two-temperature kinetic Ising model. We solve the master equation exactly for a 1x6 system, and compare the full configurational probability distribution with its equilibrium counterpart.
The Structure and Polarization Properties of the SiO Masers in the Extended Atmosphere of R Aquarii
Boboltz, David Allen Jr. (Virginia Tech, 1997-06-13)
Silicon monoxide (SiO) maser emission has been observed towards many late-type stars. The conditions necessary for the formation of SiO masers dictate that they be produced in a region which is inside the silicate dust formation point close to the surface of the star. Very Long Baseline Interferometry (VLBI) has shown that these masers do indeed lie close to the stellar surface at a distance of a few stellar radii. This extended atmosphere is a complex region dominated by stellar pulsations and permeated by circumstellar shocks. This dissertation presents the results of a multi-epoch VLBI study of the v=1, J=1-0, 43-GHz SiO maser emission towards the symbiotic binary R Aquarii. Four epochs of full-polarization observations were recorded using the Very Long Baseline Array a facility the National Radio Astronomy Observatory. The first high-resolution images of the extended atmosphere of a Mira variable in a symbiotic binary have been produced. The SiO masers towards R Aquarii have been found to exhibit a ring-like morphology ~31 mas (~6.8 AU) in diameter. The emission changes significantly over a time period of ~1-2 months with almost no similarity in structure on timescales >6 months. An analysis of the four epochs of observations has provided the first direct evidence of SiO maser proper motions. These observations, taken as the Mira variable approached maximum light, show that over a 98-day period the masers have an average inward proper motion of ~1 mas. This contraction of the maser shell implies an infall velocity of ~4 km/s for the SiO masers during this phase of the stellar pulsation cycle. In addition to the total intensity images, maps of the linear and circular polarization morphology were also produced. These images show that the SiO masers are significantly polarized, and that the polarization structure and intensity change on timescales as short as ~1-2 months. For three of the four epochs, a mean fractional circular polarization of 4% was determined implying a magnetic field strength Bsec(theta) = 13 G. For one of the four epochs, the mean fractional circular polarization was found to be ~14% indicating a magnetic field strength Bsec(theta) = 46 G. The fractional linear polarization is fairly constant for all four epochs with mean values ranging from 20.8-25.0%, and peak values as high as ~83% for isolated maser features. Maps of the linear polarization vectors show an orderly structure over large portions of the maser shell indicating a uniform magnetic field topology in these regions of the extended atmosphere of R Aquarii.
Study of Electromagnetic Scattering from Randomly Rough Ocean-Like Surfaces Using Integral-Equation-Based Numerical Technique
Toporkov, Jakov V. (Virginia Tech, 1998-04-28)
A numerical study of electromagnetic scattering by one-dimensional perfectly conducting randomly rough surfaces with an ocean-like Pierson-Moskowitz spectrum is presented. Simulations are based on solving the Magnetic Field Integral Equation (MFIE) using the numerical technique called the Method of Ordered Multiple Interactions (MOMI). The study focuses on the application and validation of this integral equation-based technique to scattering at low grazing angles and considers other aspects of numerical simulations crucial to obtaining correct results in the demanding low grazing angle regime. It was found that when the MFIE propagator matrix is used with zeros on its diagonal (as has often been the practice) the results appear to show an unexpected sensitivity to the sampling interval. This sensitivity is especially pronounced in the case of horizontal polarization and at low grazing angles. We show - both numerically and analytically - that the problem lies not with the particular numerical technique used (MOMI) but rather with how the MFIE is discretized. It is demonstrated that the inclusion of so-called "curvature terms" (terms that arise from a correct discretization procedure and are proportional to the second surface derivative) in the diagonal of the propagator matrix eliminates the problem completely. A criterion for the choice of the sampling interval used in discretizing the MFIE based on both electromagnetic wavelength and the surface spectral cutoff is established. The influence of the surface spectral cutoff value on the results of scattering simulations is investigated and a recommendation for the choice of this spectral cutoff for numerical simulation purposes is developed. Also studied is the applicability of the tapered incident field at low grazing incidence angles. It is found that when a Gaussian-like taper with fixed beam waist is used there is a characteristic pattern (anomalous jump) in the calculated average backscattered cross section at incidence angles close to grazing that indicates a failure of this approximate (non-Maxwellian) taper. This effect is very pronounced for the horizontal polarization and is not observed for vertical polarization and the differences are explained. Some distinctive features associated with the taper failure are visible in the surface current (solution to the MFIE) as well. Based on these findings we are able to refine one of the previously proposed criteria that relate the taper waist to the angle of incidence and demonstrate its robustness.
A Water Maser Flare in W49N: Amplification by a Rotating Foreground Cloud
Boboltz, David Allen Jr.; Simonetti, John H.; Dennison, Brian K.; Diamond, Philip J.; Uphoff, J. A. (IOP PUBLISHING LTD, 1998-12)
We monitored the 22 GHz H2O maser emission toward W49N from December 1989 through May 1990. During this period we observed an outburst in a component at -66.25 km s(-1). The flux density of the flaring component increased by a factor of greater than or similar to 10 to a maximum of 4020 Jy over a period of 24 days and decreased over the following 34 days to 1400 Jy on the last day of monitoring. During the flux increase the line narrowed from about 1.1 to 0.8 k s(-1); it subsequently rebroadened to 1.0 km s(-1) Most interestingly, during the flaring behavior the line center shifted by approximately 0.5 km s(-1) over the 58 day period. To explain the flare, particularly its shifting line center, we present a model of two interacting maser clouds. In the model, saturated maser radiation produced in a background cloud is amplified by an unsaturated, masing foreground cloud. Motion of the foreground cloud across the line of sight produces a flaring line, accompanied by line narrowing. We demonstrate that the observed flare in W49N may be explained by such a model where a rotating foreground cloud passes in front of a non-rotating background cloud. The differential amplification of the background cloud's radiation produces the observed increase in flux density, line narrowing, and systematic shift in center velocity.

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