Showing papers on "Radiative transfer published in 1977"

A theory of the interstellar medium - Three components regulated by supernova explosions in an inhomogeneous substrate

Abstract: Supernova explosions in a cloudy interstellar medium produce a three-component medium in which a large fraction of the volume is filled with hot, tenuous gas. In the disk of the galaxy the evolution of supernova remnants is altered by evaporation of cool clouds embedded in the hot medium. Radiative losses are enhanced by the resulting increase in density and by radiation from the conductive interfaces between clouds and hot gas. Mass balance (cloud evaporation rate=dense shell formation rate) and energy balance (supernova shock input=radiation loss) determine the density and temperature of the hot medium with (n, T) = (10/sup -2.5/, 10/sup 5.7/) being representative values. Very small clouds will be rapidly evaporated or swept up. The outer edges of ''standard'' clouds ionized by the diffuse UV and soft X-ray backgrounds provide the warm (approx.10/sup 4/ K) ionized and neutral components. A self-consistent model of the interstellar medium developed herein accounts for the observed pressure of interstellar clouds, the galactic soft X-ray background, the O VI absorption line observations, the ionization and heating of much of the interstellar medium, and the motions of the clouds. In the halo of the galaxy, where the clouds are relatively unimportant, we estimate (n, T)more » = (10/sup -3.3/, 10/sup 6.0/) below one pressure scale height. Energy input from halo supernovae is probably adequate to drive a galactic wind.« less

Interstellar bubbles. II - Structure and evolution

Abstract: The detailed structure of the interaction of a strong stellar wind with the interstellar medium is presented. First, an adiabatic similarity solution is given which is applicable at early times. Second, a similarity solution is derived which includes the effects of thermal conduction between the hot (about 1 million K) interior and the cold shell of swept-up interstellar matter. This solution is then modified to include the effects of radiative energy losses. The evolution of an interstellar bubble is calculated, including the radiative losses. The quantitative results for the outer-shell radius and velocity and the column density of highly ionized species such as O VI are within a factor 2 of the approximate results of Castor, McCray, and Weaver (1975). The effect of stellar motion on the structure of a bubble, the hydrodynamic stability of the outer shell, and the observable properties of the hot region and the outer shell are discussed.

Atmospheric Science: An Introductory Survey

Abstract: Preface. Units and Numerical Values. A brief Survey of the Atmosphere. Atmospheric Thermodynamics. Extratropical Synoptic-Scale Disturbances. Atmospheric Aerosol and Cloud Microphysical Processes. Clouds and Storms. Radiative Transfer. The Global Energy Balance. Atmospheric Dynamics. The General Circulation. Index.

A Satellite Technique for Quantitatively Mapping Rainfall Rates over the Oceans

Abstract: A theoretical model for calculating microwave radiative transfer in raining atmospheres is developed. These calculations are compared with microwave brightness temperatures at a wavelength of 1.55 cm measured on the Nimbus-5 satellite and rain rates derived from WSR-57 meteorological radar measurements. A specially designed ground based verification experiment was also performed wherein upward viewing microwave brightness temperature measurements at wavelengths of 1.55 cm and 0.81 cm were compared with directly measured rain rates.

Self‐absorption effects on the radiative lifetime in GaAs‐GaAlAs double heterostructures

Abstract: An analysis is presented of the effects of self‐absorption of spontaneously emitted photons on the recombination lifetime of injected carriers in the active region of GaAs‐GaAlAs double heterostructures. It is shown that at moderate injection levels (∼100 A/cm2) self‐absorption effects may lengthen appreciably the net radiative lifetime of the carriers in heterostructures with thick (?1 μ) active regions. Results of the analysis are in good agreement with data of Ettenberg and Kressel on variation of carrier lifetime with active region thickness. From the analysis it is inferred that the internal quantum efficiency of radiative recombination in the device of Ettenberg and Kressel is greater than 90%.

The evaporation of spherical clouds in a hot gas. II. Effects of radiation.

Abstract: The effects of radiation on the evaporation of spherical clouds in a hot medium are considered. The critical cloud radius at which radiative losses balance conductive heating is determined as a function of the external temperature and density. Smaller clouds evaporate, and larger clouds condense. The conditions under which the surfaces of the clouds may be detected are discussed. Net radiative losses for evaporating clouds are calculated, and an effective cooling function for a cloudy medium is obtained. The results may be applied to clouds in supernova remnants, in the interstellar medium, and in clusters of galaxies.

Multiple scattering in reflection nebulae. I - A Monte Carlo approach. II - Uniform plane-parallel nebulae with foreground stars. III - Nebulae with embedded illuminating stars

Abstract: A method for calculating the surface brightness distribution on a plane-parallel reflection nebula of uniform density illuminated by a star located either in front of, behind, or arbitrarily inside the scattering medium is proposed. The Monte Carlo technique is used to find solutions to the radiative transfer problem. The scattering properties of the nebular particles are parameterized by the albedo for single scattering and a three-parameter analytic phase function. Calculations are then presented for the surface brightness distribution across the face of such nebulae with (1) a foreground star and (2) immersed stars. The calculations include the full effects of multiple scattering, are independent of a particular assumed grain material or size distribution, and are applicable to any wavelength region for which observations can be obtained.

Radiative decays of the n = 2 states of He-like ions

Abstract: The relativistic random-phase approximation (RRPA) is applied to study radiative transitions from $n=2$ states along the He isoelectronic sequence. The strengths of various decay modes and the energy splittings of the $n=2$ multiplets are investigated. At low $Z$ the present results agree with earlier nonrelativistic studies, whereas, at high $Z$ our results provide new information about oscillator strengths, branching ratios, and multiplet structure for the $n=2$ states.

Solar seismology. I - The stability of the solar p-modes

Abstract: We investigate the stability of the radial p-modes of the Sun by computing nonadiabatic eigen-values and e1genfunctions for a solar envelope model which extends from an inner radius r ≈ 0.3 R_☉ out to an optical depth r ≈ 3 x 10^(-4). Our calculations take into account in a crude fashion the response of the convective flux to the oscillation. The dynamical effect of turbulence in the convection zone is parametrized in terms of a turbulent shear viscosity. The results of our calculations are as follows. If damping by turbulent viscosity is neglected, all modes with penriods longer than 6 minutes are unstable. The familiar K-mechanism, which operates in the H ionization-H^- opacity region, is the dominant source of driving of the oscillations. Modes with periods shorter than 6 minutes are stabilized by radiative damping in the solar atmosphere. When turbulent dissipation of pulsational energy is included, all modes are predicted to be stable. However, the margin of stability is very small. In view of the large uncertainty that must be assigned to our estimate of turbulent damping, we conclude that theoretical calculations cannot unequivocally resolve the question of the stability of the solar p-modes.

Microwave Emissivity and Accumulation Rate of Polar Firn

Abstract: Radiative transfer theory is formulated to permit a meaningful definition of emissivity for bulk emitting media such as snow. The emissivity in the Rayleigh-Jeans approximation is then the microwave brightness temperature T B divided by an effective physical temperature 〈T〉. The 〈T〉 is an average of the physical temperature, T(z), weighted by a radiative transfer function ƒ(z). Similarly, where e(z) is the local emittance. An approximate ƒ(z) is used to determine analytically the effects of various absorption coefficients, of scattering coefficients that vary with depth, and of the seasonal variation of T(z). It is shown that a mean emissivity, which is equal to the mean annual T B divided by the mean annual surface temperature T m, is a useful quantity for comparing theory and observations. Snow-crystal size measurements, r(z), at seven locations in Greenland and Antarctica are used to determine the Mie/Rayleigh scattering coefficient γs (z) and to calculate the mean emissivities. The observed mean emissivities are determined by a which is the average of 12 monthly Nimbus-5 (1.55 cm) microwave observations, and the Tm measured at the same locations. The calculated emissivities are about one-half of the observed values. The assumption that each snow crystal is an independent and equally effective scatterer, and the use of an approximation to ƒ(z), tend to over-estimate the effect of scattering. Therefore, a parameter multiplying γs (z) is used. The emissivities calculated with a single value of this empirical parameter for all seven locations agree well with the observed emissivities, showing that the microwave emissivity variations of dry polar urn can be characterised as a function of the crystal sizes. One optical depth corresponds to a typical fini depth of 5 m, but significant radiation emanates from up to 30 m. Since r(z) depends on the snow accumulation rate A and T m. the sensitivity of the emissivity to changes in T m or A are estimated using this semi-empirical theory. The results show that a one degree change or uncertainty in Tm is approximately equivalent to a 10% change in A, and that such a change will affect the emissivity by 0.003 to 0.014 or the T B by about 0.6 K to 3 K, depending on the location.

Calculations of impurity radiation and its effects on tokamak experiments

Abstract: The impurity radiation for typical tokamak parameters has been numerically calculated using an "average-ion model". Coronal equilibrium values for the emission of oxygen, iron, molybdenum, tungsten and gold were determined from the steady-state solutions of a set of related rate equations which included the effects of electron collisional ionization and excitation, dielectronic and radiative recombination, Δn = 0 and Δn ≠ 0 line transitions, and bremsstrahlung. The results for oxygen, iron, and molybdenum compare very well with other calculations. Since impurities diffusing in a tokamak are not expected to be in coronal equilibrium, time-dependent radiation calculations were also performed. A comparison of these non-equilibrium loss rates with those calculated under the assumption of coronal equilibrium indicates that coronal radiation calculations do not significantly underestimate the moderate- and high-Z impurity radiation losses for neoclassical diffusion velocities in large tokamaks, such as Princeton Large Torus and Tokamak Fusion Test Reactor. Finally, the detailed steady-state emission rates were used to investigate the effects of various concentrations of impurities on the neτ requirements for breakeven, ignition, and Q = 5 beam-driven reactor experiments.

X-ray sources in stellar winds

Abstract: A model of a binary X-ray source immersed in a stellar wind from its companion star is considered. The geometry of the constant ionization and temperature contours in the model is discussed, and X-ray radiative transfer through the wind is described. The results are applied to make predictions about the presence and variability of iron K-shell fluorescence lines, to explain the light curve of Cen X-3 during a 'low' to 'high' state transition in July 1972, and to predict variability with orbital phase of resonance ultraviolet lines such as N V at 1238 and 1242 A from the binary system.

Radiative characteristics of Saharan dust at solar wavelengths

Abstract: Solar radiation measurements made on Sal in the Cape Verde Islands during the 1974 Gate project are analyzed in conjunction with aerosol size spectra data in order to describe the extinction and absorption of solar radiation by Saharan dust. From optical depth measurements, supported by Mie calculations, we were able to show that near-neutral extinction must exist across most of the solar spectrum as the result of a large effective scattering diameter of the dust. Absorption estimates were also made from our composite radiation data by inverting radiative transfer equations based on the delta-Eddington model. Imaginary indices of refraction obtained by this method at three wavelengths agree satisfactorily with laboratory measurements made with Saharan dust. Total depletion of solar energy as a function of path length and optical depth is presented along with an analysis of the extinction coefficient as a function of aerosol concentrations. From the latter we determined a near-linear relationship between optical depth as measured at the ground and total aerosol loading in an atmospheric column. A brief comparison is made between aerosol extinction properties at Sal and at Barbados.

ESCA applied to polymers. XV. RF Glow-discharge modification of polymers, studied by means of ESCA in terms of a direct and radiative energy-transfer model†

Abstract: The crosslinking of an ethylene–;tetrafluoroethylene copolymer by exposure to an argon plasma, excited by an inductively coupled RF field, is studied over a wide range of pressures and power loadings. The results are interpreted in terms of a two-component, direct and radiative energy-transfer model showing that the outermost monolayer crosslinks rapidly via direct energy transfer from argon ions and metastables.

Nonisothermal Band Model Theory

Abstract: Theoretical formulations of radiation band models for general nonuniform optical paths are presented. The models are framed within the statistical band model for an array of Lorentz lines and an inverse line strength distribution. Radiative transfer for an isolated line in a general nonuniform medium and the statistical model for a uniform optical path are reviewed in order to provide the foundation required for the band model formulations. Two approaches to the development of these models are taken. The first is based on making approximations to such radiative transfer functions as transmittance or equivalent width and yields models equivalent or similar to the traditional Curtis-Godson approximation. The second treats approximations to the spatial derivatives of these functions. From the standpoint of computing line or band radiance, the spatial derivatives are more fundamental quantities than the transfer functions themselves; consequently, these latter “derivative approximations” are instrinsically more accurate than the Curtis-Godson type approximations. All of the models are formulated to give the spatial derivative of the mean equivalent width function W (s) δ in the form 1 δ d W (s) ds =c(s)p(s) k (s)y(s) , where c(s), p(s) and k(s) are the concentration, total pressure and absorption band model parameter, respectively, at the local path position s. The functional form of the derivative function y(s) is derived for each of the models. In general, y(s) is a function of both local and path-averaged (subscript e) band model parameters. These local and averaged parameters are used to define a dimensionless optical depth parameter x e = u k e β e and three nonuniformity indexes ρ= γ γ e , r= β β e and θ = δ e δ (γ is line width, ḡd is the mean line spacing band model parameter, and β = 2μ γ γ ). The explicit manner in which these parameters enter into the various functional forms for y(s) is derived and discussed. The introduction of the parameter q is an important aspect of the new models and is an explicit measure of nonisothermality along the optical path. In addition to the traditional Curtis-Godson definitions for the effective parameters ke and βe, new definitions for the path averages γe and δe are derived. These new definitions are obtained from the fundamental properties of the assumed inverse line strength distribution. A summary of all the models is given as a table of relevant equations for use in practical calculations.

Determination of Rainfall Distributions from Microwave Radiation Measured by the Nimbus 6 ESMR

Abstract: The polarization of 37 GHz microwave radiances emerging from rain clouds above land, rough and calm water surfaces was computed from the equation of radiative transfer. Scattering was assumed to be characterized by a Rayleigh phase matrix. The radiative transfer equation was solved by means of a Neumann solution. It was found that the brightness temperatures of the upward directed radiances emerging from rain clouds were relatively independent of polarization. The weak polarization of radiation emitted by rain clouds can be used to discriminate between cool brightness temperatures emerging from rain clouds and open water. The brightness temperatures of radiances emerging from rain clouds over water can be transformed into a single-valued function of the rainfall rate if polarization effects are considered. A sample of Nimbus 6 data is analyzed in accord with the results of the theoretical analysis.

An Extension to the Chahine Method of Inverting the Radiative Transfer Equation

Abstract: An extension of the Chahine relaxation method (1970) for inverting the radiative transfer equation is presented. This method is superior to the original method in that it takes into account in a realistic manner the shape of the kernel function, and its extension to nonlinear systems is much more straightforward. A comparison of the new method with a matrix method due to Twomey (1965), in a problem involving inference of vertical distribution of ozone from spectroscopic measurements in the near ultraviolet, indicates that in this situation this method is stable with errors in the input data up to 4%, whereas the matrix method breaks down at these levels. The problem of non-uniqueness of the solution, which is a property of the system of equations rather than of any particular algorithm for solving them, remains, although it takes on slightly different forms for the two algorithms.

An extension to the Chahine method of inverting the radiative transfer equation. [application to ozone distribution in atmosphere]

Abstract: Abstract An extension of the Chahine relaxation method for inverting the radiative transfer equation is presented. This method is superior to the original method in that it takes into account in a realistic manner the shape of the kernel function, and its extension to nonlinear systems is much more straightforward. A comparison of the new method with a matrix method due to Twomey (1965), in a problem involving inference of vertical distribution of ozone from spectroscopic measurements in the near ultraviolet, indicates that in this situation this method is stable with errors in the input data up to 4%, whereas the matrix method breaks down at these levels. The problem of non-uniqueness of the solution, which is a property of the system of equations rather than of any particular algorithm for solving them, remains, although it takes on slightly different forms for the two algorithms.

Inversion Methods in Atmospheric Remote Sounding

Abstract: The mathematical theory of inversion methods is applied to the remote sounding of atmospheric temperature, humidity, and aerosol constituents.

Derivation of quantum mechanics from stochastic electrodynamics

Abstract: From the equation of motion for a radiating charged particle embedded in the zero‐point radiation field we construct a stochastic Liouville equation which serves to derive, by a smoothing process, a Fokker–Planck‐type equation with infinite memory. We show that an exact alternative form of this phase‐space equation is the Schrodinger equation in configuration space, with radiative corrections. In the asymptotic, radiationless limit (when the radiative corrections became negligible), the phase‐space density reduces to Wigner’s distribution, thus confirming Weyl’s rule of correspondence. We briefly discuss several other implications of stochastic electrodynamics which are relevant for quantum theory in general.

A Model of the Effect of Aerosols on Urban Climates with Particular Applications to the Los Angeles Basin

Abstract: A one-dimensional, time-dependent model of the boundary layer has been developed to study the effects of pollutants an local meteorological variables. Radiative terms in the model are computed using a fourstream discrete-ordinate method, convective terms are parameterized at the surface using transfer coefficients, and dynamical terms are evaluated from available data. The model is compared initially to the Great Plains data with good results. The results of several model runs with varying types of pollutant concentrations are then discussed. The dominant conclusion that can be drawn from these results is that strong compensation between radiative and convective effects and between low-level heating and inversion rise act to minimize the effect of urban pollutants on surface and atmospheric temperatures. The model results also show that urban pollutants tend to warm the surface slightly and, possibly, to suppress the rate of inversion rise.

Convective and Radiative Heat Transfer in a High Swirl Direct Injection Diesel Engine

Abstract: The investigation described in this thesis is a study of instantaneous heat fluxes (total and radiative) and their variation with operating conditions in a high-swirl direct-injection diesel engine. The problem is approached experimentally and methods for prediction of convective and radiative components are suggested. Total heat fluxes were measured using a thin-film type thermocouple developed in the course of the work, while the radiant flux was measured by a pyroelectric infrared detector. The experimental observations demonstrate variations in local heat fluxes which are moderate under motored conditions but large in the fired engine. Some of the observed features of flux variation with time and with location have been shown to be qualitatively explicable in terms of probable local events during the cycle. [Continues.]

The optical surface modes of metal spheres

Abstract: A theoretical study of optical surface plasmon modes of metal spheres is made within the framework of a free-electron spatially dispersive dielectric function model which includes retardation effects. Dispersion equations for homogeneous and inhomogeneous spheres are derived for the lth spherical harmonic. The radiative properties of the modes are investigated as a function of l and surface structure which is included by means of a double step-function profile. The real part of the frequency has the same magnitude, and trend with sphere radius, as the current experimental results.