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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

The opacity of the universe and the strong equivalence principle

A possible explanation of why the advanced solutions of Maxwell's equations are not observed in nature is by way of absorption by an opaque universe. As Davies has shown, the ever expanding, general relativistic cosmological models fail to provide the needed absorption. The absorption mechanism calling for an interplay between local physics and cosmology, is usually developed adopting the strong equivalence principle, SEP, which precludes such interplay. It is shown that complete absorption of electromagnetic radiation by ionized intergalactic plasma is obtained provided a violation of the SEP, of the order of the Hubble's constant, is allowed to occur. The same degree of violation was previously found to be compatible with a large body of observational data.

Canuto, V. M.↗
NASA NTRS1985

Laboratory measurements of the microwave opacity and vapor pressure of sulfuric acid vapor under simulated conditions for the middle atmosphere of Venus

Microwave absorption measurements at wavelengths of 13.4 and 3.6 cm were made in gaseous H2SO4 in a CO2 atmosphere under simulated conditions for the Venus middle atmosphere. The results suggest that abundances of gaseous H2SO4 on the order of 15-30 ppm could account for the absorption observed by radio occultation measurements at these wavelengths. They also imply that such abundances would correspond to saturation vapor pressure existing at or above the 46-48-km range, which correlates with the observed cloud base.

Steffes, P. G.↗
NASA NTRS1986

The effects of electron scattering opacity in the broad emission-line regions of quasars

It is demonstrated that three observational puzzles in quasars, nearly symmetric Lyman-alpha profiles, weakly asymmetric C IV 1549 A, and offsets between the peaks of these high ionization lines and the systematic velocity, can all be explained by the standard model of the physical state of the broad-line region with the addition of the simplest possible kinematics. Electron scattering in the intercloud medium is the essential ingredient that had been previously neglected. It is shown that the profiles depend at least as sensitively on the run of physical conditions as on the velocity law. Approximate analytic representations of the line emissivities based on detailed photoionization models are used to describe that dependence.

Kallman, T. R.↗
NASA NTRS1986

Evaluation of the microwave spectrum of Venus in the 1.2-22 centimeter wavelength range based on laboratory measurements of constituent gas opacities

Laboratory measurements of the microwave (1.2-22.3 cm) spectrum properties of Venus' gaseous atmospheric constituents were performed using an apparatus for simulating conditions of the middle atmosphere of Venus (gaseous H2SO4 + CO2 at 1 to 6 atm). The results have shown that at wavelengths longer than 1.8 cm, gaseous H2SO4 and CO2 are the predominant microwave absorbers, while at wavelengths from 1.2 to 1.8 cm, SO2 and CO2 are the predominant absorbers. These results were used to develop a model for the microwave emission spectrum of Venus, which correlated well with microwave observations of this planet.

Steffes, Paul G.↗
NASA NTRS1989

Atomic data for opacity calculations. X - Oscillator strengths and photoionisation cross sections for O III

Energy levels and electric dipole radiative transitions were determined for O III. The wavefunctions for the bound and continuum states were derived by solving coupled integrodifferential equations in the close-coupling approximation with the aid of two methods, the R-matrix and linear algebraic methods. Configuration interaction wavefunctions are presented for eight states of the O IV target with configurations of 2s(x)sp(y) (x + y = 3). Oscillator strengths are calculated for the transitions between, and photoionization cross sections from, bound states of O III with configurations of 2s(x)2p(y)nl (for n of not greater than 10 and l of not greater than 3).

Luo, D.↗
NASA NTRS1989

Atomic data for opacity calculations. XI - The carbon isoelectronic sequence

Close-coupling calculations are carried out for radiative processes in neutral carbon and a number of carbon-like ions; energy levels, oscillator strengths, and photoionization cross sections have been computed for all bound states of the type 2 s(j)2p(k)nl with n not above 10 and 1 not above 3. The R-matrix method is employed to solve the coupled equations with a ten-state eigenfunction expansion for the parent ion C II and an eight-state expansion for the other boron-like target ions. A number of selected results for oscillator strengths are presented and compared with earlier data, as well as for photoionization cross sections with autoionizing resonance structures. Isoelectronic trends are discussed. The present results for the oscillator strengths of C I and N II are found to differ significantly from some earlier theoretical works for a number of transitions. However, the present C I f values are in excellent agreement with recent calculations and experimental results.

Luo, D.↗
NASA NTRS1991

Dust and molecular properties of the low-opacity cloud Lynds 1563

Optical, molecular, and far-infrared data are analyzed for L1563, estimated peak Ab 2.5 mag. The cloud is detected by IRAS at 12, 25, 60, and 100 microns, and with CO, (C-13)O, and H2CO molecules. A column density comparison yields an estimate of the temperature of the classical dust grains of 15.6 + or - 1 K, while the color temperature derived from the ratio I(60)/I(100) is 26 K. Both dust and color temperatures decrease toward the cloud center.

Clark, Frank O.↗
NASA NTRS1991

Opacity effects on the radiative losses of coronal loops

This paper presents calculations of radiative losses, which include the effects of geometry and optical depth, that can be used to improve the energy losses in many flux-tube calculations for loops with constant and variable cross sections. The results include the non-LTE ionization state of hydrogen and helium, thus allowing the determination of the ionization energy in the gas and the relative importance of collisional and radiative processes. These calculations show that optical depth effects are important under solar conditions in the temperature range of 8000-40,000 K.

Kuin, N. P. M.↗
NASA NTRS1992

L-shell X-ray opacity of many-electron atoms

The study presents a formulation for obtaining the L-shell photoelectric cross section (sigma sub L) for many-electron atomic systems. A scaling law is suggested for the evaluation of the contribution to sigma sub L from 2s and 2p ejection from a general system in any ionization state. The formula is expressed in terms of the effective nuclear charge (Z sub 2s and Z sub 2p) seen by the corresponding bound electrons, with Z sub 2s and Z sub 2p, obtained from improved screening constants derived in a recent investigation. A comparison is made of Hartree-Fock and hydrogenic expressions for sigma sub L with experimental results for atomic neon at X-ray photon energies, providing the basis for the scaling law and an approximate energy dependence for the cross sections. The special case of sigma sub L for atomic Fe is considered in detail. This L edge has structure because of the different threshold for 2s and 2p photoejection; the latter has fine structure.

Jung, Young-Dae↗
NASA NTRS1993

Groundbased monitoring of Martian atmospheric opacity

The amount of dust in the Martian atmosphere is variable in both space and time. The presence of aerosols in the Mars atmosphere complicates quantitative analysis of Martian surface properties. We have developed a model for Mars surface and atmospheric scattering based on equations in Hillier et al (1991). This formulation was chosen for its speed of computation and because it accounts for the spherical geometry of atmospheric scattering at high mission angles, i.e., near the planetary limb.

Herkenhoff, K. E.↗
NASA NTRS1994

Airborne CO2 coherent lidar measurements of cloud backscatter and opacity over the ocean surface

The use of an airborne CO2 lidar to obtain cloud backscatter and extinction data at a thermal infrared wavelength is described. The extinction in this spectral region is proportional to the cloud liquid water content. The use of coherent detection results in high sensitivity and narrow field of view, the latter property greatly reducing multiple-scattering effects. Backscatter measurements in absolute units are obtained through a hard target calibration methodology. For clouds of low to moderate optical thickness at the lidar wavelength, both geometric thickness and optical thickness can be measured. The sea surface reflectance signal is used to obtain estimates of the cloud optical thickness. The utility of this technique results from studies that indicate that the spatial scale of variability of the sea surface reflectance is generally large compared with that of cloud optical thickness. Selected results are presented from data taken during flights over the Pacific Ocean.

Menzies, Robert T.↗
NASA NTRS1995

Photon-photon opacity constraints for relativistically expanding gamma-ray burst sources

Five bright gamma-ray bursts (GRBs) detected by the Burst and Transient Source Experiment (BATSE) have also been detected at higher energies by EGRET. Four are consistent with power-law spectra extending to energies as high as, in the case of GRB930131, 1 GeV. The fifth, and most recent, GRB940217, has a more complex spectrum, with one photon detected at 18 GeV, the most energetic GRB photon detection to date. The optical depth to photon-photon pair production in these sources is extremely large for distances more than about 10pc away if the radiation is emitted isotropically in the observer's frame. This optical depth can be dramatically reduced if the source is moving with a relativstic bulk Lorentz factor Gamma, and recent calculations for this situation have been limited to cases of a beam with opening angle 1 Gamma, or expansions of infinitely thin spherical shells. This paper presents our extension of the pair production otpical depth calculation in relativistically expanding sources to more general geometries, including shells of finite thickness and arbitrary opening angle. We find that the minimum bulk Lorentz factors for the Energy Gamma Ray Experiment Telescope (EGRET) sources to be optically thin, i.e. display no spectral attenuation, is only moderately dependent on the shell thickness and its opening solid angle; these new limits on required velocity for given geometries will aid in placing realistic constraints on GRB source models.

Baring, M. G.↗
NASA NTRS1996

Research on Spectroscopy, Opacity, and Atmospheres

I discuss errors in theory and in interpreting observations that are produced by the failure to consider resolution in space, time, and energy. I discuss convection in stellar model atmospheres and in stars. Large errors in abundances are possible such as the factor of ten error in the Li abundance for extreme Population II stars. Finally I discuss the variation of microturbulent velocity with depth, effective temperature, gravity and abundance. These variations must be dealt with in computing models and grids and in any type of photometric calibration.

Kurucz, Robert L.↗
NASA NTRS1996

Ultraviolet Opacity and Fluorescence in Supernova Envelopes

By the time the expanding envelope of a Type 2 supernova becomes transparent in the optical continuum, most of the gamma-ray luminosity produced by radioactive Fe/Co/Ni clumps propagates into the hydrogen/helium envelope and is deposited there, if at all. The resulting fast electrons excite He 1 and H 1, the two- photon continua of which are the dominant internal sources of ultraviolet radiation. The UV radiation is blocked by scattering in thousands of resonance lines of metals and converted by fluorescence into optical and infrared emission lines that escape freely. We describe results of Monte Carlo calculations that simulate non-LTE scattering and fluorescence in more than five million allowed lines of Ca, Sc, Ti, V, Cr, Mn, Fe, Co, and Ni. For a model approximating conditions in the envelope of SN 1987A, the calculated emergent spectrum resembles the observed one. For the first 2 yr after explosion, the ultraviolet radiation (lambda less than or approximately equals 3000) is largely blocked and converted into a quasi continuum of many thousands of weak optical and infrared emission lines and some prominent emission features, such as the Ca 2 lambdalambda8600 triplet. Later, as the envelope cools and expands, it becomes more transparent, and an increasing fraction of the luminosity emerges in the UV band.

Li, Hongwei↗