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At least 631 records · Page 35
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NASA NTRS1981

Study of the ammonia ice cloud layer in the Equatorial Region of Jupiter from the infrared interferometric experiment on Voyager

Spectra from the Voyager 1 infrared interferometer spectrometer (IRIS) obtained near the time of closest approach to Jupiter were analyzed for the purpose of inferring ammonia cloud properties associated with the Equatorial Region. Comparisons of observed spectra with synthetic spectra computed from a radiative transfer formulation, that includes multiple scattering, yielded the following conclusions: (1) very few NH3 ice particles with radii less than 3 microns contribute to the cloud opacity; (2) the major source of cloud opacity arises from particles with radii in excess of 30 microns; (3) column particle densities are between 1 and 2 orders of magnitude smaller than those derived from thermochemical considerations alone, implying the presence of important atmospheric motion; and (4) another cloud system is confirmed to exist deeper in the Jovian troposphere.

Marten, A.↗
NASA NTRS1982

Atmospheric Correction for Remote Sensor Data

Mathematical model based on radiative transfer theory as applied to plane parallel, horizontally spatially homogeneous, isotropic atmosphere. Model includes effects of multiple scattering by gases and aerosols, and effects of absorption by ozone in near ultraviolet and visible parts of spectrum.

Turner, R. E.↗
NASA NTRS1982

Analytic optical potentials for nucleon-nucleus nucleus-nucleus collisions involving light and medium nuclei

Utilizing an optical model potential approximation to the exact nucleus-nucleus multiple-scattering series, optical potentials for nucleon-nucleus and nucleus-nucleus collisions are analytically derived. These expressions are applicable to light and medium cosmic ray nuclei as their single-particle density distributions are analytically determined, without approximation, from their actual harmonic well charge density distributions. Pauli correlation effects are included through the use of a simple Gaussian function to replace the usual expression obtained in the infinite nuclear matter approximation.

Bidasaria, H. B.↗
NASA NTRS1982

The Spectroscopy of Venus

Problems in the spectroscopy of the planet are discussed. Two major problems are focused on: the almost total domination of the spectrum by CO2 (including almost every conceivable isotropic combination) makes the search for other species difficult; and the knowledge that no wavelengths short of the the microwave penetrate through the Venus cloud decks, which means that UV, visible, and IR remote sensing can investigate only the middle and upper atmosphere. The problem of intense multiple scattering is also considered.

Beer, R.↗
NASA NTRS1982

Electronic levels and charge distribution near the interface of nickel

The energy levels in clusters of nickel atoms were investigated by means of a series of cluster calculations using both the multiple scattering and computational techniques (designated SSO) which avoids the muffin-tin approximation. The point group symmetry of the cluster has significant effect on the energy of levels nominally not occupied. This influences the electron transfer process during chemisorption. The SSO technique permits the approaching atom or molecule plus a small number of nickel atoms to be treated as a cluster. Specifically, molecular levels become more negative in the O atom, as well as in a CO molecule, as the metal atoms are approached. Thus, electron transfer from the nickel and bond formation is facilitated. This result is of importance in understanding chemisorption and catalytic processes.

Waber, J. T.↗
NASA NTRS1982

The peculiar UV extinction of Herschel 36

The differential extinction curve of Herschel 36 was determined from International Ultraviolet Explorer data. It is quite unusual, characterized by a distinct 2200 A peak with a very low far blue end at 5 to 7 mu to the -1 power. The star appears to be an extreme member of the group Savage drew attention to, previously consisting only of theta Ori, NU Ori, sigma Sco, and rho Oph. It appears that multiple scattering effects are needed to explain the observations.

Hecht, J.↗
NASA NTRS1981

Radiative transfer in the surfaces of atmosphereless bodies. I - Theory. II - Interpretation of phase curves

A generalized theory of radiative transfer is presented which includes single and multiple scattering of light in particulate surfaces, and is applicable to both the surfaces of atmosphereless bodies and to laboratory samples. Single scattering is described in terms of the effects of porosity and roughness, and is formulated by means of a probabilistic method. It is shown that, for low-albedo surfaces, the effects of porosity and roughness are separable, the opposition effect is caused by the former, and the slope of the linear part of the phase curve is mainly controlled by the latter. The theory, which is applicable to all albedos and phase angles, may be used in both surface brightness and integrated brightness studies. The limiting case of roughness and volume density tending to zero yields the results of classical radiative transfer theory.

Lumme, K.↗
NASA NTRS1982

Analysis of nitrogen and oxygen far ultraviolet auroral emissions

Far ultraviolet observations of the auroral and airglow provide quantitative diagnostics of atmospheric abundances, energy deposition, and excitation processes because many atmospheric species have resonance transitions in this spectral region. The spectroscopy of an active auroral arc observed above Fort Churchill on March 29, 1978, has been discussed by Feldman and Gentieu (1982). The present investigation attempts to quantify the measurements with the aid of a self-consistent approach used by Meier et al. (1980). It is shown that the primary electron spectrum has a characteristic energy of 1.75-2.5 keV, corresponding to energy deposition rates of 5-8 erg/(sq cm-sec). Atomic nitrogen emissions resulting from molecular dissociative excitation show no evidence of multiple scattering, in contrast to data from other auroras.

Meier, R. R.↗
NASA NTRS1982

Photometry and polarimetry of Titan - Pioneer 11 observations and their implications for aerosol properties

Pioneer 11 measurements of Titan's limb darkening and polarization at red and blue wavelengths are refined, and the integrated disk brightness measurements at phase angles from 22 to 96 deg, over which Titan's phase coefficient averages about 0.014 magnitudes/deg in both colors, are reduced. Comparisons of the data with vertically homogeneous multiple scattering models indicate that the single-scattering phase functions of the aerosols in both colors are flat at scattering angles between 80 and 150 deg, with a small peak at larger scattering angles. Although a vertically homogeneous model in which particles are assumed to scatter as spheres cannot simultaneously match the polarization observations of both colors for any refractive index, observed polarizations are most sensitive to the particle properties near optical depth 1/2 in each color. Models based on single scattering by spheres can therefore be successful over a range of refractive indices, if the size of the particles increases with depth and if the cross section of the particles increases sufficiently rapidly with decreasing wavelength.

Tomasko, M. G.↗
NASA NTRS1982

On the geometric nature of high energy nucleus-nucleus reaction cross sections

Within the context of a high energy double-folding optical potential approximation to the exact nucleus-nucleus multiple-scattering series, eikonal scattering theory is used to investigate the validity of geometric reaction cross sections in relativistic heavy ion collisions. The potential used includes a finite range interaction and nuclear single-particle densities extracted from nuclear charge distributions by unfolding the finite proton charge distribution. Pauli correlation effects are also included in an approximate way. The sensitivity of the predictions to the assumed interaction, Pauli correlation approximation, and nuclear density distributions is investigated. These results are in agreement with early predictions concerning the geometric nature of relativistic heavy ion collisions and in disagreement with a recent analysis, utilizing the zero range approximation, which suggested otherwise. Reasons for the lack of agreement between the analyses are also presented. Finally, approximate applicability limits for geometric reaction cross sections are determined.

Townsend, L. W.↗
NASA NTRS1983

Heavy-ion total and absorption cross sections above 25 MeV/nucleon

Within the context of a double-folding optical potential approximation to the exact nucleus-nucleus multiple-scattering series, eikonal scattering theory is used to generate tables of heavy ion total and absorption cross sections at incident kinetic energies above 25 MeV/nucleon for use in cosmic ray high-energy heavy ion transport and shielding studies. Comparisons of predictions with nucleus-nucleus experimental data show excellent agreement except at the lowest energies, where the eikonal approximation may not be completely valid. Even at the lowest energies, however, agreement is typically within 20 percent.

Townsend, L. W.↗
NASA NTRS1983

Development of a nuclear technique for monitoring water levels in pressurized vehicles

A new technique for monitoring water levels in pressurized stainless steel cylinders was developed. It is based on differences in attenuation coefficients of water and air for Cs137 (662 keV) gamma rays. Experimentally observed gamma ray counting rates with and without water in model reservoir cylinder were compared with corresponding calculated values for two different gamma ray detection theshold energies. Calculated values include the effects of multiple scattering and attendant gamma ray energy reductions. The agreement between the measured and calculated values is reasonably good. Computer programs for calculating angular and spectral distributions of scattered radition in various media are included.

Singh, J. J.↗
NASA NTRS1982

The photochemistry and dynamics of a dusty cometary atmosphere

The solving of a simultaneous set of differential equations representing conservation of number density, momentum, and energy together with solar radiation transfer in the streams which result in photolytic processes and the heating of the nucleus yields a self-consistent solution of the dynamical and thermal structure of an H2O-dominated two-phase dusty gas cometary atmosphere. Two models are considered for the transfer of solar radiation through the circumnuclear dust halo. The first considers only the direct extinction by the dust, and in the second the diffuse radiation field due to multiple scattering by the dust halo, which compensates for radiation removed by direct absorption when the optical depth is near unity, is approximated by neglecting the attenuation of radiation given off by the dust. It is shown that while dust attenuation has a strong effect on the H2O production rate, it also increases the electron density in the inner coma over the unattenuated case.

Marconi, M. L.↗
NASA NTRS1982

Photoelectric photometry of asteroids 33 Polyhymnia and 386 Siegena

The photometric parameters of 33 Polyhymnia and 386 Siegena were determined in the course of an international campaign performed during August 15-September 14, 1980 at Table Mountain Observatory, European Southern Observatory, and Catania Astrophysical Observatory. The rotational periods and amplitudes were determined, and the multiple-scattering factor was found to be 0.15 plus or minus 0.06 for 386 and 0.26 plus or minus 0.33 for 33, implying higher albedos in each case than expected according to their taxonomic classes, C and S, respectively. The color indices B-V and U-B were measured and found to differ significantly from those given by Bowell et al. (1979).

Zappala, V.↗
NASA NTRS1982

Cloud height differences on Saturn

Spectrophotometry from 0.6-2.5 micron at 1.5% spectral resolution of Saturn's equator and visibly dark South Equatorial Belt near a 1980 ring-plane crossing shows substantially less absorption by methane over the equator than over the SEB. Model fits using Appleby's model atmosphere and a multiple scattering model lead to the conclusion that the pressure levels of the clouds in the two regions differ by 20%. The region of high clouds is coincident with the location of the equatorial fast jet observed by Voyager.

Apt, J.↗
NASA NTRS1983

The interplanetary hydrogen and helium glow and the inferred interstellar gas properties

Observations of the interplanetary hydrogen and helium glow have been obtained by a number of spacecraft and rocket experiments during the past fifteen years. Important results have been established on the temperature, density, velocity, spatial dependence, and hydrogen to helium ratio. However, only four spacecraft launched to date are investigating the outer solar system and of these four the Pioneer 10 spacecraft is the farthest out at 28 A.U. Observations from this spacecraft at great distances have permitted an improved analysis of the effects which are only evident at large distances from the Sun. Perhaps the most significant result in this regard is the clear evidence of the importance of multiple scattering of solar Ly-alpha; an effect which has not been observed in earlier work. Ignoring this effect can lead to a gross overestimate of the local galactic glow. Current best estimates of the galactic glow and the local interstellar wind parameters obtained by the Pioneer 10 photometer at great distances are presented, in addition to complementary experimental observations of particular interest.

Judge, D. L.↗
NASA NTRS1984

An abrasion-ablation model description of galactic heavy-ion fragmentation

The fragmentation of high-energy galactic heavy ions by nuclear interactions with arbitrary target nuclei is described within the context of a simple abrasion-ablation fragmentation model. The abrasion part of the theory utilizes a quantum-mechanical formalism based upon an optical model potential approximation to the exact nucleus-nucleus multiple-scattering series. Nuclear charge distributions of the excited prefragments are calculated using either a hypergeometric distribution or a method based upon the zero-point oscillations of the giant dipole resonance. The excitation energy of the prefragment is estimated from the geometric clean-cut abrasion-ablation model. The decay probabilities for the various particle emission channels, in the ablation stage of the fragmentation, are obtained from the EVAP-4 Monte Carlo computer program. Elemental production cross sections for 1.88-GeV/nucleon iron colliding with carbon, silver, and lead targets are calculated and compared with experimental data and with the predictions from the semiempirical relations of Silberberg and Tsao.

Townsend, L. W.↗
NASA NTRS1984

Voyager 1 imaging and IRIS observations of Jovian methane absorption and thermal emission: Implications for cloud structure

Images from three filters of the Voyager 1 wide angle camera are used to measure the continuum reflectivity and spectral gradient near 6000 A and the 6190 A band methane/continuum ratio for a variety of cloud features in Jupiter's atmosphere. The dark barge features in the North Equatorial Belt have anomalously strong positive continuum spectral gradients suggesting unique composition. Methane absorption is shown at unprecedented spatial scales for the Great Red Spot and its immediate environment, for a dark barge feature in the North Equatorial Belt, and for two hot spot and plume regions in the North Equatorial Belt. Methane absorption and five micrometer emission are correlated in the vicinity of the Great Red Spot but are anticorrelated in one of the plume hot spot regions. Methane absorption and simultaneous maps of five micrometer brightness temperature is quantitatively compared to realistic cloud structure models which include multiple scattering at five micrometer as well as in the visible. Variability in H2 quadrupole lines are also investigated.

West, R. A.↗