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  • Other Sources  (19)
  • 1
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    IR heating of the cometary atmosphere (1986)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The transfer of radiation, solar and IR dust, in the cometary atmosphere is analyzed using a six-band radiative transfer model. The first five bands correlate with the strongest IR band of H2O and the sixth corresponds to the bulk of solar energy. The radiation field for each band is calculated as a function of the cometocentric distance using the three-stream approximation. It is observed that the mean intensity of the IR radiation field generated by the dust and the cometary nucleus drops from a very large value of about 40,000 erg/sq cm sec at the surface to 400 erg/sq cm sec at 7 km, rises to a peak value of 500 erg/sq cm sec at 100 km and then falls off monotonically. The causes of these changes in radiation field intensity are discussed. The effect of variation of the IR radiation field on the gas temperature is studied.
    Keywords: ASTROPHYSICS
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  • 2
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    A multi-fluid model of an H2O-dominated dusty cometary atmosphere (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The solution of the simultaneous set of differential equations representing conservation of number density, momentum, and energy, together with solar radiation transfer in streams that are responsible for major photolytic processes and nucleus heating, yields a self-consistent multifluid solution for the thermal and dynamical structure of an H2O-dominated, two-phase dusty gas cometary atmosphere, whose validity is restricted to the collision-dominated region where ions and neutrals are assumed to achieve a common temperature and velocity. The model is transonic, due to the dust in the inner coma, and involves the subsonic expansion of the ions and neutrals from the nucleus until, in the outermost part of the collision-dominated coma, temperatures of 300-400 K are reached due to UV pyrolytic heating.
    Keywords: ASTROPHYSICS
    Type: Moon and the Planets; 27; Dec. 198
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  • 3
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    The physics of comets (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: This volume of the 'Fundamentals of Cosmic Physics' is concerned with the physics of comets. Aspects regarding the nucleus are discussed, taking into account observations, radius and albedo, mass, rotation, splitting and disruption, the temperature and sublimation of the cometary nucleus, the chemical composition, the electrostatic charging of the cometary nucleus, and the structure and evolution of the cometary nucleus. Other topics explored are related to the atmosphere and its interaction with the solar wind, the plasma tail, and the dust tail. Attention is given to excitation mechanisms, atmospheric abundances and production rates, chemical models of the inner coma, collisional processes and the multifluid approach, radiative transfer, the ionization processes, models of the inner layer, the outer coma, acceleration mechanisms in the plasma tail, the structure of the plasma tail, the cometary magnetosphere, dust grain orbits, gas-dust interaction in the inner coma, and the nature of the cometary dust.
    Keywords: ASTROPHYSICS
    Type: Fundamentals of Cosmic Physics (ISSN 0094-5846); 10; 1-4
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  • 4
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    The effects of the diffuse radiation fields due to multiple scattering and thermal reradiation by dust on the dynamics and thermodynamics of a dusty cometary atmosphere (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A self-consistent model of a dirty, clathrate cometary nucleus is extended to account for diffuse radiation fields caused by multiple scattering and thermal reradiation of the solar continuum by dust. The model is configured to fit conditions expected for the various spacecraft which will encounter Halley's comet at 0.89 AU. The atmosphere is assumed a chemically reactive dust-gas mixture in quasi-steady spherically symmetric expansion. The effect of electron-neutral ion collisions is accounted for, along with rate constants of the various species of clathrate ice particles and radiative transfer of the solar input in the UV, visible and near-IR intervals. The opacity of the circumnuclear dust is projected to be an order of magnitude greater than previous estimates, which severely impacts the potential visibility of the nucleus to the cameras of the flyby satellites.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 287; 445-454
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  • 5
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    The atmosphere of a dirty-clathrate cometary nucleus - A two-phase, multifluid model (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The dynamical and thermal structure of a dirty-clathrate cometary nucleus' gas atmosphere is presently given a self-consistent, transonic multifluid solution in which, although the heavy neutron and ion species are treated as a single fluid in the collision-dominated region, the photoproduced H is treated separately. The thermal profile of the atmosphere thus obtained is entirely different from those predicted by the earlier, single-fluid models as well as the multifluid models which assumed equipartition of energy between electrons and ions. While the electron gas, like the neutrals and the ions, cools due to expansion, its main mode of energy loss in the inner coma is by way of inelastic collisions with the predominant H2O molecule. The high electron temperature in the outer coma also decreases the efficiency of electron removal by dissociative recombination, thereby increasing electron density throughout the coma.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 273; 381-396
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  • 6
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    The photochemistry and dynamics of a dusty cometary atmosphere (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: 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.
    Keywords: ASTROPHYSICS
    Type: Moon and the Planets; 27; Aug. 198
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  • 7
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    Non-radial flow near the cometary surface (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: It is shown that, as a consequence of the non-uniform temperature distribution of the cometary nucleus, large lateral pressure gradients are set up, which in turn drive strong lateral flows. However, at small heliocentric distances the onset of turbulence within a thin boundary layer destroys these steady lateral flows and the eventual outflow of gas from within the outer boundary of this layer is expected to be more or less radial. On the other hand, at large heliocentric distances, turbulence is unlikely to set in, and the lateral flows that are set up, may persist. Consequently, it is expected that the gas flow out of the cometary nucleus at these large distances to be highly non-radial.
    Keywords: ASTROPHYSICS
    Type: Moon and the Planets; 26; June 198
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  • 8
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    The photochemical heating of the cometary atmosphere (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A self-consistent solution of the thermodynamic structure of an H2O-dominated multispecies cometary atmosphere at 1 AU is obtained by solving the simultaneous set of differential equations representing conservation of number density, momentum, energy, and the flux of solar UV radiation in the three streams responsible for the major photolytic processes. The relative efficiency of the expansion and infrared cooling by H2O over the photolytic heating by the UV radiation in the inner coma brings about a rapid decrease in the temperature there, reaching a minimum of only about 5.4 K at a nuclear distance of approximately 54 km. Thereafter, the temperature is found to increase to about 810 K at a nuclear distance of 25,000 km, owing to higher efficiency of the photolytic heating over the cooling process. The expansion in the inner coma is seen to be highly supersonic, attaining a maximum Mach number of about 15 at the distance of temperature minimum. After that, the Mach number steadily decreases, reaching a value of about 2.3 at a nuclear distance of 25,000 km.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 260
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  • 9
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    Studies of the Gas Tori of Titan and Triton (1997)
    In:  CASI
    Details
    Publication Date: 2019-08-17
    Description: A model for the spatial distribution of hydrogen in the Saturn system including a Titan source, an interior source for the rings and inner icy satellites, and a Saturn source has been applied to the best available Voyager 1 and 2 UVS Lyman-alpha observations presented by Shemansky and Hall. Although the model-data comparison is limited by the quality of the observational data, source rates for a Titan source of 3.3 - 4.8 x 10(exp 27) H atoms/s and, for the first time, source rates larger by about a factor of four for the interior source of 1.4 - 1.9 x 10(exp 27) H atoms/s were determined. Outside the immediate location of the planet, the Saturn source is only a minor contribution of hydrogen. A paper describing this research in more detail has been submitted to The Astrophysical Journal for publication and is included in the Appendix. Limited progress in the development of a model for the collisional gas tori of Triton is also discussed.
    Keywords: Lunar and Planetary Exploration
    Type: NASA-CR-205039 , NAS 1.26:205039 , P-498
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  • 10
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    Analysis of hydrogen H-alpha observations of the coma of Comet P/Halley (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The Monte Carlo Particle Trajectory Model of Combi and Smyth (1988) is used here to analyze observations of the H-alpha coma of Comet Halley. The solar excitation mechanism for the H-alpha emissions line is described. The H2O production rates derived for the H-alpha brightness measurements are shown to be very consistent with the H2O production rates determined from other Comet Halley observations of the H, O, and OH comae. Revised H2O production rates determined from 6300 A brightness measurements are presented.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 413; 2; p. 757-763.
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