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  • 1990-1994  (9)
  • 1
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    Analysis of hydrogen H-alpha observations of the coma of Comet P/Halley (1993)
    Institute of Physics
    Details
    Publication Date: 1993-08-01
    Print ISSN: 0004-637X
    Electronic ISSN: 1538-4357
    Topics: Physics
    Published by Institute of Physics
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    PAPER CURRENT
  • 2
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    The sodium zenocorona (1991)
    American Geophysical Union
    Details
    Publication Date: 1991-01-01
    Print ISSN: 0148-0227
    Electronic ISSN: 2156-2202
    Topics: Geosciences
    Published by American Geophysical Union
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  • 3
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    The Nature of the Hydrogen Tori of Titan and Triton (1993)
    Elsevier
    Details
    Publication Date: 1993-01-01
    Print ISSN: 0019-1035
    Electronic ISSN: 1090-2643
    Topics: Physics
    Published by Elsevier
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  • 4
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    The sodium zenocorona (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: A recent narrow-band-filtered CCD image by Mendillo et al. (1990) has shown that a sodium corona, produced near Io, extends at least 400 Jupiter radii in the planet's equatorial plane. Isophotes indicate that the polar to equatorial extents are in about 1 to 3 proportions. The image can be reproduced by a model which includes both a high- and an intermediate-speed distribution, with source rates of 2.2 and 1.1 x 10 exp 26 atoms/s, respectively. The high-speed distribution was ejected from Io with a velocity tangential to the satellite orbit of 57 km/s (about 74 km/s relative to Jupiter) plus an isotropic Maxwellian velocity distribution of about 25 km/s. This distribution likely corresponds to a charge exchange source of plasma torus sodium ions which are neutralized in the near-Io atmosphere and are ejected relative to Jupiter with a corotational velocity (74 km/s) plus a thermal ion (25 km/s) Maxwellian distribution. The intermediate speed distribution was ejected from Io with a tangential speed near 20 km/s (37 km/s relative to Jupiter) plus an isotropic Maxwellian velocity distribution of about 12 km/s. This distribution corresponds to the same nonthermal sodium atoms earlier identified near Io in the sodium directional features (Pilcher et al., 1984).
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 22
    Format: text
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  • 5
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    Analysis of hydrogen H-alpha observations of the coma of Comet P/Halley (1993)
    In:  Other Sources
    Details
    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.
    Format: text
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  • 6
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    The OH distribution in cometary atmospheres - A collisional Monte Carlo model for heavy species (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: The study presents an extension of the cometary atmosphere Monte Carlo particle trajectory model formalism which makes it both physically correct for heavy species and yet computationally reasonable. The derivation accounts for the collision path and scattering redirection of a heavy radical traveling through a fluid coma with a given radial distribution in outflow speed and temperature. The revised model verifies that the earlier fast-H atom approximations used in earlier work are valid, and it is applied to a case where the heavy radical formalism is necessary: the OH distribution. It is found that a steeper variation of water production rate with heliocentric distance is required for a water coma which is consistent with the velocity-resolved observations of Comet P/Halley.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 408; 2; p. 668-677.
    Format: text
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  • 7
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    The nature of the hydrogen tori of Titan and Triton (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: The nature of the hydrogen tori of Titan and Triton is examined. Critical time scales of the two tori are discussed. For the Titan torus, where atom-atom collisions are not important, the time scale for solar radiation pressure to act on the system is shown to be comparable to the hydrogen lifetime due to ionization and charge exchange losses by solar, magnetospheric, and solar wind processes. The solar radiation pressure then provides a mechanism which destroys the initial azimuthal symmetry of the hydrogen atom orbits about the planet and causes atom orbits to move inward and to collide with the planet on its dusk side. For Triton, the atom-atom collision time scale dominates all other time scales in the system. The evolution of the torus is then an inherently nonlinear problem that depends upon the collisional redistribution of atom-orbit velocities in the presence of a planetary gravitational force field. This nonlinear process introduces an expansion mechanism into the torus problem which dramatically alters its structure.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Icarus (ISSN 0019-1035); 101; 1; p. 18-32.
    Format: text
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  • 8
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    Extended atmospheres of comets and outer planet-satellite systems (1992)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: For the hydrogen coma of comet P/Halley, both a Lyman-alpha image and extensive Lyman-alpha scan data obtained by the Pioneer Venus Orbiter Ultraviolet spectrometer as well as H-alpha ground-based spectral observations obtained by the University of Wisconsin Space Physics Group were successfully interpreted and analyzed with our Monte Carlo particle trajectory model. The excellent fit of the model and the Halley data and the water production rate determined near perihelion (9 Feb. 1986) from 13 Dec. 1985 to 13 Jan. 1986 and from 1 Feb. to 7 Mar. 1986 are discussed. Studies for the circumplanetary distribution of atomic hydrogen in the Saturn and Neptune systems were undertaken for escape of H atoms from Titan and Triton, respectively. The discovery of a new mechanism which can dramatically change the normal cylindrically symmetric distribution of hydrogen about the planet is discussed. The implications for the Titan-Saturn and Triton-Neptune are summarized.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA-CR-190142 , NAS 1.26:190142
    Format: application/pdf
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  • 9
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    Studies for Io's extended atmosphere and neutral clouds and their impact on the local satellite atmosphere and on the planetary magnetosphere (1993)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The research performed in this project is divided in two main investigations: (1) the synthesis and analysis of a collection of independent observations for Io's sodium corona, its sodium extended atmosphere, and the sodium cloud, and (2) the analysis of a (System III longitude correlated) space-time 'bite-out' near western elongation in the 1981 sodium cloud images from the JPL Table Mountain Sodium Cloud Data Set. For the first investigation, modeling analysis of the collective observed spatial profiles has shown that they are reproduced by adopting at Io's exobase a modified sputtering flux speed distribution function which is peaked near 0.5 km/s and has a small high-speed (15-20 km/s) nonisotropic component. The nonisotropic high-speed component is consistent with earlier modeling of the trailing directional feature. For the second investigation, modeling analysis of the 'bite-out' observed near western elongation (but not eastern elongation) has shown that it is reproduced in model calculation by adopting a plasma torus description for the sodium lifetime that is inherently asymmetric in System III longitudes of the active sector and that also has an east-west asymmetry. The east-west and System III longitude asymmetries were determined from independent observations for the plasma torus in 1981. The presence of the 'bite-out' feature only near western elongation may be understood in terms of the relative value for sodium of its lifetime and its transport time through the System III enhanced plasma torus region.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA-CR-195255 , NAS 1.26:195255
    Format: application/pdf
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