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  • 1
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    Investigation of the composition of the Luna 16 lunar sample (1977)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: The concentrations of aluminum, manganese, sodium, chromium, iron, cobalt, and 12 rare earth elements were determined by neutron activation analysis using slow neutrons. Oxygen and silicon were determined using a fast neutron generator. Mossbauer spectroscopy was used to investigate iron compounds in Luna 16 regolith samples from the upper part of the core.
    Keywords: ASTROPHYSICS
    Type: NASA, Washington The Soviet-Am. Conf. on Cosmochem. of the Moon and Planets, pt. 1; p 277-280
    Format: application/pdf
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  • 2
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    A model of inner cometary ionospheres (1986)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A hydrodynamical model to describe the movement of the thermalized charged components in the inner ionosphere of comet Halley is presented. Photoelectrons are included by applying a two-stream-type approach. The numerical scheme describes shock transitions in a natural way. Solutions are obtained for a number of different assumptions concerning electron heating rates but all show that the electron temperature increases sharply where the collisional electron neutral coupling becomes unimportant. This temperature increase is accompanied by an increase in the plasma pressure and in its associated polarization electric field, and causes the plasma flow to go subsonic. In certain cases this transition occurs as an inner shock which may explain the observed ion pile-up in Halley's comet.
    Keywords: ASTROPHYSICS
    Type: ESA Proceedings of the 20th ESLAB Symposium on the Exploration of Halley's Comet. Volume 1: Plasma and Gas; p 235-239
    Format: text
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  • 3
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    Charge exchange in solar wind-cometary interactions (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A simple model of a cometary spherically symmetrical atmosphere and ionosphere is considered. An analytic solution of the governing equations describing the radial distribution of the neutral and ion densities is found. The new solution is compared to the well-known solution of the equations containing only ionization terms. Neglecting recombination causes a significant overestimate of the ion density in the vicinity of the comet. An axisymmetric model of the solar wind-cometary interaction is considered, taking into account the loss of solar wind ions due to charge exchange. The calculations predict that for active comets, solar wind absorption due to charge exchange becomes important at a few thousand kilometers from the nucleus, and a surface separating the shocked solar wind from the cometary ionosphere develops in this region. These calculations are in reasonable agreement with the few observations available for the ionopause location at comets.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 268; May 15
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  • 4
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    The friable sponge model of a cometary nucleus (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The mantle/core model of cometary nuclei, first suggested by Whipple and subsequently developed by Mendis and Brin, is modified and extended. New terms are added to the heat conduction equation for the mantle, which is solved in order to obtain the temperature distribution in the mantle and the gas production rate as a function of mantle thickness and heliocentric distance. These results are then combined with some specific assumptions about the mantle structure (the friable sponge model) in order to make predictions for the variation of gas production rate and mantle thickness as functions of heliocentric distance for different comets. A solution of the time-dependent heat conduction equation is presented in order to check some of the assumptions.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 278; 449-455
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  • 5
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    Time-dependent dusty gasdynamical flow near cometary nuclei (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: This paper presents time-dependent solutions to the coupled dusty hydrodynamics equations describing the spherically symmetric expansion of cometary neutral gas in the vicinity of a cometary nucleus. The sublimation process is repressented by gas outflow from a dust-covered reservoir containing stationary gas whose pressure and density values are determined by the sublimating (Ts) and surface (T0) temperatures. The model resolves earlier ambiguities in determining gas production rates and provides analytic relations between Ts, T0, and the gas parameters at the sonic point. The time evolution of a cometary outburst was modeled. It was found that, as a result of the strong gas-dust interaction in the inner coma region, a 'slow' disturbance in both the dust and gas parameters will be created in addition to the familiar gas blast-wave solution. This new 'slow' disturbance, which propagates with a velocity of about 0.2 km/s, might be responsible for some of the observed slowly expanding cometary halos, such as the one which was recently identified using 1910 Mount Wilson high-resolution comet Halley photographs.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 293; 328-341
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  • 6
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    The sheath/ionosphere boundary layer at Venus (1995)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: At Venus the interaction of the shocked solar wind and cold planetary ions takes place in the dayside mantle. The shocked solar wind is a warm, drifting Maxwellian plasma whereas the planetary plasma is cold; the plasma in the mantle is strongly magnetized. The coexistence of these two populations is unstable, and it leads to wave excitations that organize the energy and momentum exchange between the shocked solar wind and the plasma of planetary origin. The source of the free energy is the solar wind. The intensive wave activity seen in the 100 Hz channel of the wave instrument onboard the Pioneer-Venus Orbiter in the dayside mantle region of Venus can be identified as almost electrostatic VLF waves excited by the kinetic branch of the modified two-stream lower hybrid instability. The waves interact with the particles, and the planetary plasma is heated and accelerated outside the ionosphere, close to its upper boundary. This way solar wind scavenges the ionosphere, and planetary ions leave the planetary magnetosphere. A portion of the wave energy is capable of penetrating directly into the ionosphere and heating it.
    Keywords: ASTROPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 16; 4; p. (4)71-(4)74
    Format: text
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  • 7
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    Dust and neutral gas modeling of the inner atmospheres of comets (1986)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: This paper summarizes the present, preencounter understanding of the physical and chemical processes controlling the inner (r less than 1000 km) region of cometary atmospheres. Special emphasis was attached to compiling a self-consistent set of governing equations. This review is aimed at readers who want to understand the present status of the mantle and coma regions and/or who want to develop new, next generation models which will be needed as the large volume of new observational data will become available in the near future.
    Keywords: ASTROPHYSICS
    Type: Reviews of Geophysics (ISSN 8755-1209); 24; 667-700
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  • 8
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    Electron impact ionization in the vicinity of comets (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The solar wind interacts very strongly with the extensive cometary coma, and the various interaction processes are initiated by the ionization of cometary neutrals. The main ionization mechanism far outside the cometary bow shock is photoionization by solar extreme ultraviolet radiation.Electron distributions measured in the vicinity of comets Halley and Giacobini-Zinner by instruments on the VEGA and ICE spacecraft, respectively, are used to calculate electron impact ionization frequencies. Ionization by electrons is of comparable importance to photoionization in the magnetosheaths of Comets Halley and Giacobini-Zinner. The ionization frequency in the inner part of the cometary plasma region of comet Halley is several times greater than the photoionization value. Tables of ionization frequencies as functions of electron temperature are presented for H2O, CO2, CO, O, N2, and H.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 92; 7341-735
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  • 9
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    A new model of cometary ionospheres (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The coupled continuity, momentum, and energy equations were solved for ionospheric conditions appropriate for Comet Halley at 1 AU. The numerical scheme used is such that any shock transition appears naturally in the solution and no a priori assumptions are necessary. Solutions were obtained for a number of different assumptions concerning electron heating rates, but all showed that the electron temperatures increase rapidly and significantly at a distance from the nucleus where collisional electron-neutral cooling becomes unimportant. This temperature increase is accompanied by a sharp increase in both the plasma pressure and its associated polarization electric field, causing the supersonic plasma flow to go subsonic. It is not clear at this time whether or not this sonic transition is accompanied by a shock.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 92; 733l-734
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