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  • 1
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    Ion flow at comet Halley (1986)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: The three-dimensional positive ion analyzer aboard the Giotto spacecraft has been used to study the interaction between protons and alpha-particles in the solar wind and positive ions from comet Halley. Although the first impression of the overall structure is that the plasma flow evolves smoothly as the nucleus is approached, three sharp transitions of relatively small amplitude can be identified on both the inbound and outbound legs of the trajectory. The outermost one, at about one million km from the nucleus, appears to be a multiple crossing of a weak bow shock. The innermost one, at 80,000 km, is the boundary where the flowing plasma becomes depleted. On a microscopic scale, the turbulence created by the interaction between the two ion populations extends to a distance of several million km from the nucleus. At Giotto's closest approach to the nucleus, the plasma produced around the spacecraft by dust and gas impacts was much more energetic than had been expected.
    Keywords: ASTROPHYSICS
    Type: Nature (ISSN 0028-0836); 321; 344-347
    Format: text
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  • 2
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    A kinematographic study of the tail of comet Kohoutek (1973f) (1976)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: Observations of comet Kohoutek made in the Southwest U.S., Alaska and Hawaii were combined to produce a movie of the cometary tail. Plasma tail motions seen in the movie are described. Features of the plasma tail discussed include: tail rays; wave trains; and condensations and kinks.
    Keywords: ASTROPHYSICS
    Type: The Study of Comets, Part 1; p 370-377
    Format: application/pdf
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  • 3
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    Velocity space diffusion of pickup ions from the water group at Comet Halley (1989)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The diffusion in velocity space of cometary ions was studied using the distributions of ions measured by the implanted ion spectrometer in Giotto during the inbound pass. The measurements were transformed into a frame comoving with the solar wind and oriented with the magnetic field. The observations show the evolution of the pitch angle distribution in the solar wind turbulence to form a shell from the initial ring. Diffusion in energy takes place simultaneously but on a longer time scale. Comparison with theory is inhibited by the lack of a suitable spatial model, but the simple arguments that can be made indicate that pitch angle diffusion and the process of parallel pickup take place more slowly than theory suggests.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 94; 9983-999
    Format: text
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  • 4
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    Bulk properties and velocity distributions of water group ions at Comet Halley - Giotto measurements (1990)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: In the region upstream of Comet Halley, pickup heavy ions of cometary origin were directly observed by the implanted ion spectrometer on Giotto. Diffusion of this population in pitch angle and in energy, during the approach to the comet and on the outbound leg is discussed. The two data sets are compared and qualitative ideas on scattering timescales are inferred. In addition the bulk parameters of these distributions have been computed and a comparison of the observed speed in the solar wind frame and the observed density with expectations is presented. Pitch angle scattering occurs more slowly than expected with filled shells appearing at 2,500,000 km, and significant energy diffusion does not occur until the bow shock region. Also the shell distributions downstream of the shock flow at the bispherical bulk speed (related to the Alfven speed) along the magnetic field with respect to the solar wind in accordance with conservation of energy between the pickup ions and the wave turbulence.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 95; 10249-10
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  • 5
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    In-situ observations of a bi-modal ion distribution in the outer coma of comet P/Halley (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Observations obtained by the Johnstone Plasma Analyzer on the Giotto fly-by of comet Halley showed a fairly sudden decrease in the count rate of energetic (about 30 KeV) water-group ions inside about 500,000 km from the nucleus. This decrease was accompanied by the appearance of a new water-group ion population at slightly lower energies (less than 10 KeV). Close inspection reveals that this lower-energy peak was also present somewhat earlier in the postshock flow but only became prominent near the sudden transition just described. It is shown that the observed bimodal ion distribution is well explained in terms of the velocity history of the accreting solar wind flow in the outer coma. The decline in count rate of the energetic pick-up distribution is due to a relatively sudden slowing of the bulk flow there and not to a loss of particles. Hence, charge-exchange cooling of the flow is probably not important at these distances from the nucleus. The observations suggest that pitch-angle scattering is fairly efficient at least after the bow shock, but that energy diffusion is probably not very efficient.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 187; 1-2
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  • 6
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    Pickup water group ions at Comet Grigg-Skjellerup (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-27
    Description: The density and velocity distribution of cometary water group ions was measured by the Giotto spacecraft in the regions upstream and downstream of the 'bow shock' at Comet Grigg-Skjellerup. The results show that the distributions of ions are ring-like until quite close to the shock, the timescales for pitch angle and energy diffusion appear similar and the ion density follows a r exp -2 dependence.
    Keywords: ASTROPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 20; 6; p. 483-486.
    Format: text
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