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  • 1
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    Ion measurements during Pioneer Venus reentry: Implications for solar cycle variation of ion composition and dynamics (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: During the final, low solar activity phase of the Pioneer Venus (PV) mission, the Orbiter Ion Mass Spectrometer (OIMS) measurements found all ion species, in the midnight-dusk sector, reduced in concentration relative to that observed at solar maximum. Molecular ion species comprised a greater part of the total ion concentration as O(+) and H(+) had the greatest depletions. The nightside ionospheric states were strikingly similar to the isolated solar maximum 'disappearing' ionospheres. Both are very dynamic states characterized by a rapidly drifting plasma and 30-100 eV superthermal O(+) ions.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 20; 23; p. 2735-2738
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  • 2
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    On the lower altitude limit of the Venusian ionopause (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: It has been observed from the plasma experiments on the Pioneer Venus Orbiter that the altitude of the upper boundary of the ionosphere decreases in response to increasing solar wind dynamic pressure. However, at pressures above about 2.5 x 10 to the -8th dynes/sq cm, the further decrease in the ionopause height is rather small. Following the model of Cloutier et al. (1969), it is suggested that during high solar wind conditions, when the ionopause is formed at lower altitudes, the solar wind induces vertical and horizontal flows which sweep away the ionospheric plasma that is produced locally by photoionization. As a result, a disturbed photodynamical ionosphere is formed which has the scale height of the ionizable neutral constituent. It is shown that such a photodynamical ionosphere is observed at the subsolar ionopause under these conditions. As a consequence of this interaction, the ionopause altitude is observed to follow the small-scale height of the ionizable species, atomic oxygen, showing only small changes with solar wind pressure.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 16; 759-762
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  • 3
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    Venus nightside ionospheric troughs - Implications for evidence of lightning and volcanism (1985)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The inferred presence of lightning has been a potentially very important result from the in situ exploration of the environment of Venus by the Pioneer Venus Orbiter (PVO). The evidence for lightning has been derived from impulsive low-frequency plasma wave events recorded by the Orbiter electric field detector. The present paper is concerned with an alternative interpretation of the plasma results. It is shown that prominent examples of the plasma waves which have previously been specifically attributed to lightning are associated with distinct nightside ionization troughs. It is suspected that many of the plasma wave disturbances are not due to lightning but rather result from energetic and dynamic processes to be expected in the vicinity of the magnetic field and plasma configurations involved.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 90; 7415-742
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  • 4
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    Response of earth and Venus ionospheres to corotating solar wind stream of 3 July 1979 (1985)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Corotating solar wind stream interactions are examined for the earth and Venus in light of data from the plasma detectors aboard ISEE-3, the Pioneer Venus Orbiter (PVO), and Helios-A, as well as in situ ion composition measurements taken by the mass spectrometers aboard the PVO and Atmosphere Explorer-E spacecraft. During May-July 1979, a sequence of distinct, recurrent coronal regions developed at the sun; their analysis indicates a corresponding sequence of corotating streams. Although the planetary environments are distinctly different, it is noted that pronounced and analogous ionospheric responses to the stream passage were observed at both the earth and Venus. The response to the intercepted stream is consistent with independent investigations showing the importance of the variability of the solar wind momentum flux in the solar wind-ionosphere interaction at both planets.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Earth, Moon and Planets (ISSN 0167-9295); 32; 275-290
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  • 5
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    Modification of the Martian ionosphere by the solar wind (1969)
    In:  Other Sources
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    Publication Date: 2011-08-16
    Description: Dynamic model for Martian ionosphere modification by solar wind, assuming negligible Mars magnetic moment and neutral atmosphere
    Keywords: SPACE SCIENCES
    Type: ; ACE(
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  • 6
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    Global observations of the composition and dynamics of the ionosphere of Venus - Implications for the solar wind interaction (1980)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: The in-situ measurements of the global composition and Venus ionosphere dynamics recorded by the Bennett ion mass spectrometer on the Pioneer Venus orbiter during Dec. 1978-Aug. 1979 are presented. The observations of three plasma regimes show the bowshock-ionosheath region, the thermal ionosphere, and a superthermal flowing ion layer contacting the ionosphere at the ionopause and extending outward to different heights above the planet. An abundant ionosphere dominated by O(+) above 200 km and by O2(+) down to the typical periapsis altitudes of 160 km occur during quiet periods; less disturbed data shows strong day to night changes in the distributions of ions including O(+), O2(+), CO2(+), and N(+). The ionopause is located near the subpolar point at 250-400 km; under disturbed nighttime conditions it may have randomly spaced concentration gradients in the dusk region.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research; 85; Dec. 30
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  • 7
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    High resolution measurements of nightside ion troughs at Venus - Evidence of electrodynamic perturbations (1982)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: The Bennett rf ion mass spectrometer of the Pioneer Venus Orbiter was expressly designed to provide variable temporal resolution for measurements of thermal ion composition and density. The Explore-Adapt mode is used to obtain priority for measuring the most prominent ion species; in the 2/16 configuration, the two dominant ions within the available range of 16 species are selectively sampled at the highest rate of 0.2 sec/sample. The high-resolution measurements are combined with independent observations from the magnetic field, neutral mass spectrometer, and electron temperature experiments in investigating sharply structured troughs in the low-altitude nightside ion concentrations. The results suggest a close correlation between the structure in the ion distributions and the structured configuration of the magnetic field that is draped about the planet. In the regions of the ion depletions, sharp fluctuations in electron temperature and anomalous increases in the density of neutral gases suggest that the ion depletion may be associated with dynamic perturbation in the ion and neutral flows and/or local joule heating.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: (ISSN 0273-1177)
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  • 8
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    Ionosphere of Venus - First observations of the dayside ion composition near dawn and dusk (1979)
    In:  Other Sources
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    Publication Date: 2011-08-17
    Description: Independent Bennett radio-frequency ion mass spectrometers on the Pioneer Venus bus and orbiter spacecraft obtained in situ measurements of the composition of the ionosphere of Venus. The spectrometer on the bus explored the dawn region while the spectrometer on the orbiter explored the duskside region. Information on the ion composition in the topside, the lower ionosphere, and the upper ionosphere is presented. Below the O(+) peak near 200 km, the ions are found to exhibit scale heights consistent with a neutral gas temperature of about 180 K near the terminator. In the upper ionosphere, scale heights of all species reflect the effects of plasma transport.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Science; 203; Feb. 23
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  • 9
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    Ionosphere of Venus - First observations of the effects of dynamics on the dayside ion composition (1979)
    In:  Other Sources
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    Publication Date: 2011-08-17
    Description: Data obtained by Bennett radio-frequency ion mass spectrometers indicate that the ionosphere envelope, dominated above 200 km by O(+), responds dramatically to variations in the solar wind pressure. The pressure compresses the thermal ion distributions from heights as great as 1800 km inward to 280 km. At the thermal ion boundary, or ionopause, the ambient ions are swept away by the solar wind, while at higher altitudes energetic ion currents are detected. Within the ionosphere, ion convection stimulated by the solar wind interaction causes pass-to-pass differences in the ion scale heights.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Science; 203; Feb. 23
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  • 10
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    Ionosphere of Venus - First observations of day-night variations of the ion composition (1979)
    In:  Other Sources
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    Publication Date: 2011-08-17
    Description: Preliminary observations of day-night variations in the ion composition of the ionosphere of Venus, obtained by the Pioneer Venus Orbiter ion mass spectrometer experiment, are reported. A remarkable abundance and extent of ionization in the deep regions of the nightside ionosphere was observed, in spite of the long Venus night. A comparison of dayside and nightside ion distributions reveals a nightside composition similar in several respects to that of the dayside, with the ions O(+) and O2(+) forming the nightside F 2 and F 1 regions, respectively, as in the dayside. Important differences include a greater abundance of low-latitude ionization in the nightside, a significant increase of H(+) and NO(+) ions with increasing solar zenith angle, and extreme dynamic variability of the nightside region above 160 km. Ion composition data support the view that the nightside ionosphere can be maintained by the transport of ionization from the dayside.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Science; 205; July 6
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