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  • 1
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    The Venus ionosphere and solar wind interaction (1977)
    In:  Other Sources
    Details
    Publication Date: 2011-08-17
    Description: The current state of knowledge of the chemistry, dynamics and energetics of the upper atmosphere and ionosphere of Venus is reviewed together with the nature of the solar wind-Venus interaction. Because of the weak, though perhaps not negligible, intrinsic magnetic field of Venus, the mutual effects between these regions are probably strong and unique in the solar system. The ability of the Pioneer Venus Bus and Orbiter experiments to provide the required data to answer the questions outstanding is discussed in detail.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Space Science Reviews; 20; June 197
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  • 2
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    Structure and dynamics of the ionosphere (1982)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: The structure of the Venus ionosphere and the major processes occurring within it are summarized. The daytime ionosphere is created by solar EUV radiation incident on the thermosphere; it is in photochemical equilibrium near its peak at about 142 km, where O2(+) is the major ion, and near diffusive equilibrium in its upper regions, where the major ion is O(+). The day-to-night plasma pressure gradient across the terminator drives a nightward ion flow which, together with electron precipitation, contributes to the formation of the nighttime ionosphere. Large-scale radial holes or plasma depletions extending downwards to nearly the ionization peak in the antisolar region are also observed which are associated with regions of strong radial magnetic fields. The ionopause is a highly dynamic and complex surface, extending from an average altitude of 290 km at the subsolar point to about 1000 km at the terminator and from 200 to over 3000 km on the nightside. A variety of solar wind interaction products are observed in the mantle, a transition region between the ionospheric plasma and the flowing shocked solar wind.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Nature; 296; Mar. 4
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  • 3
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    The ancient oxygen exosphere of Mars: Implications for atmosphere evolution (1992)
    In:  CASI
    Details
    Publication Date: 2013-08-29
    Description: This study involves the calculation, by the 2-stream method of Nagy and Cravens, of 'hot' oxygen exosphere density profiles for 'ancient' atmospheres and ionospheres (e.g., different extreme ultraviolet fluxes) and the associated escaping fluxes. We computed the total production rates above different 'nominal' ionopause altitudes (not taking into account the fact that some will reenter the atmosphere). We do not consider the additional neutral escape due to the sputtering process described by Luhmann and Kozyra. The results presented here thus represent conservative estimations of the neutral escape fluxes, but generous estimates of ion loss rates (except that here we do not consider charge exchange and impact ionization ion production processes). Further work along the lines of Luhmann and Kozyra can lead to estimates of sputtering losses over time and the roles played by impact ionization and charge exchange.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Workshop on the Martian Surface and Atmosphere Through Time; p 175
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  • 4
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    History of oxygen and carbon escape from the Martian atmosphere (1992)
    In:  CASI
    Details
    Publication Date: 2013-08-29
    Description: A fraction of the oxygen in the Martian atmosphere continually escapes to space because dissociative recombination of the O2(+) ions in the ionosphere can impart sufficient energy to the product O atoms. In addition, ionization of the extended atomic oxygen corona resulting from the above process adds to escape since the solar wind can carry away O(+) ions born above a few hundred km altitude. A further by-product of this ion-pickup by the solar wind is an additional population of escaping oxygen atoms that are sputtered from the atmosphere near the exobase by pickup ions that are on reentry rather than escaping trajectories. This sputtering process can also remove carbon in the form of intact or dissociated CO2 since all atoms and molecules in the 'target' gas are subject to the collisional energy transfer that characterizes sputtering. We have estimated the present rates of escape of oxygen and carbon due to these mechanisms, as well as the rates at several epochs in the history of the solar system.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Papers Presented to the Workshop on the Evolution of the Martian Atmosphere; p 19-20
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  • 5
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    Escape of Mars atmospheric carbon through time by photochemical means (1993)
    In:  CASI
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    Publication Date: 2013-08-31
    Description: Luhmann et al. recently suggested that sputtering of the Martian atmosphere by re-entering O(+) pickup ions could have provided a significant route of escape for CO2 and its products throughout Mars' history. They estimated that the equivalent of C in an approximately 140-mbar CO2 atmosphere should have been lost this way if the Sun and solar wind evolved according to available models. Another source of escaping C (and O) that is potentially important is the dissociative recombination of ionospheric CO(+) near the exobase. We have evaluated the loss rates due to this process for 'ancient' solar EUV radiation fluxes of 1, 3, and 6 times the present flux in order to calculate the possible cumulative loss over the last 3.5 Gyr.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Mars: Past, Present, and Future. Results from the MSATT Program, Part 1; p 27
    Format: application/pdf
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  • 6
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    Electron temperatures and densities in the Venus ionosphere - Pioneer Venus orbiter electron temperature probe results (1979)
    In:  Other Sources
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    Publication Date: 2011-08-17
    Description: The Pioneer Venus orbiter electron temperature probe was used to obtain altitude profiles of electron temperature and density in the ionosphere of Venus. Elevated temperatures at times of low solar wind flux might indicate support for a certain model. According to this model, less than 5% of the solar wind energy is deposited at the ionopause and is conducted downward through an unmagnetized ionosphere to the region below 200 km where electron cooling to the neutral atmosphere proceeds rapidly. The patterns of electron temperatures and densities at higher solar wind fluxes are considered, the variability of the ionopause height in the late afternoon is noted, and the role of an induced magnetic barrier in the neighborhood of the ionopause is discussed.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Science; 203; Feb. 23
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  • 7
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    Venus nighttime hydrogen bulge (1980)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: The concentration of atomic hydrogen in the Venus thermosphere near 165 km altitude and approximately 18 deg north latitude has been derived from Pioneer Venus in situ measurements of H(+), O(+), O and CO2 concentrations, under the assumption of chemical equilibrium. Altitude profiles of derived H concentration suggest that chemical equilibrium prevails to an altitude of at least 200 km on the dayside and to 165 km on the nightside. Measurements below these limits were made by the ion and neutral mass spectrometers on the orbiter spacecraft between December 1978 and July 1979, while periapsis traversed a complete diurnal cycle. The hydrogen concentration is found to rise sharply at both terminators from a dayside value of approximately 50,000/cu cm, and to exhibit an asymmetric nightside distribution with a peak density in the predawn sector approximately 400 times greater than the dayside value. Analysis suggests that wind-induced diffusion, combined with exospheric return flow, can account for the observed hydrogen behavior. The large day-night temperature contrast enhances advective transport, which produces the large H concentration diurnal variation; the shift of the H concentration nighttime maximum toward dawn is caused by atmospheric superrotation.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters; 7; Nov. 198
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  • 8
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    Empirical models of the electron temperature and density in the nightside Venus ionosphere (1979)
    In:  Other Sources
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    Publication Date: 2011-08-17
    Description: Empirical models of the electron temperature and density of the late afternoon and nightside Venus ionosphere based on the Pioneer Venus measurements are presented. They describe the ionosphere conditions near 18 deg latitude between 150 and 700 km altitude for solar zenith angles of 80 to 180 deg, with a 10-fold decrease beyond 90 deg and a gradual decrease between 120 and 180 deg. The nightside electron density profile, the ion transport process, and electron precipitation are discussed. The high nocturnal temperatures and the well defined nightside ionopause suggest that energetic processes occur across the top of the entire nightside ionosphere, maintaining elevated temperatures.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Science; 205; July 6
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  • 9
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    On the configuration of the nightside Venus ionopause (1979)
    In:  Other Sources
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    Publication Date: 2011-08-17
    Description: Nightside measurements of electron and ion concentrations by instruments on the Pioneer Venus Orbiter have revealed an ionopause which is highly dynamic and whose global configuration is surprisingly complex. Inbound crossing which occur at high northern latitudes encounter the ionopause at an average altitude of 870 km in the late afternoon sector. The average ionopause altitude increases slightly near 100 deg solar zenith angle (SZA) followed by a nightside compression to well below 500 km. The outbound crossings of the ionopause occur at about 20 deg south of the equator. The ionospause at this latitude is found at an average altitude of 800 km in the afternoon-evening sector between 65 and 110 deg SZA, followed by a pronounced rise to altitudes as high as 3000 km in a pronounced bulge near 130 deg SZA.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters; 6; May 1979
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  • 10
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    The ionosphere of Venus - Observations and their interpretation (1983)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: The implications of Soviet and U.S. observations of the Venus ionosphere's density, temperature, composition, motion, and magnetic structure are discussed, in view of the strong influence exerted on nearly all ionospheric parameters by the solar wind. The IMF conveys solar wind pressure to the ionosphere, compressing, accelerating, heating and removing plasma, forming the ionopause and inducing a nightward convection of plasma. Within the ionosphere, the main electron density peak is at an altitude of about 140 km on the day side, and is believed to be formed by local production and loss analogous to the earth's E region. Throughout most of the ionosphere, the nightward ion flow is primarily driven by the day-to-night pressure gradient, and electron precipitation also contributes to the nightside ionization. The lower atmosphere is dominated by O2(+), except at the lowest altitudes at night, where NO(+) and CO2(+) become significant ions.
    Keywords: LUNAR AND PLANETARY EXPLORATION
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