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  • 1
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    Superthermal ions detected in Venus' dayside ionosheath, ionopause, and magnetic barrier regions (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Results of a comprehensive morphological analysis of the dayside superthermal ion presence in the dayside ionopause region of Venus made using the Orbiter Ion Mass Spectrometer measurements are presented. There is a strong inbound-outbound difference in both the number and intensity of the superthermal ions with most events detected on the inbound crossing of the ionopause. The ambient energy of the detected ions is comparable to the ram energy of cold ions relative to the moving spacecraft. A comparison of the locations of the superthermal O(+) ion occurrences with respect to other parameters shows that the superthermals are a characteristic of the ionopause transition separating the essentially stationary ionosphere plasma from the flowing ionosheath plasma as deduced by Taylor et al. (1980) from individual examples.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; E5; p. 9055-9064.
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  • 2
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    Solar EUV index for aeronomical studies at earth from Langmuir probe photoelectron measurements on the Pioneer Venus orbiter (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: A solar EUV index for aeronomical studies at earth, obtained from the Langmuir probe measurement of photoelectron current on the Pioneer Venus orbiter, is presented. To examine the potential of E sub EUV as a solar EUV flux index, the behavior of ionospheric parameters f sub 0 E, f sub 0 F1, and f sub 0 F2 are studied at midlatitude stations, and their relationship with E sub EUV and with the 10.7-cm solar radio flux is compared. f sub 0 F1 and f sub 0 F2 are found to be better correlated with E sub EUV than with the 10.7-cm flux. F sub 0 E is better correlated with the 10.7-cm flux, because the 10.7-cm flux is also a proxy for soft X-rays, which are an important ionizing source in the E region. A table is also presented for the EUV index, E sub EUV, for the period February 12, 1979, through most of 1991.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 97; A7, J; 10
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  • 3
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    On-board landmark navigation and attitude reference parallel processor system (1978)
    In:  CASI
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    Publication Date: 2006-01-12
    Description: An approach to autonomous navigation and attitude reference for earth observing spacecraft is described along with the landmark identification technique based on a sequential similarity detection algorithm (SSDA). Laboratory experiments undertaken to determine if better than one pixel accuracy in registration can be achieved consistent with onboard processor timing and capacity constraints are included. The SSDA is implemented using a multi-microprocessor system including synchronization logic and chip library. The data is processed in parallel stages, effectively reducing the time to match the small known image within a larger image as seen by the onboard image system. Shared memory is incorporated in the system to help communicate intermediate results among microprocessors. The functions include finding mean values and summation of absolute differences over the image search area. The hardware is a low power, compact unit suitable to onboard application with the flexibility to provide for different parameters depending upon the environment.
    Keywords: ASTRODYNAMICS
    Type: NASA. Goddard Space Flight Center Flight Mechanics(Estimation Theory Symp.; p 153-177
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  • 4
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    A reconsideration of the effects of terminator ionopause height on the nightward ion transport at Venus (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The ionopause altitude near the terminator is a crucial parameter for studies dealing with the maintenance of the nightside ionosphere of Venus. It is generally thought that, during high solar wind dynamic pressures (P(SW)) or during solar minimum conditions, the ionopause comes down to very low altitudes so that the dayside ionosphere is not able to supply sufficient plasma to maintain the observed nightside densities. However, there are a number of workable definitions of the ionopause. Near the terminator, the altitude of the ionopause differs considerably depending upon the definition. The ionopause deduced from the radio occultation experiment as well as the pressure ionopause can be significantly lower than the density ionopause deduced from the Langmuir probe at these locations. The latter refers to the altitude where the electron density falls to 100/cu cm. Using in situ data from the Pioneer Venus Orbiter, it is shown that the density ionopause remains fairly high even for high P(SW) conditions. Simple quantitative estimates indicate that significant flow of plasma is still possible under these conditions. Thus, nightward transport of plasma during high P(SW) conditions may be more efficient than has been assumed so far. Since such conditions are more prevalent during solar cycle minimum, it is argued that transport may be relevant in the maintenance of nightside ionosphere at that time also.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 97; A9, S; 13
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  • 5
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    On the response of exospheric temperature on Venus to solar wind conditions (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Exospheric temperatures, T(ex), inferred from in situ measurements of neutral densities by the Pioneer Venus Orbiter have been analyzed to examine any possible dependence on solar wind dynamic pressure, P(sw). No linear relationship was found between P(sw) and T(ex). However, on one occasion when P(sw) was unusually high for a prolonged period, an unmistakable temperature enhancement was observed. Minor enhancement of T(ex) were also observed in few other cases, with sustained but moderately high P(sw).
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 7901-790
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  • 6
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    Mars ionopause during solar minimum - A lesson from Venus (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The ion densities measured by the Viking landers (Hanson et al., 1977) do not show an abrupt falloff with height, giving the false impression that Mars has no ionopause. On the basis of knowledge gained from the solar wind interaction at Venus during solar minimum, it is demonstrated that the observed O2 profile above about 160 km on Mars is a distributed photodynamical ionosphere and can produce an ionopause at around 325 km, similar to that observed on Venus during solar minimum. It is concluded that the solar wind interacts directly with the Mars ionosphere, suggesting that the planet does not have an intrinsic magnetic field of any consequence.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 95; 8265-826
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  • 7
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    A comparative study of Venus and Mars - Upper atmospheres, ionospheres and solar wind interactions (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The neutral atmospheres of Mars and Venus are discussed. A comparative study is presented of the upper atmospheres, ionospheres, and solar wind interactions of these two planets. The review is mainly concerned with the region about 100 km above the surface of the planets.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Indian Journal of Radio and Space Physics (ISSN 0367-8393); 19; 444-465
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  • 8
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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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  • 9
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    Seasonal variation in the F2 region (1971)
    In:  Other Sources
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    Publication Date: 2011-08-16
    Description: Seasonal variations in F 2 region, using critical frequencies during descending solar activity phase
    Keywords: GEOPHYSICS
    Type: ; YAL SOCIETY (
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  • 10
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    A polar maximum of electron concentration at 1000 km altitude (1970)
    In:  Other Sources
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    Publication Date: 2011-08-16
    Description: Upper ionosphere electron concentration enhancement in polar cap from satellite probe measurements
    Keywords: GEOPHYSICS
    Type: ; YAL SOCIETY (
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