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  • Other Sources  (13)
  • 1985-1989  (13)
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  • 1
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    Analysis of Pioneer Venus Orbiter ultraviolet spectrometer Lyman alpha data from near the subsolar region (1988)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Pioneer Venus Orbiter ultraviolet spectrometer data from 20 orbits which span 3 years have been analyzed to determine the atomic hydrogen number density and vertical flux at the exobase as a function of solar zenith angle, F(10.7) index, and spacecraft latitude. From 1979 through 1981, the exobase number density n(c) and flux phi(c) are remarkably constant at n(c) = 6.0 + or - 1.5 x 10 to the 4th/cu cm and phi(e) = 7.5 + or - 1.5 x 10 to the 7th/sq cm per s in the subsolar region. The integrated vertical column density above 110 km is 3.6 + or - 1 x 10 to the 13th/sq cm. An empirical relationship is determined between the line center solar flux at H Lyman alpha, piF(0), and the F(10.7) index.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 93; 1766-177
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  • 2
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    Solar cycle study of interplanetary Lyman-alpha variations - Pioneer Venus Orbiter sky background results (1987)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: PVO observations of the interplanetary Ly-alpha (IPL) background, obtained over an entire solar cycle (SC) from 1979 to 1985, are compiled and analyzed statistically, along with data from other instruments and earlier solar cycles. The results are presented in extensive tables and graphs and characterized in detail. Findings reported include SC variation of 1.8 for the longitudinally averaged IPL intensity (in agreement with the variation of the 27-d disk-averaged integrated solar Ly-alpha flux), yearly averaged ecliptic H-atom lifetime at 1 AU equal to 1.0 Ms at solar minimum and 1.5 Ms at solar maximum, interplanetary H density equal to 0.07 + or - 0.01/cu cm, and interplanetary H/He within the heliopause but far from the sun of 7 + or - 3.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 317; 964-986
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  • 3
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    The prediction of blade wake interaction noise based on a turbulent vortex model (1989)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Blade wake interaction is defined as the broadband noise generated by the interaction of helicopter rotor blades with their own wake. Experimental observations have shown that this is a strong function of advance ratio and tip path plane angle. This paper describes how this noise source can be associated with the blade vortex interactions in the forward sector of the rotor. Measured levels of turbulence in the vortex core are used to predict the broadband noise levels with some success. However, more detailed information on the turbulence spectrum and the trajectory of the shed vortices is required before more accurate noise predictions can be made.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 89-1134
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  • 4
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    The prediction of helicopter tail rotor noise on the flight path (1987)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Experimental measurements on a model scale helicopter in a wind tunnel have shown that the levels radiated in the direction of flight are strongly dependent on the fuselage angle to the mean flow. Here a theoretical model is derived to explain these measurements using unsteady thickness noise as the principle source mechanism. The model includes the effect of fuselage wake turbulence which is convected through only the lower sector of the rotor disk. This causes spectral peaks which do not occur at blade passing frequencies, but rather at frequencies associated with convected flow inhomogeneities. The results are compared with measurements and show good agreement over a range of fuselage angles and mean flow speeds.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 87-2747
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  • 5
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    Analysis and interpretation of observations of airglow at 297 nm in the Venus thermosphere (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Data obtained by the Pioneer Venus Orbiter's UV spectrometer for the limb profiles near periapsis, with the instrument's grating set to a bandpass-center wavelength of 297 nm, are analyzed to identify the sources giving rise to the observed signals, which are forbidden O I 2972 A and C I 2967 A. Structure in the forbidden O I 2972 A emission profile in the lower thermosphere is explained in terms of large temperature excursions altering the quenching rate of O(1S) by CO2.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 94; 208-216
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  • 6
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    Sound radiation from an airfoil encountering an oblique gust in its plane of motion (1989)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: An evaluation is conducted of the acoustic field radiated when an airfoil encounters a gust in the plane of motion, with a view to the prediction of helicopter tail rotor noise. The analysis is based on the unsteady thickness noise theory of Glegg (1987). Sound radiation is shown to be dominated by the transient effect of the blade tip moving into the velocity deficit, so that the effects of blade rotation are not important.
    Keywords: ACOUSTICS
    Type: AIAA Journal (ISSN 0001-1452); 27; 1820-182
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  • 7
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    Pioneer Venus Lyman-alpha observations of Comet P/Giacobini-Zinner and the life expectancy of cometary hydrogen (1986)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Results are given of the Pioneer Venus observation of the hydrogen coma of Comet P/Giacobini-Zinner on September 11, 1985 during the fly-by conducted by the International Cometary Explorer (ICE). Analysis of the data with a time-dependent 3-D particle-trajectory model implies a water production rate of 2.2 x 10 to the 28 per second on that day. The model includes the irregular variations in the H lifetime and the H Lyman-alpha fluorescence rate determined from simultaneous measurements of the solar wind by ICE and of the solar UV by the Solar Mesosphere Explorer (SME), respectively. The H lifetime varied from 2 x 10 to the 5 to 3 x 10 to the 6 seconds during the 42 days preceding the observation.
    Keywords: ASTROPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 13; 385-388
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  • 8
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    Sulfur dioxide at the Venus cloud tops, 1978-1986 (1988)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Ultraviolet spectroscopy from the Pioneer Venus Orbiter shows a decline in the cloud top abundance of SO2 from about 100 ppb to about 10 ppb in the period 1978-1986. A consistent decline in polar haze has occurred over the same period, with the correlation coefficient between these two observables of r = 0.8. Star calibrations determine the instrument sensitivity to within 10 percent which rules out the possibility that this is an instrumental effect. Systematic errors could increase the SO2 abundance to twice the inferred values in later orbits. Tracking of SO2 features and power spectral analysis give rotation periods for the longer-lived features of 3.6-5.2 days, consistent with cloud-tracked winds observed at other wavelengths. The behavior of SO2 and polar haze can be plausibly explained by episodic injection of SO2 into the cloud top regions, for example, by active volcanism.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 93; 5267-527
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  • 9
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    The Venus ultraviolet aurora - Observations at 130.4 nm (1986)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Continuous and highly variable emissions at 130.4 nm are observed on the night-side of Venus by the Pioneer Venus Orbiter ultraviolet spectrometer. The emissions have a typical intensity of 10-20 R, occasionally exceed 100 R, and have a dawn-dusk symmetry in brightness during periods of unusual intensity. Possible energy source for this 'aurora' is the precipitation of suprathermal electrons. Field and particle observations suggest an additional source for the brightest emissions which is associated with solar flares and/or interplanetary shocks.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 13; 1047-105
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  • 10
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    Pioneer Venus measurements of H, O, and C production in comet P/Halley near perihelion (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The Pioneer Venus Orbiter Ultraviolet Spectrometer measured the production of H, O, and C in comet Halley from late December 1985 to early March 1986. Water production rates obtained from the H data rose from 3.3 x 10 to the 29th/s at 1 AU inbound to 1.2 x 10 to the 30th/s at perihelion; there was a further increase to 1.6 x 10 to the 30th/s followed by a slow decline to 1.4 x 10 to the 30th/s at the time of the Vega-1 encounter. The H measurements show a periodic behavior that imply variations in the water production rate having structure within a 7.4-day sidereal period, in phase with similar variations seen in radical and dust production in March and April. Average H:O:C ratios of 1.0:0.7:0.07 were measured from 10 to 26 days after perihelion.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 187; 1-2
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