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  • Other Sources  (3)
  • LUNAR AND PLANETARY EXPLORATION  (2)
  • Earth Resources and Remote Sensing  (1)
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  • Other Sources  (3)
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  • 1
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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
    Format: text
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    NASA TECHNICAL REPORTS
  • 2
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    Variability of dayside electron temperature at Venus (1994)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Langmuir probe measurements on Pioneer Venus Orbiter show that electron temperature (Te) profiles exhibit two distinct regions. The lower, but more extended region is in the main ionosphere where Te increases slowly with altitude. The other, less extended region is in the ionopause, where Te rise sharply with altitude. If horizontal magnetic fields and flux ropes in the ionosphere inhibit vertical thermal conductivity sufficiently, then the observed Te profile could be explained with EUV as the major heat source (Cravens et al., 1980). The rise in Te in the ionopause region has generally been attributed to solar wind heating (Brace and Kliore, 1991). We suggest that this sharp rise in Te is due primarily to the steep fall in electron density, Ne. If the heating rate is essentially unchanged and heat conduction is not of primary importance, then a steep rise in Te will maintain a constant electron cooling rate for a steeply falling Ne. We have observed large orbit to orbit variations in Te in the ionopause region which are found to be inversely related to changes in Ne. Variations in solar wind dynamic pressure do not seem to have a direct effect on Te, rather the effect is indirect coming through the sharp decrease in Ne.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: PAPER-93GL03384 , Geophysical Research Letters (ISSN 0094-8276); 21; 1; p. 77-80
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    NASA TECHNICAL REPORTS
  • 3
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    The Development of Kolmogoroff-Like Power Spectra in the Expanding Solar Wind (1998)
    In:  Other Sources
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    Publication Date: 2019-07-17
    Description: Power spectra of the solar wind fluctuations consistently exhibit a -5/3 power-law slope consistent with the idea that the medium is undergoing a turbulent cascade as seen in ordinary fluids. This is surprising both because the radial streams and the magnetic field threading the plasma will induce anisotropies and because the expansion of the wind will tend to lead to the suppression of nonlinear cascades. These conditions violate the assumptions used by Kolmogoroff to derive the -5/3 law. We have studied this issue using a compressible IMHD code in spherical coordinates and have shown that a -5/3 spectrum results from a broad-band flat-spectrum input condition that is sheared and distorted by a current sheet as the wind expands. We determine spectra from time series taken at selected points in the domain as is done with observational spacecraft data. The spectra are very like those we have seen in nonexpanding runs and exhibit evolution and compressive characteristics very similar to those seen in observations. We will report on these results in addition to a new set of runs intended to constrain the necessary and sufficient conditions for the spectra to have this form. The simulation also allows us to examine the anisotropy for the spectra to attempt to determine why the result of an isotropic magnetofluid is obtained in a highly anisotropic situation.
    Keywords: Earth Resources and Remote Sensing
    Type: Solar Wind 9; Oct 05, 1998 - Oct 09, 1998; Nantucket, RI; United States
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