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  • Other Sources  (8)
  • 1990-1994  (8)
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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
    Details
    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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    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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    NASA TECHNICAL REPORTS
  • 3
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    Light ion flow in the nightside ionosphere of Venus (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The flow characteristics of the light ions H(+) and He(+) have been studied in the midnight region of the ionosphere of Venus. Measurements of ion composition, electron and ion temperatures and magnetic fields by instruments onboard the Pioneer Venus Orbiter have been used in rite electron and ion equations of conservation of mass and momentum to derive the vertical flow velocities of H(+) and He(+). When average height profiles of the measured quantities were used, H(+) was found to flow upward, accelerating to speeds of almost 1 km/s at the ion-exobase. In a similar fashion, He(+) was found to flow downward into the neutral atmosphere where it is readily quenched by charge transfer reactions. The polarization electric field played an important role in forcing H(+) upward, but did not contribute enough to the He(+) force balance to produce upward flow. At the ion-exobase, the outward electric polarization force on H(+) was shown to be five times the gravitational force. Using an analogy with the terrestrial ion-exosphere, H(+) was inferred to flow upward into the ionotail of Venus and accelerate to escape speeds. A planet averaged escape flux of 1.4 x 10 exp 7/sq cm/s was calculated, which is comparable to hydrogen loss rates estimated by other investigators.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; E4; p. 7437-7445.
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  • 4
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    Upward ion flow in ionospheric holes on Venus (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The nature of ion flow within two ionospheric holes on the nightside of Venus was investigated using ion composition measurements made by the ion mass spectrometer on the Pioneer Venus Orbiter. A comparison of the altitude profiles of the observed ion densities with those expected under diffusive equilibrium conditions indicates that the major ions O(+), NO(+), and O2(+), as well as the minor ions H(+) and He(+) flow upward and away from Venus along the axes of the holes. This result agrees with a quantitative evaluation of the ion flow speeds appearing in expressions derived from the equations for conservation of mass and momentum of the ions and electrons. The analysis shows that all ion species flow upward in the holes because the upward force produced by the plasma pressure gradient exceeds all downward forces. However, the nature of the ion source required to maintain such flow is not known.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 95; 31-37
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  • 5
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    Initial PVO Evidence of Electron Depletion Signatures Downstream of Venus (1993)
    In:  CASI
    Details
    Publication Date: 2019-08-17
    Description: This first analysis of Pioneer Venus Orbiter (PVO) plasma analyzer electron measurements obtained in early 1992 during the PVO entry phase of the mission indicates the presence downstream from the terminator of a depletion or "bite out" of energetic ionosheath electrons similar to that observed on Mariner 10. There is more than one possible explanation for this energetic electron depletion. If it is due to atmospheric scattering, the electrons traveling along draped magnetic flux tubes that thread through the Venus neutral atmosphere would lose energy from impact ionization with oxygen. The cross-section for such electron impact ionization of oxygen has a peak near 100 eV, and it remains high above this energy, so atmospheric loss could provide a natural process for electrons at these energies to be selectively removed. In this case, our results are consistent with the Kar et al. (1994) study of PVO atmospheric entry ion mass spectrometer data which indicates that electron impact plays a significant role in maintaining the nightside ionosphere. Although it is appealing to interpret the energetic electron depletion in terms of direct atmospheric scattering, alternatively it could result from strong draping which connects the depletion region magnetically to the weak downstream bow shock and thereby reduces the electron source strength.
    Keywords: Lunar and Planetary Exploration
    Type: Paper 93GL02483 , Geophysical Research Letters. Selected Papers on Pioneer Venus Orbiter: Entry Phase; 20; 23; 2779-2782; NASA-TM-112700
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  • 6
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    Initial PVO evidence of electron depletion signatures downstream of Venus (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: This first analysis of Pioneer Venus Orbiter (PVO) plasma analyzer electron measurements obtained in early 1992 during teh PVO entry phase of the mission indicates the presence downstream from the terminator of a depletion or 'bite out' of energetic ionosheath electrons similar to that observed on Mariner 10. There is more than one possible explanation for this energetic electron depletion. If it is due to atmospheric scattering, the electrons traveling along draped magnetic flux tubes that thread through the Venus neutral atmosphere would lose energy from impact ionization with oxygen. The cross-section for such electron impact ionization of oxygen has a peak near 100 eV, and it remains high above this energy, so atmospheric loss could provde a natural process for electrons at these energies to be selectively removed. In this case, our results are consistent with the Kar et al. (1994) study of PVO atmospheric entry ion mass spectrometer data, which indicates that electron impact plays a significant role in maintaining the nightside ionosphere. Although it is appealing to interpret the energetic electron depletion in terms of direct atmospheric scattering, alternatively it could result from strong draping which connects the depletion region magnetically to the weak downstream bow shock and thereby reduces the electron source strength.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 20; 23; p. 2779-2782
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  • 7
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    Ion Measurements During Pioneer Venus Reentry: Implications for Solar Cycle Variation of Ion Composition and Dynamics (1993)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: During the final, low solar activity phase of the Pioneer Venus mission, the Orbiter Ion Mass Spectrometer 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: Paper 93GL02239 , Geophysical Research Letters. Selected Papers on Pioneer Venus Orbiter: Entry Phase; 20; 23; 2735-2738; NASA-TM-112700
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  • 8
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    Evidence for ion transport and molecular ion dominance in the Venus ionotail (1994)
    In:  Other Sources
    Details
    Publication Date: 2019-08-27
    Description: We present analyses from the five Pioneer Venus Orbiter plasma experiments and the plasma wave experiment when a patch of plasma with enhanced densities was encountered in the near-Venus ionotail during atmospheric entry at an altitude of approximately 1100 km in the nightside ionosphere. Our analyses of the thermal and superthermal ion measurements in this plasma feature provides the first evidence that at times molecular ions in the 28-32 amu mass range are dominant over atomic mass species thus yielding evidence for a transport mechanism that reaches into the lower ionosphere. Analysis of plasma analyzer (OPA) observations at this time indicates the presence of ions measured in the rest frame of the spacecraft at approximately 27 and 37 volt energy per unit charge steps. In the rest frame of the planet these superthermal ions are flowing from the dawn direction at speeds (assuming they are O2(+)) of approximately 8 km/s and with a flow component downward (perpendicular to the ecliptic plane) at speeds of approximately 2 km/s. OPA analyses also determine the ion number flux, energy, flow angles, and angular distributions. Plasma wave bursts appear to indicate that plasma density decreases within and on the equatorward edge of the patch of enhanced plasma densities are associated with ion acoustic waves and relative ion streaming.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 99; A9; p. 17,413-17,420
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