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  • Other Sources  (29)
  • 1980-1984  (29)
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  • 1
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    On the stability of the ionopause of Venus (1984)
    In:  Other Sources
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    Publication Date: 2011-08-18
    Description: The stability of the Venus ionopause is examined in light of the importance of gravitation and curvature. Using a one-fluid approximation for the equation of motion of the plasma, and ignoring the effects of neutrals, a dispersion relation is obtained that includes the effects of the magnetic field, sheared plasma flow, buoyancy, centrifugal force and magnetic tension due to boundary curvature. It is found that buoyancy acts to neutralize the flute instability. As expected, the Kelvin-Helmholtz mode is the dominant instability over most of the dayside ionopause. The expected growth times of this mode are short in comparison with the wave-convection time over the boundary; the waves can grow and saturate quickly, producing a turbulent boundary that may affect electrodynamic coupling between the solar wind and ionospheric plasmas.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 89; 997-1002
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  • 2
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    Nightward ion flow in the Venus ionosphere - Implications of momentum balance (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Using global empirical models of Venus ionospheric conditions, the plasma flow field consistent with the horizontal momentum equation in both viscid and inviscid forms is solved for. It is found that plasma viscosity is negligible except at low altitudes and that the observed plasma flows are consistent with the inviscid solution above 300 km but are larger than the calculated flows at lower altitudes. This is probably due to downward momentum advection. The strong vertical shear in the calculated and observed flows just beyond the terminator may produce turbulence there, manifested as the observed transterminator waves in plasma density and magnetic field. The implications of ionospheric superrotation are discussed.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 11; 1007-101
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  • 3
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    Time scales for the decay of induced large-scale magnetic fields in the Venus ionosphere (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Observations made with the aid of a magnetometer on the Pioneer Venus Orbiter have shown large-scale horizontal magnetic fields in the dayside ionosphere of Venus. According to Cloutier and Daniell (1981), the observed magnetic structures may be quasi-steady features produced by an ionospheric current system driven by solar wind interaction. Russell et al. (1983) have suggested that the altitude profiles of the horizontal field on different orbits exhibit a pattern which can be interpreted as phases in the temporal evolution of an initial state in which the ionosphere was permeated with magnetosheath-like fields. The present investigation is concerned with the argument in favor of a temporal versus spatial explanation for some of the observed field structure. A calculation indicates that the diffusion time for ionospheric fields is long enough to justify attributing the observed fields to the 'memory' of the Venus ionosphere in certain regions.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 89; 362-368
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  • 4
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    Effects of large-scale magnetic fields in the Venus ionosphere (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Theoretical models of the ionosphere of Venus have been constructed in the past without due consideration of the fact that the ionosphere is sometimes magnetized. This paper examines some differences between the magnetized and unmagnetized dayside Venus ionosphere using the Pioneer Venus Orbiter Langmuir probe and magnetometer data. Particular attention is given to the evaluation of the altitude profiles of the thermal electron heating and comparison of the magnitude of the magnetic force with other forces in the ionosphere. Several examples illustrate how heating profiles are different in the magnetized ionosphere with effective heating below 200 km altitude reduced by orders of magnitude compared to the field-free ionosphere. The force associated with the magnetic field is comparable to other forces in the magnetized ionosphere. The measured plasma density, electron temperature and magnetic field thus suggest that large-scale magnetic fields should be included in future ionosphere models.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: (ISSN 0273-1177)
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  • 5
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    Evidence for helical kink instability in the Venus magnetic flux ropes (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Empirical models of the magnetic field structure of flux ropes found in the Venus ionosphere are seen as suggesting that the ropes are unstable to long-wavelength (more than 100 km) helical-kink perturbations. The onset of such an instability can explain the apparent volume distribution of flux ropes with altitude, as well as their orientation as a function of altitude. In the subsolar region, the fraction of volume occupied by flux ropes increases from approximately 20 percent at high altitudes to more than 50 percent at low altitudes; this is a greater increase than would be expected if ropes convect downward as simple straight horizontal cylinders. The helical kink instability raises the fractional volume occupied by ropes by turning the originally straight, horizontal flux tubes into corkscrew-shaped structures as they convect to lower altitudes. It is noted that this instability also explains why high altitude ropes tend to be horizontal and low altitude ropes appear to have almost any orientation.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 10; June 198
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  • 6
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    Magnetic flux ropes in the Venus ionosphere - Observations and models (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Pioneer Venus Orbiter data are used as evidence of naturally occurring magnetic field filamentary structures which can be described by a flux rope model. The solar wind is interpreted as piling up a magnetic field on the Venus ionosphere, with the incident ram pressure being expressed as magnetic field pressure. Currents flowing at the ionopause shield out the field, allowing magnetic excursions to be observed with magnitudes of tens of nT over an interval of a few seconds. A quantitative assessment is made of the signature expected from a flux rope. It is noted that each excursion of the magnetic field detected by the Orbiter magnetometer was correlated with variations in the three components of the field. A coordinate system is devised which shows that the Venus data is indicative of the presence of flux ropes whose parameters are the coordinates of the system and would yield the excursions observed in the spacecraft crossings of the fields.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research; 88; Jan. 1
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  • 7
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    Global characteristics of magnetic flux ropes in the Venus ionosphere (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: An examination is undertaken of global characteristics of Venus ionosphere magnetic flux ropes, whose maximum spatial occurrence at 165-km altitude occupies more than half of the ionospheric volume. Ropes above 200 km altitude in the low zenith angle regions appear to have quasi-horizontal orientations, while those below that altitude tend to be quasi-vertical. High zenith angle cases tend to be horizontal above 300 km, and randomly oriented below that altitude. Ropes may be more tightly 'twisted' at low than at high altitudes, especially in the low zenith angle regions. Rope field strengths are highest near the altitudes where their occurrence is greatest, and scale with the square root of the ambient thermal pressure. The global polarities of flux rope field-aligned currents seem to be random and do not support a steady, nonturbulent global formation mechanism.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research; 88; Apr. 1
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  • 8
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    Large-scale current systems in the dayside Venus ionosphere (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The occasional observation of large-scale horizontal magnetic fields within the dayside ionosphere of Venus by the flux gate magnetometer on the Pioneer Venus orbiter suggests the presence of large-scale current systems. Using the measured altitude profiles of the magnetic field and the electron density and temperature, together with the previously reported neutral atmosphere density and composition, it is found that the local ionosphere can be described at these times by a simple steady state model which treats the unobserved quantities, such as the electric field, as parameters. When the model is appropriate, the altitude profiles of the ion and electron velocities and the currents along the satellite trajectory can be inferred. These results elucidate the configurations and sources of the ionospheric current systems which produce the observed large-scale magnetic fields, and in particular illustrate the effect of ion-neutral coupling in the determination of the current system at low altitudes.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research; 86; May 1
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  • 9
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    On the role of the magnetic field in the solar wind interaction with Venus - Expectations versus observations (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Observations of the magnetic field near Venus suggest that elements of three different models (direct interaction, tangential discontinuity, magnetic barrier) are present. A bow shock is found to occur at an altitude of about 0.3 Venus radii at the subsolar point. The compression of the decelerated solar wind plasma behind the bow shock causes interplanetary field lines to 'pile up'. The magnetic field inside the bow shock increases from approximately twice the IMF strength at the bow shock to values in the range of approximately 40-100 gammas at altitudes between about 200 and 1,200 km. The maximum value of the piled up field, which is correlated with the dynamic pressure of the solar wind outside the bow shock, is found at lower altitudes for larger field strengths. Just Venus-ward of the maximum field, the pressure of the cold plasma increases to a level balancing the pressure of the external magnetic field. Hence, to a first approximation, the ionosphere has a diamagnetic response excluding the magnetosheath field. However, strong magnetic fields are found at times throughout the ionosphere.
    Keywords: LUNAR AND PLANETARY EXPLORATION
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  • 10
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    Observation of the Venus mantle, the boundary region between solar wind and ionosphere (1980)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: For three orbit paths of the Pioneer Venus orbiter the interaction between the solar wind and the Venusian ionosphere has been studied. Results of the retarding potential analyzer and the magnetometer are described for the boundary region between the solar wind and the planetary ionosphere. These are the first measurements that show that a transition region exists between the two plasmas of different origin. The observed magnetic field and current system producing it appear strong enough to stop the solar wind flow in front of the ionosphere and to separate the shocked solar wind from the ionosphere. The transition region between the ionosheath and the ionosphere is called the 'mantle'. The observed mantle electron energy spectra close to the ionopause show ionospheric character. With increasing height the number of electrons that have ionospheric energies decreases, and the number of electrons that have solar wind energies gradually increases toward the ionosheath boundary, where only solar wind energy spectra are observed. The mantle surrounds the frontside of the ionosphere and extends probably more than eight Venus radii downstream.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research; 85; Dec. 30
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