Venus Strategy for Exploration (1970) / Chapter Skim
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3 UPPER ATMOSPHERE AND PLASMA INTERACTIONS
3 UPPER ATMOSPHERE AND PLASMA INTERACTIONS
Pages 17-29
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From page 17... ...
An isotopic analysis of hydrogen compounds in the lower atmosphere is more difficult and does not appear feasible with mass spectroscopy in the present state of the art. The interaction of the planet with the solar wind is an intriguing phenomenon in own right.
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From page 18... ...
THERMAL STRUCTURE OF NEUTRAL ATMOSPHERE At present, observational data relating to the thermal struc ture of the upper atmosphere of Venus are extremely limited. They consist of a single profile of Lyman-a emission and a single dayside electron density profile provided by Mariner 5.
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From page 19... ...
Although solar ultraviolet radiation dissociates CO2 into CO and 0, above the cloud tops o.f Venus the concentration of CO is only about 2 parts in 105 and that of °2 is less than about 10 parts in 105. The ultraviolet .airglow e~periments aboard the Venera probes indicate that the atomic oxygen density at 300 kIn in the upper atmosphere late in Venus's night is only 10-5 times the density at the same altitude on earth.
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From page 20... ...
A model of a "runaway greenhouse" has been proposed in which an ocean might evaporate rapidly when the solar radia tion is as strong as it is at Venus. Water vapor would be dissociated by sunlight, and H atoms would escape thermally from the upper atmosphere.
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From page 21... ...
Because helium is likely to playa major role in the formation of the topside ionosphere and plasma tail, and because the total abundance might contain information concerning the formation of the planet itself, observations of neutral and ionized helium are of prime importance. Minor neutral constituents should be measured throughout the upper atmosphere with a neutral mass spectrometer.
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From page 22... ...
It is necessary to measure at least the relative concentrations of two inert constituents at two altitudes, one in lower atmosphere below the turbo pause and one in the upper atmosphere where molecular diffu sion is dominant. Then, because it can be assumed that the relative abundance of two constituents is uniform below the turbopause, it is possible to estimate the altitude of the turbopause from considerations of diffusive separation in the upper atmosphere.
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From page 23... ...
Both the peak density and the scale height near the peak are satisfactorily described by photochemical equilibrium, if the major neutral gas is CO2. The identity of the major positive ion is, however, unknown.
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From page 24... ...
If these problems are to be resolved, it is evident that we need observations of the bulk motion of the plasma in the topside ionosphere and measurements of its composition, den sity, and temperature, both on the sunlit hemispheres of the planet and in ,the tail. In addition, we require measurements of the magnetic field in the ionosphere and of the abundances of H, D, and He in the neutral atmosphere.
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From page 25... ...
The precise mechanism could be clarified by in situ measurement of ion composition and temperature and by direct measurement of possible ionization sources. An additional phenomenon, of great interest on the nightside, is the mysterious ashen light reported by many visual observers.
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From page 26... ...
This surface was located much more precisely by the Mariner 5 dual-frequency radio occultation da~a, which 10 cm-3 abrupt drop in electron density from at this 10 cm-3 to gave an over a distance of less than 100 km about elevation over the sunward side, clearly a transition from ionospheric to solar-wind conditions. Except for over-all scale, the observations of Mariner 5 and of Venera 4 show no difference between the bow shocks at earth and Venus and no essential difference in magnetic and plasma conditions out side and just inside the bow shock.
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From page 27... ...
If this had a temperature of the order of 1000 K, it would have a reasonable scale height, and along the roughly cylindrical boundary between the wake a~d the solar wind there would be an approximate pressure balance between the magnetic pressure of the field observed just outside the wake and kinetic gas pressure of the presumablY stagnant gas inside. It must be emphasized that this model represents a large structure erected on the basis of a very small amount of somewhat uncertain data.
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From page 28... ...
Electrons in this energy range are also emitted by the sun following solar flares; be cause absorption will cause the flux in any magnetic field line that enters the atmosphere to become anisotropic, it should be possible to obtain information concerning the struc ture of the magnetic field around Venus from observations of the streaming direction of these particles. Although observations of galactic and solar cosmic rays near the orbit of Venus are of considerable interest, there is much less interest in observations in the vicinity of the planet (e.g., from an orbiter)
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From page 29... ...
must produce an extensive lower ionosphere somewhat akin to the terrestrial C and D regions.
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