Altitude profile of H in the atmosphere of Venus from Lyman α observations of Venera 11 and Venera 12 and origin of the hot exospheric component
Abstract
Two extreme ultraviolet (EUV) spectrophotometers flown in December 1978 on Venera 11 and Venera 12 measured the hydrogen Lyman α emission resonantly scattered in the atmosphere of Venus. Measurements were obtained across the dayside of the disk, and in the exosphere up to 50,000 km. They were analyzed with spherically symmetric models for which the radiative transfer equation was solved. The H content of the Venus atmosphere varies from optically thin to moderately thick regions. A shape fit at the bright limb allows one to determine the exospheric temperature Tc and the number density nc independently of the calibration of the instrument or the exact value of the solar flux. The dayside exospheric temperature was measured for the first time in the polar regions, with Tc = 300 ± 25°K for Venera 11 (79°S) and Tc = 275 ± 25°K (59°S) for Venera 12. At the same place, the density is nc = 4 -2+3 × 10 4 atom.cm -3, and the integrated number density Nt from 250 to 110 km (the level of CO 2 absorption) is 2.1 × 10 12 atom.cm -2, a factor of 3 to 6 lower than that predicted in aeronomical models. This probably indicates that the models should be revised in the content of H-bearing molecules and should include the effect of dynamics. Across the disk the value of Nt decreases smoothly with a total variation of two from the morning side to the afternoon side. Alternately it could be a latitude effect, with less hydrogen in the polar regions. The nonthermal component if clearly seen up to 40,000 km of altitude. It is twice as abundant as at the time of Mariner 10 (solar minimum). Its radial distribution above 4000 km can be simulated by an exospheric distribution with T = 10 30K and n = 10 3 atom.cm -3 at the exobase level. However, there are less hot atoms between 2000 and 4000 km than predicted by an ionospheric source. A by-product of the analysis is a determination of a very high solar Lyman α flux of 7.6 × 10 11 photons (cm 2 sec Å) -1 at line center (1 AU) in December 1978.
- Publication:
-
Icarus
- Pub Date:
- November 1982
- DOI:
- 10.1016/0019-1035(82)90110-5
- Bibcode:
- 1982Icar...52..221B
- Keywords:
-
- Exosphere;
- Hydrogen Atoms;
- Lyman Alpha Radiation;
- Venus Atmosphere;
- Vertical Distribution;
- Altitude;
- Atmospheric Density;
- Atmospheric Temperature;
- Optical Thickness;
- Resonance Scattering;
- Ultraviolet Spectrophotometers;
- Venera 11 Satellite;
- Venera 12 Satellite