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The Institute of Space and Astronautical Science Report

2. NEUTRAL ATMOSPHERE

The first in-situ measurernents of the Venus upper neutral atmosphere (above 100 km) were made by the orbiter neutral mass spectrometer, ONMS (Niemann et al., 1980a). During the day, carbon dioxide was found to predominate up to about 150 km, atomic oxygen from 150 km to 250 km and Helium above 250 km. These transitions occur at somewhat lower altitudes during the night. Figure 3 shows typical altitude profiles for the important neutral constituents for dayside and nightside conditions and is an update by Kasprazk et al. (1997) of earlier measurernents of Niemann et al. (1980b) during solar maximum. It includes H from Brinton et al. (1980) and Grebowsky et al. (1995) and mass density from Keating et al. (1980). The ONMS densities were raised by a factor of 1.63 to bring agreement between the satellite drag and neutral mass spectrometer measurements. Figure 4 shows the composition data from the pre-entry measurements at solar medium conditions for 2.2 hr local solar time. Values for H density have again been taken from Grebowsky et al. (1995) and for the mass density from Keating and Hsu (1993). No significant change seems to have occurred from solar maximum (Figure.3) to solar medium (Figure.4) for nighttime composition and densities. Scale heights of the neutral constituents indicate exospheric temperature of about 300 K during the day and 100 K during the night. The diurnal variations of all species, except H, D and He, have maximum near noon with very steep gradients at the terminators, leading to much lower densities on the nightside than on the dayside. Predawn H, He and D bulges have been observed, which indicate that the upper thermosphere superrotates (Mayr et al., 1980).

Fig.3:Neutral composition measurements by ONMS during sola maximam conditions. CO2 dominates in the lower thermosphere,O and He dominate at higher altitudes(from Kasprazk et al., 1997).

Fig.4: Pre-entry measuremnt of neutral composition at medium solar activity condition.
Composition and densities for the nightside have not changed
between solar maximum and solar medium(from Kasprazk et al.,1997).

The PVO data from 1978-1980 has been analyzed by various workers to study the variations due to solar rotation in the upper atmosphere at solar maximum (Fl0.7~200) and a response of 0.14 to 0.19K/Fl0.7 unit in the global mean temperature has been found (Kasprazk et al., 1997). This is about ten times smaller than seen in the Earth's thermosphere. The ONMS pre-entry data and drag measurernent of Magellan orbiter (MGN) and PVO spacecrafts suggest a rather very small change in the dayside and nightside thermosphere density and temperature with solar activity on the long term scale. The exospheric temperature varies by about 60 K for the dayside and by less than 15 K for the night, from solar maxirnum to solar minimum as can be noted from Figure 5. The weak dayside response has been explained by the CO2 15

m-emission cooling which approximately balances the EUV and UV heating (see Kasprazk et al., 1997 and references therein). In contrast, hydrogen and deuterium densities in the bulge region increase with decrease in solar activity (Figure 6). This has been attributed to the reduction in the exospheric escape of H and D with decreasing solar activity (Hartle et al., 1996).

Fig.5:Dayside and near midnight exospheric temperature meaured over a solar cycle.
The temperature measured over a solar cycle.
The temperature changed by 60 K for the day and less
than 15 K for the night from solar maximum to solar medium
(from Kasprazk et al., 1997).

Fig.6:Variation of H,D and He densities with solar activity for local solar time 0 to 4 hr.
Densities decrease with increase in solar activity (from Kasprazk et al., 1997).



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