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S. Engel, S. W. Bougher (LPL/Univ. of Arizona)
Recent aerobraking maneuvers in the Martian thermosphere demonstrated the need to understand the structure and dynamics of these upper atmospheres. 3-dimensional circulation models (MTGCM for Mars and VTGCM for Venus) are used to calculate relevant physical and chemical processes. In this study we focus on the thermal, compositional and dynamical responses of these upper atmospheres to solar EUV/UV flux variability. Standard EUV/UV solar flux data sets are adopted for minimum, moderate, and maximum solar conditions. The thermospheres of Mars and Venus are compared to each other, applying the same processes and parameters, however their responses to solar flux changes are quite different.
We found that dynamics are mostly responsible for cooling the Mars thermosphere. An increase in temperature due to solar cycle variations (min to max) causes an increase in upwelling and downwelling winds, which regulate the overall thermospheric temperature increase. The temperature in the upper thermosphere changes by about 100 K over the solar cycle. The O/CO2 mixing ratio does change significantly, but the resultant CO2 cooling is secondary in regulating temperatures.
For Venus, strong O-CO2 cooling serves as an effective thermostat that gives rise to small variations of thermospheric temperatures over the solar cycle. This is reflected in the increase of the noontime O/CO2 mixing ratio from 7-8 the upper thermosphere the change is about ~ 80 K, which is small considering Venus' close proximity to the Sun.
Fundamental planetary parameters are at the root of the differences observed in these Venus and Mars upper atmospheric responses to solar EUV/UV variability.
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The author(s) of this abstract have provided an email address for comments about the abstract: sengel@lpl.arizona.edu