The principle ionizing radiation responsible for the heating of the upper planetary atmospheres and the formation of planetary ionospheres from stellar or solar sources is ionizing X-rays and EUV radiation (XUV). Above the mesopause of any planet XUV radiation is absorbed and a part of its energy goes to heat leading to a positive temperature gradient. This region is the thermosphere. In the lower thermosphere convection is the principal heat transport process, while in the upper thermosphere heat is transported by conduction leading to an isothermal region.

The XUV heating of Earth's thermosphere yields an average exospheric temperature of about 1000 - 1200 K (e.g., Jacchia, 1977; Crowley, 1991), while on a comparable terrestrial-type planet, Venus, which is even closer to the Sun but has a 96% CO₂ atmosphere, the average exospheric temperature on the dayside is only about 270 K (e.g., Nieman et al., 1979a,b; von Zahn et al., 1980; Hedin et al., 1983).

The main reason for the cooler Venusian thermospheric-exospheric environment compared to a much warmer upper atmosphere of Earth is cooling by the IR radiation in the 15µm CO₂ fundamental band (e.g., Gordiets and Kulikov, 1985; Bougher et al., 1999, 2000; Kulikov et al., 2006). This indicates that upper atmospheres of terrestrial exoplanets that have higher CO₂ mixing ratios are better protected from expansion due to XUV heating and enhanced atmospheric loss arising from it. For modelling the Venusian thermosphere over the planetary history Kulikov et al. (2006) used observational data from the Sun in Time project and applied a diffusive-gravitational equilibrium and thermal balance model and investigated the heating of an early CO₂-rich Venusian thermosphere by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by CO₂ IR emission in the 15 µm band. Figures 1, 2 and 3 compare the results of Kulikov's model for the Venusian He, O, N₂, CO₂ density and thermospheric temperature at present time low, medium and high solar activity conditions with spacecraft observations and mode results of Hedin et al. (1983) and von Zahn et al. (1980).