Comparative terrestrial planet thermospheres: 1. Solar cycle variation of global mean temperatures

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Abstract
Global average models for the thermospheres of Venus, Earth, and Mars are used to calculate the solar cycle variations of the global mean temperatures using the compositional profiles for each planet. All models use the same absolute solar EUV and UV fluxes but scaled for planetary distances. Photochemistry appropriate to each planet is used (or assumed) with similar rate coefficients for common physical and chemical processes. In particular, the CO2 15‐μm cooling rates for all three planets are determined using a common rate coefficient for collisional excitation of the CO2 bending mode by atomic oxygen (relaxation rate 10−12 cm³ s−1). Solar EUV and UV heating efficiencies are calculated self‐consistently for Earth, but prescribed for Venus (15/22%) and Mars (18/22%) according to independent calculations. Eddy diffusion profiles are prescribed for each planet in accord with previous studies that compared model predictions with available satellite observations. The global mean models are run to steady state for both solar minimum (F10.7 = 70) and solar maximum (F10.7 = 240) conditions. The results show that the solar cycle global mean exospheric temperature variation is about 76 K for Venus (172 to 248 K), 518 K for Earth (737 to 1255 K), and 110 K for Mars (180 to 290 K). A thermal balance analysis shows that the small exospheric temperature variation on Venus occurs because of the strong radiative damping by CO2 15‐μm cooling. The peak CO2 cooling occurs at the altitude of maximum solar heating, and efficiently radiates it to space. On Earth, the increased solar heating occurs at a higher altitude than the peak in the infrared cooling. Consequently, it must be thermally conducted down to the altitude of the peak infrared cooling before it is radiated to space. An increase in the thermally conducted heat flux requires an increase in the vertical temperature gradient which results in a larger exospheric temperature variation. On Mars, the increased solar heating also occurs at a higher altitude than the peak cooling and likewise must be conducted downward before radiating to space. Furthermore, CO2 cooling is not as effective on Mars as it is on Venus because of lower O/CO2 ratios. These two factors yield a Mars solar cycle variation of global mean temperatures that is larger than for Venus.