Comparative terrestrial planet thermospheres 2. Solar cycle variation of global structure and winds at equinox

Abstract

The present maturity of available planetary databases and modeling capabilities now permits us to extend the comparison of terrestrial planetary thermospheres beyond the limited capability of one-dimensional models to global multidimensional models [e.g., Bougher and Roble, 1997]. This effort focuses upon the comparison of the solar cycle responses of the thermospheres of Venus, Earth, and Mars using three-dimensional global models that couple the energetics, dynamics, and neutral-ion composition above ∼100 km for each planet. Standard solar EUV and UV fluxes are adopted for use in these simulations. The Venus, Earth, and Mars Thermosphere General Circulation Models (TGCMs) each share a common formulation scheme and development heritage making use of the computing facilities of the National Center for Atmospheric Research. The motivation of this research is not only to simulate the observed responses of these individual planets to solar EUV/UV flux variations but also to understand the relative importance of common processes that regulate this unique behavior. The role of O-CO2 enchanced 15-μm cooling is investigated in the context of global dynamics and its effect on atomic-O distributions. It is found that CO2 cooling is an effective thermostat for control of the Venus dayside temperatures, while Mars and Earth are only moderately affected. By contrast, the role of global dynamics in controlling temperature distributions is most pronounced for Mars and the Venus nightside but negligible for Earth. The net effect of these radiative and dynamical processes is to determine that Venus and Mars thermospheres respond rather quickly to solar flux variations (much less than an Earth day), while the Earth thermosphere is more sluggish in its behavior. This work confirms the relative importance of CO2 cooling in the Earth's lower thermosphere. Furthermore, the value of the CO2-O deactivation rate near 300 K is rather well constrained by these planetary comparisons. Copyright 1999 by the American Geophysical Union.