Department of Aerospace Engineering Sciences, University of
Colorado Boulder, Colorado, USA
In this paper the responses of Earth, Mars and Venus to periodic solar forcing in the form of of atmospheric heating are considered in a comparative planetary context. Three mechanisms for inducing periodicities in thermal forcing are considered: (1) Rotation of the planet; (2) Rotation of the Sun; and (3) Rotation of the planet around the Sun. Rotation of the planet gives rise to thermal tides that are subharmonics of a solar day, which can propagate vertically and thus transfer energy and momentum from low altitudes to planetary thermospheres and even exospheres. As the Sun rotates, active regions can induce periodicities (mainly near 27 days) in the EUV fluxes entering the atmosphere; this changing heat source primarily affects planetary thermospheres, ionospheres and exospheres since solar radiation at these wavelengths do not penetrate far into atmospheres. Finally, the eccentricity of the planetary orbit around the Sun induces annual variations in the solar forcing at all levels; this effect is most important in Marsí atmosphere. The comparative planetary responses to solar periodic forcing will be discussed in terms of the main controlling factors, namely, distance from the Sun, surface topography and thermal properties, chemical composition, planetary rotation rate, and radiative cooling processes.