Circulation and Thermal Structure of the Venusian Thermosphere

Abstract

Absorption of solar radiation in the dayside Venusian thermosphere forces a circulation cell with vertical motions upward on the dayside and downward on the nightside with maximum amplitude greater than a meter per second and horizontal velocities away from the subsolar point with amplitudes up to several hundred meters per second. The first harmonic in temperature determines a several-hundred-degree temperature decrease from dayside to nightside. These conclusions follow from the numerical integration of a dynamic model which includes realistic stratification and temperature-dependent radiative damping. The large day-to-night temperature contrast is a consequence of the addition of extreme ultraviolet (EUV) heating at sufficiently high levels that it must he conducted downward to lower levels before adiabatic and 15 μ cooling can balance it. The observed exospheric temperature of ∼650K near the subsolar point is reproduced with an EUV heating efficiency of 0.3. The calculated nightside exospheric temperature is below 300K for this efficiency.