Numerical Solution for the Composition of a Thermosphere in the Presence of a Steady Subsolar to-Antisolar Circulation with Application to Venus

Abstract

We consider the thermosphere of a nonrotating planet with a large-scale circulation from dayside to nightside driven by differential solar heating. Return flow is assumed to occur only below the region considered. The motion is moixotonically upward on the dayside and downward on the nightside. A numerical model is developed and a method of solution derived for the distribution of N components in the presence of sources and sinks due to photodissociation. These components, any of which way be major species, are transported vertically by molecular diffusion and carried horizontally and vertically by the large-scale circulation. The model is integrated for parameters appropriate to the Venusian upper atmosphere, assuming only CO2 is carried upward on the dayside through the bottom boundary at 0.1 mb. This boundary condition requires that photodissociation fragments carried out at the bottom on the nightside essentially recombine into CO2 before they are carried back upward into the dayside integration region. Under these conditions, the calculations show that large-scale circulation alone can keep the relative number concentrations of O and CO at the Fl peak to ?2%. Atomic oxygen concentrations of this magnitude have interesting consequences for the topside Venusian ionosphere.