Structure, luminosity, and dynamics of the Venus thermosphere

Abstract

We review here observations and models related to the chemical and thermal structures, airglow and auroral emissions and dynamics of the Venus thermosphere, and compare empirical models of the neutral densities based in large part on in situ measurements obtained by the Pioneer Venus spacecraft.Observations of the intensities of emissions are important as a diagnostic tool for understanding the chemical and physical processes taking place in the Venus thermosphere. Measurements, ground-based and from rockets, satellites, and spacecraft, and model predictions of atomic, molecular and ionic emissions, are presented and the most important sources are elucidated. Coronas of hot hydrogen and hot oxygen have been observed to surround the terrestrial planets. We discuss the observations of and production mechanisms for the extended exospheres and models for the escape of lighter species from the atmosphere. Over the last decade and a half, models have attempted to explain the unexpectedly cold temperatures in the Venus thermosphere; recently considerable progress has been made, although some controversies remain. We review the history of these models and discuss the heating and cooling mechanisms that are presently considered to be the most important in determining the thermal structure. Finally, we discuss major aspects of the circulation and dynamics of the thermosphere: the sub-solar to anti-solar circulation, superrotation, and turbulent processes.

Modelling the Venusian airglow

“…Moreover, the CO + 2 doublet can be seen at 2883−2896 Å. The nightglow spectrum of these planets consists mainly of O 2 and NO emissions (Fox & Bougher 1991), although other lines and bands are visible. Lummerzheim & Lilensten (1994).…”

“…The dayglow spectrum of Mars and Venus, which are planets with an atmosphere mainly composed of CO 2 , is dominated by the CO Cameron and Fourth Positive bands below 2700 Å (Leblanc et al 2006;Fox & Bougher 1991). The other constituents of the spectrum are H, O and C lines, and some N 2 bands.…”

“…Moreover, the 1304 Å line is optically thick for Venus (Fox & Bougher 1991). Kassal (1975Kassal ( , 1976 showed that the (9, 0) Fourth Positive bands of CO, contaminate the O( 3 S) triplet.…”

“…They are generated by the CO(a 3 Π) → CO(X 1 Σ) transition. Fluorescent scattering is negligible, and it is a metastable state (lifetime 8 ms, Fox & Bougher (1991) and references therein). We therefore only consider deactivation radiation for this state.…”

“…A review of the modelling and observations of these emissions, can be found in Fox & Bougher (1991). The oxygen green line was discovered in the nightside of Venus (Slanger et al 2001), opening a controversy about its origin.…”