Observations and theory of the solar semidiurnal tide in the mesosphere of Venus

Abstract

The Orbiter Infrared Radiometer experiment, aboard the 1978 Pioneer Venus mission, produced strong evidence for the existence of a semidiurnal solar tide in the mesosphere of Venus¹. At the same time, measurements in situ of radiative fluxes and cloud particle distribution, in other Pioneer Venus experiments, have provided much information about the structure of the solar heating and infared cooling there^2–4, making it possible to calculate the thermal tidal forcing functions with confidence, and also allowing good estimates of the radiative dissipation rates to be made. Because the structure of tidal fields depends sensitively on that of the background zonal mean wind velocity, U(θ, z), a knowledge of the sources and sinks of tidal energy and of the actual structure of the tide should allow one to infer the behaviour of U. We show here that a physically reasonable mean flow can be found that leads to theoretically predicted tides that are in excellent agreement with those observed. Our zonal mean flow shows a maximum velocity of ∼130 ms⁻¹ at 70 km, decreasing dramatically to ∼0 at 88 km, and a jet-like meridional structure that deviates strongly from solid-body rotation.