The thermosphere of Titan simulated by a global three-dimensional time-dependent model

Abstract

We present three-dimensional numerical simulations for dynamics and energetics of Titan's thermosphere. In so doing, we distinguish between the dynamics driven by solar insolation and those driven by vertical coupling to winds in Titan's middle atmosphere. Our calculations reveal that the solar-driven thermospheric dynamics are characterized by the balance between pressure gradients and viscosity, while the super-rotating zonal winds detected in Titan's stratosphere set up a balance between the pressure gradients, curvature and Coriolis forces. The day to night temperature gradients in the upper thermosphere (around 1300 km) typically lie around 20 (10) K for solar maximum (minimum), with peak solar-driven winds of around 60 (30) m/s. This difference decreases with height and virtually disappears below 1000 km as a result of dayside adiabatic cooling and nightside adiabatic heating. The model highlights unique features about the thermosphere on Titan, such as the important nighttime heating from mid-latitudes to high-latitudes caused by the relatively small size of the planet's shadow, leading to features in the wind profiles which are not found on Earth. Although the lack of measurement constraints prevents us from making predictions of actual wind profiles on Titan, the model does illustrate the physical processes driving the dynamics and suggests that anticipated thermospheric measurements from the Cassini spacecraft may provide constraints also for the dynamics at lower altitudes. Copyright 2000 by the American Geophysical Union.