Gravity Waves in Titan's Atmosphere: A Comparison Between Linearized Wave Model Calculations and HASI Observations

Abstract

The Huygens Atmospheric Structure Instrument (HASI) detected wavelike temperature fluctuations from 500 to 1,000 km on Titan. However, these fluctuation structures have not been satisfactorily reproduced by any theoretical model to date. In this study, we construct a full-wave model to simulate the observed gravity wave structure. The model includes dissipation processes due to molecular viscosity, which increases exponentially with altitude, and eddy viscosity, which dominates the lower atmosphere. Using our model, we reproduced the observed temperature perturbations with the superposition of two gravity wave modes, one with λ_z = 600 km and λ_x = 1,600 km while the other with λ_z = 140 km and λ_x = 8,000 km. We estimate the thermal effects introduced by gravity waves and find that gravity waves below 900 km may significantly modify temperature structure of the upper atmosphere by tens of Kelvins. The wave-induced thermal effect is sensitive to the eddy viscosity, which controls the dissipation and thermal conductivity below 900 km. When the eddy viscosity is increased by a factor of five, the thermal effect of gravity waves changes from −89 K cooling to nearly zero. This heating mechanism may contribute to the large temperature variability of 60 K in Titan's upper atmosphere.