Temperature Variability in Titan's Upper Atmosphere: The Role of Wave Dissipation

The Cassini spacecraft detected a surprisingly large temporal temperature variability of 60 Kin Titan's upper atmosphere during multiple flybys. Previous efforts examining such a variability focused on the role of radiative heating but were unable to explain the observations. Analytic estimates of the wave energy fluxes have suggested that wave heating might be an important process affecting the thermal structure of the upper atmosphere. However, approaches to date have been highly idealized and have not described wave propagation rigorously. Here, we implement an anelastic linearized wave model adopting the Wentzel-Kramers-Brillouin approximation that adequately describes wave propagation in Titan's upper atmosphere, where the observed vertical wavelengths are several times larger than the density scale height. Our results show that the wave heating and cooling rates generated by molecular diffusion of monochromatic waves are larger than those found in previous studies. The energy fluxes associated with wave dissipation can exceed that of the combined solar extreme ultraviolet (EUV) heating and HCN rotational line cooling. Compared to the wave-free, mean-state temperature, the wave energy fluxes associated with certain wave modes can produce a temperature variability as large as 20 K, which is larger than that driven by magnetospheric particle precipitation but still smaller than that observed. Our results suggest that wave heating and cooling are important processes that can modify the thermal structure of Titan's upper atmosphere and also suggest that additional processes such as wave breaking and molecular diffusion of a spectrum of waves should be considered in future studies.