Investigating the Long-Term and Short-Term trends in the thermal structure of Titans upper atmosphere.

In this investigation, we present three-dimensional (3-D) numerical simulations of Titans upper atmosphere, ranging from 500 km altitude up to the exobase at ~1500 km using the Titan Global Ionosphere-Thermosphere Model T-GITM (Bell et al. [2010]). For the longer-term trends, our goal is to simulate the global thermal and dynamical structure of this moons upper atmosphere for different seasonal, solar cycle, and orbital conditions. We validate model results with in-situ Cassini Ion-Neutral Mass Spectrometer (Cassini/INMS) and Voyager remote measured neutral mass densities, ion densities, and composition mixing ratios (where available). For short-term trends, we examine the response of the upper atmosphere thermal structure due to three major energy sources: (1) daily variations in Solar EUV/UV flux taken from the Flare Irradiance Spectral Model (FISM), (2) specified wave heating (e.g., Snowden et al. [2013]), and (3) specified plasma heating due to precipitating Oxygen ions and pickup ion heating from Saturns magnetosphere (e.g., Westlake et al. [2011], Bell et al. [2014]). Our results reveal that, while Titan can manifest relatively rapid short-term variations on an orbit-to-orbit basis, the mean state of Titans atmosphere is surprisingly consistent throughout the solar cycle, seasons, and orbital variations.