Titan’s chaotic upper atmosphere Abstract: Saturn’s largest moon Titan has been observed systematically since the arrival of Cassini/Huygens in 2004. While the landing of the Huygens probe in January 2005 provided amongst other a continuous, yet single vertical profile of atmospheric properties from the thermosphere down to the surface, regular in-situ observations by the Cassini orbiter’s Ion Neutral Mass Spectrometer (INMS) above 950 km provided information on global and time variations in Titan’s thermosphere. Infrared remote sensing onboard Cassini provided further details on Titan’s stratosphere and troposphere, while UV occultations measured the mesosphere and lower thermosphere.   After 10 years of regular observations, Titan has proven to possess one of the most enigmatic atmospheres explored to-date. Density variations observed in the thermosphere show dramatic variations by orders of magnitude between observations and temperatures can vary by 100 K from one flyby to another - rates which are difficult to interpret physically. Equally, horizontal structures are counter-intuitive, suggesting a slightly hotter dayside than nightside on Titan. Work over the past years has attempted and largely failed to systematically correlate these atmosphere structures with changing conditions in Saturn’s magnetosphere, another potential energy source.  Most recently, our work has explored the potential influence of vertical coupling in the atmosphere, of Titan’s lower atmosphere driving changes above. By coupling the Titan WRF stratosphere model with our Titan Thermosphere General Circulation Model, we have found small and short term variations in the stratosphere to drive considerable variability in the thermosphere as well, remarkably consistent with observations. Our findings highlight the broader principle of processes which play a secondary role in atmospheres of bodies closer to the Sun becoming primary in nature in atmospheres further out in the solar system where solar heating is no longer a dominant thermal energy source. [mehr]