Observed Seasonal Change in Titan's Thermosphere

Titan's upper thermosphere has exhibited structural changes throughout the Cassini mission to date. Some thermospheric structure has been tied to magnetospheric forcing (Westlake et al. 2011) and wave activity (Snowden et al. 2013). Now, after several years of observation we have evidence of clear solar forcing of Titan's thermosphere. During the time between TA (2004) and T86 (2012) N2 and CH4 have declined in density resulting in a factor of 3 reduction of N2 and a factor of 5.6 reduction in CH4 at 1000 km. This decline is in response to the solar conditions at Titan, either through a change in distance from the Sun (Titan has receded nearly 1AU from the Sun since 2004), a change in the solar activity level, or a change in the sub-solar point (season). In the most recent flybys Titan's methane has declined at an enhanced rate. We postulate that this decline is a direct response to the solar maximum conditions in 2012. Titan's N2 responds primarily to changes in the thermal structure over a timescale of months to years, while CH4 responds to changes in solar forcing on shorter timescales (Bell et al. 2011). We illustrate this process though analysis of the INMS data since TA and modeling studies using the T-GITM global circulation model.