  The temperature structure of Titan's upper atmosphere exhibits large
variability resulting from numerous spatially and temporally irregular external
energy sources, seasonal changes, and the influence of molecular species
produced via photochemistry. In particular, Titan's relatively abundant HCN is
thought to provide substantial cooling to the upper atmosphere through
rotational emission, balancing UV/EUV heating and thermal conduction. Here, we
present the analysis of ALMA observations of Titan from 2012, 2014, 2015, and
2017, corresponding to planetocentric solar longitudes of ~34-89$^{\circ}$,
including vertical HCN and temperature profiles retrieved from the lower
mesosphere through the thermosphere (~350-1200 km;
$3\times10^{-2}$-$2\times10^{-8}$ mbar). Throughout the atmosphere, temperature
profiles differ by 10 to 30 K between observations approximately one Earth year
apart, particularly from 600-900 km. We find evidence for a large imbalance in
Titan's upper atmospheric energy budget between 2014 and 2015, where the
mesospheric thermal structure changes significantly and marks the transition
between a mesopause located at ~600 km ($2\times10^{-4}$ mbar) and at ~800 km
($3\times10^{-6}$ mbar). The retrieved HCN abundances vary dramatically during
the 2012 to 2017 time period as well, showing close to 2 orders of magnitude
difference in abundance at 1000 km. However, the change in HCN abundance does
not appear to fully account for the variation in mesospheric temperatures over
the $L_S\sim$34-89$^{\circ}$ period. These measurements provide additional
insight into the variability of Titan's mesospheric composition and thermal
structure following its 2009 vernal equinox, and motivate continued
investigation of the origins of such rapid changes in Titan's atmosphere
throughout its seasonal cycle.
