Sub-Seasonal Variation in Neptune's Mid-Infrared Emission.

We present an analysis of all currently available ground-based imaging of Neptune in the mid-infrared. Dating between 2003 and 2020, the images reveal changes in Neptune's mid-infrared (~8-25 micron) emission over time. Images sensitive to stratospheric ethane (~12 micron), methane (~8 micron), and CH3D (~9 micron) display significant sub-seasonal temporal variation on regional and global scales. Comparison with hydrogen-quadrupole (~17-micron H2 S(1)) spectra suggests these changes are primarily related to stratospheric temperature changes. The stratosphere appears to have cooled between 2003 and 2010 across multiple filtered wavelengths, followed by a dramatic warming of the south pole between 2018 and 2020. Conversely, upper tropospheric temperatures appear invariant during this period, except for the south pole, which appeared warmest between 2003 and 2006. We discuss the observed variability in the context of seasonal forcing, tropospheric meteorology, and the solar cycle. Collectively, these data provide the strongest evidence to date that processes produce sub-seasonal variation on both global and regional scales in Neptune's stratosphere.