  Between 2001 and 2023, we obtained high spectral resolution mid-infrared
observations of Io using the TEXES instrument at NASA's Infrared Telescope
Facility. These observations were centered at 529.8 cm-1 (18.88 {\mu}m) and
include several SO2 absorption lines. By modeling the shapes and strengths of
these absorption lines, we are able to determine how Io's SO2 atmospheric
density varies over the 22-year time period, covering nearly two Jovian years.
Previous analysis has shown that the density of Io's atmosphere on the
anti-Jovian hemisphere exhibits clear seasonal temporal variability, which can
be modeled as the sum of a seasonally-varying frost sublimation component and a
constant component, assumed to be volcanic. The new data show that the seasonal
pattern repeats during the second Jovian year, confirming the importance of
sublimation support. The considerable longitudinal variability in Io's
atmospheric density found in previous work is also stable over the second
Jovian year with the SO2 column density on the Jupiter-facing hemisphere being
5--8 times lower than the anti-Jovian hemisphere. For the first time, we detect
seasonal variability on the Jupiter-facing hemisphere as well. This can also be
modeled as a combination of sublimation and a small constant source. The lower
atmospheric density on the Jupiter-facing hemisphere can plausibly be explained
by the daily Jupiter eclipses, which decrease the surface temperature and
therefore reduce the sublimation-driven component of the atmosphere, combined
with a lower level of volcanic activity directly emitting SO2 into the
atmosphere.
