  Though the global atmospheres of hot Jupiters have been extensively studied
using phase curve observations, the level of time variability in these data is
not well constrained. To investigate possible time variability in a planetary
phase curve, we observed two full-orbit phase curves of the hot Jupiter
WASP-43b at 4.5 microns using the Spitzer Space Telescope, and reanalyzed a
previous 4.5 micron phase curve from Stevenson et al. (2017). We find no
significant time variability between these three phase curves, which span
timescales of weeks to years. The three observations are best fit by a single
phase curve with an eclipse depth of 3907 +- 85 ppm, a dayside-integrated
brightness temperature of 1479 +- 13 K, a nightside-integrated brightness
temperature of 755 +- 46 K, and an eastward-shifted peak of 10.4 +- 1.8
degrees. To model our observations, we performed 3D general circulation model
simulations of WASP-43b with simple cloud models of various vertical extents.
In comparing these simulations to our observations, we find that WASP-43b
likely has a cloudy nightside that transitions to a relatively cloud-free
dayside. We estimate that any change in WASP-43bs vertical cloud thickness of
more than three pressure scale heights is inconsistent with our observed upper
limit on variation. These observations, therefore, indicate that WASP-43bs
clouds are stable in their vertical and spatial extent over timescales up to
several years. These results strongly suggest that atmospheric properties
derived from previous, single Spitzer phase curve observations of hot Jupiters
likely show us the equilibrium properties of these atmospheres.
