  NASA's Spitzer Infrared Spectrometer (IRS) acquired mid-infrared (5-37
microns) disc-averaged spectra of Uranus very near to its equinox in December
2007. A mean spectrum was constructed from observations of multiple central
meridian longitudes, spaced equally around the planet, which has provided the
opportunity for the most comprehensive globally-averaged characterisation of
Uranus' temperature and composition ever obtained (Orton et al., 2014 a
[arXiv:1407.2120], b [arXiv:1407.2118]). In this work we analyse the
disc-averaged spectra at four separate central meridian longitudes to reveal
significant longitudinal variability in thermal emission occurring in Uranus'
stratosphere during the 2007 equinox. We detect a variability of up to 15% at
wavelengths sensitive to stratospheric methane, ethane and acetylene at the
~0.1-mbar level. The tropospheric hydrogen-helium continuum and deuterated
methane absorption exhibit a negligible variation (less than 2%), constraining
the phenomenon to the stratosphere. Building on the forward-modelling analysis
of the global average study, we present full optimal estimation inversions
(using the NEMESIS retrieval algorithm, Irwin et al., 2008
[10.1016/j.jqsrt.2007.11.006]) of the Uranus-2007 spectra at each longitude to
distinguish between thermal and compositional variability. We found that the
variations can be explained by a temperature change of less than 3 K in the
stratosphere. Near-infrared observations from Keck II NIRC2 in December 2007
(Sromovsky et al., 2009 [arXiv:1503.01957], de Pater et al., 2011
[10.1016/j.icarus.2011.06.022]), and mid-infrared observations from VLT/VISIR
in 2009 (Roman et al., 2020 [arXiv:1911.12830]), help to localise the potential
sources to either large scale uplift or stratospheric wave phenomena.
