  Beyond our solar system, aurorae have been inferred from radio observations
of isolated brown dwarfs (e.g. Hallinan et al. 2006; Kao et al. 2023). Within
our solar system, giant planets have auroral emission with signatures across
the electromagnetic spectrum including infrared emission of H3+ and methane.
Isolated brown dwarfs with auroral signatures in the radio have been searched
for corresponding infrared features but have only had null detections (e.g.
Gibbs et al. 2022). CWISEP J193518.59-154620.3. (W1935 for short) is an
isolated brown dwarf with a temperature of ~482 K. Here we report JWST
observations of strong methane emission from W1935 at 3.326 microns.
Atmospheric modeling leads us to conclude that a temperature inversion of ~300
K centered at 1-10 millibar replicates the feature. This represents an
atmospheric temperature inversion for a Jupiter-like atmosphere without
irradiation from a host star. A plausible explanation for the strong inversion
is heating by auroral processes, although other internal and/or external
dynamical processes cannot be ruled out. The best fit model rules out the
contribution of H3+ emission which is prominent in solar system gas giants
however this is consistent with rapid destruction of H3+ at the higher pressure
where the W1935 emission originates (e.g. Helling et al. 2019).
