  We characterise the origin and evolution of a mesoscale wave pattern in
Jupiter's North Equatorial Belt (NEB), detected for the first time at 5 $\mu$m
using a 2016-17 campaign of `lucky imaging' from the VISIR instrument on the
Very Large Telescope and the NIRI instrument on the Gemini observatory, coupled
with M-band imaging from Juno's JIRAM instrument during the first seven Juno
orbits. The wave is compact, with a $1.1-1.4^\circ$ longitude wavelength
(wavelength 1,300-1,600 km, wavenumber 260-330) that is stable over time, with
wave crests aligned largely north-south between $14$ and $17^\circ$N
(planetographic). The waves were initially identified in small ($10^\circ$
longitude) packets immediately west of cyclones in the NEB at $16^\circ$N, but
extended to span wider longitude ranges over time. The waves exhibit a 7-10 K
brightness temperature amplitude on top of a $\sim210$-K background at 5
$\mu$m. The thermal structure of the NEB allows for both inertio-gravity waves
and gravity waves. Despite detection at 5 $\mu$m, this does not necessarily
imply a deep location for the waves, and an upper tropospheric aerosol layer
near 400-800 mbar could feature a gravity wave pattern modulating the
visible-light reflectivity and attenuating the 5-$\mu$m radiance originating
from deeper levels. Strong rifting activity appears to obliterate the pattern,
which can change on timescales of weeks. The NEB underwent a new expansion and
contraction episode in 2016-17 with associated cyclone-anticyclone formation,
which could explain why the mesoscale wave pattern was more vivid in 2017 than
ever before.
