Jupiter's Mesoscale Waves Observed at 5 μm by Ground-Based Observations and Juno JIRAM

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.