  Small-scale waves were observed along the boundary between Jupiter's North
Equatorial Belt and North Tropical Zone, ~16.5{\deg} N planetographic latitude
in Hubble Space Telescope data in 2012 and throughout 2015 to 2018, observable
at all wavelengths from the UV to the near IR. At peak visibility, the waves
have sufficient contrast (~10%) to be observed from ground-based telescopes.
They have a typical wavelength of about 1.2{\deg} (1400 km), variable-length
wave trains, and westward phase speeds of a few m/s or less. New analysis of
Voyager 2 data shows similar wave trains over at least 300 hours. Some waves
appear curved when over cyclones and anticyclones, but most are straight, but
tilted, shifting in latitude as they pass vortices. Based on their wavelengths,
phase speeds, and faint appearance at high-altitude sensitive passbands, the
observed NEB waves are consistent with inertia-gravity waves at the 500-mbar
pressure level, though formation altitude is not well constrained. Preliminary
General Circulation Model simulations generate inertia-gravity waves from
vortices interacting with the environment and can reproduce the observed
wavelengths and orientations. Several mechanisms can generate these waves, and
all may contribute: geostrophic adjustment of cyclones; cyclone/anticyclone
interactions; wind interactions with obstructions or heat pulses from
convection; or changing vertical wind shear. However, observations also show
that the presence of vortices and/or regions of convection are not sufficient
by themselves for wave formation, implying that a change in vertical structure
may affect their stability, or that changes in haze properties may affect their
visibility.
