Supersonic winds in Jupiter's aurorae

Jupiter has a giant magnetosphere that is coupled to the planet's upper atm osphere; as the planet rotates, its magnetic field drags a dense ionized eq uatorial sheet of plasma, which must interact with the upper atmosphere. Ju piter's aurorae are much more powerful(1,2) than the Earths, and cause sign ificant local heating of the upper atmosphere. Auroral electrojets-ion wind s that race around Jupiter's auroral ovals-play a key role in theoretical m odels of how Jupiter's rotational energy is transferred to the plasma sheet (3,4) and how winds may transport energy from auroral heating to lower lati tudes(5-7). But there has hitherto been no direct observational evidence fo r the existence of such electrojets, Here we report observations of electro jets that have winds approaching or in excess of the local speed of sound. The energy produced by these electrojets could heat the whole upper atmosph ere, if the auroral regions couple efficiently with the rest of the planet.