Review of M-I coupling and auroral emissions at the outer planets

The bulk morphology of Jupiter's and Saturn's UV aurorae is conveniently divided into three components: 1) the main emission (main oval), 2) the satellites auroral footprints (equatorward of the main emission): Io, Europa and Ganymede for Jupiter and Enceladus for Saturn, 3) the polar emissions (poleward of the main emission).This schematic view is already providing useful information on the giant planets aurorae. However, a quick inspection of HST and Cassini UV images directly shows that this simplified classification does not really match the complex morphology of the auroral emissions. As an example, in the case of Jupiter's UV aurorae, it appears that the main emission is not forming an oval, not even a closed shape; it is far from uniform and its position and size vary with time and depend on the viewing geometry. A secondary emission appears equatorward of the main emission and many small scale features regularly appear, some of them periodically. Satellites auroral footprints themselves appear to be much more intricate than predicted by present models. These footprints are actually multiple, their location and number vary periodically with time and with the satellites orbital longitude. The polar emissions are also much more complicated than predicted. Each of the above effects is related to a specific physical phenomenon in the atmosphere, the magnetosphere, or even in the planet's interior. For instance, the spatial distribution of the satellites auroral footprints made it possible to demonstrate the existence of a magnetic field anomaly near the surface of Jupiter's northern hemisphere. Observations which might appear insignificant, like the multiplicity of the satellites footprints or their periodicity are actually extremely valuable because they reveal the complexity of the interaction, in this case between a moon and the magnetospheric plasma of the planet around which it is orbiting. The same applies to small scale auroral structures which depict crucial magnetospheric processes like hot plasma injection, flux-tube interchange or magnetic reconnection mechanisms.

The growing HST and Cassini databases are shedding new light on the origin of Jupiter's and Saturn's aurorae. Mechanisms that we thought could be taken for granted may even be challenged.