Auroral Processes at Earth, Jupiter and Saturn.

We review the main characteristics of the auroral ultraviolet emissions at Earth, Jupiter and Saturn. Based on auroral morphology considerations, we discuss and compare the different solar wind - magnetosphere - ionosphere coupling processes giving rise to these emissions. Earth's magnetosphere is usually described as 'open', meaning that its field reconnects with the interplanetary magnetic field (IMF) frozen in the solar wind. This reconnection process allows solar-wind plasma and energy to be transferred to the magnetosphere and to provide the main driving force for the auroral emissions. Different cases of solar-wind plasma conditions have been recognized to give rise to different types of auroral features. Jupiter is opposed to Earth, with a 'closed' magnetosphere. Its larger distance to the Sun and its enormous magnetic field make it difficult for the reconnection process with the IMF to occur efficiently. Io's volcanism is considered to be the prime (internal) plasma source for the magnetosphere, and corotation enforcement of this outward moving plasma is the likely process generating field aligned currents, responsible for the main auroral emissions. Saturn's aurora has not been as extensively studied as Earth's and Jupiter's. Owing to fainter magnetic field and internal plasma source than Jupiter, it has been expected to be intermediate between the cases of Earth and Jupiter. Recent detailed analysis of the Terrestrial, Jovian and Saturnian auroral morphology and dynamics suggests that the simple open/closed/open-closed magnetosphere picture is somewhat oversimplified. They show a much more complex situation with, for example, auroral activity without solar-wind reconnection at Earth, Earth-like reconnection signatures at Jupiter, or extreme auroral variability at Saturn.