Jovian-like aurorae on Saturn

Saturn's other aurora: Jovian-like origins A new study of Saturn's auroral emissions has uncovered a secondary auroral oval, a quarter as bright as the main one. The main auroral oval was first seen in Hubble Space Telescope images more than a decade ago and its morphology has since been determined in detail, yet there is continued debate about its origins. One theory was that Saturn's auroral emissions were a hybrid between those seen on Earth, primarily formed by interaction with the solar wind, and those on Jupiter, formed by interaction with plasma flows. But the properties of the secondary oval at Saturn show it to be a weak equivalent of Jupiter's main oval, its relative dimness due to the lack of as large a source of ions as Jupiter's volcanic moon Io. Now it seems that the underlying aurora formation processes on Saturn and Jupiter are very similar, with scaling differences accounting for the differences in appearance. Planetary aurorae are formed by energetic charged particles streaming along the planet's magnetic field lines into the upper atmosphere from the surrounding space environment. At Saturn only the main auroral oval has been observed, but this paper reports the discovery of a secondary oval, ∼25 per cent as bright as the main oval. Planetary aurorae are formed by energetic charged particles streaming along the planet’s magnetic field lines into the upper atmosphere from the surrounding space environment. Earth’s main auroral oval is formed through interactions with the solar wind 1 , whereas that at Jupiter is formed through interactions with plasma from the moon Io inside its magnetic field (although other processes form aurorae at both planets 2 , 3 ). At Saturn, only the main auroral oval has previously been observed and there remains much debate over its origin. Here we report the discovery of a secondary oval at Saturn that is ∼25 per cent as bright as the main oval, and we show this to be caused by interaction with the middle magnetosphere around the planet. This is a weak equivalent of Jupiter’s main oval, its relative dimness being due to the lack of as large a source of ions as Jupiter’s volcanic moon Io. This result suggests that differences seen in the auroral emissions from Saturn and Jupiter are due to scaling differences in the conditions at each of these two planets, whereas the underlying formation processes are the same.