Auroral emissions, the natural light displays in the sky, are the optical manifestations of a chain of interactions involving the planetary atmosphere, its moons, interplanetary space and the Sun. The Ultraviolet Imaging Spectrograph (UVIS) instrument onboard Cassini allowed us to discover a striking auroral feature of Saturn: the ‘nightside polar arc’ (pictured left, indicated by the arrow), which strongly resembles a terrestrial transpolar arc (pictured right, indicated by the arrow).
Transpolar auroral arcs are some of the most spectacular and puzzling auroral emissions at Earth. They are also known as ‘theta aurora’ because when they are seen from above they look like the Greek letter theta (like an oval with a line crossing through the centre). They extend from the nightside auroral oval into the open magnetic field line region and often reach the dayside auroral oval. Nightside polar arcs, which are regularly observed at Earth, represent optical signatures of the interaction of the deep planetary magnetosphere with a stream of charged particles emitted by the Sun, through the process of magnetic reconnection.
Contrary to Earth, Saturn is a fast rotator. In addition, its magnetospheric plasma mainly originates from internal sources (such as Enceladus, neutral clouds and rings). These ingredients constitute a magnetosphere whose deepest region is not supposed to be vulnerable to the solar wind. The Cassini discovery shows that Earth-like polar arcs can exceptionally occur in a fast rotational and internally influenced magnetosphere such as Saturn’s. One possible scenario could be that the polar auroral arc at Saturn is formed by solar-wind-driven magnetic reconnection in Saturn’s magnetotail, like at Earth. This suggests that solar wind might have a stronger influence on Saturn’s deep magnetotail region than we thought. This is the first and only observation that reveals the presence of such an arc at Saturn. The rarity of its appearance demonstrates that the conditions for its formation are rarely met at Saturn.