1. Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
2. Atmospheric Physics Laboratory, Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
3. Department of Space and Climate Physics, Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
4. Space and Atmospheric Physics Group, Department of Physics, Imperial College of Science, Technology and Medicine, South Kensington Campus, London SW7 2AZ, UK
5. Lunar and Planetary Laboratory and Steward Observatory, University of Arizona, Tucson, Arizona 85721-0092, USA
6. Jet Propulsion Laboratory, California Institute of Technology, M/S 183-601, 4800 Oak Grove Drive, Pasadena, California 91109-8099, USA
7. US Geological Survey, Denver, Colorado 80225, USA
8. Cornell University, Astronomy Department, Ithaca, New York 14853, USA
9. Observatoire de Paris, Meudon 92195, France

Correspondence to: Tom Stallard¹ Correspondence and requests for materials should be addressed to T.S. (Email: tss@ion.le.ac.uk).

The majority of planetary aurorae are produced by electrical currents flowing between the ionosphere and the magnetosphere which accelerate energetic charged particles that hit the upper atmosphere. At Saturn, these processes collisionally excite hydrogen, causing ultraviolet emission^{1, 2, 3, 4, 5, 6, 7, 8}, and ionize the hydrogen, leading to H₃ ⁺ infrared emission^{9, 10, 11, 12, 13, 14, 15}. Although the morphology of these aurorae is affected by changes in the solar wind^{6, 11}, the source of the currents which produce them is a matter of debate^{16, 17}. Recent models predict only weak emission away from the main auroral oval¹⁸. Here we report images that show emission both poleward and equatorward of the main oval (separated by a region of low emission). The extensive polar emission is highly variable with time, and disappears when the main oval has a spiral morphology; this suggests that although the polar emission may be associated with minor increases in the dynamic pressure from the solar wind, it is not directly linked to strong magnetospheric compressions. This aurora appears to be unique to Saturn and cannot be explained using our current understanding of Saturn's magnetosphere. The equatorward arc of emission exists only on the nightside of the planet, and arises from internal magnetospheric processes that are currently unknown.

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