Letter

Nature 453, 1083-1085 (19 June 2008) | doi:10.1038/nature07077; Received 4 December 2007; Accepted 29 April 2008

Jovian-like aurorae on Saturn

Tom Stallard1, Steve Miller2, Henrik Melin3, Makenzie Lystrup2, Stan W. H. Cowley1, Emma J. Bunce1, Nicholas Achilleos2 & Michele Dougherty4

  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. Space Environment Technologies, Planetary and Space Science Division, 320 N. Halstead Street, Suite 110, Pasadena, California 91107, USA
  4. Space and Atmospheric Physics Group, Department of Physics, Imperial College of Science, Technology and Medicine, London SW7 2BW, UK

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

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 wind1, 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 planets2, 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 approx25 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.

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