1. Boston University, 725 Commonwealth Avenue, Boston, Massachusetts 02215, USA
2. Université de Liège, Allée du 6 Aout - Sart Tilman, B4000 Liège, Belgium
3. NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
4. Southwest Research Institute, Culebra Road, San Antonio, Texas 78228, USA
5. Imperial College, London SW7 2AZ, UK
6. University of Iowa, Iowa City, Iowa 52242, USA
7. University of Leicester, Leicester LEI 7RH, UK
8. Rice University, Houston, Texas 77005, USA
9. School of Civil and Environmental Engineering, Yonsei University, Seoul 120-749, Korea

Correspondence to: J. T. Clarke¹ Correspondence and requests for materials should be addressed to J.T.C. (Email: jclarke@bu.edu).

Abstract

It has often been stated that Saturn's magnetosphere and aurorae are intermediate between those of Earth, where the dominant processes are solar wind driven¹, and those of Jupiter, where processes are driven by a large source of internal plasma^{2, 3, 4}. But this view is based on information about Saturn that is far inferior to what is now available. Here we report ultraviolet images of Saturn, which, when combined with simultaneous Cassini measurements of the solar wind⁵ and Saturn kilometric radio emission⁶, demonstrate that its aurorae differ morphologically from those of both Earth and Jupiter. Saturn's auroral emissions vary slowly; some features appear in partial corotation whereas others are fixed to the solar wind direction; the auroral oval shifts quickly in latitude; and the aurora is often not centred on the magnetic pole nor closed on itself. In response to a large increase in solar wind dynamic pressure⁵ Saturn's aurora brightened dramatically, the brightest auroral emissions moved to higher latitudes, and the dawn side polar regions were filled with intense emissions. The brightening is reminiscent of terrestrial aurorae, but the other two variations are not. Rather than being intermediate between the Earth and Jupiter, Saturn's auroral emissions behave fundamentally differently from those at the other planets.

1. Boston University, 725 Commonwealth Avenue, Boston, Massachusetts 02215, USA
2. Université de Liège, Allée du 6 Aout - Sart Tilman, B4000 Liège, Belgium
3. NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
4. Southwest Research Institute, Culebra Road, San Antonio, Texas 78228, USA
5. Imperial College, London SW7 2AZ, UK
6. University of Iowa, Iowa City, Iowa 52242, USA
7. University of Leicester, Leicester LEI 7RH, UK
8. Rice University, Houston, Texas 77005, USA
9. School of Civil and Environmental Engineering, Yonsei University, Seoul 120-749, Korea

Correspondence to: J. T. Clarke¹ Correspondence and requests for materials should be addressed to J.T.C. (Email: jclarke@bu.edu).

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