1. Southwest Research Institute, Culebra Road, San Antonio, Texas 78288, USA
2. Boston University, 725 Commonwealth Avenue, Boston, Massachusetts 02215, USA
3. Blackett Laboratory, Imperial College of Science and Technology, London SW7 2BZ, UK
4. The University of Michigan, Space Research Building, Ann Arbor, Michigan 48109, USA
5. Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
6. University College London, Mullard Space Science Laboratory, Surrey RH5 6NT, UK
7. Université de Liège, 17 avenue du 6 aout, B4000 Liège, Belgium
8. Department of Physics and Astronomy, The University of Iowa, Iowa City, Iowa 52242, USA
9. Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, Maryland 20723, USA

Correspondence to: F. J. Crary¹ Correspondence and requests for materials should be addressed to F.J.C. (Email: fcrary@swri.edu).

Abstract

The interaction of the solar wind with Earth's magnetosphere gives rise to the bright polar aurorae and to geomagnetic storms¹, but the relation between the solar wind and the dynamics of the outer planets' magnetospheres is poorly understood. Jupiter's magnetospheric dynamics and aurorae are dominated by processes internal to the jovian system², whereas Saturn's magnetosphere has generally been considered to have both internal and solar-wind-driven processes. This hypothesis, however, is tentative because of limited simultaneous solar wind and magnetospheric measurements. Here we report solar wind measurements, immediately upstream of Saturn, over a one-month period. When combined with simultaneous ultraviolet imaging³ we find that, unlike Jupiter, Saturn's aurorae respond strongly to solar wind conditions. But in contrast to Earth, the main controlling factor appears to be solar wind dynamic pressure and electric field, with the orientation of the interplanetary magnetic field playing a much more limited role. Saturn's magnetosphere is, therefore, strongly driven by the solar wind, but the solar wind conditions that drive it differ from those that drive the Earth's magnetosphere.

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