Solar wind dynamic pressure and electric field as the main factors controlling Saturn's aurorae

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Abstract
The cover shows Hubble Space Telescope (HST) images of Saturn and its polar auroral emissions on 24, 26 and 28 January 2004. Visible wavelength images are combined with ultraviolet images of the south polar region. The strong brightening of the aurora on 28 January corresponded with a large disturbance in the solar wind. These images were obtained during a campaign by the Cassini spacecraft to measure the solar wind approaching Saturn and the Saturn kilometric emissions, and the combined results are presented in three letters in this issue. Saturn's aurorae differ in shape and form from those seen on Earth and on Jupiter, and they also differ in their response to the solar wind. Saturn's strongest radio emissions appear to be closely tied to the polar aurora. 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.