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Tethys and Dione as sources of outward-flowing plasma in Saturn’s magnetosphere

Abstract

Rotating at over twice the angular speed of Earth, Saturn imposes a rapid spin on its magnetosphere. As a result, cold, dense plasma is believed to be flung outward from the inner magnetosphere by centrifugal force and replaced by hotter, more tenuous plasma from the outer magnetosphere. The centrifugal interchange1 of plasmas in rotating magnetospheres was predicted many years ago2,3,4 and was conclusively demonstrated by observations in Jupiter’s magnetosphere5,6,7, which—like that of Saturn (but unlike that of Earth)—is rotationally dominated. Recent observations in Saturn’s magnetosphere8,9,10 have revealed narrow injections of hot, tenuous plasma believed to be the inward-moving portion of the centrifugal interchange cycle. Here we report observations of the distribution of the angle between the electron velocity vector and the magnetic field vector (‘pitch angle’) obtained in the cold, dense plasma adjacent to these inward injection regions. The observed pitch-angle distributions are indicative of outward plasma flow and consistent with centrifugal interchange in Saturn’s magnetosphere. Further, we conclude that the observed double-peaked (‘butterfly’) pitch-angle distributions result from the transport of plasma from regions near the orbits of Dione and Tethys, supporting the idea of distinct plasma tori associated with these moons11,12,13.

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Figure 1: Electron and magnetic field data obtained by Cassini near the equatorial plane during the outbound leg of the orbit on 28 October 2004 (day 302).
Figure 2: Scatter plots showing the correlation of the pitch angle of the peak electron flux with electron energy.
Figure 3: Scatter plots of electron counts versus pitch angle for two electron energies measured at the orbit of Dione.
Figure 4: Comparison of observed and modelled peak-flux pitch angles for butterfly distributions versus radial distance from the centre of Saturn.

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Acknowledgements

We gratefully acknowledge comments by F. Crary, B. Mauk, E. Sittler, M. Thomsen, T. Hill, and H. Waite.

Author Contributions J.L.B. developed the interpretation of the Cassini data and is lead author of this paper. J.G. and W.S.L. were responsible for data analysis and, respectively, for figure preparation and preparation of the text. D.T.Y, A.J.C. and M.K.D. are, respectively, the CAPS principal investigator, the CAPS Electron Spectrometer lead and the Cassini MAG principal investigator, and provided both data and analysis. N.A. contributed to the data analysis and modelling.

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Correspondence to W. S. Lewis.

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Supplementary Information

This file contains Supplementary Figures 1-3, including illustrative sketch and Supplementary Discussion with detail of pitch angle production and critical pitch angle effect in outflowing plasma. (PDF 1283 kb)

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Burch, J., Goldstein, J., Lewis, W. et al. Tethys and Dione as sources of outward-flowing plasma in Saturn’s magnetosphere. Nature 447, 833–835 (2007). https://doi.org/10.1038/nature05906

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