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


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.


  1. Kruskal, M. D. & Schwarzschild, M. Some instabilities of a completely ionized plasma. Proc. R. Soc. Lond. A 223, 348–360 (1954)

    ADS  MathSciNet  Article  Google Scholar 

  2. Michel, F. C. & Sturrock, P. A. Centrifugal instability of the jovian magnetosphere and its interaction with the solar wind. Planet. Space Sci. 22, 1501–1510 (1974)

    ADS  Article  Google Scholar 

  3. Hill, T. W. Interchange stability of a rapidly rotating magnetosphere. Planet. Space Sci. 24, 1151–1154 (1976)

    ADS  Article  Google Scholar 

  4. Pontius, D. H., Hill, T. W. & Rassbach, M. E. Steady state plasma transport in a corotation-dominated magnetosphere. Geophys. Res. Lett. 13, 1097–1100 (1986)

    ADS  Article  Google Scholar 

  5. Bolton, S. J. et al. Enhanced whistler-mode emissions: signatures of interchange motion in the Io torus. Geophys. Res. Lett. 24, 2123–2126 (1997)

    ADS  Article  Google Scholar 

  6. Thorne, R. M. et al. Galileo evidence for rapid inward transport in the Io torus. Geophys. Res. Lett. 24, 2131–2134 (1997)

    ADS  Article  Google Scholar 

  7. Frank, L. A. & Paterson, W. R. Observations of plasmas in the Io torus with the Galileo spacecraft. J. Geophys. Res. 105, 16,017–16,034 (2000)

    ADS  Article  Google Scholar 

  8. Burch, J. L. et al. Properties of local plasma injections in Saturn’s magnetosphere. Geophys. Res. Lett. 32 L14S02 doi: 10.1029/2005GL022611 (2005)

    ADS  Article  Google Scholar 

  9. Hill, T. W. et al. Evidence for rotationally driven plasma transport in Saturn’s magnetosphere. Geophys. Res. Lett. 32 L14S10 doi: 10.1029/2005GL022620 (2005)

    Article  Google Scholar 

  10. Mauk, B. H. et al. Energetic particle injections in Saturn’s magnetosphere. Geophys. Res. Lett. 32 L14S05 doi: 10.1029/2005GL022485 (2005)

    Article  Google Scholar 

  11. Frank, L. A. et al. Plasma in Saturn’s magnetosphere. J. Geophys. Res. 85, 5695–5708 (1980)

    ADS  CAS  Article  Google Scholar 

  12. Richardson, J. D., Eviatar, A. & Siscoe, G. L. Satellite tori at Saturn. J. Geophys. Res. 91, 8749–8755 (1986)

    ADS  CAS  Article  Google Scholar 

  13. Wahlund, J.-E. et al. The inner magnetosphere of Saturn: Cassini RPWS cold plasma results from the first encounter. Geophys. Res. Lett. 332 L20S09 doi: 10.1029/2005GL022699 (2005)

    Article  Google Scholar 

  14. Thorne, R. M., Williams, D. J., Zhang, L. D. & Stone, S. Energetic electron butterfly distributions near Io. J. Geophys. Res. 104, 14755–14766 (1999)

    ADS  Article  Google Scholar 

  15. Rymer, A. M. et al. Electron sources in Saturn’s magnetosphere. J. Geophys. Res. 112 A02201 doi: 10.1029/2006JA013017 (2007)

    ADS  Article  Google Scholar 

  16. Connerney, J. E. P., Ness, N. F. & Acuna, M. H. Zonal harmonic model of Saturn’s magnetic field from Voyager 1 and 2 observations. Nature 298, 44–46 (1982)

    ADS  Article  Google Scholar 

  17. Alexeev, I. I. et al. A global magnetic model of Saturn's magnetosphere and a comparison with Cassini SOI data. Geophys. Res. Lett. 33 L08101 doi: 10.1029/2006GL025896 (2006)

    ADS  Article  Google Scholar 

  18. André, N. Magnetic signatures of plasma-depleted flux tubes in the Saturnian inner magnetosphere. Geophys. Res. Lett. (submitted)

  19. Sittler, E. C., Ogilvie, K. W. & Scudder, J. D. Survey of low-energy plasma electrons in Saturn’s magnetosphere: Voyagers 1 and 2. J. Geophys. Res. 88, 8847–8870 (1983)

    ADS  CAS  Article  Google Scholar 

  20. Sittler, E. C. et al. Cassini observations of Saturn’s inner plasmasphere: Saturn orbit insertion results. Planet. Space Sci. 54, 1197–1210 (2006)

    ADS  CAS  Article  Google Scholar 

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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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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).

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