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Modulation of Saturn’s radio clock by solar wind speed

Nature volume 450, pages 265–267 (2007)Cite this article

Abstract

The internal rotation rates of the giant planets can be estimated by cloud motions, but such an approach is not very precise because absolute wind speeds are not known a priori and depend on latitude1: periodicities in the radio emissions, thought to be tied to the internal planetary magnetic field, are used instead2,3,4,5. Saturn, despite an apparently axisymmetric magnetic field6, emits kilometre-wavelength (radio) photons from auroral sources. This emission is modulated at a period initially identified as 10 h 39 min 24 ± 7 s, and this has been adopted as Saturn’s rotation period3. Subsequent observations7,8, however, revealed that this period varies by ±6 min on a timescale of several months to years. Here we report that the kilometric radiation period varies systematically by ±1% with a characteristic timescale of 20–30 days. Here we show that these fluctuations are correlated with solar wind speed at Saturn, meaning that Saturn’s radio clock is controlled, at least in part, by conditions external to the planet’s magnetosphere. No correlation is found with the solar wind density, dynamic pressure or magnetic field; the solar wind speed therefore has a special function. We also show that the long-term fluctuations are simply an average of the short-term ones, and therefore the long-term variations are probably also driven by changes in the solar wind.

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Figure 1: Evidence of short-term variations in P SKR and their relation to long-term variations.
Figure 2: ‘Short-term’ variations in P SKR compared with solar wind speed at Saturn and with variations in orbital parameters of Cassini.
Figure 3: Comparison of SKR period variations with solar wind speed and dynamic pressure at Saturn.

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Acknowledgements

We thank D. Southwood for a seminar at Meudon Observatory; F. Mottez, S. Hess and J.-M. Griessmeier for early inputs; B. Kurth for comments on the manuscript; D. Pelat for advice on the statistical analysis; Cassini Radio and Plasma Wave Science (RPWS) engineers at the University of Iowa and the Laboratoire d’Études Spatiales et d’Instrumentation en Astrophysique for support on instrumental questions and ephemeris; and N. Letourneur for processing RPWS data. The French co-authors acknowledge support from the Centre National d’Études Spatiales.

Author Contributions P.Z., L.L., B.C. and R.P. contributed equally to this work. H.O.R. contributed to the projection of solar wind data. P.Z. wrote the paper. All authors discussed the results and commented on the manuscript.

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Authors and Affiliations

  1. Laboratoire d’Études Spatiales et d’Instrumentation en Astrophysique, Observatoire de Paris, Centre National de la Recherche Scientifique, Université Pierre et Marie Curie, Université Paris Diderot, 92190 Meudon, France

    Philippe Zarka, Laurent Lamy, Baptiste Cecconi & Renée Prangé

  2. Space Research Institute, Austrian Academy of Sciences, A-8042 Graz, Austria ,

    Helmut O. Rucker

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  1. Philippe Zarka
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Correspondence to Philippe Zarka.

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Zarka, P., Lamy, L., Cecconi, B. et al. Modulation of Saturn’s radio clock by solar wind speed. Nature 450, 265–267 (2007). https://doi.org/10.1038/nature06237

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