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Anti-planetward auroral electron beams at Saturn

Nature volume 439, pages 699–702 (2006)Cite this article

Abstract

Strong discrete aurorae on Earth are excited by electrons, which are accelerated along magnetic field lines towards the planet1. Surprisingly, electrons accelerated in the opposite direction have been recently observed2,3,4,5,6. The mechanisms and significance of this anti-earthward acceleration are highly uncertain because only earthward acceleration was traditionally considered, and observations remain limited. It is also unclear whether upward acceleration of the electrons is a necessary part of the auroral process or simply a special feature of Earth's complex space environment. Here we report anti-planetward acceleration of electron beams in Saturn's magnetosphere along field lines that statistically map into regions of aurora. The energy spectrum of these beams is qualitatively similar to the ones observed at Earth, and the energy fluxes in the observed beams are comparable with the energies required to excite Saturn's aurora. These beams, along with the observations at Earth2,3,4,5,6 and the barely understood electron beams in Jupiter's magnetosphere7,8, demonstrate that anti-planetward acceleration is a universal feature of aurorae. The energy contained in the beams shows that upward acceleration is an essential part of the overall auroral process.

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Figure 1: Segments of Cassini's orbit in Saturn's magnetosphere show electron distributions that contain magnetic-field-aligned electron beams.
Figure 2: Electron beams in Saturn's magnetosphere.
Figure 3: Four electron beam energy spectra during Cassini's inbound orbit 0a in 2004.
Figure 4: Saturn's electron beams and aurorae.

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Acknowledgements

We thank D. Grodent for helpful discussions.

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Author notes

  1. J. Saur

    Present address: Institut für Geophysik und Meteorologie, Universität zu Köln, Albertus-Magnus-Platz, 50923, Köln, Germany

Authors and Affiliations

  1. Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Maryland, 20723, Laurel, USA

    J. Saur, B. H. Mauk, D. G. Mitchell, S. Livi, S. M. Krimigis, P. T. Newell, D. J. Williams & P. C. Brandt

  2. Max-Planck-Institut für Sonnensystemforschung, 37191, Katlenburg-Lindau, Germany

    N. Krupp, A. Lagg & E. Roussos

  3. Institute of Geophysics and Planetary Physics, UCLA, California, 90095, Los Angeles, USA

    K. K. Khurana

  4. Blackett Laboratory, Imperial College, SW7 2AZ, London, UK

    M. K. Dougherty

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Correspondence to J. Saur.

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Saur, J., Mauk, B., Mitchell, D. et al. Anti-planetward auroral electron beams at Saturn. Nature 439, 699–702 (2006). https://doi.org/10.1038/nature04401

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