Letter | Published:

The evolution of Saturn’s radiation belts modulated by changes in radial diffusion

Nature Astronomyvolume 1pages872877 (2017) | Download Citation


Globally magnetized planets, such as the Earth1 and Saturn2, are surrounded by radiation belts of protons and electrons with kinetic energies well into the million electronvolt range. The Earth’s proton belt is supplied locally from galactic cosmic rays interacting with the atmosphere3, as well as from slow inward radial transport4. Its intensity shows a relationship with the solar cycle4,5 and abrupt dropouts due to geomagnetic storms6,7. Saturn’s proton belts are simpler than the Earth’s because cosmic rays are the principal source of energetic protons8 with virtually no contribution from inward transport, and these belts can therefore act as a prototype to understand more complex radiation belts. However, the time dependence of Saturn’s proton belts had not been observed over sufficiently long timescales to test the driving mechanisms unambiguously. Here we analyse the evolution of Saturn’s proton belts over a solar cycle using in-situ measurements from the Cassini Saturn orbiter and a numerical model. We find that the intensity in Saturn’s proton radiation belts usually rises over time, interrupted by periods that last over a year for which the intensity is gradually dropping. These observations are inconsistent with predictions based on a modulation in the cosmic-ray source, as could be expected4,9 based on the evolution of the Earth’s proton belts. We demonstrate that Saturn’s intensity dropouts result instead from losses due to abrupt changes in magnetospheric radial diffusion.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.


  1. 1.

    Van Allen, J. A. & Frank, L. A. Radiation around the Earth to a radial distance of 107,400 km. Nature 183, 430–434 (1959).

  2. 2.

    Van Allen, J. A., Randall, B. A. & Thomsen, M. F. Sources and sinks of energetic electrons and protons in Saturn’s magnetosphere. J. Geophys. Res. 85, 5679–5694 (1980).

  3. 3.

    Singer, S. F. Trapped albedo theory of the radiation belt. Phys. Rev. Lett. 1, 181–183 (1958).

  4. 4.

    Selesnick, R. S., Looper, M. D. & Mewaldt, R. A. A theoretical model of the inner proton radiation belt. Space Weather 5, 4003 (2007).

  5. 5.

    Li, X., Baker, D. N., Kanekal, S. G., Looper, M. & Temerin, M. Long term measurements of radiation belts by SAMPEX and their variations. Geophys. Res. Lett. 28, 3827–3830 (2001).

  6. 6.

    Lorentzen, K. R., Mazur, J. E., Looper, M. D., Fennell, J. F. & Blake, J. B. Multisatellite observations of MeV ion injections during storms. J. Geophys. Res. 107, 1231 (2002).

  7. 7.

    Selesnick, R. S., Hudson, M. K. & Kress, B. T. Direct observation of the CRAND proton radiation belt source. J. Geophys. Res. 118, 7532–7537 (2013).

  8. 8.

    Cooper, J. F. Nuclear cascades in Saturn’s rings: cosmic ray albedo neutron decay and origins of trapped protons in the inner magnetosphere. J. Geophys. Res. 88, 3945–3954 (1983).

  9. 9.

    Roussos, E. et al. Long- and short-term variability of Saturn’s ionic radiation belts. J. Geophys. Res. 116, A02217 (2011).

  10. 10.

    Walt, M. Introduction to Geomagnetically Trapped Radiation. 1st edn, (Cambridge Univ. Press, Cambridge, 1994).

  11. 11.

    Hood, L. L. Radial diffusion in Saturn’s radiation belts: a modeling analysis assuming satellite and ring E absorption. J. Geophys. Res. 88, 808–818 (1983).

  12. 12.

    Sauer, H. H. On Saturnian cosmic ray cutoff rigidities. Geophys. Res. Lett. 7, 215–217 (1980).

  13. 13.

    Usoskin, I. G., Alanko-Huotari, K., Kovaltsov, G. A. & Mursula, K. Heliospheric modulation of cosmic rays: monthly reconstruction for 1951–2004. J. Geophys. Res. 110, 12108 (2005).

  14. 14.

    Qin, M. et al. Solar cycle variations of trapped proton flux in the inner radiation belt. J. Geophys. Res. 119, 9658–9669 (2014).

  15. 15.

    Gombosi, T. I. et al. in Saturn from Cassini-Huygens 1st edn (eds. Dougherty, M. K. et al.) 203–255 (Springer Science+Business Media, Heidelberg, 2009).

  16. 16.

    Mauk, B. H. et al. in Saturn from Cassini-Huygens 1st edn (eds. Dougherty, M. K. et al.) 281–331 (Springer Science+Business Media,Heidelberg, 2009).

  17. 17.

    Southwood, D. J. & Kivelson, M. G. Magnetospheric interchange motions. J. Geophys. Res. 94, 299–308 (1989).

  18. 18.

    Paranicas, C. et al. Sources and losses of energetic protons in Saturn’s magnetosphere. Icarus 197, 519–525 (2008).

  19. 19.

    Hedman, M. M. et al. An observed correlation between plume activity and tidal stresses on Enceladus. Nature 500, 182–184 (2013).

  20. 20.

    Elrod, M. K., Tseng, W.-L., Woodson, A. K. & Johnson, R. E. Seasonal and radial trends in Saturn’s thermal plasma between the main rings and Enceladus. Icarus 242, 130–137 (2014).

  21. 21.

    Kollmann, P., Roussos, E., Paranicas, C., Krupp, N. & Haggerty, D. K. Processes forming and sustaining Saturn’s proton radiation belts. Icarus 222, 323–341 (2013).

  22. 22.

    Bunce, E. J. et al. Cassini observations of the variation of Saturn’s ring current parameters with system size. J. Geophys. Res. (Space Phys.) 112, A10202 (2007).

  23. 23.

    Sergis, N. et al. Radial and local time structure of the Saturnian ring current, revealed by Cassini. J. Geophys. Res. (Space Phys.) 122, 1803–1815 (2017).

  24. 24.

    Roussos, E. et al. Discovery of a transient radiation belt at Saturn. Geophys. Res. Lett. 35, 22106 (2008).

  25. 25.

    Roussos, E. et al. The variable extension of Saturn’s electron radiation belts. Planet. Space Sci. 104, 3–17 (2014).

  26. 26.

    Santos-Costa, D., Blanc, M., Maurice, S. & Bolton, S. J. Modeling the electron and proton radiation belts of Saturn. Geophys. Res. Lett. 30, 2059 (2003).

  27. 27.

    Meeks, Z., Simon, S. & Kabanovic, S. A comprehensive analysis of ion cyclotron waves in the equatorial magnetosphere of Saturn. Planet. Space Sci. 129,47–60 (2016).

  28. 28.

    Tsuchiya, F., Misawa, H., Imai, K. & Morioka, A. Short-term changes in Jupiter’s synchrotron radiation at 325 MHz: enhanced radial diffusion in Jupiter’s radiation belt driven by solar UV/EUV heating. J. Geophys. Res. 116, A09202 (2011).

  29. 29.

    Hill, T. W. Inertial limit on corotation. J. Geophys. Res. 84, 6554–6558 (1979).

  30. 30.

    Thomsen, M. F. et al. Saturn’s inner magnetospheric convection pattern: Further evidence. J. Geophys. Res. 117, 9208 (2012).

  31. 31.

    Brice, N. M. & McDonough, T. R. Jupiter’s radiation belts. Icarus 18, 206–219 (1973).

  32. 32.

    Krimigis, S. M. et al. Magnetosphere Imaging Instrument (MIMI) on the Cassini Mission to Saturn/Titan. Space Sci. Rev. 114,233–329 (2004).

  33. 33.

    Vandegriff, J. et al. Cassini/MIMI Instrument Data User Guide. NASA’s Planetary Data System (2013); http://pds-atmospheres.nmsu.edu/data_and_services/atmospheres_data/Cassini/mimi.html

  34. 34.

    Roederer, J. G. Dynamics of Geomagnetically Trapped Radiation (Springer, Heidelberg, 1970).

  35. 35.

    Dougherty, M. K. et al. The Cassini magnetic field investigation. Space Sci. Rev. 114, 331–383 (2004).

  36. 36.

    Burton, M. E., Dougherty, M. K. & Russell, C. T. Saturn’s internal planetary magnetic field. Geophys. Res. Lett. 37, 24105 (2010).

  37. 37.

    Kollmann, P. et al. Energetic particle phase space densities at Saturn: Cassini observations and interpretations. J. Geophys. Res. 116, A05222 (2011).

  38. 38.

    Vos, E. E. & Potgieter, M. S. Global gradients for cosmic-ray protons in the heliosphere during the solar minimum of cycle 23/24. Sol. Phys. 291, 2181–2195 (2016).

  39. 39.

    Kotova, A. Energetic Charged Particles Tracing Techniques and their Application in the Magnetosphere of Saturn. PhD thesis, Univ. Paul Sabatier Toulouse III (2016).

  40. 40.

    Woods, T. et al. TIMED Solar EUV experiment. Phys. Chem. Earth C 25, 393–396 (2000).

  41. 41.

    NIST Chemistry WebBook National Institute of Standards and Technology, 2015); http://webbook.nist.gov/chemistry.

  42. 42.

    Waite, J. H. et al. Electron precipitation and related aeronomy of the Jovian thermosphere and ionosphere. J. Geophys. Res. 88, 6143–6163 (1983).

  43. 43.

    Dragt, A. J., Austin, M. M. & White, R. S. Cosmic ray and solar proton albedo neutron decay injection. J. Geophys. Res. 71, 1293–1304 (1966).

  44. 44.

    Van Allen, J. A. In Saturn (eds Gerhels, T. & Matthews, M. S.) 281–317 (Univ. Arizona Press, Tucson, 1984).

  45. 45.

    Roussos, E. et al. Solar energetic particles (SEP) and galactic cosmic rays (GCR) as tracers of solar wind conditions near Saturn: event lists and applications. Icarus 300, 47–71 (2018).

Download references


The Johns Hopkins University Applied Physics Laboratory (JHU/APL) authors were partially supported by NASA Cassini Data Analysis grant NNX13AG05G (FG3TK) and by the NASA Office of Space Science under task order 003 of contract NAS5-97271 between NASA/GSFC and JHU. The Max Planck Institute authors were partially supported by the German Space Agency (DLR) under contract 50 OH 1502, the Max Planck Society and the Max Planck Institute for Solar System Research (MPS). The authors thank A. Lagg (MPS) for analysis software support, and J. Vandegriff (JHU/APL) and M. Kusterer (JHU/APL) for data reduction.

Author information

Author notes

  1. P. Kollmann and E. Roussos contributed equally to this work.


  1. Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD, 20723-6099, USA

    • P. Kollmann
    •  & C. Paranicas
  2. Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077, Göttingen, Germany

    • E. Roussos
    • , A. Kotova
    •  & N. Krupp
  3. L’Institut de Recherche en Astrophysique et Planétologie, 9, avenue du Colonel Roche,BP 44346, 31028, Toulouse Cedex 4, France

    • A. Kotova


  1. Search for P. Kollmann in:

  2. Search for E. Roussos in:

  3. Search for A. Kotova in:

  4. Search for C. Paranicas in:

  5. Search for N. Krupp in:


All authors contributed to the interpretation of the data and writing of the manuscript. P.K. and E.R. both performed the data analysis. E.R. developed the study concept. P.K. performed the modelling. A.K. performed the cosmic ray tracing. C.P. and N.K. administered the project on the US and German sides, respectively.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to P. Kollmann.

About this article

Publication history




Issue Date