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

Abstract

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.

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Fig. 1: Spatial structure of Saturn’s proton radiation belts.
Fig. 2: The time dependence of Saturn’s radiation belts differs from that of the Earth.
Fig. 3: Time variability of Saturn’s proton radiation belts is driven by radial diffusion.

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Acknowledgements

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.

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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.

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Correspondence to P. Kollmann.

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Kollmann, P., Roussos, E., Kotova, A. et al. The evolution of Saturn’s radiation belts modulated by changes in radial diffusion. Nat Astron 1, 872–877 (2017). https://doi.org/10.1038/s41550-017-0287-x

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