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A dynamic, rotating ring current around Saturn


The concept of an electrical current encircling the Earth at high altitudes was first proposed in 1917 to explain the depression of the horizontal component of the Earth’s magnetic field during geomagnetic storms1,2,3,4. In situ measurements of the extent and composition of this current were made some 50 years later5 and an image was obtained in 2001 (ref. 6). Ring currents of a different nature were observed at Jupiter7,8 and their presence inferred at Saturn9,10. Here we report images of the ring current at Saturn, together with a day–night pressure asymmetry and tilt of the planet’s plasma sheet, based on measurements using the magnetospheric imaging instrument (MIMI) on board Cassini. The ring current can be highly variable with strong longitudinal asymmetries that corotate nearly rigidly with the planet. This contrasts with the Earth’s ring current, where there is no rotational modulation and initial asymmetries are organized by local time effects.

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Figure 1: ENA image of the ring current as viewed from above the northern hemisphere.
Figure 2: Typical Cassini pass through the day–night plasma sheet in early 2007.
Figure 3: Observed particle pressure profile (colour scale) for all non-equatorial Cassini orbits.
Figure 4: An artist’s concept of Saturn’s plasma sheet and embedded ring current, consistent with the data shown in Fig. 3 .
Figure 5: Sequence of six ENA images in neutral hydrogen taken by INCA.


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We thank M. Kusterer (The Johns Hopkins University Applied Physics Laboratory) for assistance with the data reduction. We are grateful to colleagues on the MIMI team who provided comments that have improved the presentation. Work at The Johns Hopkins University Applied Physics Laboratory was supported by NASA and by subcontracts at the University of Maryland and the Office of Space Research and Technology of the Academy of Athens. The German contribution of MIMI/LEMMS was financed in part by the Bundesministerium für Bildung und Forschung (BMBF) through the Deutsches Zentrum für Luft-und Raumfahrt e.V. (DLR) and by the Max-Planck-Gesellschaft.

Author Contributions S.M.K. is the MIMI Principal Investigator and contributed most of the text; N.S. analysed the in situ pressure data; D.G.M. is the INCA lead investigator and analysed the ENA images; D.C.H. is the lead investigator of CHEMS, while N.K. oversees the LEMMS data analyses.

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Correspondence to S. M. Krimigis.

Supplementary information

Supplementary Information

The file contains Supplementary Figure 1 with Legend and Legend to Supplementary Video 1. The Supplementary Figure shows the three-dimensional distribution of the increased energetic particle pressure as measured during the first three years of Cassini in orbit around Saturn (July 2004 to July 2007). (PDF 2041 kb)

Supplementary Video 1

The Supplementary Video 1 shows consecutive INCA images obtained March 16, 2004 UTC through March 18, 0404 UTC (days 75–77, 2007) as mentioned in Figure 5. (MOV 3955 kb)

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Krimigis, S., Sergis, N., Mitchell, D. et al. A dynamic, rotating ring current around Saturn. Nature 450, 1050–1053 (2007).

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