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Rotationally driven magnetic reconnection in Saturn’s dayside

Abstract

Magnetic reconnection is a key process that explosively accelerates charged particles, generating phenomena such as nebular flares1, solar flares2 and stunning aurorae3. In planetary magnetospheres, magnetic reconnection has often been identified on the dayside magnetopause and in the nightside magnetodisc, where thin-current-sheet conditions are conducive to reconnection4. The dayside magnetodisc is usually considered thicker than the nightside due to the compression of solar wind, and is therefore not an ideal environment for reconnection. In contrast, a recent statistical study of magnetic flux circulation strongly suggests that magnetic reconnection must occur throughout Saturn’s dayside magnetosphere5. Additionally, the source of energetic plasma can be present in the noon sector of giant planetary magnetospheres6. However, so far, dayside magnetic reconnection has only been identified at the magnetopause. Here, we report direct evidence of near-noon reconnection within Saturn’s magnetodisc using measurements from the Cassini spacecraft. The measured energetic electrons and ions (ranging from tens to hundreds of keV) and the estimated energy flux of ~2.6 mW m2 within the reconnection region are sufficient to power aurorae. We suggest that dayside magnetodisc reconnection can explain bursty phenomena in the dayside magnetospheres of giant planets, which can potentially advance our understanding of quasi-periodic injections of relativistic electrons6 and auroral pulsations7.

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Fig. 1: Overview of the magnetopause and dayside magnetosphere crossing near the magnetic reconnection event on 30 September 2008.
Fig. 2: Magnetic and plasma measurements in the magnetic reconnection region on 30 September 2008.
Fig. 3: Trajectory of Cassini relative to the geometry of the reconnection diffusion region and detected electron pitch-angle distribution at different regions.

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Acknowledgements

This work was supported by the National Science Foundation of China (41525016, 41474155, 41704169, 41274167 and 41621063). Z.H.Y. is a Marie Curie COFUND research fellow, cofunded by the EU. Cassini operations are supported by NASA (managed by the Jet Propulsion Laboratory) and European Space Agency (ESA). R.L.G. is supported by the opening fund of the Lunar and Planetary Science Laboratory (a partner laboratory of the Key Laboratory of Lunar and Deep Space Exploration) (Macau FDCT grant 039/2013/A2). I.J.R. is supported in part by Science and Technology Facilities Council (STFC) grant ST/N000722/1. Z.H.Y., B.P. and D.G. are supported by the PRODEX programme managed by ESA in collaboration with the Belgian Federal Science Policy Office. W.R.D. is supported by an STFC research grant to University College London, an SAO fellowship to the Harvard–Smithsonian Centre for Astrophysics and ESA contract 4000120752/17/NL/MH. A.J.C. is supported by STFC Consolidated Grants to UCL-MSSL (ST/K000977/1 and ST/N000722/1).

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All authors were involved in writing the paper. R.L.G., Z.H.Y. and Y.W. led the work and conducted most of the analysis for the Cassini measurements. L.C.R. provided knowledge of planetary magnetospheric dynamics and critical review of the techniques applied, along with extensive paper writing and data analysis. I.J.R., C.S.A., P.A.D., Z.Y.P. and J.L.B. provided expertise on auroral drivers and magnetospheric processes. N.S. and P.K. provided crucial support in using ion data, as well as insight on magnetospheric dynamics. A.J.C., D.G., W.R.D., J.H.W., B.P. and M.K.D. provided detailed knowledge of planetary magnetospheres.

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Correspondence to Z. H. Yao.

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Guo, R.L., Yao, Z.H., Wei, Y. et al. Rotationally driven magnetic reconnection in Saturn’s dayside. Nat Astron 2, 640–645 (2018). https://doi.org/10.1038/s41550-018-0461-9

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