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Rotationally driven magnetic reconnection in Saturn’s dayside


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.


  1. 1.

    Clausen-Brown, E. & Lyutikov, M. Crab nebula gamma-ray flares as relativistic reconnection minijets. Mon. Not. R. Astron. Soc. 426, 1374–1384 (2012).

    Article  ADS  Google Scholar 

  2. 2.

    Parker, E. N. The solar-flare phenomenon and the theory of reconnection and annihilation of magnetic fields. Astrophys. J. Suppl. Ser. 8, 177 (1963).

    Article  ADS  Google Scholar 

  3. 3.

    Dungey, J. W. Interplanetary magnetic field and the auroral zones. Phys. Rev. Lett. 6, 47 (1961).

    Article  ADS  Google Scholar 

  4. 4.

    Paschmann, G. et al. Plasma acceleration at the Earth’s magnetopause: evidence for reconnection. Nature 282, 243–246 (1979).

    Article  ADS  Google Scholar 

  5. 5.

    Delamere, P., Otto, A., Ma, X., Bagenal, F. & Wilson, R. Magnetic flux circulation in the rotationally driven giant magnetospheres. J. Geophys. Res. Space Phys. 120, 4229–4245 (2015).

    Article  ADS  Google Scholar 

  6. 6.

    Roussos, E. et al. Quasi-periodic injections of relativistic electrons in Saturn’s outer magnetosphere. Icarus 263, 101–116 (2016).

    Article  ADS  Google Scholar 

  7. 7.

    Mitchell, D. et al. Recurrent pulsations in Saturn’s high latitude magnetosphere. Icarus 263, 94–100 (2016).

    Article  ADS  Google Scholar 

  8. 8.

    Vasyliunas, V. Plasma distribution and flow. Phys. Jovian Magnetos. 1, 395–453 (1983).

    Article  ADS  Google Scholar 

  9. 9.

    Burkholder, B. et al. Local time asymmetry of Saturn’s magnetosheath flows.Geophys. Res. Lett. 44, 5877–5883 2017).

    Article  ADS  Google Scholar 

  10. 10.

    Yao, Z. et al. Corotating magnetic reconnection site in Saturn’s magnetosphere.Astrophys. J. Lett. 846, L25 (2017).

    Article  ADS  Google Scholar 

  11. 11.

    Angelopoulos, V. et al. Tail reconnection triggering substorm onset. Science 321, 931–935 (2008).

    Article  ADS  Google Scholar 

  12. 12.

    Arridge, C. S. et al. Cassini in situ observations of long-duration magnetic reconnection in Saturn’s magnetotail.Nat. Phys. 12, 268–271 2016).

    Article  Google Scholar 

  13. 13.

    Kronberg, E., Kasahara, S., Krupp, N. & Woch, J. Field-aligned beams and reconnection in the jovian magnetotail. Icarus 217, 55–65 (2012).

    Article  ADS  Google Scholar 

  14. 14.

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

    Article  ADS  Google Scholar 

  15. 15.

    Young, D. et al. Cassini plasma spectrometer investigation. Space Sci. Rev. 114, 1–112 (2004).

    Article  ADS  Google Scholar 

  16. 16.

    Nagai, T. et al. Geotail observations of the Hall current system: evidence of magnetic reconnection in the magnetotail. J. Geophys. Res. Space Phys. 106, 25929–25949 (2001).

    Article  ADS  Google Scholar 

  17. 17.

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

    Article  ADS  Google Scholar 

  18. 18.

    Lindstedt, T. et al. Separatrix regions of magnetic reconnection at the magnetopause.Ann. Geophys. 27, 4039–4056 (2009).

    Article  ADS  Google Scholar 

  19. 19.

    Mauk, B. et al. Transient aurora on Jupiter from injections of magnetospheric electrons. Nature 415, 1003 (2002).

    Article  ADS  Google Scholar 

  20. 20.

    Wang, S. et al. Electron heating in the exhaust of magnetic reconnection with negligible guide field. J. Geophys. Res. Space Phys. 121, 2104–2130 (2016).

    Article  ADS  Google Scholar 

  21. 21.

    Birn, J. et al. Geospace environmental modeling (GEM) magnetic reconnection challenge. J. Geophys. Res. Space Phys. 106, 3715–3719 (2001).

    Article  ADS  Google Scholar 

  22. 22.

    Palmaerts, B. et al. Statistical analysis and multi-instrument overview of the quasi-periodic 1-hour pulsations in Saturn’s outer magnetosphere. Icarus 271, 1–18 (2016).

    Article  ADS  Google Scholar 

  23. 23.

    Radioti, A. et al. Auroral signatures of multiple magnetopause reconnection at Saturn. Geophys. Res. Lett. 40, 4498–4502 (2013).

    Article  ADS  Google Scholar 

  24. 24.

    Gérard, J. C. et al. Altitude of Saturn’s aurora and its implications for the characteristic energy of precipitated electrons. Geophys. Res. Lett. 36, L02202 (2009).

    Article  ADS  Google Scholar 

  25. 25.

    Grodent, D., Gérard, J. C., Clarke, J., Gladstone, G. & Waite, J.A possible auroral signature of a magnetotail reconnection process on Jupiter.J. Geophys. Res. Space Phys. 109, A05201 (2004).

    Article  ADS  Google Scholar 

  26. 26.

    Badman, S. V. et al. Bursty magnetic reconnection at Saturn’s magnetopause. Geophys. Res. Lett. 40, 1027–1031 (2013).

    Article  ADS  Google Scholar 

  27. 27.

    Gladstone, G. et al. A pulsating auroral X-ray hot spot on Jupiter. Nature 415, 1000–1003 (2002).

    Article  ADS  Google Scholar 

  28. 28.

    Fu, H. S., Khotyaintsev, Y. V., Vaivads, A., Retino, A. & Andre, M. Energetic electron acceleration by unsteady magnetic reconnection. Nat. Phys. 9, 426–430 (2013).

    Article  Google Scholar 

  29. 29.

    Kanani, S. J. et al. A new form of Saturn’s magnetopause using a dynamic pressure balance model, based on in situ, multi-instrument Cassini measurements. J. Geophys. Res. Space Phys. 115, A06207 (2010).

    Article  ADS  Google Scholar 

  30. 30.

    Tao, C., Kataoka, R., Fukunishi, H., Takahashi, Y. & Yokoyama, T.Magnetic field variations in the Jovian magnetotail induced by solar wind dynamic pressure enhancements.J. Geophys. Res. 110, A11208 (2005).

    Article  ADS  Google Scholar 

  31. 31.

    Arridge, C. et al. Saturn’s magnetodisc current sheet. J. Geophys. Res. Space Phys. 113, A04214(2008).

    ADS  Google Scholar 

  32. 32.

    Arridge, C. S. et al. Periodic motion of Saturn’s nightside plasma sheet.J. Geophys. Res. Space Phys. 116, A11205 (2011).

    Article  ADS  Google Scholar 

  33. 33.

    Sergeev, V. et al. Current sheet flapping motion and structure observed by Cluster. Geophys. Res. Lett. 30, 1327–1324 (2003).

    ADS  Google Scholar 

  34. 34.

    Runov, A. et al. Electric current and magnetic field geometry in flapping magnetotail current sheets.Ann. Geophys. 23, 1391–1403 (2005).

    Article  ADS  Google Scholar 

  35. 35.

    Delamere, P. A., Wilson, R. J. & Masters, A.Kelvin–Helmholtz instability at Saturn’s magnetopause: hybrid simulations.J. Geophys. Res. Space Phys. 116, A10222 (2011).

    Article  ADS  Google Scholar 

  36. 36.

    Masters, A. et al. Cassini observations of a Kelvin–Helmholtz vortex in Saturn’s outer magnetosphere.J. Geophys. Res. Space Phys. 115, A07225 (2010).

    Article  ADS  Google Scholar 

  37. 37.

    Sweet, P. A. The production of high energy particles in solar flares. Nuovo Cimento 8, 188–196 (1958).

    Article  Google Scholar 

  38. 38.

    Parker, E. N. Sweet’s mechanism for merging magnetic fields in conducting fluids. J. Geophys. Res. 62, 509–520 (1957).

    Article  ADS  Google Scholar 

  39. 39.

    Thomsen, M. F. et al. Survey of ion plasma parameters in Saturn’s magnetosphere.J. Geophys. Res. Space Phys. 115, A10220 (2010).

    Article  ADS  Google Scholar 

  40. 40.

    Korovinskiy, D. B., Semenov, V. S., Erkaev, N. V., Divin, A. V. & Biernat, H. K. The 2.5-D analytical model of steady-state Hall magnetic reconnection. J. Geophys. Res. Space Phys. 113, A04205 (2008).

    Article  ADS  Google Scholar 

  41. 41.

    Nichols, J. D. et al. Saturn’s equinoctial auroras.Geophys. Res. Lett. 36, L24102 (2009).

    Article  ADS  Google Scholar 

  42. 42.

    Yao, Z. H. et al. Mechanisms of Saturn’s near-noon transient aurora: in situ evidence from Cassini measurements.Geophys. Res. Lett. 44, 217–228 (2017).

    Google Scholar 

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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).

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