Discrete and broadband electron acceleration in Jupiter’s powerful aurora


The most intense auroral emissions from Earth’s polar regions, called discrete for their sharply defined spatial configurations, are generated by a process involving coherent acceleration of electrons by slowly evolving, powerful electric fields directed along the magnetic field lines that connect Earth’s space environment to its polar regions1,2. In contrast, Earth’s less intense auroras are generally caused by wave scattering of magnetically trapped populations of hot electrons (in the case of diffuse aurora) or by the turbulent or stochastic downward acceleration of electrons along magnetic field lines by waves during transitory periods (in the case of broadband or Alfvénic aurora)3,4. Jupiter’s relatively steady main aurora has a power density that is so much larger than Earth’s that it has been taken for granted that it must be generated primarily by the discrete auroral process5,6,7. However, preliminary in situ measurements of Jupiter’s auroral regions yielded no evidence of such a process8,9,10. Here we report observations of distinct, high-energy, downward, discrete electron acceleration in Jupiter’s auroral polar regions. We also infer upward magnetic-field-aligned electric potentials of up to 400 kiloelectronvolts, an order of magnitude larger than the largest potentials observed at Earth11. Despite the magnitude of these upward electric potentials and the expectations from observations at Earth, the downward energy flux from discrete acceleration is less at Jupiter than that caused by broadband or stochastic processes, with broadband and stochastic characteristics that are substantially different from those at Earth.

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Figure 1: Energetic electron data from JEDI during Juno’s first perijove (PJ1) encounter with Jupiter (27 August 2016).
Figure 2: Ultraviolet auroral images of Jupiter from the Juno Ultraviolet Spectrograph (UVS) instrument.
Figure 3: Energetic electron data during Juno’s fourth perijove (PJ4) encounter with Jupiter’s low-altitude polar regions (2 February 2017).
Figure 4: Energetic electron data during Juno’s third perijove (PJ3) encounter with Jupiter’s low-altitude polar regions (11 December 2016).


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We are grateful to NASA and contributing institutions that helped to make the Juno mission possible. This work was funded by NASA’s New Frontiers Program for Juno via subcontract with the Southwest Research Institute.

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B.H.M. is lead investigator for the Juno JEDI investigation, created Figs 1, 3 and 4 and analysed the data. D.K.H. operated the JEDI instruments during the Juno polar passes. C.P. coordinated planning of the instruments for the Juno polar passes. G.C. determined the JEDI efficiencies for quantifying the intensities of the particle fluxes. P.K. determined the out-of-band sensitivities of the JEDI instrument. A.M.R. developed the procedure for removing the minimum ionizing contamination of the electron spectra due to high-energy penetrators. S.J.B. is the principal investigator of the Juno mission. S.M.L. is a project scientist on the Juno mission. A.A. is lead investigator for the Juno Infra-Red Auroral Imager (JIRAM), helped to plan the Juno polar encounters and interpreted the auroral images. F.A. is a member of the Juno JADE plasmas sensor, helped to plan the Juno auroral encounters and interpreted the particle measurements. F.B. helped to plan the Juno polar encounters and performed the magnetic mapping of the Juno trajectory to the auroral atmosphere. B.B. is a member of the UVS imaging team and interpreted the Juno UVS auroral images. J.E.P.C. is lead investigator of the Juno magnetometer instrument and provided the magnetic field data necessary for ordering the particle data in Figs 1, 3 and 4. G.R.G. is lead investigator of the Juno UVS instrument and generated Fig. 2. W.S.K. is lead investigator of the Juno Waves investigation and helped to plan and interpret the observations of Jupiter’s polar regions. D.J.M. is the lead developer of the Juno JADE plasma instrument and helped to interpret the electron measurements. P.V. is the lead investigator for the Juno JADE instrument, and helped to plan the Juno polar encounters and interpret the electron measurements.

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Correspondence to B. H. Mauk.

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Reviewer Information Nature thanks J. Clarke, T. Cravens and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Energetic electron data during the PJ3 encounter with Jupiter’s polar regions (11 December 2016).

Same as Fig. 4b–d, but showing a greatly extended period of time that encompasses the entire northern polar pass. The horizontal brightening centred near 150 keV in b, particularly in the radiation belts, is the minimum ionizing feature discussed in Methods.

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Mauk, B., Haggerty, D., Paranicas, C. et al. Discrete and broadband electron acceleration in Jupiter’s powerful aurora. Nature 549, 66–69 (2017). https://doi.org/10.1038/nature23648

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