Electrons accelerated on Earth by a rich variety of wave-scattering or stochastic processes1,2 generate hard, non-thermal X-ray bremsstrahlung up to ~1 MeV (refs. 3,4) and power Earth’s various types of aurorae. Although Jupiter’s magnetic field is an order of magnitude larger than Earth’s, space-based telescopes have previously detected X-rays only up to ~7 keV (ref. 5). On the basis of theoretical models of the Jovian auroral X-ray production6,7,8, X-ray emission in the ~2–7 keV band has been interpreted as thermal (arising from electrons characterized by a Maxwell–Boltzmann distribution) bremsstrahlung5,9. Here we report the observation of hard X-rays in the 8–20 keV band from the Jovian aurorae, obtained with the NuSTAR X-ray observatory. The X-rays fit to a flat power-law model with slope of 0.60 ± 0.22—a spectral signature of non-thermal, hard X-ray bremsstrahlung. We determine the electron flux and spectral shape in the kiloelectronvolt to megaelectronvolt energy range using coeval in situ measurements taken by the Juno spacecraft’s JADE and JEDI instruments. Jovian electron spectra of the form we observe have previously been interpreted as arising in stochastic acceleration, rather than coherent acceleration by electric fields10. We reproduce the X-ray spectral shape and approximate flux observed by NuSTAR, and explain the non-detection of hard X-rays by Ulysses11, by simulating the non-thermal population of electrons undergoing precipitating electron energy loss, secondary electron generation and bremsstrahlung emission in a model Jovian atmosphere. The results highlight the similarities between the processes generating hard X-ray aurorae on Earth and Jupiter, which may be occurring on Saturn, too.
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The NuSTAR and XMM-Newton data are archived at NASA’s HEASARC website (https://heasarc.gsfc.nasa.gov). The JEDI, JADE and MAG data are available at the Planetary Data System(https://pds-ppi.igpp.ucla.edu). The magnetic footprint of JUNO is available through the LASP MOP group’s website (https://lasp.colorado.edu/home/mop/missions/juno/trajectory-information/).
NuSTAR and XMM-Newton data were analysed by HEASOFT version 6.28 and SAS analysis software version 16.1.0, respectively. GEANT4 is publicly available at https://geant4.web.cern.ch. The simulation and data reduction code is available from the corresponding author upon request.
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We thank R. Wilson for his help in analysing JADE data. We also thank the MOP group of LASP for their work on Juno’s trajectory and magnetic footprint. We acknowledge D. Wik for his help in analysing NuSTAR background data. Support for this work by K.M. was provided by NASA through NuSTAR Cycle 3 Guest Observer Program grant number NNH16ZDA001N. Support for this work by C.J. at DIAS was supported by the Science Foundation Ireland grant number 18/FRL/6199.
The authors declare no competing interests.
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Counts of escaping X-rays as a function of the altitude [km] (which is measured from 1 Rj). The counts drop sharply below h ~ 200 [km] as X-rays are heavily absorbed. The altitude range shown in the plot cover Region 1 and 2 described in the Method. Note that most of escaping X-rays are from Region 1 (h < 430 km).
Magnetic footprint of Juno as viewed from above the north (left) and south (right) poles in PJ12. Black ovals represent the UV ovals39. The color bar indicates the net electron counts obtained by JADE across all energy channels and look directions (in logarithmic scale). A region in the Juno orbit (indicated by black), where background counts exceeded signal counts, was removed from our analysis. We obtained coordinates for the auroral ovals and JRM09 magnetic footprint20 from the MOP LASP website. All coordinates are SYS III.
NuSTAR observations of Jupiter. aNet count rates in the 3–20 keV band. Both FPMA and FPMB counts are combined. Source and background counts were extracted from an r = 45” circle and an r = 60–75” annular region around the Jovian center, respectively.
Extended Data Table 2 Juno orbits coincident with NuSTAR observations and electron spectral parameters.
Juno orbits coincident with NuSTAR observations and electron spectral parameters. Note: αe is the best-fit spectral index of JADE + JEDI electron spectra in each Juno passage. The last column lists the downward electron flux in the 3keV – 1 MeV band.
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Mori, K., Hailey, C., Bridges, G. et al. Observation and origin of non-thermal hard X-rays from Jupiter. Nat Astron 6, 442–448 (2022). https://doi.org/10.1038/s41550-021-01594-8