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.
This is a preview of subscription content
Subscribe to Nature+
Get immediate online access to the entire Nature family of 50+ journals
Subscribe to Journal
Get full journal access for 1 year
only $8.25 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Get time limited or full article access on ReadCube.
All prices are NET prices.
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.
Li, Z. et al. Investigation of EMIC wave scattering as the cause for the BARREL 17 January 2013 relativistic electron precipitation event: a quantitative comparison of simulation with observations. Geophys. Res. Lett. 41, 8722–8729 (2014).
Lorentzen, K. R. et al. Precipitation of relativistic electrons by interaction with electromagnetic ion cyclotron waves. J. Geophys. Res. 105, 5381–5390 (2000).
Millan, R. M., Lin, R. P., Smith, D. M., Lorentzen, K. R. & McCarthy, M. P. X-ray observations of MeV electron precipitation with a balloon-borne germanium spectrometer. Geophys. Res. Lett. 29, 2194 (2002).
Foat, J. E. et al. First detection of a terrestrial MeV X-ray burst. Geophys. Res. Lett. 25, 4109–4112 (1998).
Branduardi-Raymont, G. et al. A study of Jupiter’s aurorae with XMM-Newton. Astron. Astrophys. 463, 761–774 (2007).
Barbosa, D. D. Bremsstrahlung X rays from Jovian auroral electrons. J. Geophys. Res. 95, 14969–14976 (1990).
Waite, J. J. H., Boice, D. C., Hurley, K. C., Stern, S. A. & Sommer, M. Jovian Bremsstrahlung X rays: a Ulysses prediction. Geophys. Res. Lett. 19, 83–86 (1992).
Singhal, R. P., Chakravarty, S. C., Bhardwaj, A. & Prasad, B. Energetic electron precipitation in Jupiter’s upper atmosphere. J. Geophys. Res. 97, 18245–18256 (1992).
Wibisono, A. D. et al. Temporal and spectral studies by XMM-Newton of Jupiter’s X-ray auroras during a compression event. J. Geophys. Res. Space Phys. 125, e27676 (2020).
Mauk, B. H. et al. Discrete and broadband electron acceleration in Jupiter’s powerful aurora. Nature 549, 66–69 (2017).
Hurley, K., Sommer, M. & Waite, J. H. Upper limits to Jovian hard X radiation from the Ulysses gamma ray burst experiment. J. Geophys. Res. 98, 21217–21220 (1993).
Harrison, F. A. et al. The Nuclear Spectroscopic Telescope Array (NuSTAR) high-energy X-ray mission. Astrophys. J. 770, 103 (2013). 1301.7307.
Dunn, W. R. et al. The impact of an ICME on the Jovian X-ray aurora. J. Geophys. Res. Space Phys. 121, 2274–2307 (2016).
Jackman, C. M. et al. Assessing quasi-periodicities in Jovian X-ray emissions: techniques and heritage survey. J. Geophys. Res. Space Phys. 123, 9204–9221 (2018).
Cowley, S. W. H. & Bunce, E. J. Origin of the main auroral oval in Jupiter’s coupled magnetosphere-ionosphere system. Planet. Space Sci. 49, 1067–1088 (2001).
Dunn, W. R. et al. The independent pulsations of Jupiter’s northern and southern X-ray auroras. Nat. Astron. 1, 758–764 (2017).
Kotsiaros, S. et al. Birkeland currents in Jupiter’s magnetosphere observed by the polar-orbiting Juno spacecraft. Nat. Astron. 3, 904–909 (2019).
Clark, G. et al. Energetic particle signatures of magnetic field-aligned potentials over Jupiter’s polar regions. Geophys. Res. Lett. 44, 8703–8711 (2017).
Allegrini, F. et al. Electron beams and loss cones in the auroral regions of Jupiter. Geophys. Res. Lett 44, 7131–7139 (2017).
Connerney, J. E. P. et al. A new model of Jupiter’s magnetic field from Juno’s first nine orbits. Geophys. Res. Lett. 45, 2590–2596 (2018).
Clark, G. et al. Precipitating electron energy flux and characteristic energies in Jupiter’s main auroral region as measured by Juno/JEDI. J. Geophys. Res. Space Phys. 123, 7554–7567 (2018).
Agostinelli, S. et al. GEANT4—a simulation toolkit. Nucl. Instrum. Methods Phys. Res. Sect. A 506, 250–303 (2003).
Atreya, S., Mahaffy, P., Niemann, H., Wong, M. & Owen, T. Composition and origin of the atmosphere of Jupiter—an update, and implications for the extrasolar giant planets. Planet. Space Sci. 51, 105–112 (2003).
Atreya, S. K., Donahue, T. M. & Festou, M. Jupiter: structure and composition of the upper atmosphere. Astrophys. J. Lett. 247, L43–L47 (1981).
Woodger, L. A. et al. A summary of the BARREL campaigns: technique for studying electron precipitation. J. Geophys. Res. Space Phys. 120, 4922–4935 (2015).
Saur, J. et al. Anti-planetward auroral electron beams at Saturn. Nature 439, 699–702 (2006).
Giorgini, J.D. et al. JPL’s On-Line Solar System Data Service. AAS 28, 1158 (1996).
Abazajian, K. N. et al. The seventh data release of the Sloan Digital Sky Survey. Astrophys. J. Suppl. Ser. 182, 543–558 (2009).
Wik, D. R. et al. NuSTAR observations of the Bullet Cluster: constraints on inverse Compton emission. Astrophys. J. 792, 48 (2014).
Scargle, J. D., Norris, J. P., Jackson, B. & Chiang, J. Studies in astronomical time series analysis. VI. Bayesian block representations. Astrophys. J. 764, 167 (2013).
McComas, D. et al. The Jovian Auroral Distributions Experiment (JADE) on the Juno mission to Jupiter. Space Sci. Rev. 213, 547–643 (2017).
Mauk, B. et al. The Jupiter Energetic Particle Detector Instrument (JEDI) investigation for the Juno mission. Space Sci. Rev. 98, 98– (2013).
Allegrini, F., Wilson, R., Ebert, R. & Loeffler, C. JUNO JADE Calibrated Science Data (NASA Planetary Data System, 2019); https://pds-ppi.igpp.ucla.edu/data/JNO-J_SW-JAD-3-CALIBRATED-V1.0/
JUNO JEDI Jupiter Standard Calibrated Products (NASA Planetary Data System, 2020); https://pds-ppi.igpp.ucla.edu/data/JNO-J-JED-3-CDR-V1.0/
Connerney, J. JUNO Magnetometer Jupiter Archive (NASA Planetary Data System, 2020); https://pds-ppi.igpp.ucla.edu/data/JNO-J-3-FGM-CAL-V1.0/
Acton, C., Bachman, N., Semenov, B. & Wright, E. A look towards the future in the handling of space science mission geometry. Planet. Space Sci. 150, 9–12 (2018).
Allegrini, F. et al. Energy flux and characteristic energy of electrons over Jupiter’s main auroral emission. J. Geophys. Res. Space Phys 125, e2019JA027693 (2020).
Bonfond, B. et al. Auroral evidence of Io’s control over the magnetosphere of Jupiter. Geophys. Res. Lett 39, L01105 (2012).
Allison, J. et al. Geant4 developments and applications. IEEE Trans. Nucl. Sci. 53, 270–278 (2006).
Pandola, L., Andenna, C. & Caccia, B. Validation of the GEANT4 simulation of bremsstrahlung from thick targets below 3 MeV. Nucl. Instrum. Methods Phys. Res B 350, 41–48 (2015).
Livengood, T. A., Strobel, D. F. & Moos, H. W. Long-term study of longitudinal dependence in primary particle precipitation in the north Jovian aurora. J. Geophys. Res. Space Phys 95, 10375–10388 (1990).
Valek, P. W. et al. Jovian high-latitude ionospheric ions: Juno in situ observations. Geophys. Res. Lett 46, 8663–8670 (2019).
Ozak, N., Schultz, D. R., Cravens, T. E., Kharchenko, V. & Hui, Y. W. Auroral X-ray emission at Jupiter: depth effects. J. Geophys. Res. Space Phys. 115, A11306 (2010).
Branduardi-Raymont, G. et al. Spectral morphology of the X-ray emission from Jupiter’s aurorae. J. Geophys. Res. Space Phys. 113, A02202 (2008).
Paranicas, C. et al. Intervals of intense energetic electron beams over Jupiter’s poles. J. Geophys. Res. Space Phys. 123, 1989–1999 (2018).
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.
Peer review information
Nature Astronomy thanks Tomoki Kimura and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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.
About this article
Cite this article
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