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A brightening of Jupiter’s auroral 7.8-μm CH4 emission during a solar-wind compression

Abstract

Enhanced mid-infrared emission from CH4 and other stratospheric hydrocarbons has been observed coincident with Jupiter’s ultraviolet auroral emission1,2,3. This suggests that auroral processes and the neutral stratosphere of Jupiter are coupled; however, the exact nature of this coupling is unknown. Here we present a time series of Subaru-COMICS images of Jupiter measured at a wavelength of 7.80 μm on 11–14 January, 4–5 February and 17–20 May 2017. These data show that both the morphology and magnitude of the auroral CH4 emission vary on daily timescales in relation to external solar-wind conditions. The southern auroral CH4 emission increased in brightness temperature by about 3.8 K between 15:50 ut, 11 January and 12:57 ut, 12 January, during a predicted solar-wind compression. During the same compression, the northern auroral emission exhibited a duskside brightening, which mimics the morphology observed in the ultraviolet auroral emission during periods of enhanced solar-wind pressure4,5. These results suggest that changes in external solar-wind conditions perturb the Jovian magnetosphere in such a way that energetic particles are accelerated into the planet’s atmosphere, deposit their energy as deep as the neutral stratosphere, and modify the thermal structure, the abundance of CH4 or the population of energy states of CH4. We also find that the northern and southern auroral CH4 emission evolved independently between the January, February and May images, as has been observed at X-ray wavelengths over shorter timescales6 and at mid-infrared wavelengths over longer timescales7.

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Fig. 1: Southern polar projections of Jupiter’s 7.80-μm CH4 emission.
Fig. 2: Northern polar projections of Jupiter’s 7.80-μm CH4 emission.
Fig. 3: Polar projections and regions chosen for analysis.
Fig. 4: Auroral–quiescent residual over time.

Data availability

The COMICS images presented here are publicly available on the SMOKA (Subaru Mitaka Okayama-Kiso Archive) system (https://smoka.nao.ac.jp/). Reduced and calibrated images may be requested from J.A.S. The Data Reduction Manager is a suite of IDL software designed for reduction and processing of planetary images and is available in compressed format from G.S.O. on request (glenn.s.orton@jpl.nasa.gov). The ionosphere-to-magnetosphere mapping calculation is also written in IDL and is available from M.F.V. on request (mvogt@bu.edu). Results of the solar-wind propagation model in a specific time period may be requested from C.T. (chihiro.tao@nict.go.jp). The NEMESIS forward model and retrieval tool is written in Fortran and is available as a GitHub repository; a user account for this repository may be requested from P.G.J.I. (patrick.irwin@physics.ox.ac.uk).

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Acknowledgements

All data presented were obtained at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. COMICS observations obtained on 11, 12 January and 19, 20 May were proposed by and awarded to Y.K. using Subaru classical time. COMICS observations on 13, 14 January, 4, 5 February and 17, 18 May were proposed by and awarded to G.S.O. through the Keck-Subaru time exchange programme. We acknowledge the W. M. Keck Observatory, which is operated as a scientific partnership between California Institute of Technology, the University of California and NASA and supported financially by the W. M. Keck Foundation. We recognize and acknowledge the very important cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. The research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. We thank the NASA Postdoctoral and Caltech programmes for funding and supporting J.A.S. during this research. G.S.O. was supported by grants from NASA to the Jet Propulsion Laboratory/California Institute of Technology.

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J.A.S. led the analysis of the observations and the preparation of this Letter. G.S.O. and Y.K. were principal investigators of the awarded telescope time. J.A.S., G.S.O., Y.K., T.M.S. and T.F. participated in the measurements at the Subaru Telescope. J.F. performed the reduction and calibration of the images. C.T. and M.F.V. provided model output for the interpretation of the results. P.G.J.I. is the lead developer of the NEMESIS code. All remaining authors contributed to the interpretation of the results and the preparation of the Letter.

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Correspondence to J. A. Sinclair.

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Sinclair, J.A., Orton, G.S., Fernandes, J. et al. A brightening of Jupiter’s auroral 7.8-μm CH4 emission during a solar-wind compression. Nat Astron 3, 607–613 (2019). https://doi.org/10.1038/s41550-019-0743-x

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