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Far-ultraviolet aurora identified at comet 67P/Churyumov-Gerasimenko

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An Author Correction to this article was published on 04 May 2021

Abstract

Having a nucleus darker than charcoal, comets are usually detected from Earth through the emissions from their coma. The coma is an envelope of gas that forms through the sublimation of ices from the nucleus as the comet gets closer to the Sun. In the far-ultraviolet portion of the spectrum, observations of comae have revealed the presence of atomic hydrogen and oxygen emissions. When observed over large spatial scales as seen from Earth, such emissions are dominated by resonance fluorescence pumped by solar radiation. Here, we analyse atomic emissions acquired close to the cometary nucleus by the Rosetta spacecraft and reveal their auroral nature. To identify their origin, we undertake a quantitative multi-instrument analysis of these emissions by combining coincident neutral gas, electron and far-ultraviolet observations. We establish that the atomic emissions detected from Rosetta around comet 67P/Churyumov-Gerasimenko at large heliocentric distances result from the dissociative excitation of cometary molecules by accelerated solar-wind electrons (and not by electrons produced from photo-ionization of cometary molecules). Like the discrete aurorae at Earth and Mars, this cometary aurora is driven by the interaction of the solar wind with the local environment. We also highlight how the oxygen line O i at wavelength 1,356 Å could be used as a tracer of solar-wind electron variability.

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• The Plasma Environment of Comet 67P/Churyumov-Gerasimenko

Space Science Reviews Open Access 10 November 2022

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Data availability

The Rosetta data that support the plots within this paper and other findings of this study are available from the ESA–PSA archive (https://www.cosmos.esa.int/web/psa/rosetta) or the NASA PDS archive (https://pdssbn.astro.umd.edu/data_sb/missions/rosetta/index.shtml) Source data are provided with this paper.

Code availability

iPIC3D is publicly available on GitHub (https://github.com/iPIC3D/iPIC3D; Apache License 2.0).

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Acknowledgements

Rosetta is a European Space Agency (ESA) mission with contributions from its member states and the National Aeronautics and Space Administration (NASA). We acknowledge the continuous support of the Rosetta teams at the European Space Operations Centre in Darmstadt and at the European Space Astronomy Centre. We acknowledge the staff of Centre de Données de la Physique des Plasmas (CDPP) and Imperial College for the use of Automated Multi-Dataset Analysis (AMDA) and the RPC Quicklook database. This work has benefited from discussions within International Team 402: Plasma Environment of Comet 67P after Rosetta at the International Space Science Institute (ISSI) (Bern, Switzerland). We thank N. Fougere for his help and advice using the ICES models. We are grateful to M. Taylor for his constructive feedback. Work at Imperial College London was supported by the STFC of the UK under grants ST/N000692/1 and ST/S505432/1, by Imperial College London through a President’s Scholarship, and by ESA under contract number 4000119035/16/ES/JD. The Alice team acknowledges support from NASA’s Jet Propulsion Laboratory through contract 1336850 to the Southwest Research Institute. M.R. acknowledges the support of the State of Bern and the Swiss National Science Foundation (200021_165869, 200020_182418). J.D. acknowledges support from NASA’s Rosetta Data Analysis Program, grant number 80NSSC19K1305, NASA’s Solar System Exploration Research Virtual Institute (SSERVI): Institute for Modeling Plasmas, Atmosphere, and Cosmic Dust (IMPACT), and the computational resources provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center. We acknowledge PRACE for awarding us access to Curie at GENCI@CEA, France. Work at LPC2E/CNRS was supported by CNES and by ANR under the financial agreement ANR-15-CE31-0009-01. VIRTIS was built by a consortium, which includes Italy, France and Germany, under the scientific responsibility of the Istituto di Astrofisica e Planetologia Spaziali of INAF, Italy, which also guides the scientific operations. The VIRTIS instrument development, led by the prime contractor Leonardo-Finmeccanica (Florence, Italy), has been funded and managed by ASI, with contributions from Observatoire de Meudon financed by CNES, and from DLR. The VIRTIS calibrated data will be available through the ESA’s Planetary Science Archive (PSA) website (http://www.rssd.esa.int) and is available upon request until posted to the archive. We thank the following institutions and agencies for support of this work: Italian Space Agency (ASI, Italy) contract number I/024/12/1, Centre National d’Etudes Spatiales (CNES, France), DLR (Germany), NASA (USA) Rosetta Program, and Science and Technology Facilities Council (UK). All ROSINA data are the work of the international ROSINA team (scientists, engineers and technicians from Switzerland, France, Germany, Belgium and the USA) over the past 25 years, which we gratefully acknowledge. We acknowledge the contributions of the entire MIRO team in enabling collection of some of the data used in this study.

Author information

Authors

Contributions

M.G. led the study, performed the multi-instrument analysis, generated Figs. 2 and 3, and wrote the manuscript. P.D.F. identified times of interest for Alice, analysed the FUV dataset, advised on the different emission source mechanisms, and estimated the interplanetary medium contribution. D.B.-M. and Y.-C.C. analysed the VIRTIS-H dataset. N.B. analysed the MIRO dataset. G.R. analysed the VIRTIS-M dataset. M.R. and K.A. (the principal investigator of the ROSINA instrument) provided the ROSINA dataset. They all provided guidance on the interpretation of their respective dataset. J.D. generated Fig. 4 on the basis of the output of a particle-in-cell (PiC) simulation. J.D. and P.H. provided guidance on the PiC simulation interpretation. A.B., P.S. and K.L.H. provided feedback on the multi-instrument analysis. A.B. generated Fig. 1. J.Wm.P. (the principal investigator of the Alice instrument) contributed to the interpretation of the Alice dataset. C.C., A.I.E. and J.B. (all principal investigators of RPC) provided guidance on the interpretation of the RPC dataset. A.I.E. provided the RPC-LAP dataset. All authors contributed to the interpretation of the results and commented on this manuscript.

Corresponding author

Correspondence to M. Galand.

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Extended data

Extended Data Fig. 1 Differential electron flux as a function of energy.

Examples of differential electron fluxes measured by the RPC–IES electron spectrometer and used in the multi-instrument analysis. The fluxes were observed at 11:47 UT on 29 March 2015 (orange crosses) and at 08:35 UT on 26 December 2015 (red plus symbols) during two nadir-viewing FUV observation periods. The fluxes are corrected for the spacecraft potential (Galand et al.30; −2 V). By integration, the number density and mean energy of electrons with energies between 10 eV and 200 eV are derived and given in Extended Data Fig. 2.

Extended Data Fig. 2 Examples of Rosetta simultaneous measurements.

This dataset has been used for calculating the FUV atomic emission brightnesses at two times during FUV nadir-viewing observation periods (fourth and fifth cases in Figure 2): (1) the differential electron flux Je (see Extended Data Fig.1) measured by the RPC-IES electron spectrometer at the selected day and start time tIES (first and second columns), at a cometocentric distance rR (third column), and associated with a number density $${n}_{e}^{{\mathrm{IES}}}$$ (fourth column) and mean energy $${E}_{e}^{{\mathrm{IES}}}$$ (fifth column) of electrons with energies between 10 eV and 200 eV; (2) the total neutral density $${n}_{{\mathrm{tot}}}^{{\rm{COPS}}}$$ measured by the ROSINA-COPS pressure gauge (sixth column) from which the column density CCOPS is derived (seventh column); (3) the neutral composition measured by the ROSINA-DFMS neutral mass spectrometer and given in terms of volume mixing ratio υn of the four major neutral species (eighth column). a The number density $${n}_{e}^{{\mathrm{IES}}}$$ and mean energy $${E}_{e}^{{\mathrm{IES}}}$$ of electrons with energies between 10 eV and 200 eV are derived by integrating the differential electron flux Je (corrected for the spacecraft potential) over the velocity space. These quantities are given for information; only Je, not its moments, is used in the calculation of the modelled FUV brightnesses. b The total column density is derived from the total neutral density ntot assuming a mean cometocentric distance for the nucleus' surface of 1.7 km50 (see Eq. 4). c The volume mixing ratio for the four major neutral species is obtained from the ROSINA/DFMS mass spectrometer (other species are neglected).

Source data

Source Data Fig. 2

Excel file: Observed and modelled brightnesses for the seven nadir-viewing cases presented for H Lyβ, O i 1,304 Å, and O i 1,356 Å (in Rayleigh).

Source Data Fig. 3

Excel file: Observed and modelled H Lyβ brightnesses for the two sets presented for limb viewing (in Rayleigh).

Source Data Fig. 4

ASCII file (VTK format): Includes glow (shown in violet), electron trajectory (lines colour-coded by energy) and ambipolar electric field (- Eambi green arrows).

Source Data Extended Data Fig. 1

Excel file: Two sets of differential electron fluxes as a function of energy, derived from RPC-IES after correction for the spacecraft potential.

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Galand, M., Feldman, P.D., Bockelée-Morvan, D. et al. Far-ultraviolet aurora identified at comet 67P/Churyumov-Gerasimenko. Nat Astron 4, 1084–1091 (2020). https://doi.org/10.1038/s41550-020-1171-7

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• DOI: https://doi.org/10.1038/s41550-020-1171-7

• The Plasma Environment of Comet 67P/Churyumov-Gerasimenko

• Charlotte Goetz
• Etienne Behar
• Martin Volwerk

Space Science Reviews (2022)