The oxygen emission at 557.7 nm is a ubiquitous component of the spectrum of the terrestrial polar aurora and the reason for its usual green colour1. It is also observed as a thin layer of glow surrounding the Earth near 90 km altitude in the dayside atmosphere2,3 but it has so far eluded detection in other planets. Here we report dayglow observations of the green line outside the Earth. They have been performed with the Nadir and Occultation for Mars Discovery ultraviolet and visible spectrometer instrument on board the European Space Agency’s ExoMars Trace Gas Orbiter. Using a special observation mode, scans of the dayside limb provide the altitude distribution of the intensity of the 557.7 nm line and its variability. Two intensity peaks are observed near 80 and 120 km altitude, corresponding to photodissociation of CO2 by solar Lyman α and extreme ultraviolet radiation, respectively. A weaker emission, originating from the same upper level of the oxygen atom, is observed in the near ultraviolet at 297.2 nm. These simultaneous measurements of both oxygen lines make it possible to directly derive a ratio of 16.5 between the visible and ultraviolet emissions, and thereby clarify a controversy between discordant ab initio calculations and atmospheric measurements that has persisted despite multiple efforts. This ratio is considered a standard for measurements connecting the ultraviolet and visible spectral regions. This result has consequences for the study of auroral and airglow processes and for spectral calibration.
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The data that support the plots within this paper and other findings of this study are available from the BIRA-IASB repository at https://repository.aeronomie.be/?doi=10.18758/71021055 or from the corresponding author on reasonable request.
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B.H. is research associate and S.A. is postdoctoral researcher of the Belgian Fund for Scientific Research (FNRS). ExoMars is a space mission of the ESA and Roscosmos. The NOMAD experiment is led by the IASB-BIRA, assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS) and the United Kingdom (The Open University). This project acknowledges funding from BELSPO, with the financial and contractual coordination by the ESA Prodex Office (PEA grant numbers 4000103401 and 4000121493). M.A.L.-V. and J.-J.L.-M. were supported by grant number PGC2018-101836-B-100 (MCIU/AEI/FEDER, EU) and by the Spanish Science Ministry Centro de Excelencia Severo Ochoa Program under grant number SEV-2017-0709. We also acknowledge support from the UK Space Agency through grant numbers ST/R005761/1, ST/P001262/1 and ST/S00145X/1 and the Italian Space Agency through grant number 2018-2-HH.0. We thank the ESA TGO team and its project scientist H. Svedhem for supporting observations in this special mode.
The authors declare no competing interests.
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Gérard, JC., Aoki, S., Willame, Y. et al. Detection of green line emission in the dayside atmosphere of Mars from NOMAD-TGO observations. Nat Astron 4, 1049–1052 (2020). https://doi.org/10.1038/s41550-020-1123-2