Abstract :
[en] 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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