Structure, variability, and origin of the low-latitude nightglow continuum between 300 and 1,800 nm: Evidence for HO_2 emission in the near-infrared
arxiv(2023)
摘要
The Earth's mesopause region between about 75 and 105 km is characterised by
chemiluminescent emission from various lines of different molecules and atoms.
This emission was and is important for the study of the chemistry and dynamics
in this altitude region at nighttime. However, our understanding of molecular
emissions with low intensities and high line densities is still very limited.
Based on 10 years of data from the astronomical X-shooter echelle spectrograph
at Cerro Paranal in Chile, we have characterised in detail this nightglow
(pseudo-)continuum in the wavelength range from 300 to 1,800 nm. We studied the
spectral features, derived continuum components with similar variability,
calculated climatologies, studied the response to solar activity, and even
estimated the effective emission heights. The results indicate that the
nightglow continuum at Cerro Paranal essentially consists of only two
components, which exhibit very different properties. The main structures of
these components peak at 595 and 1,510 nm. While the former was previously
identified as the main peak of the FeO 'orange arc' bands, the latter is a new
discovery. Laboratory data and theory indicate that this feature and other
structures between about 800 and at least 1,800 nm are caused by emission from
HO_2. We performed runs with the Whole Atmosphere Community Climate Model
(WACCM) with modified chemistry and found that the total intensity, layer
profile, and variability indeed support this interpretation, where the excited
HO_2 radicals are mostly produced from the termolecular recombination of H
and O_2. The WACCM results for the continuum at visual wavelengths show good
agreement for FeO from the reaction of Fe and O_3. However, the simulated
total emission appears to be too low, which would require additional mechanisms
where the variability is dominated by O_3.
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