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The Earth's mesopause region between about 75 and 105 km is characterised bychemiluminescent emission from various lines of different molecules and atoms.This emission was and is important for the study of the chemistry and dynamicsin this altitude region at nighttime. However, our understanding of molecularemissions with low intensities and high line densities is still very limited.Based on 10 years of data from the astronomical X-shooter echelle spectrographat 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 thespectral features, derived continuum components with similar variability,calculated climatologies, studied the response to solar activity, and evenestimated the effective emission heights. The results indicate that thenightglow continuum at Cerro Paranal essentially consists of only twocomponents, which exhibit very different properties. The main structures ofthese components peak at 595 and 1,510 nm. While the former was previouslyidentified as the main peak of the FeO 'orange arc' bands, the latter is a newdiscovery. Laboratory data and theory indicate that this feature and otherstructures between about 800 and at least 1,800 nm are caused by emission fromHO$_2$. We performed runs with the Whole Atmosphere Community Climate Model(WACCM) with modified chemistry and found that the total intensity, layerprofile, and variability indeed support this interpretation, where the excitedHO$_2$ radicals are mostly produced from the termolecular recombination of Hand O$_2$. The WACCM results for the continuum at visual wavelengths show goodagreement for FeO from the reaction of Fe and O$_3$. However, the simulatedtotal emission appears to be too low, which would require additional mechanismswhere the variability is dominated by O$_3$.

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