Abstract
Abstract. Measurements of the OH Meinel emissions in the terrestrial nightglow are one of the standard ground-based techniques to retrieve upper mesospheric temperatures. It is often assumed that the emission peak altitudes are not strongly dependent on the vibrational level, although this assumption is not based on convincing experimental evidence. In this study we use Envisat/SCIAMACHY (Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY) observations in the near-IR spectral range to retrieve vertical volume emission rate profiles of the OH(3-1), OH(6-2) and OH(8-3) Meinel bands in order to investigate whether systematic differences in emission peak altitudes can be observed between the different OH Meinel bands. The results indicate that the emission peak altitudes are different for the different vibrational levels, with bands originating from higher vibrational levels having higher emission peak altitudes. It is shown that this finding is consistent with the majority of the previously published results. The SCIAMACHY observations yield differences in emission peak altitudes of up to about 4 km between the OH(3-1) and the OH(8-3) band. The observations are complemented by model simulations of the fractional population of the different vibrational levels and of the vibrational level dependence of the emission peak altitude. The model simulations reproduce the observed vibrational level dependence of the emission peak altitude well – both qualitatively and quantitatively – if quenching by atomic oxygen as well as multi-quantum collisional relaxation by O2 is considered. If a linear relationship between emission peak altitude and vibrational level is assumed, then a peak altitude difference of roughly 0.5 km per vibrational level is inferred from both the SCIAMACHY observations and the model simulations.
Highlights
It is generally assumed that the exothermic reaction of H and O3,H + O3 −→ OH(v ≤ 9) + O2 (R1)is the main source of vibrationally excited OH near the mesopause, producing an emission layer – first identified by Meinel (1950) – that is centered at about 87 km with a full width at half maximum (FWHM) of about 8–10 km (e.g. Baker and Stair, 1988)
In order to allow for an easier comparison of the inverted volume emission rate profiles of the three OH-Meinel bands analyzed, the volume emission rate profiles are normalized relative to their respective peak value for the following discussions
The normalized volume emission rate profiles for all three OH Meinel bands and for July 2005 are shown in Fig. 2 for 10◦ latitude bands between −30◦ S and 10◦ N
Summary
Is the main source of vibrationally excited OH near the mesopause, producing an emission layer – first identified by Meinel (1950) – that is centered at about 87 km with a full width at half maximum (FWHM) of about 8–10 km (e.g. Baker and Stair, 1988). The height of the OH emission layer is affected by characteristic tidal variations (Ward, 1999) with maximum changes of about 3 km during the course of the night (e.g., Yee et al, 1997) Another potential difficulty for the interpretation of ground-based OH rotational temperature measurements is related to the possibility that Meinel bands originating from different vibrational levels may have different peak emission altitudes. Xu et al (2012) recently provided a comparison of SABER OH emission observations and model results that showed evidence for a vibrational level dependence of the OH emission altitude with emissions from higher vibrational levels occurring at higher altitudes In this context it is relevant to mention that according to Cosby and Slanger (2007) significant changes in the relative population of the OH vibrational levels occur throughout the night.
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