Comparison of VLT/X-shooter OH and O<sub>2</sub> rotational temperatures with consideration of TIMED/SABER emission and temperature profiles

Abstract

Rotational temperatures T rot derived from lines of the same OH band are an important method to study the dynamics and long-term trends in the mesopause region near 87 km. To measure realistic temperatures, the rotational level populations have to be in local thermodynamic equilibrium (LTE). However, this might not be fulfilled, especially at high emission altitudes. In order to quantify possible non-LTE contributions to the OH T rot as a function of the upper vibrational level v ′ , we studied a sample of 343 echelle spectra taken with the X-shooter spectrograph at the Very Large Telescope at Cerro Paranal in Chile. These data allowed us to analyse 25 OH bands in each spectrum. Moreover, we could measure lines of O 2 b(0-1), which peaks at about 94 to 95 km, and O 2 a(0-0) with an emission peak at about 90 km. The latter altitude is reached in the second half of the night after a rise of several km because of the decay of a daytime population of excited O 2 . Since the radiative lifetimes for the upper levels of the two O 2 bands are relatively long, the derived T rot are not significantly affected by non-LTE contributions. These bands are well suited for a comparison with OH if the differences in the emission profiles are corrected. For different sample averages, we made these corrections by using OH emission, O 2 a(0-0) emission, and CO 2 -based temperature profile data from the multi-channel radiometer SABER on the TIMED satellite. The procedure relies on differences of profile-weighted SABER temperatures. For an O 2 a(0-0)based reference profile at 90 km, we found a good agreement of the O 2 with the SABER-related temperatures, whereas the OH temperatures, especially for the high and even v ′ , showed significant excesses with a maximum of more than 10 K for v ′ = 8. The exact value depends on the selected lines and molecular parameters. We could also find a nocturnal trend towards higher non-LTE effects, particularly for high v ′ . The amplitude of these variations can be about 2 K or less, which tends to be significantly smaller than the total amount of the non-LTE contributions. The variations revealed may be important for dynamical studies based on T rot derived from OH bands with high v ′ .

The OH<sup>*</sup>(3–1) layer emission altitude cannot be determined unambiguously from temperature comparison with lidars

“…The GRIPS 9 data were taken with a one-minute resolution, from which I computed a nightly mean temperature. This is similar to the data shown in Noll et al (2016), justifying this simplification of a…”

A comparison of ground-based hydroxyl airglow temperatures with SABER/TIMED measurements over 23° N, India

“…Figures 3 and 4 while OH (5, 3) and (4, 2) band emissions are mainly collected at 1.6 µm (Baker et al, 2007). As OH emissions from different υ peak at different heights (von Savigny, 2015;Noll et al, 2016), the difference in OH 2.0 and 1.6 µm emission peaks is observed. Generally, the 2.0 µm emission peak lies above that of the 1.6 µm channel by ∼ 1.6 km at about 89 ± 2 km, thereby resulting in different values of two OHequivalent temperatures.…”

Mesospheric OH layer altitude at midlatitudes: variability over the Sierra Nevada Observatory in Granada, Spain (37° N, 3° W)

“…The work of von Savigny et al (2012a) showed that the peak altitude difference increases by 0.6 km per increasing vibrational level, assuming linear dependence. Noll et al (2016) reported shifts of 0.4 km for adjacent levels. Therefore, there should be offsets of roughly +1 and −1.5 km between the peak altitude in SATI and in SABER 1.6 and 2.0 µm channels, respectively.…”