Comparison of VLT/X-shooter OH and O2 rotational temperatures with consideration of TIMED/SABER emission and temperature profiles

摘要

Rotational temperatures derived from lines of thesame OH band are an important method to study the dynamicsand long-term trends in the mesopause region near 87 km. Tomeasure realistic temperatures, the rotational level populationshave to be in local thermodynamic equilibrium (LTE). However, thismight not be fulfilled, especially at high emission altitudes. Inorder to quantify possible non-LTE contributions to the OH as a function of the upper vibrational level′, we studied a sample of 343 echelle spectra taken withthe X-shooter spectrograph at the Very Large Telescope at CerroParanal in Chile. These data allowed us to analyse 25 OHbands in each spectrum. Moreover, we could measure lines ofO(0-1), which peaks at about 94 to95 km, and O(0-0) with an emission peakat about 90 km. The latter altitude is reached in the secondhalf of the night after a rise of several km because of thedecay of a daytime population of excited O. Since theradiative lifetimes for the upper levels of the two O bandsare relatively long, the derived are notsignificantly affected by non-LTE contributions. These bands arewell suited for a comparison with OH if the differences inthe emission profiles are corrected. For different sample averages,we made these corrections by using OH emission,O(0-0) emission, and CO-basedtemperature profile data from the multi-channel radiometer SABER onthe TIMED satellite. The procedure relies on differences ofprofile-weighted SABER temperatures. Foran O(0-0)-based reference profile at90 km, we found a good agreement of the O with theSABER-related temperatures, whereas the OH temperatures,especially for the high and even ′, showed significantexcesses with a maximum of more than 10 K for ′ = 8. The exact value depends on the selected lines and molecularparameters. We could also find a nocturnal trend towards highernon-LTE effects, particularly for high ′. The amplitudeof these variations can be about 2 K or less, which tends tobe significantly smaller than the total amount of the non-LTEcontributions. The variations revealed may be important fordynamical studies based on derived from OHbands with high ′.