Coherent and Non-Coherent Components of Mesoscale Variations of Hydroxyl Rotational Temperature near the Mesopause.

Mesoscale variations of the rotational temperature of excited hydroxyl (OH*) are studied at altitudes 85 – 90  km using the data of spectral measurements of nightglow emission at Russian observatories Zvenigorod (56 ° N, 37°E.) in years 2004  –  2016, Tory (52 ° N, 103°E) in  2012  –  2017 and Maimaga (63° N,  130° E) in  2014 - 2019. The filtering of mesoscale variations was made by calculations of the differences between the measured values of OH* rotational temperature separated with time intervals of dt ~ 0.5 - 2 hr. Comparisons of monthly variances of the temperature differences for various dt allow us to estimate coherent and non-coherent in time components of the mesoscale temperature perturbations. The first component can be associated with mesoscale waves near the mesopause. The non-coherent component may be produced by instrument errors and atmospheric turbulence. The results allow us correcting the observed mesoscale temperature variances at all listed sites for contributions of instrumental and turbulent errors. Seasonal and interannual changes in the coherent component of mesoscale variances of the temperature at the observational sites are studied, which may reflect respective changes in the intensity of mesoscale internal gravity waves in the mesosphere and lower thermosphere region.

     The analysis of nightglows data was supported by the grant #19-35-90130 of the Russian Foundation for Basic Research. Hydroxyl nightglow data at the Tory site were obtained with the equipment of the Center for Common Use «Angara» http://ckp-rf.ru/ckp/3056/ at the ISTP SB RAS within budgetary funding from the Basic Research Program (Project 0278-2021-0003). Data of the “Geomodel” Resource Center of Saint-Petersburg State University were used.