Spectral Structure of Temperature Variations in the Midlatitude Mesopause Region

Long-term series of midnight temperature in the mesopause region have been obtained from spectral observations of hydroxyl airglow emission (OH(6-2) lambda 840 nm band) at the Tory station (52A degrees N, 103A degrees E) in 2008-2016 and Zvenigorod (56A degrees N, 37A degrees E) station in 2000-2016. On their basis, the Lomb-Scargle spectra of the variations in the period range from similar to 12 days to similar to 11 years have been determined. Estimates of the amplitudes of statistically significant temperature fluctuations are made. The dominant oscillations are the first and second harmonics of the annual variation, the amplitudes of which are 23-24 K and 4-7 K, respectively. The remaining variations, the number of which was 16 for the Tory and 22 for Zvenigorod stations, have small amplitudes (0.5-3 K). Oscillations with combinational frequencies, which arise from modulation of the annual variation harmonics, are observed in a structure of the variation spectra in addition to interannual oscillations (periods from similar to 2 to similar to 11 years) and harmonics of the annual variation (up to its tenth harmonic).