Multi-year behaviour of the midnight OH∗ temperature according to observations at Zvenigorod over 2000-2016

Using spectral measurements of the hydroxyl airglow at the Zvenigorod station (56° N, 37° E), Moscow region, over 2000-2016, we obtained the long-term set of data comprising 1822 midnight values of the OH∗ temperature in the mesopause region. These data revealed a 17-year series of its mean annual values, as well as amplitudes and phases of the first two harmonics of its annual variation. The obtained parameters were analyzed to determine statistically relevant characteristics of their long-term variations. As a result, we found that the long-term behaviour of the mean annual OH∗ temperature features a small negative linear trend (−0.07 ± 0.03 K/year) over the addressed period. Besides, its dependence on solar activity is shown to be 4.1 ± 0.5 K/100 SFU. Regarding the long-term behaviour of the mean annual OH∗ temperature, we revealed the existence of two oscillations with 3-year (the amplitude being 1.3 ± 0.2 K) and 4.1-year (the amplitude being 0.6 ± 0.2 K) periods. We obtained empirical relations describing year-to-year variations in the amplitudes and phases of the annual and semi-annual harmonics.