Ground-based observations of periodic temperature fluctuations in the mesopause region with periods longer than 2 d

Abstract. We analysed more than 30 years (1988–2021) of OH*(3,1) rotational temperatures observed from Wuppertal, Germany, with respect to periodic fluctuations (2 to 60 d) using the Lomb-Scargle periodogram. The main type of fluctuation observed in the last decades shows a period of about 28 d. Other periods which are frequently found in the observations lie in the period ranges around 2, 5 to 6, 8 to 12 d, and around 15 d and can likely be assigned to the quasi-2-day, the quasi-5-day, the quasi-10-day, and the quasi-16-day wave, respectively. The occurrence frequency of the waves is typically higher in winter time than in summer time because of the different wave filtering in summer and winter. This winter to summer difference holds for waves with longer periods, but it breaks off in the case of shorter periods below about 20 d. The occurrence frequency of these waves with periods below 20 d exhibit two smaller maxima around the equinoxes. A further division of these observations shows that waves with periods below 10 d account for the majority of observations in the month April to September. Waves with periods between 10 and 20 d are more equally observed throughout the year except for summer. The long-term behaviour of the wave activity indicates a quasi-bidecadal, which is likely driven by the amplitude of the waves.