## 2 Observations and Method for Data Analysis
### Climatology, Seasonal and Interannual Variability of the Nonmigrating Diurnal Tides
The DE3 is symmetrical tide about the equator, while June DE2 maximum is located at \(\sim\)10\({}^{\circ}\) S, and that in December is at \(\sim\)10\({}^{\circ}\) N. The climatology of DW2 reveals enhancements around the equator, and at \(\pm\)50\({}^{\circ}\) with the largest amplitudes in November (\(\sim\)5.2 k) at the equator, in August (\(\sim\)4.5 K) at 50\({}^{\circ}\) N, and in December (\(\sim\)4.8 k) at (40-50\({}^{\circ}\)) S. Figure 1b demonstrates the altitude-latitude structures of the tidal amplitudes (upper row of plots, in K) and phases (bottom row of plots, in degrees)  
Figure 1: **(a)** Latitude-time structures of the climatologically mean (2002–2022) DE3 (**left** plot, in K), DE2 (**middle** plot), and DW2 (**right** plot) tidal amplitude calculated at altitude of 110 km; (**b)** altitude-latitude structures of the amplitudes (**upper** row of plots, in K) and phases (**bottom** row of plots, in degrees) of the above-mentioned diurnal tides arranged in the same way as in (**a**); DE3 is presented in August, while DE2 and DW2 is presented in June and December, respectively. of the above-mentioned diurnal tides arranged in the same way as in Figure 1a; DE3 is presented in August, while DE2 and DW2 are presented in June and December, respectively. All these tides are vertically upward-propagating and reach the largest amplitudes at or slightly above 110 km height, with an exception of DW2 tide over the equator, particularly in December, which maximizes around 105 km with amplitude of 6 K.  
The next step in our analysis is to look at the distribution of the monthly mean tidal amplitudes of the above considered nonmigrating diurnal tides. Figure 2 presents the latitude-time structures of the monthly mean DE3 (upper plot), DE2 (middle plot), and DW2 (bottom plot) tidal amplitudes at altitude of 110 km; the time is actually the number of months started from January 2002. It is seen that during the considered period of 21 years the monthly mean DE3 tidal amplitude reaches 22.2 K, while those of DE2 and DW2 are, respectively, 12.1 K and 9.8 K, i.e., these nonmigrating tides have a large contribution to the neutral dynamics of the lower thermosphere. Furthermore, the monthly mean representation shown in Figure 2 allows for both the highlighting of the main seasonal components and the rapid visualization of the interannual variations if they exist. It can be distinguished that for DE3 tide, the main seasonal component is the annual one, while for DE2 and DW2 there is also a well-seen semi-annual component. Moreover, for DE2, the semi-annual component is better delineated than the annual component, while in some years for DW2, a 4-month component can also be distinguished. Figure 2 reveals very clearly the presence of interannual variations in all considered tides. The quasi-2-year variability can be noticed at once in DE3 and DE2, particularly in the June maximum; some signature of this variability can be distinguished in DW2 as well, especially in the December maximum at southern middle latitudes. The results in Figure 2 demonstrate that it is necessary to investigate in detail the seasonal components of the considered nonmigrating tides in the low thermosphere. The reason is that it is in the seasonal components that the interannual variability is best manifested, and this will be done as a next step of the tidal analysis. Usually, the seasonal variability is described by the 12- and 6-month components, i.e., AC and SAC, but in this study, the shorter 4- and 3-month components are also included. Besides the above-mentioned seasonal components, an important parameter is the annual mean (AM) as well. All these components are separated from the monthly mean tidal amplitudes by a decomposition applied to a 12-month window that is moved through the time series with a step of 1 month in order to derive the monthly values of the seasonal characteristics.