## 4 Results  
### Temperature  
The mesopause is colder in summer than in winter. It is related to atmospheric circulation at the mesopause altitudes causing the upwelling at the summer pole and downwelling at the winter pole. The ascending air expands adiabatically and cools down, which leads to the cold summer mesopause and vice versa--the air descent leads to air compression and temperature rise in the winter mesopause related to it [23]. In the mesosphere, annual circulation is associated with the dissipation of gravity waves driving the meridional circulation [24].  
Figure 3 presents annual variations in average nighttime effective temperature in the mesopause region as obtained from the SABER, FPI and NRLMSIS model for 2017-2021 at the altitudes of 85-100 km over the south of East Siberia. Figure 4 shows annual variations in the monthly average nighttime effective temperatures in the mesopause region, the same as Figure 3.  
Figure 2: Scatter diagram (blue points) and regression line (green line) from FPI and SABER data of I 557.7 nm.  
Figures 3 and 4 show that the lowest temperatures are seen in summer (\(\sim\)160-180 K), and the highest are seen in winter (\(\sim\)200 K) from empirical and model data. However, according to model data, the seasonal temperature decrease is smoother, while satellite and ground data show higher rates of temperature decrease. The temperature decrease from interferometer data near the hundredth day of the year is followed by strong variations, during which some nighttime means can exceed winter temperatures (Figure 3). These variations in the daily averaged values of I 557.7 according to the FPI lead to the fact that instead of a seasonal decrease in I 557.7 from winter to summer, it slightly increases in April. Minimal monthly average effective temperatures in the mesopause region are reached in May from SABER data (175 K) and in July from NRLMSIS data (178 K), after which the temperature begins to rise. According to FPI data, the lowest temperatures are reached in July and remain almost the same (165 K) until September (Figure 4). Due to data dispersion, these points in time are difficult to detect from Figure 3, but the difference between temperatures in the summer-autumn period is well-defined. The rates of temperature rise from model and satellite data in the summer-autumn period are approximately the same, while according to interferometer data, temperature begins to rise significantly only in autumn. The difference in temperature from different sources amounts to \(\sim\)20 K and occurs in September-October. According to interferometer data, in autumn, temperature growth begins in October, the growth rate being significantly higher than in model and satellite  
Figure 4: Monthly average effective temperatures and deviations from 2017 to 2021 from SABER (red), FPI (green) and NRLMSIS model (blue) data.  
Figure 3: Annual average effective temperature from 2017 to 2021. SABER (red), FPI (green) and NRLMSIS model (blue).  
data. As a result, the effective temperatures reach the same values for all three instruments in wintertime.  
Figure 5 presents the comparison of temperature minimum altitudes from data by SABER and the NRLMSIS model (there are no interferometer data on the graph since it measures integral parameters of the atmosphere). The annual dynamics of the temperature minimum position from height profiles from the satellite instrument and model are consistent. It should be noted that according to SABER, the amplitude of variations in nighttime mesopause altitude is a little less than 10 km, and according to the model, this value change is 15 km.  
Thus, the temperature dynamics and the temperature profile (temperature minimum) in general correspond to the modern concept of the mesopause temperature regime. The revealed differences are discussed below.