The low thermospheric sodium layer (LTSL) is the separate sodium atom layer above 105 km. Based on 11,607 h of lidar observations from Yanqing (40.5° N, 116.0° E) from 2010 to 2016, we found 38 LTSLs wherein the peak densities were more than five percent above those of the main sodium layers. This work presents the peak altitudes, peak local times and peak densities of the LTSLs as well as the long-term characteristics of the seasonal and inter-annual variations of LTSLs. We analyzed the correlation between the LTSL and sporadic E layer (Es). The seasonal variation trends of the occurrences of LTSL and Es are similar, and the results showed that 95% of the LTSLs were accompanied by Es. We also found that 69% of the LTSL cases exhibited apparent downward phase progressions, while the descending rates of the LTSLs are consistent with the phase speeds of the tide.

Based on 139 nights of observational data of the Rayleigh lidar site in Beijing, China (40.5° N, 116.2° E), typical lower MIL cases and their temperature inversion evolution process were reported and compared with the SABER data from the same time. Meanwhile, the seasonal distribution of lower MIL cases over North China was also statistically analyzed. The average inversion temperature of the low MIL is 23.4 K, and the average layer thickness is 4.78 km with an average MIL bottom altitude of 68.2 km. Meanwhile, 65% of the MIL propagates vertically, most of which goes downward. These results show the temperature behavior properties of the lower MIL over North China, which may be helpful for us to further understand middle atmosphere chemical and dynamics processes.