Lidar observations of thermospheric Na layers over Beijing

The thermospheric layers reported are located in the E and F regions of the ionosphere, which can be used as unique tracers for studies of interaction processes between neutral and ionized components in 105 similar to 200 km. In order to enhance our understanding of the origin and formation mechanisms of thermospheric layers, this study analyzes the observed sodium (Na) data by a high-sensitivity resonance fluorescence lidar at Yanqing Station, Beijing (40.42 degrees N, 116.02 degrees E). According to their morphological characteristics, occurrence frequency, and referring to the previous reports, the thermospheric Na atomic layers observed from the station are mainly classified into the following four types: lower thermosphere sporadic Na layers, dawn thermosphereionosphere Na layers, midnight thermosphere-ionosphere Na layers and mid-latitude thermosphereionosphere Na layers (Mid-TINa). Then we analyze the last thermospheric Na layer in detail. Based on totaling 415 nights and approximately 3914 hours of observation from 2018 to 2020, there were only about 17 Mid-TINa events found (with an occurrence rate of only 4. 1%, mostly in winter). Among the 14 complete events analyzed, only about 35. 7% (5/14) of the events' occurrence time is similar but earlier than the Sporadic E (Es) layers observed at the adjacent ground-based station. The remaining 9 events occurred between 2. 5 to 8. 6 hours after the latest Es layers. This weak correlation suggests that Mid-TINa layers may be triggered by other possible formation mechanisms.