Abstract: Objective　Metallic layers in the mesosphere and lower thermosphere (MLT) , containing various metal elements such as sodium, iron, and potassium, originate predominantly from meteoric ablation. Among these metal species, sodium atoms, owing to their strong resonance fluorescence, serve as sensitive tracers for dynamical and chemical processes in this altitude region. One of the purposes of this study is to systematically investigate the seasonal behavior of the sodium layer over a typical mid latitude site (Beijing) and the factors governing double sodium layer (DSL) events. The DSL event refers to a scenario where a secondary layer appears above 105 km and is clearly separated from the main layer by a gap of near zero density. The second purpose is to evaluate whether these DSL events are linked to ionospheric sporadic E layers (Es), which is important for understanding the coupling between the neutral atmosphere and the ionosphere. Methods　Observational data were from a sodium fluorescence lidar located at Yanqing, Beijing (40.20° N, 116° E), spanning 251 nights (2305 h) between January 2021 and December 2022. The lidar system employed a Nd:YAG pumped dye laser operating at 589 nm, with a pulse energy of approximately 45 mJ, a repetition rate of 50 Hz, and a pulse width of 8 ns. Backscattered fluorescence photons were collected using a 1 m diameter telescope, and the resulting sodium density profiles were retrieved at a vertical resolution of 1 km. For each retrieved sodium density profile, the column density, centroid height, root mean square (RMS) width, and peak density of the sodium layer were evaluated using the standard moment method. The seasonal, annual, and semi annual variations of these parameters were quantified through least squares harmonic fitting. DSL events were identified by the occurrence of a distinct secondary sodium layer (SeSL) above 105 km that was separated from the main layer by a region of negligible sodium density. For each identified DSL event, the peak density, peak altitude, full width at half maximum (FWHM) of SeSL, and the ratio of the SeSL peak density to the main sodium layer peak density were determined. To investigate the possible association between DSL event and Es, ionosonde observations from the nearby Changping station (39.5°N, 116.2°E) were examined together with the lidar measurements. The ionosonde could provide the critical frequency of Es (foEs) and virtual height of Es (h'Es), enabling direct temporal and altitudinal comparison of DSL events. Results and Discussion　During the two year observation period, the mean Na column density over Beijing was 4.25 × 109 cm−2, consistent with the previously reported values at approximately 40°N. The column density exhibited a pronounced annual cycle, characterized by higher values in winter and lower values in summer. The maximum monthly value reached 1.01 × 1010 cm−2 in February 2022, whereas the minimum decreased to 1.54 × 109 cm−2 in July 2022. The analysis showed that the amplitude of the annual variation component of column density accounted for about 29%–30% of the annual mean value, whereas the semi annual component accounted for approximately 15%– 19%. The mean centroid height and RMS layer width were 90.75 km and 4.76 km, respectively, and both parameters exhibited significant semi annual variation characteristics. The RMS width of Na layer was generally narrower in spring and autumn and broader in winter and summer. At the same time, peak density exhibited a seasonal pattern similar to that of column density, with higher values in winter and lower values in summer. The correlation analysis between the variation of Na column density and sunspot numbers showed that there was a positive correlation between the two in winter, while the correlation in summer was significantly weak, indicating that solar modulated photoionization and temperature-driven chemistry both affect sodium abundance, and the relative importance varies with the season. Over the 251 observation nights, 13 DSL events were identified, with an occurrence rate of 5.1% in the Beijing area. Notably, all 13 DSL events occurred between May and July, with none detected in any other month. The SeSL peak density ranged from 89 to 2081 cm−3. The highest value, exceeding 2000 cm−3, was recorded on 17 May 2021, with a relative peak density ratio of 88.9% to the main layer. This was the first time such a strong event was documented on this site. The SeSL peaks were mainly clustered between 109 and 114 km in height, and their FWHM ranged from 2 to 11 km, indicating considerable variability in the vertical extent of SeSLs. The possible relationship between DSL and Es was examined in detail based on the observation on the night of 18 June 2021, when complete simultaneous lidar and ionosonde observations were available. It was found that the three consecutive DSL episodes detected by the lidar that night corresponded one-to-one in time with the three Es events recorded by the ionosonde. The time difference between each DSL and its corresponding Es was about 60 minutes, and the average height difference between the SeSL peak and h'Es was less than 7 km. This close spatiotemporal coincidence suggests that DSLs and Es share common driving mechanisms, most likely wind shear and ion convergence in the lower thermosphere. The seasonal confinement of DSLs to late spring and early summer also coincides with the peak season of Es at this latitude, further supporting the coupling hypothesis. Conclusions　The observations demonstrate that the sodium layer over Beijing exhibits strong seasonal variation, characterized by annual oscillations in column density and semi annual oscillations in centroid height and width. These variations are mainly driven by temperature dependent chemical reaction rates and modulated by solar irradiance cycles. The frequency of DSL events is only 5.1%, with a clear tendency to occur in summer and a very wide dynamic range of peak density (89–2081 cm−3), including the first recorded local event with peak density exceeding 2000 cm−3. The close temporal and altitudinal correspondence between DSLs and Es, revealed through simultaneous lidar and ionosonde measurements, suggests that both phenomena may be governed by common upper atmospheric forcing mechanisms, such as wind shear, gravity wave breaking, or electric fields. These results not only expand the long term observational database for mid latitude metal layers but also provide quantitative constraints for future modelling studies aimed at elucidating the formation and evolution of DSLs and their role in neutral ion coupling. Continued multi instrument campaigns and chemistry climate modelling are needed to establish the causal chain and to assess the response of these events to solar and geomagnetic variability.

Key words: lidar, sodium layer structure, seasonal variations, double sodium layer, sporadic E layer