Based on the full-diurnal-cycle sodium (Na) lidar observations at Beijing (40.41∘ N, 116.01∘ E), we report pronounced
downward extensions of the Na layer bottomside to below 75 km near
mid-December 2014. Considerable Na atoms were observed even as low as
∼ 72 km, where Na atoms are short-lived. More interestingly, an unprecedented Na density of ∼ 2500 atoms cm−3 around 75 km
was observed on 17 December 2014. Such high Na atoms concentration was 2
orders of magnitude larger than that normally observed at the similar
altitude region. The variations of Na density on the layer bottom were found to be accompanied by warming temperature anomalies and considerable
perturbations of minor chemical species (H, O, O3) in the upper
mesosphere. Different from the previous reported metal layer bottom
enhancements mainly contributed by photolysis after sunrise, these
observational results suggest more critical contributions were made by the
Na neutral chemical reactions to the Na layer bottom extensions reported
here. The time–longitudinal variations of background atmospheric parameters
in the upper mesosphere and stratosphere from global satellite observations
and ERA reanalysis data indicated that the anomalous structures observed
near the lidar site in mid-December 2014 were associated with planetary
wave (PW) activities. The anomalies of temperature and O3 perturbation
showed opposite phase in the altitude range of 70–75 and
35–45 km. This implied that the vertical coupling between the
mesosphere and stratosphere, possibly driven by the interactions of PW
activities with background atmosphere and modulation of gravity wave (GW)
filtering by stratospheric wind, contributed to the perturbations of
background atmosphere. Furthermore, the bottom enhancement on 17 December
2014 was also accompanied by clear wavy signatures in the main layer. The
strong downwelling regions are likely due to the superposition of tide and
GW, suggesting the wave-induced adiabatic vertical motion of the air parcel
contributed greatly to the formation of the much stronger Na layer bottom
enhancement on 17 December 2014. These results provide a clear
observational evidence for the Na layer bottom response to the
planetary-scale atmospheric perturbations in addition to tide and GW through affecting the chemical balance. The results of this paper also have
implications for the response of the metal layer to vertical coupling
between the lower atmosphere and the mesosphere.