Significant extension of the mesospheric Na layer bottom observed by a full-diurnal-cycle lidar

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 is short-lived. More interestingly, an unprecedented Na density of ~2500 atoms/cm³ around 75 km was observed on December 17, 2014. Such high Na atoms concentration was two orders of magnitude larger than that normally observed at the similar altitude region. Liberation of Na atoms from its reservoir (e.g., NaHCO₃) near the Na layer bottom via neutral chemical reactions, which are accelerated by the largely increased temperature and concentrations of atomic H and O, is suggested to be the critical production mechanism of the enhanced Na layer below 75 km. The diurnal lidar measurements of the Na layer, zonal wind results from a nearby meteor radar, global satellite observations as well as reanalysis data presented here reveal the close correlation between the variation of Na layer bottom and planetary scale atmospheric processes. The longitudinal distributions of geopotential amplitudes of PW show that there exists unusual development of the amplitude of PW2, and the stratosphere near the lidar location is dominated by PW2 trough in mid-December. The out-of-phase temperature anomalies in the upper stratosphere and upper mesosphere are likely due to the modulation of GW filtering by stratosphere wind. The strong eastward wind in the upper stratosphere provides a favorable condition for the vertical propagation of westward GWs. Westward forcing could induce a poleward flow and drive downward circulation in the mesosphere, leading to adiabatic heating. Furthermore, the bottom enhancement on December 17, 2014 was also accompanied by clear wavy signatures in the main layer. The unprecedented Na density of ~2500 cm^-3 near 75 km observed on December 17, 2014 is also greatly contributed by the adiabatic vertical motion of air parcel forced by the superposition of tide and GW.

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. These results also have implications for the response of the metal layer to vertical coupling between the lower atmosphere and the mesosphere.