Lidar Observations Of Thermospheric Na Layers

Neutral metal atom layers in the upper atmosphere can be detected by resonance fluorescence lidar. They provide tracers for exploring the neutral properties of the space-atmosphere intergration region (SAIR). The neutral metal atom layers are normally confined to altitude of 75-110km, which is usually referred to as the mesosphere and lower thermosphere region. The detection range of resonant lidars is correspondingly confined in this region according to the distribution of neutral metal atom layers.New discoveries in recent years have radically changed our view of the range limit of neutral metal layers. Chu et al. (2011) reported the first lidar observations of neutral Fe layers with gravity wave signatures in the thermosphere up to 155 km at McMurdo (77.83°S,166.66°E), Antarctica. Several observations of thermospheric neutral metal layers have been reported since then. Thermospheric metal layers provide good tracers in observing the the neutral properties up to 200 km with high spatial and temporal resolution using lidar. These layers are significant in studying the coupling of thermosphere and ionosphere, and the chemical reactions in the region of 100-200 km, serving as a natural lab for studying various physical and chemical processes.However, by the end of 2013, Na layers have not been reported above 140 km, well below the records of 170 km for Fe (Chu et al.,2013) and 155 km for K (Friedman et al.,2013). Are the unusually high Fe layers observed unique to McMurdo or a property of Fe itself? Can other species, like Na, occur above 140 km at locations other than Antarctica? These questions are important because the answers to them will provide useful insight into the origin of these neutral metal layers and their formation mechanisms.In this paper, we report the first lidar observations of thermospheric Na layers up to 170 km with a descending tidal phase at Lijiang (geomagnetic 21.6°N,171.8°E), China in March, April and December 2012. This dissertation has conducted qualitative analysis on the formation mechanism of these thermospheric Na layers. The Na densities inside the layers are low, ranging from ～1 to ～6 cm-3 at altitudes of 130-170 km, about three orders of magnitude smaller than the Na peak density in the mesopause region. All of these layers exhibit an apparent downward phase progression with a descending rate of 11-12 km/h or ～3 m/s, consistent with the vertical phase speed of semidiurnal tides around 140 km. We have identified at least 12 events from the total 37 nights of lidar observations, giving an occurrence frequency of ～33% over Lijiang. These thermospheric layer events correspond to strong to moderate equatorial fountain effects, bolstering our hypothesis that the deposit of metallic ions from the equatorial region to low latitudes via the fountain effect provides the Na+ ions in the thermosphere over Lijiang. Adopting the theory by Chu et al. [2011] and the hypothesis by Tsuda et al. [2015], we further hypothesize that the thermospherie Na layers are formed through the neutralization of the tidal-wind-shear-converged Na+layers via direct electron-Na+ recombination Na++e-â†'Na+hv. An envelope calculation using reasonable ion and electron densities shows good consistency with the observations.We also report the obseravations by the Na resonance fluorescence lidar chain of the Chinese Meridian Project. These observations demonstrate the significance of detection limit in detecting thermospheric metal layers. The equatorial fountain effect plays an important role in the formation of neutral metal atom layers above 130 km through transporting ions at low latitudes. The mechanism of the formation of the thermospheric Na layers at Lijiang should be generalized to low latitudes.