Lidar observations of thermospheric Na layers up to 170km with a descending tidal phase at Lijiang (26.7°N, 100.0°E), China

We report the first lidar observations of thermospheric Na layers up to 170km at Lijiang (geomagnetic 21.6 degrees N, 171.8 degrees E), China, in March, April, and December 2012. The Na densities inside the layers are low, ranging from similar to 1 to similar to 6cm(-3) at altitudes of 130-170km, about 3 orders of magnitude lower than the Na peak density in the mesopause region. All of these layers exhibit an apparent downward phase progression with a descending rate of 11-12km/h or similar to 3m/s, consistent with the vertical phase speed of semidiurnal tides around 140km. We have identified at least 12 events from the total 37 nights of lidar observations with four shown in this report, giving an occurrence frequency of similar to 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 thermospheric Na layers are formed through the neutralization of the tidal-wind-shear-converged Na+ layers via direct electron-Na+ recombination Na++e(-)Na+h. An envelope calculation using reasonable ion and electron densities shows good consistency with the observations.