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In 2011, Chu et al. reported the observation of neutral Fe layers reaching 155km. Since then, there are five additional reports of layers extending above 130km to as high as 170km, none from central middle latitudes (35 degrees-55 degrees), a large atmospheric region of considerable importance. Here we report the first concurrent observations of thermospheric Na layers from two nearby lidar stations so located, Yanqing (40.5 degrees N, 116.0 degrees E) and Pingquan (41.0 degrees N, 118.7 degrees E). From one-year data set, we found four such layers, including an unprecedented one reaching 200km with highest density of 35cm(-3) and fastest descending rate. While the main Na layers were comparable, in three nights, these thermospheric layers were observed only in one station (Yanqing), suggesting that these layers often occur locally with a horizontal scale less than similar to 250km. We tabulate, compare, and discuss the principle characteristics of all the reported thermospheric layers.

Plain Language Summary The neutral Na atomic layers are mainly distributed from 80 to 105km and they have been used as tracers to study atmospheric chemistry and dynamical processes. Based on four thermospheric Na layers above 130km observed by two central middle-latitude lidar stations simultaneously, the paper analyzes the horizontal dimension of the observed thermospheric Na layers, and reports an unprecedented layer reaching an altitude of 200km with a peak density of 35cm(-3) and a descending rate of 9.3m/s. The fact high peak density is possible around 150km opens up the potential for tripling the altitude range for probing temperature and wind by a resonance fluorescence lidar, thus advancing our understanding of dynamics and chemistry in the upper atmosphere.