We report a flat spectral Faraday anomalous dispersion optical filter (FS-FADOF) for sodium lidar. The physical and technical considerations for obtaining a FS-FADOF with a $3.5\mathrm{GHz}$ flat spectral transmission function are presented. It was found that the effective transmission of this filter was much higher ($>94\%$) and more uniform than that of the ultranarrowband FADOF, and therefore were less sensitive to laser-frequency drift. Thus, the FS-FADOF can improve lidar efficiency and precision.

A double‐laser beam lidar was successfully developed to simultaneously measure K and Na layers at Beijing (40.6°N, 116.2°E) in 2010. Statistical analysis of the parameters of sporadic K (K_s) and sporadic Na (Nas) layers was performed over 2 years of lidar data, and different characteristics of them were found. The average K_s occurrence (2.9%) was lower than that of Nas (5.9%); the Nas occurrence had a maximum (19.3%) in May–June months and a minimum (1.6%) in January–February months, while the K_s occurrence had a maximum (4.9%) in January–February months and a minimum (1.0%) in September–October months; most K_s peaks tended to occur around 93 km, which was ~2 km lower than that of Nas (~95 km); the K_s peak density was often at least 1 order of magnitude lower than that of Nas; notably, two K_s with high peak densities (>1000 cm⁻³) were observed, which were much higher than K density (15–300 cm⁻³) reported before. The ascending time of K_s was often longer than its descending time, but an opposite trend occurred for Nas. During the 152 cases of joint observation for the K and Na layers, 21% (32/152) were cases in which K_s and Nas events simultaneously occurred, while 79% (120/152) were cases in which only one layer (K or Na) exhibited a strong K_s or Nas.

A solid-state sodium (Na) Doppler lidar developed at YanQing Station, Beijing, China (40°N, 116°E) aiming to simultaneous wind and temperature measurement of mesopause region was reported. The 589 nm pulse laser was produced by two injection seeded 1064 nm and 1319 nm Nd:YAG pulse lasers using the sum-frequency generation (SFG) technique. A fiber amplifier is implemented to boost the seed power at 1064 nm, enabling a robust, all-fiber-coupled design for seeding laser unit, absolute laser frequency locking, and cyclic three-frequency switching necessary for simultaneous temperature and wind measurements. The all-fiber-coupled injection seeding configuration together with the solid-state Nd:YAG lasers make the Na Doppler lidar more compact and greatly reduce the system maintenance, which is conducive to transportable and unattended operation. A preliminary observational result obtained with this solid-state sodium Doppler lidar was also reported in this paper.

In 2011, Chu et al. reported the observation of neutral Fe layers reaching 155 km. Since then, there are five additional reports of layers extending above 130 km to as high as 170 km, none from central middle latitudes (35°–55°), 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°N, 116.0°E) and Pingquan (41.0°N, 118.7°E). From one‐year data set, we found four such layers, including an unprecedented one reaching 200 km with highest density of 35 cm⁻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 ~250 km. We tabulate, compare, and discuss the principle characteristics of all the reported thermospheric layers.