Seasonal Variation Characteristics of Sodium Layer in Low Latitude by Lidar

Sodium lidar observations of sodium layer from 2018 to 2019 at a low-latitude location (Haikou, China, 20°N, 110.3°E) are reported in this paper. The purpose is to reveal characteristics of seasonal variations of the sodium layer, including density distribution characteristics, seasonal variation characteristics and special events of sodium layer. According to statistical analysis, there is a great correlation between the variation of sodium layer and seasons. Average density of sodium layer presents a basically symmetrical Gaussian distribution, with good morphological symmetry in autumn. Column density and peak value are small in spring and summer and vice versa in autumn and winter. Peak position is high in summer and low in autumn. There is no clear seasonal variation about the centroid height of the sodium layer. Full Width at Half Maximum (FWHM) and Root Mean Square (RMS) width of sodium layer are both small in autumn and large in winter. Sporadic Sodium Layers (SSLs) change obviously with seasons and the growth, decline and duration time are short in summer and long in spring. Peak value has the minimum in spring and maximum in autumn, respectively. Peak position tends to be higher during autumn and winter but lower during spring and summer. The intensity is the largest in summer. Except autumn, peak value of other seasons is mostly in the latter half of the night. With respect to the correlation with sporadic E, combining with the ionosphere data obtained by the nearest ionosonde at Danzhou, China ($ 19.5°\mathrm{N}, 109.1°\mathrm{E} $), there is a considerable correlation between sporadic E layers (Es) and SSLs above 97 km at low latitude, which is embodied in the time, height and intensity. By statistical analysis of special events, it is found that the probability of SSLs in winter is high and the Double Sodium Layer (DSL) mostly appears in spring and summer at Haikou.