Investigation on the Distinct Nocturnal Secondary Sodium Layer Behavior Above 95 km in Winter and Summer Over Logan, UT (41.7°N, 112°W) and Arecibo Observatory, PR (18.3°N, 67°W)

Recent studies reveal that the Secondary sodium layer (SeSL) occurs more frequently in summer than in winter at midlatitudes. However, the physical mechanism underlying such seasonal difference is still a mystery due to the complexity of the process that involves both chemistry and neutral dynamics. In this paper, we undertake a statistical study based on seven-year Na lidar observations at Utah State University in Logan, Utah (41.7 degrees N, 112 degrees W), and 12-year Na lidar observations at Arecibo Observatory, Puerto Rico (18.3 degrees N, 67 degrees W), along with a two-dimensional numerical model simulations, to investigate the mechanism that drives such summer-winter difference of the SeSL occurrence rate in the middle latitude. The numerical simulations reveal that the Na number density, width of the mesospheric Na main layer, and the behavior of Na+ all strongly influence the winter-summer occurrence of the nocturnal SeSL at Utah State University. And this mechanism is also able to explain the winter-summer SeSL difference over other midlatitude locations in China, and that observed in the mid-low-latitude station at Arecibo Observatory.