The E<inf>s</inf> occurrence rate during the simultaneous observation period of Na layer over Qingdao (36&#x00B0;N, 120&#x00B0;E), China
Sporadic E (E s ) layers have been detected by an ionosonde during 2008 to 2011 at Qingdao (36°N, 120°E), China. At the same time, we have observed nocturnal Na layers by a lidar with a horizontal distance of only ∼30 m away from the ionosonde. In order to study the correlation between the occurrences of E s and sporadic Na (Na s ) layers, we can compare the E s occurrence rate during the period of Na s layers with that during the entire observation period of Na layer. It should be noted that the E s occurrence rate during the observation period of Na layer could be different with the nighttime mean E s occurrence rate. Based on the four years simultaneous and closely colocated observations at Qingdao, it is found that the temporal coverage of Na data possessed remarkably seasonal and nocturnal distribution asymmetry. Consequently the E s occurrence probability during Na data period (11%) was different with the nighttime mean E s occurrence probability (20%).