First report of sporadic Na layers at Qingdao (36° N, 120° E), China
This paper reports, for the first time, observational results of mesopause sporadic Na (Na s) layers by a ground-based lidar at Qingdao (36° N, 120° E), China. Based on ~ 430 h of observational data on 95 nights from December 2007 to June 2012, we have selected a total of 53 Na s layer events. It is found that characteristics of Na s layers over Qingdao have general similarity with those over nearby sites, Wuhan (30° N, 114° E) and Hefei (32° N, 117° E), but not those over the site Hachioji (35° N, 139° E) at nearly the same latitude as Qingdao. At the same time, parameters of sporadic E (E s) layers were recorded by an ionosonde. The fact that E s layer occurrence probabilities of 19, 22, and 18% in time intervals before, during, and after the Na s layers are very close to the average occurrence ratio of the nocturnal E s layer (21%), may reveal a general independence between Na s and E s layers over Qingdao. Only those strong Na s layers above the peak altitude of the main Na layer might have a significant correlation with E s layers. In addition, a total of 11 high-altitude (above 105 km) Na s layer events have been surveyed specially. It is found that these high-altitude Na s layers were usually weak. And they possessed long-duration (> 147 min) and broad-layer width (4.0 km) compared with Na s layers below 105 km (> 96 min and 2.4 km). These characteristics are in accord with observational results at Wuhan. It is suggested that there is little correlation between this kind of Na s layers and E s layers. Finally, the summer topside enhancement phenomenon of Na atoms observed at Qingdao is in accord with several earlier observational results at different sites (18, 30, and 54° N) in the Northern Hemisphere.