Abstract
Abstract. This paper reports, for the first time, observational results of mesopause sporadic Na (Nas) 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 Nas layer events. It is found that characteristics of Nas 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 (Es) layers were recorded by an ionosonde. The fact that Es layer occurrence probabilities of 19, 22, and 18% in time intervals before, during, and after the Nas layers are very close to the average occurrence ratio of the nocturnal Es layer (21%), may reveal a general independence between Nas and Es layers over Qingdao. Only those strong Nas layers above the peak altitude of the main Na layer might have a significant correlation with Es layers. In addition, a total of 11 high-altitude (above 105 km) Nas layer events have been surveyed specially. It is found that these high-altitude Nas layers were usually weak. And they possessed long-duration (> 147 min) and broad-layer width (4.0 km) compared with Nas 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 Nas layers and Es 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.
Highlights
Free metal atoms are present in Earth’s mesopause region between 75 and 110 km
Besides this normal layer, Na atoms can be sporadically enhanced within a narrow height range, which has been defined as a sporadic Na (Nas) layer (Hansen and von Zahn, 1990; Clemesha et al, 1999)
In this study we present observational results of Nas layers obtained by ground-based lidar at 36◦ N, as well as the statistical results of correlation between Nas and Es layers
Summary
Free metal atoms are present in Earth’s mesopause region between 75 and 110 km. Among Na, Fe, K, and Ca, Na is most widely observed. A high correlation between the occurrences of Nas layers and Es layers has led to suggesting the neutralization of the Na ion reservoir in the Es layers as a source of the neutral Na atoms (von Zahn and Hansen, 1988; Beatty et al, 1989; Kane and Gardner, 1993; Cox and Plane, 1998; Clemesha et al, 1999; Shibata et al, 2006; Williams et al, 2006, 2007; Delgado et al, 2012; Yuan et al, 2013; Dou et al, 2013). It has been suggested that the high-altitude sporadic metal layers might belong to a different population and have different origin (Hansen and von Zahn, 1990; Kane and Gardner, 1993; Clemesha, 1995).
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