Double sporadic metal layers as observed by colocated Fe and Na lidars at Wuhan, China

In this paper, we report a set of double sporadic layer events observed by Fe and Na lidars over Wuhan, China. The two sporadic metal layers above normal layer were named as upper and middle sporadic metal layers, respectively. In these events, the upper, middle, and normal Fe layers presented altitude separately. There were nine double sporadic Fe events observed in 163 nights during 2010–2013. Eight of the nine events were observed in summer. The maximum ratios of peak density for upper and middle sporadic Fe layers to normal Fe layer were up to ~375% and ~225%, respectively. The peak altitudes of upper (middle) sporadic Fe layers were in the range of 102–107 km (95–98.5 km). The double sporadic Fe layers lasted more than 2 h. Interestingly, we found that density enhancement occurred simultaneously in upper, middle, and normal Fe layers on two events. On the nine Fe events, there existed five nights of colocated Na lidar observations. We found that double sporadic Na and Fe layers simultaneously appeared. They presented similar structures, altitudes, and temporal variations in all five compared events. A little different from Fe, the middle sporadic Na layer was not separated from Na main layer maybe for the wide altitude range of Na main layer. The ratios of upper (middle) sporadic Fe and Na peak values were in the range of 6.6–52 (0.57–6.58). While the exact formation mechanism responsible for double sporadic metal layers is still unclear, some possible explanations and corresponding observations are discussed.