Simultaneous and common-volume three-lidar observations of sporadic metal layers in the mesopause region

We report on the first simultaneous three-lidar observations of sporadic metal layers in the mesopause region. The case studies indicate that the sporadic layering events can be observed generally in three neutral metal atom species (Na, Fe and Ca) or two neutral atom and one ion species (Na, Fe and Ca+). The density enhancements of all the sporadic metal atom and ion species occurred in overlapping altitude range and moved following almost the same track, indicating that the sporadic metal layers are usually a mixture of multiple metal atom and ion species, meanwhile suggesting that all these metal species in a mixture are product of the same or similar source processes. Some strong multi-metal sporadic layering events were found to manifest a regular altitude relation that the Nas is highest, the Fes is a few to tens of hundreds of meters lower than the Nas, while the Cas is a few hundred meters lower than the Fes, and the Cas+ is lowest. This altitude sequence coincides well with the boiling-point dependent differential ablation in the thermal ablation theory. For those weak multi-metal sporadic events, the altitude relation becomes complicated. Profiles from two short sequences of a single night show that the formation/variation of the Cas+ layer features appeared to follow the related Nas and Fes layer features with a delayed period of ∼5 min. At altitudes above 105 km, weak single-species Cas+ layers were occasionally observed. This appears to reflect a feature of the sputtering ablation. For the explanation of our observations the formation mechanism issues of sporadic metal layers are discussed in detail.