Sporadic metal layers in the upper mesosphere

Abstract

One of the most interesting current problems in the aeronomy of the mesosphere is the origin of sporadic Na (Nas) and sporadic Fe (Fes) layers. These are very thin (ca. 1 km), dense layers of atomic Na and Fe which often form explosively in a matter of minutes, may extend horizontally for several thousand kilometres, persist for several hours, and then disappear rapidly. While the typical concentrations of atomic Na and Fe in the upper mesosphere are respectively, 4 × 103 and 104 cm–3, maximum densities exceeding 105 cm–3 have been observed for both species during sporadic layer events. Incoherent scatter radar and lidar observations at Arecibo have shown conclusively that many Nas layers are linked to the formation of sporadic E (Es) layers, while rocket and lidar observations at Andoya have shown a similar link between Fes and Es. Recent lidar observations at Urbana and Hawaii have revealed the simultaneous formation of Nas, Fes, and Ca+ layers and the occurrence of substantial temperature enhancements and wind shears during some Nas events. We present new observations of Nas obtained with a Na wind/temperature (W/T) lidar during the recent ALOHA-93 (1993 airborne lidar and observations of the Hawaiian airglow) Campaign in Hawaii. These observations are used to characterize the wind and temperature structure associated with several of the major Nas events observed during ALOHA-93.