Mid‐latitude lidar observations of large sporadic sodium layers

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Abstract
During the early morning of October 31, 1988 two large sporadic Na (Nas) layers were observed near the mesopause above Urbana, IL (40°N, 88°W) with a Na lidar system. The layers began forming near 102 km at 0026 LST and 0110 LST and moved downward with vertical velocities as high as 4 ms−1 before dissipating between 94 and 96 km. The duration of each layer was approximately 80 min. The layers were narrow (∼ 1 km FWHM) and dense with maximum densities approaching 7800 cm−3. The characteristics of these two Nas layers are very similar to those of similar phenomena observed recently at Andoya, Norway [von Zahn et al., 1987] and Mauna Kea, Hawaii [Kwon et al., 1988]. Lidar observations of the mesospheric Na layer have been conducted routinely by several groups at mid‐latitudes for almost 20 years. Although large Nas layers now appear to be relatively common at low‐ and high‐latitudes, to our knowledge the two layers described in this letter are only the second observation of this puzzling phenomenon at mid‐latitudes.