CEDAR lidar observations of sporadic Na layers at Urbana, Illinois

In March and April of 1988 five short‐lived sporadic Na layers were observed in the late evening above Urbana, IL (40°N, 88°W) with the new CEDAR lidar instrument. The layers were very narrow (∼140 m FWHM) and very dense with maximum densities exceeding 24,000 cm−3. The layers formed in the late evening between 2100 LST and midnight at altitudes between 82 and 89 km. The average duration of the layers was 100 s. Some of the layers moved rapidly, both upward and downward, with velocities as high as 9 ms−1. Compared to similar phenomena observed at Andoya, Norway [von Zahn et al., 1987] and Mauna Kea, Hawaii [Kwon et al., 1988], the sporadic layers above Urbana appear to occur much less frequently at generally lower altitudes, contain less Na and are of much shorter duration. The five sporadic Na layers described in this letter may have resulted from the advection of meteor trails across the lidar field‐of‐view.