First lidar observations of mesospheric hydroxyl

Ground-based lidars have been used to detect and identify ground-state (v" =0) hydroxyl radicals (OH) in the mesosphere between about 75 and 85 km altitude. These lidars operate near 308 nm and OH is observed through laser-induced-fluorescence on the A ²∑ ⁺ -X ²II(0, 0) band. The results expose a valuable global set of nighttime OH observations, since existing long-term lidar data at several NDSC sites contain the (serendipitous) OH information. Results of lidar observations are presented from two mid-latitude sites, one in each hemisphere: Table Mountain (34°N), California, and Lauder (45°S), New Zealand. They show observations of a geometrically thin (∼3 km) nocturnal layer of OH near 80 km. For the Table Mountain observations, the derived values for the OH density at 80 km typically are 2 - 4 × 10 ⁵ cm ^-3 which is in accordance with model predictions [Dodd et al., 1994]. The temporal behavior of the mesospheric OH signal, following sunset, that was found, supports previous model predictions [Allen et al., 1984] in a qualitative fashion.