First lidar observations of mesospheric hydroxyl

Abstract

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 ²Σ + − X ²Π(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.