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 an d 85 km altitude. These lidars operate near 308 nm and OH is observed through laser-induced-fluorescence on the A(2) Sigma(+)-X-2 Pi(0, 0) b and. The results expose a valuable global set of nighttime OH observat ions, since existing long-term lidar data at several NDSC sites contai n the (serendipitous) OH information. Results of lidar observations ar e presented from two mid-latitude sites, one in each hemisphere: Table Mountain (34 degrees N), California, and Lauder (45 degrees S), New Z ealand. They show observations of a geometrically thin (similar to 3 k m) nocturnal layer of OH near 80 km. For the Table Mountain observatio ns, the derived values for the OH density at 80 km typically are 2 - 4 x 10(5) cm(-3) which is in accordance with model predictions [Dodd et al., 1994]. The temporal behavior of the mesospheric OH signal, follo wing sunset; that was found, supports previous model predictions [Alie n et al., 1984] in a qualitative fashion.