Structure of the mesospheric Na layer at 40 degrees N latitude: Seasonal and diurnal variations

Lidar observations obtained throughout the diurnal and annual cycles at Urb ana, Illinois (40 degrees N, 88 degrees W), are used to examine the structu re of mesospheric Na between 76 and 108 km. The Na layer is strongly influe nced by seasonal and diurnal variations in solar illumination and by season al variations of mesospheric temperatures. Although there is little evidenc e of direct tidal perturbations in Na density, 24 hour oscillations dominat e the diurnal variations. Na abundance and densities above 90 km are contro lled by photoionization and charge exchange reactions during the day and re combination at night. The abundance is maximum at sunrise, and the peak-to- peak diurnal variation averages more than 30%. Below 85 km, density variati ons are controlled by photochemistry which interrupts the conversion of Na to the bicarbonate reservoir, and by wanner temperatures which enhance the liberation of Na from the reservoir. In this region the peak-to-peak diurna l Na variations can exceed 200%. The combined effects of photoionization ab ove 90 km sind photochemistry below 85 km induce a strong 24 hour oscillati on in the layer centroid height. The peak-to-peak variation exceeds 1 km. T he lowest centroid height is at local noon when Na densities below 85 km ar e maximum and photoionization above 90 km is strongest. The seasonal variat ions in Na densities and abundance are influenced primarily by changes in m esopause region temperatures which are coldest during midsummer. Below 95 k m, reactions leading to the bicarbonate sink and the liberation of Na from this reservoir dominate Na chemistry. These reactions and their temperature dependencies are responsible for the large annual variation in Na abundanc e (2.3 x 10(9) to 5.3 x 10(9) cm(-2)). The measured correlation between Na abundance and the layer weighted temperature is +0.89. The ratio of the abu ndance to temperature variations is 1.9 x 108 cm(-2) K-1. The seasonal Na d ensity and temperature perturbations are well-correlated (+0.6 to +0.9) bel ow 98 km and negatively correlated above (-0.2 to -0.5).