Iron and Sodium Doppler/temperature LIDAR Studies of the Upper Mesosphere

Lidar measurements of mesospheric Fe were conducted at Urbana, Illinois, during four nights in late October 1989. The average Fe abundances, layer centroid heights and rms widths varied between 1.0-2.0 times 10^{10} cm ^{-2}, 89.0-90.5 km and 3.2-4.1 km, respectively. The peak densities of the layer near 90 km varied between 15-25 times 10^3 cm^{-3 }. On three nights the formation and dissipation of three large sporadic Fe (Fe_{rm s}) layers were observed. A new two-frequency lidar technique for measuring mesospheric Na temperature profiles is described. This system uses a stabilized cw single-mode dye laser oscillator (rms frequency jitter <1 MHz) followed by a pulsed dye power amplifier (140 MHz FWHM linewidth) which is pumped by an injection-seeded Nd:YAG laser. The laser oscillator is tuned to the two operating frequencies by observing the Doppler-free structure of the Na D _2 fluorescence spectrum in a vapor cell. The lidar technique and the initial observations are described. Absolute temperature accuracies at the Na layer peak of better than +/-3 K with a vertical resolution of 1 km and an integration period of approximately 5 min were achieved in this initial experiment. Fourteen nights of Na temperature lidar data spanning 6 months were collected at Urbana, Illinois, using the two-frequency technique. The temperature structure over the 80-100 km region was highly variable due to the influence of gravity waves, tides, and sporadic layers (Na _{rm s}). Extreme temperatures of 140 K and 255 K were measured. A mesopause cooling rate of 9 K/month was exhibited from winter to summer at the mesopause height localized between 85 and 90 km. The temperature dependence of the Na column abundance is analyzed, and a 9 times 10^7 atoms/cm^2/K Na abundance rate of change was observed from winter to summer. The rms perturbations, wave dynamics, and Brunt-Vaisala periods are also investigated. The first observations of vertically resolved Na density, temperature, and wind profiles in the mesopause region were made using a new four-frequency Doppler/temperature lidar. The initial zonal wind and temperature observations were obtained with a profile resolution of 30 min and 1.5 km and rms accuracies of 12 m/s and 2 K. These measurements were conducted simultaneously with the medium frequency (MF) radar at the Urbana Atmospheric Observatory (40 ^circN, 88^circ W) during the night of 14-15 March, 1991. Although both the lidar and radar wind measurements exhibited the same general trends, the velocity estimates differed by as much as 40 m/s at some times during the night.