First Mesopause Temperature Profiles From A Fixed Lidar Station At 23º S

The INPE sodium lidar, which has been in operation since 1972, was recently modified so as to enable measurements to be made of the Doppler temperature of the Na atoms in the atmospheric sodium layer. A relatively simple modification to the dye laser transmitter causes it to produce a multi-line output with 1.98 pm spacing between individual lines, each of which has a bandwidth of less than 0.05 pm. By changing the gas pressure in the Fabry-Perot interferometer which produces this line structure, it is possible to switch between a laser output where the lines exactly coincide in wavelength with the Na D2 hyperfine lines, and one where the central laser line is exactly equidistant from the D2 lines. Doppler temperature is measured by determining the change in the effective backscattering cross-section of atmospheric Na atoms when the laser is switched between these two positions. Measurements were made on a total of 15 nights from July to October, 1998, providing temperature profiles between heights of about 83 and 105 km. Almost all measurements showed a mesopause temperature structure strongly perturbed by tides and gravity waves, with peaks in sodium density occurring at almost the same height as the temperature maxima on the bottom side of the sodium layer. The lowest temperatures, between 170 and 200 K, typically occur above 100 km, and the average profile for all our measurements is similar to the winter profile seen at mid-latitudes, with a mesopause temperature of 190 K at 103 km.