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