Saturation effects in Na lidar temperature measurements

Na atoms residing in the 80-110 km altitude region can be used to probe the Doppler-broadened hyperfine structure (hfs) of their D2 resonance by ground-bases lidar and hence to deduce atmospheric temperatures. In principle, two different methods may be employed: (1) wavelength scanning of the hfs with a narrow-band laser and signal detection with a broad-band receiver, and (2) use of a broad-band laser and wavelength scanning of the hfs with a narrow-band receiver. These two methods are affected in different ways by laser-induced saturation in the Na layer, the effect on the measurements of sodium densities and of atmospheric temperatures being quite different. Density measurements are affected by the absolute level of saturation. Temperature measurements, however, are affected by the difference in saturation at the scanned wavelengths. If, additionally, observed signal levels are taken into account, method 1 is more efficient than method 2 for both types of measurements at nighttime, whereas a modified method 2 surpasses method 1 for temperature measurements at daytime