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Chiao-Yao (Joe) SheProfessor 
Laser Techniques for Atmospheric MeasurementsWe have been interested in new laser spectroscopic techniques for investigating basic as well as applied problems. In recent years, in collaboration with Prof. David Krueger, we have concentrated on new laser techniques that could provide reliable measurements to impact important atmospheric issues. Thus, the thrusts of our research are: developing new laser-based technique for atmospheric measurements and using the unique data set we collect to study important geophysical problems.Temperature and thermal structure of the atmosphere is of obvious importance. We are demonstrating a ground-based lidar (laser radar) station that could monitor atmospheric temperatures from the earth's surface to the mesopause region some 100 km up. The existing lidar technique for temperature measurements relies on Rayleigh scattering for measuring air-density profile from which the atmospheric temperatures are calculated by assuming hydrostatic and local thermal equilibrium. This approach fails for altitudes below 20 km and above 80 km due, respectively, to the interference of aerosol particles and to the lack of Rayleigh signal. We remedy these short-comings by inventing new techniques. For altitudes below 20 km, we use a narrowband laser and a narrowband atomic vapor filter to separate molecular scattering from aerosol scattering so that air temperatures can be measured directly. We call this a high spectral resolution lidar. The feasibility of this method has been proven recently, and we are improving its accuracy. For altitudes above 80 km, we use a narrowband tunable laser to induce fluorescence emission of natural Na atoms existed in the mesopause region (between 80 to 110 km, a region too high for airplane and balloon and too low for satellite) for temperature measurements. The photo, left, shows our Na lidar at work, recently modified to measure wind and temperature simultaneously. The Na temperature lidar, which has been recently installed by researchers in Illinois and Canada as well, has been in regular operation for more than two years. The data collected reveal the thermal structure of a mid-latitude mesopause for the first time. We found two temperature minima in this region. The measured seasonal temperature variations of the dominant (lower) minimum at 86+2.5 km is shown in the figure above, where the vertical bars are not measurement errors but standard deviations from daily mean (data point) and Julian Day - starts from January 1, 1991. Notice that up there summer is cooler (minimum at summer solstice) than winter by 40 K! Long-term monitoring of the temperature in the mesopause region should provide reliable information on global circulation, solar-terrestrial interaction and global change. In addition, we are also developing new techniques for measuring winds and other atmospheric parameters, using molecular filter, Na vapor Faraday filter, and acoustoopitic frequency shifter, all conceived and developed in our laboratory. Work on coherent Rayleigh-Brillouin gain spectroscopy which we invented in the mid-eighties is still continuing. Currently, we are investigating the dynamics of simple liquids under high pressures using this technique. |