An investigation of atmospheric turbulence in the 100-105 km region over Cerro Pachón, Chile

In this study, turbulence activity is measured from 100 to 105 km altitude using a sodium (Na) resonance-fluorescence lidar system. Previous approaches have been limited to altitudes below 100 km due to sparse Na layer density above 97 km. Using the lidar system at the Andes Lidar Observatory in Cerro Pachón, Chile (30.3ºS, 70.7ºW), a novel reconstruction algorithm measuring turbulence fluctuation power at smaller signal-to-noise ratios (SNRs) than existing methods is derived and validated. Results are compared against instability, Na mixing ratio, and total constituent diffusion coefficient measurements. Twenty-seven (27) nights of lidar data spanning 2500 hours in the zenith and 2375 hours in the off-zenith directions at 25 m, 6 s resolution were analyzed to determine mean turbulence and instability trends in the mesosphere and lower thermosphere (MLT) region. It was found that average turbulence activity increases in a log-scale linear fashion between 93 and 100 km altitude, maximizing at 100 km. Above 100 km, turbulence power decays to a local minimum at 102.5 km. Richardson number and Na mixing ratio profiles can successfully identify regions of increased turbulence activity in the 85-105 km region, but fail to determine the relative turbulence power of the regions. Between 100 and 105 km, the atmosphere shifts from eddy diffusion-dominated to molecular diffusion-dominated. Turbulence activity modulates constituent (eddy) diffusion rates, affecting the balance between eddy and molecular diffusion. Fluctuations in this balance impact the diffusion rates of minor species at higher altitudes, altering middle- and upper-atmospheric composition. Measurements of turbulence activity in the 100-105 km region improve understanding and modeling of atmospheric variability