Properties and dynamics of mesospheric sodium and the impact on sodium LGS AO systems

Adaptive optics (AO) systems correct for atmospheric distortion in real time in order to produce sharper images and enhance scientific capabilities. Sky coverage is greatly increased by the use of laser guide stars generated by resonant excitation of mesospheric sodium atoms. However, physical processes in the upper atmosphere affect the performance of such systems. Competing processes of meteoric ablation that produce, and chemical reactions that remove, sodium atoms, govern the extent and density of the sodium region. The structure of this region is affected by gravity waves, wind shear and turbulence that cause temporal and spatial density variations and thus affect the mean sodium altitude and consequently produce focus-induced wavefront errors, degrading AO performance. In order to better quantify these effects, we developed a high-performance sodium lidar system for the 6-m Large Zenith Telescope. With a power-aperture product more than two orders of magnitude greater than most atmospheric lidar systems, the facility is capable of recording sodium density profiles with sub-second and meter-scale resolution. A recent upgrade enables us to simulate laser guide star asterisms and directly measure horizontal structure variations by chopping a pulsed laser beam horizontally within the 1 arcmin field of view. The 2d mean altitude structure function shows the variance in variation is hence linked to differential focus errors, an important aspect for multi-laser guide star facilities and multi-object adaptive optics.