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


T. Pfrommer and P. Hickson




Adaptive optics 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, the performance of such systems is affected by physical process in the upper atmosphere. The extent and density of the sodium region is governed by competing processes of meteoric ablation that produce, and chemical reactions that remove, sodium atoms. The structure of this region is affected by gravity waves, wind shear and turbulence. Temporal and spatial variations in the mean sodium altitude produce focus-induced wavefront errors, degrading performance. Spatial density variations result in laser guide star image structure that reduces wavefront sensing accuracy. 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. Results from three years of observations will be presented. These reveal striking variability in the sodium region due to dynamical processes, strong nightly variations, and sporadic events. Meteor ablation trails produce strong spikes in sodium density on sub-second time scales. A recent upgrade now enables us to simulate laser asterisms and directly measure horizontal structure variations. Such variations can produce differential focus errors, an important aspect for multi-laser guide star facilities and multi-object adaptive optics.

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