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The use of artificial laser guide stars (LGS) eliminates the need for a bright natural guide star (NGS) in adaptive optics (AO) systems. High-power lasers are essential components of most modern AO systems. LGS creation depends on exciting sodium (Na) atoms in the upper atmosphere, making the sodium vertical column density (CNa) a critical parameter. Understanding the return flux from LGSs is vital for designing AO systems, ensuring optimal and stable performance by accurately specifying both the LGS and the AO system. Various studies have focused on sodium availability in the upper atmosphere, showing a strong dependence on the observatory site. It's well-known that CNa varies over different timescales, including hours, nights, months, seasons, and years. Given that some of the world's largest telescopes are located in the Atacama Desert in northern Chile, our goal is to provide CNa statistics relevant to this region. We use telemetry data from the AO systems at Paranal Observatory (24.6°S, 70.4°W): GALACSI (Ground Atmospheric Layer Adaptive Corrector for Spectroscopic Imaging) and GRAAL (Ground Layer Adaptive Optics system Assisted by Lasers). These data are combined with measurements from two space instruments, SCIA-MACHY (SCanning Imaging Absorption Spectrometer for Atmospheric CHartographY) and OSIRIS (Optical Spectrograph and Infrared Imaging System). We carefully analyse and compare these datasets to develop a statistical model for the temporal variations of CNa. We confirm that the AO telemetry data from Paranal systems can be used to determine CNa. Continuous measurements from mid-2017 enabled us to identify monthly and yearly variations and availability of Na in the mesopause region. Throughout the full years of measurement, the annual and semi-annual variations consistently align with previously documented findings in atmospheric studies. By meticulously comparing and fitting various long-term datasets, we created a model illustrating CNa's evolution over time. Our data processing and model are validated, and the results for Paranal's latitude show notable agreement with findings from other studies.
Haguenauer et al. (2024) studied this question.