Most recent Laser Guide Star (LGS) systems for astronomical Adaptive Optics (AO) makes use of mesospheric scattering of a sodium laser (at the Multi-Mirror Telescope (MMT), e.g. Lloyd-Hart et al. 1998, at Lick Observatory, e.g. Olivier et al. 1996, or Calar Alto, e.g. Davies et al. 1998), whereby an artificial reference star, which can be used as reference for the AO wavefront sensor, is created at the altitude of the mesospheric sodium layer (around 95 km in average).
The optimisation of adaptive optics systems using Na guide stars requires a detailed knowledge of the temporal and spatial characteristics of the atmospheric Na layer on both long and short-term scale. Experiments to measure the column density and details of the excitation and scattering properties of sodium atoms in the mesospheric layer are thus very important to refine the design parameters of lasers for laser guide star systems. Indeed the intensity of the artificial star depends strongly on the amount of sodium present in the atmosphere.
Seasonal variations of both the column density and the altitude of the mesospheric Na, which impact respectively on the necessary LGS power and its focus altitude, can be studied in a statistical way. The daily and hourly variations also bring information on the variation of the averaged column density and height, but on short timescale and therefore become important for queue scheduling of LGS-AO observations, as planed e.g. at the European Southern Observatory (ESO) Very Large Telescope (VLT).
Study of the atmospheric sodium is considered here under two different aspects:
This paper aims at reviewing the current knowledge on atmospheric sodium and its impact on laser guide star adaptive optics system operations. The sodium characteristics that have a direct impact on LGS AO observations are presented in Sect. 2. Section 3 will illustrate our on-going work among statistical studies. Section 4 concentrates on the monitoring problem and states some possibilities for "on-line'' study of mesospheric sodium.
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