2000A&AS..144..533A

Laser Guide Star Adaptive Optics (AO) makes use of a laser to excite the mesospheric sodium (Na) layer and thereby creates an artificial star that can be used as reference for the AO wavefront sensor. The intensity of the created star depends strongly on the amount of Na present in the atmosphere. Experiments to measure the column density and details of the excitation and scattering properties of sodium atoms in the corresponding atmospheric layer are thus very important to refine the design parameters of laser and assess the power requirement. This article reviews the current knowledge on the mesospheric sodium derived from both astronomical and atmospherical studies. It then presents possible methods to study the sodium layer characteristics relevant for adaptive optics laser guide star systems.