The sodium layer considered here is situated in the mesosphere at 91.5±10 km and has a mean column density of 3×10 9 cm – 2 , value that is strongly latitude dependent. However this layer ‘suffers’ from seasonal, daily and short-term variations. The seasonal variations are ‘sinusoidal’ and affect the sodium column density, the average centroid position of the layer and its thickness (see e.g. 1 Papen et al. 1996).

The sodium chemistry is known to be a sensitive function of temperature and the seasonal temperature variations appear to be largely responsible for the seasonal variations in the Na abundance which is maximum in winter (i.e. July-August in Chile). Papen et al. (1996) quote a mean sodium column density (at 40 degrees latitude North) of 4.3×10 9 cm – 2 (with variations from 1 to 8×10 9 cm – 2 ) and a mean layer altitude of 91.7 ± 2 km. Measurements of sodium column density presently available for La Silla show variations from 1 to 4.5×10 9 cm – 2.

Variations of the centroid position of the layer have a direct impact on the focus for laser guide star. 2 Ageorges et al. (2000) quote variations of the centroid altitude position of the layer of up to 400m in 1 to 2 minutes (at 37 degrees North). For Laser Guide Star Adaptive Optics, the short-term variations of the atmospheric sodium are the most worrisome. These variations can be classified in two types: the daily and ‘hourly’ ones.Gravity waves are believed to be responsible for the daily modification of the Na layer, even though it is not yet clear how. Moreover 3 Qian et al. (1998) conclude that these waves play an important role in the formation of sporadic Na layers (Na s ). These are very thin (0.5 to 2 km thick) Na layers superposed to the mean mesospheric sodium layers. They are characterized by a rapid increase in sodium density over a narrow altitude range. They can last few seconds but in average few minutes up to few hours. The ‘hourly’ variations of the mesospheric sodium layer, mentioned above, are clearly dominated by these sporadic layers. Sporadics have been detected more frequently at high and low latitude than mid-latitude sites. It has long been recognized that many Na s are associated with sporadic E layers. It has therefore been suggested that the apparition of these layers might be related to magnetic latitude more than to geographic latitude ( 4 Kwon et al. 1998). With the exception of 4 Kwon et al. 1988, none of the other authors mentioning sporadics did see any ‘time’ correlation for their occurrence; neither does it appear to have significant seasonal difference in occurrence frequency.

However many groups have measured enhancement of the sodium concentration during meteor showers (see e.g. 5 Michaille et al. 2001). This can be understood since meteorite ablation is considered as the main source of mesospheric sodium. Sporadics Na layers will be the most affecting effect for LGS AO; see 6 Clemesha et al. 1995 for a review of this still puzzling effect. Moreover, a sidereal motion is required for the LGS to follow the science object, which produces an horizontal displacement of the probed area in the sodium layer, and thus a possible increase of high frequency density variations which have to be studied.

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