| Abstract
| - Aims. To aid the design of laser guide star (LGS) assisted adaptive optics (AO) systems, we present an analysis of the statistics of the mesospheric sodium layer based on long-term observations (35 years). Methods. We analyze measurements of the Na-layer characteristics covering a long period (1973-2008), acquired at latitude $23\degr$ south, in São José dos Compos, São Paulo, Brazil. We note that Paranal (Chile) is located at latitude $24\degr$ south, approximately the same latitude as São Paulo. Results. This study allowed us to assess the availability of LGS-assisted AO systems depending on the sodium layer properties. We also present an analysis of the LGSs spot elongation over the year, as well as the nocturnal and the seasonal variation in the mesospheric sodium layer parameters. Conclusions. The average values of the sodium layer parameters are 92.09 km for the centroid height, 11.37 km for the layer thickness, and $5\times10^{13}~\rm {\rm m}^{-2}$ for the column abundance. Assuming a laser of sufficient power to produce an adequate photon return flux for an AO system with a column abundance of $4\times 10^{13} ~\rm {\rm m}^{-2}$, a telescope could observe at low geographic latitudes with the sodium LGS more than 250 days per year. Increasing this power by 20%, we could observe throughout the entire year.
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