Preliminary results of the 2001-2002 Gemini sodium monitoring campaign at Cerro Tololo, Chile

In the near future several astronomical observatories in Chile are planning to use sodium laser guide stars to increase the sky coverage provided by their adaptive optics facilities. Knowledge of the mesospheric sodium layer behavior is crucial to predict the performance of future laser guide star adaptive optics systems. Whereas the sodium layer has been observed quite extensively at several locations, many of them in the Northern Hemisphere, very little measurements have been made in Chile. The Gemini Observatory therefore initiated a year-long sodium monitoring campaign at the Cerro Tololo Inter-American Observatory located only a few kilometers away from the Gemini South telescope where a conventional laser guide star facility will be offered to the community in 2005, soon to be upgraded to a multi-conjugate adaptive optics system with five laser guide stars. This paper reports on the laser-based sodium monitoring experimental set up and data reduction techniques, and presents some preliminary results on the sodium column density and layer altitude variations observed from February 2001 to February 2002. Implications for the Gemini South Adaptive Optics system expected performance are presented as well.