  Detailed knowledge of the mesopheric sodium layer characteristics is crucial
to estimate and optimize the performance of Laser Guide Star (LGS) assisted
Adaptive Optics (AO) systems. In this paper, we present an analysis of two sets
of data on the mesospheric sodium layer. The first set comes from a laser
experiment that was carried out at Cerro Tololo to monitor the abundance and
altitude of the mesospheric sodium in 2001, during six runs covering a period
of one year. This data is used to derive the mesospheric sodium column density,
the sodium layer thickness and the temporal behavior of the sodium layer mean
altitude. The second set of data was gathered during the first year of the
Gemini MCAO System (GeMS) commissioning and operations. GeMS uses five LGS to
measure and compensate for atmospheric distortions. Analysis of the LGS
wavefront sensor data provides information about the sodium photon return and
the spot elongation seen by the WFS. All these parameters show large variations
on a yearly, nightly and hourly basis, affecting the LGS brightness, shape and
mean altitude. The sodium photon return varies by a factor of three to four
over a year, and can change by a factor of two over a night. In addition, the
comparison of the photon returns obtained in 2001 with those measured a decade
later using GeMS shows a significant difference in laser format efficiencies.
We find that the temporal power spectrum of the sodium mean altitude follows a
linear trend, in good agreement with the results reported by Pfrommer & Hickson
(2010).
