Long-term variations of the nighttime upper-atmosphere sodium emission

Based on systematic measurements from 1957 until 1992, we determine charact eristics of the regular variations in monthly mean intensities of the mesos pheric sodium emission at nighttime. Seasonal correlations with solar activ ity and with the long-term trend have been found. Estimation of the tempera ture variations at heights of similar to 92 km shows that the long-term tre nd is nearly zero in summer, in agreement with data for other emissions, OH and OI 557.7 nm.