Long-term variations in the centroid height of the atmospheric sodium layer

In earlier studies, an analysis of the long-term variation in the centroid height of the atmospheric sodium layer suggested the existence of a secular decrease in the height of the layer. The analysis of lidar data obtained between 1972 and 1994 indicated a fall of 740 m over this 22 year time period. A possible solar cycle variation with an amplitude of 170 m was also detected, although with low statistical significance. A new analysis, in which care was taken to exclude days in which sporadic sodium layers were present, and including measurements made up to the end of 2001, shows that the linear trend has not continued. The fall in centroid height observed in the early data is found to be compensated by a higher values observed since 1988, and the overall linear trend for 1972 to the end of 2001 is now only 93 53 metres/decade. This new result strongly suggests that the previously detected trend was not a consequence of long-term global cooling of the upper atmosphere. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.