Long-term and solar cycle changes in the atmospheric sodium layer

A new analysis of atmospheric sodium measurements, made at Sao Jose dos Campos (23 degrees S, 46 degrees W) since 1972, confirms the previously detected trend in the centroid height of the layer (near 92 km altitude) and shows the existence of a 10-yr solar-cycle related oscillation. The centroid height fell at an average rate of 37 +/- 9 m yr(-1) between 1972 and 1994, and the 10-yr cycle has an amplitude of 170 +/- 110 m. An analysis of the vertical distribution of atmospheric sodium shows that the fall in height of the centroid is not caused by a simple vertical displacement of the sodium profile, but by the growth of a bulge on the bottomside of the layer. (C) 1997 Elsevier Science Ltd.