The nocturnal intermediate layer over South Georgia

Detailed analyses of ionograms from South Georgia (54°S, 37°W; L = 1.9) show that a nocturnal intermediate layer is frequently seen between the E- and F-regions in the height range 130–180 km. The occurrence of the intermediate layer is almost totally restricted to the winter months and between the local hours of 1930 and 2300. The intermediate layer event is characterized by a prior downward movement of the F-layer, followed by the formation of the intermediate layer and its subsequent drift downwards to about 140 km. Initially, the layer is thick (~ 10–20 km), but gradually thins as it evolves. The occurrence of the intermediate layer appears to be independent of the level of magnetic activity. Observations of intermediate layers from other localities, which are briefly reviewed, are compared with those made at South Georgia. Some similarities and differences are identified. Mechanisms which account for the formation of the intermediate layer at South Georgia are considered, It is shown that the main features of the diurnal and seasonal variations of occurrence can be adequately explained by the theory of convergence of ionization driven by the solar semi-diurnal tidal winds. Some limitations of this theory are discussed, as it cannot account for all the features seen at this southern midlatitude observatory.