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The increase of solar activity from the sunspot minimum of 1933 to 1938 resulted in large increases in the ionization of the upper atmosphere. During this time the ionization densities of the E layer increased by a ratio of approximately 1.55 to 1 and those of the F, layer by about 4 to 1. Superposed on this longperiod increase were the more or less regular diurnal and seasonal variations. Graphs indicating the long-time variations for the different layers at several different times of day are shown.
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Smith et al. (1938) studied this question.
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