Abstract
Observations of sporadic- E ( Es) activity have been carried out during 1970–1974 by using a 25 MHz sounder operating at Iioka, Japan (36°N). Earth backscatter echoes were presented on range-azimuth-intensity plots and recorded on facsimile paper. Examination of individual Es clouds has shown the variations in sizes and lifetimes with time of day and season. Es clouds drifted predominantly toward the west-southwest. Analysis of drift vectors revealed the presence of a north-south variation of the drift direction with a semi-diurnal period. The observed Es cloud motions are consistent with the interpretation that Es clouds move with the neutral winds in the lower E-region. The seasonal dependence of azimuthal occurrence of Es echoes revealed the north-south and east-west difference over Japan. The north-south difference is found to be due to the seasonal movement of the latitudinal position of the maximum Es activity. The east-west difference which is mainly caused by the difference in backscatter coefficient between land and sea was found during only a few non-summer months.
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