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The day-to-day variability observed in the structure of the equatorial anomaly during a period of high solar activity is investigated. Using hourly values of ƒ0F2 observed at a number of ionosonde locations in the American and Japanese longitude sectors during 1958, the structure of the equatorial F2 region has been obtained on an hourly basis for an entire year. It is observed that the equatorial anomaly shows considerable day-to-day fluctuations in both its structure and its diurnal development.

The anomaly is generally best developed in the evening hours, and day-to-day changes in the time of best development are typically on the order of 2 to 4 hr. It is observed that the time when the anomaly attains its maximum development is not the same on both sides of the magnetic equator, although this time is closer to coincidence during the equinoctial months than during the solstices. Also during the equinoctial months, the anomaly crests tend to be more symmetric about the equator than during the solstices. The relative magnitudes of the crests display a noticeable changeover with time during the northern solstice in both longitude sectors. The crest on the winter side of the magnetic equator is larger during the day, and the summer crest is larger at night. The exact time that this changeover occurs varies on the order of 5 to 7 hr on a day-to-day basis. During the southern solstice this crest changeover is observed only in the Japanese sector.