The F2-region equatorial anomaly during solstice periods at sunspot maximum

Abstract

The diurnal development of the F2-region equatorial anomaly is studied for the African, American, and East Asian sectors during quiet days about the June 1958 and December 1958 solstices. It is found that marked asymmetries in the daytime anomaly can occur with the ƒ 0F2 crest on the winter side being larger than on the summer side, and h m F2 at the winter crest being about 100 km lower than at the summer crest. Such asymmetries are observed only in the American sector during the June solstice period, and in the African and East Asian sectors during the December solstice period. At these times the sub-solar point is far removed from the magnetic equator. The daytime anomaly in ƒ 0F2 shows little or no asymmetry in the American sector during the December period or the African and East Asian sectors during the June period, but in each case h m F2 at the winter crest is higher than at the summer crest. Furthermore, during the subsequent evening hours in these sectors a marked enhancement of the winter crest occurs and h m F2 at the winter crest becomes lower than that at the summer crest. These results are briefly discussed in terms of ionization transport by neutral winds, differences in electron temperature, and other phenomena believed to contribute to the observed asymmetries.