The geomorphology of spreadF

An analysis has been made of reliable spread-F data obtained from IGY f plots for ionosonde stations grouped about longitudes 75°W and 120°E. The temporal variations of occurrence of the frequency-spreading component of spread F are found to change with latitude, these changes having a certain symmetry about the geomagnetic equator rather than about the geographic or dip equators. Four regions with different occurrence characteristics appear: two are regions of high activity, the auroral and equatorial regions; and the other two are regions of lower activity, the middle latitude and polar regions. The season of minimum occurrence in the equatorial region changes from the northern summer solstice in the American zone (75°W) to the southern summer solstice in the Far Eastern zone (120°E). This longitude effect, which led Reber to postulate the existence of a spread-F equator, is shown to be due to the interaction of some form of direct or indirect solar control with the magnetic control of the incidence of the phenomenon.