Variations of spread-F occurrence rates at near equatorial stations in the Australasia zone

The occurrence rates of spreadF for selected years between 1964 and 1972 are presented for three Australian stations lying between 10° and 25° magnetic latitude. Spreading is mainly of the rangespreading type. Two types of spread F are observed: one occurs during the solstices and correlates negatively with sunspot number; the other occurs during the equinoxes and correlates positively with sunspot number and negatively with magnetic activity. The relative importance of the two types is found to vary with latitude.