The morphology of spread-Foccurrence over half a sunspot cycle

An analysis has been made of bottomside spread-F data obtained at sunspot minimum at stations separated in latitude but grouped about 80° west longitude. At this part of the solar cycle, the temporal variations of the occurrence of the frequency-spreading component of spread F change with geomagnetic latitude in a manner comparable to that at sunspot maximum, the main differences being the following. The low-latitude region of high occurrence expands to higher latitudes as the sunspot number decreases. The high-latitude region of high occurrence is not so strong in sunspot minimum as in sunspot maximum. A third region of high occurrence about 10° latitude wide centered on 50° geomagnetic latitude, which is not so obvious at sunspot maximum, appears at sunspot minimum. This midlatitude region of occurrence on the bottomside is identified with the topside spread F attributed to the ducting process. A comparison of the high- and low-latitude regions of high bottomside frequency-spreading occurrence with their topside counterparts results in the conclusion that, at these latitudes, the occurrence properties of topside ducting are not substantially different from those of topside scattering.