On the basis of 331 equatorial passes of two Defense Meteorological Satellite Program satellites at 840 km, we have determined that two distinct types of topside ion depletions occur in the evening sector during equinoctial periods. First, during the early evening (1854 LT) a smooth type of depletion occurs with over 80% probability of observation per pass. This feature is aligned parallel to the magnetic equator and is probably related to the equatorial fountain effect and general F region dynamics. Second, we have seen irregular depletions later in the evening (1936-2154 hours) which are the topside signatures of spread F. Their total probability of occurrence per pass increases from ~20% at 1936 hours to ~60% at 2154 hours. A statistical study of the entire data set has shown that spread F depletions occur only when the topside ion density surpasses a seasonally controlled threshold. Therefore longitudinal and local time variations of topside ion density are reflected in similar variations in the occurrence probability of spread F as reported both here and earlier. A detection model including orbit-feature intersection geometry, the above mentioned threshold, and Poisson statistics has been empirically verified.