A recently proposed means of combining models of ionospheric F-layer peak electron density and irregularity incremental electron density (ΔN) so as to simulate the global occurrence probability of the frequency-spreading component of spread-F is discussed. This procedure is then used to model experimental spread-F occurrence results. It is found possible to readily simulate the sunspot-maximum results, independently of season, with only small adjustments to the amplitudes of the empirical expressions used to model ΔN in the several latitude regimes. However, at sunspot minimum and for each season, the ΔN model requires modification in the equatorial and midlatitude regions of high irregularity incidence, before successful simulations of the spread-F data can be obtained. These modifications, which include a broadening of the equatorial region and a poleward shift of the midlatitude region with decreasing sunspot number, are discussed in detail. It is concluded that the scintillation data base, from which the original ΔN model derives, is not sufficiently representative with regard to sunspot number and magnetic index. The use of the spread-F adaptation of the ΔN model, as well as its original scintillation version, to rectify these failings of the ΔN model are also discussed.
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