Occurrence of equatorial F region irregularities: Evidence for tropospheric seeding

We present a new gap-free version of the seasonal and longitudinal (s/l) va riations of P-EFI, the equatorial F region irregularity (EFI) occurrence pr obability, based on data from the AE-E spacecraft. The agreement of this an d three earlier partial P-EFI patterns verifies all four. We reinterpret an other earlier gap-ridden pattern, that of (D) over bar(RSF), a topside iono gram index of average darkening by range spread F. We compare it with P-EFI and, using ionosonde radio science considerations, we conclude that (D) ov er bar(RSF) = P-EFI times a factor depending on the average number of topsi de plasma bubbles visible to the ionosonde. The s/l variations of (D) over bar(RSF) thus imply s/l variations in the average spacing of bubbles, whose seeds have an occurrence probability pattern Pseed For discussion We assum e P-EFI = PinstPseed, where P-inst is the pattern of F region instability. The PEFI pattern, which is by definition independent of seed and/or bubble spacing, is far too complex to be explained by the dominant paradigm, that of changes in P-inst by simple changes in the F region altitude and/or nort h-south asymmetry. We examine evidence behind this dominance, and find it u nconvincing. Both the asymmetry and sunset-node/altitude hypotheses of 1984 and 1985, respectively, seem to be partly based on misunderstood data, and their features appear displaced in time and space from those of our repeat able P-EFI pattern. In contrast, if Pseed variations influence the P-EFI pa ttern and depend on thermospheric gravity waves from tropospheric convectio n near the dip equator, then the seasonal maxima (minima) of P-EFI could be explained, since they all occur above relatively warm (cold) surface featu res, where convection is maximal (minimal). Also, the hypothesis of the dom inance of the Pseed term could explain an unusual December/January P-EFI ma ximum in the deep, wide, normal Pacific minimum in the one data set obtaine d in El Nino years. Based on the experiments we consider, we predict that t he s/l variations of Pseed will be found to be similar to those of P-EFI, a nd largely to explain them. Finally, we find reasons, based on the similari ty of the (D) over bar(RSF) variations to s/l patterns of the average scint illation index, for not using, as is commonly done, such scintillation patt erns as substitutes for P-EFI or P-inst patterns.