Occurrence of equatorial F region irregularities: Evidence for tropospheric seeding

Abstract

We present a new gap‐free version of the seasonal and longitudinal (s/l) variations of P EFI, the equatorial F region irregularity (EFI) occurrence probability, based on data from the AE‐E spacecraft. The agreement of this and three earlier partial P EFI patterns verifies all four. We reinterpret another earlier gap‐ridden pattern, that of a topside ionogram index of average darkening by range spread F. We comparent with P EFI and, using ionosonde radio science considerations, we conclude that = P EFI times a factor depending on the average number of topside plasma bubbles visible to the ionosonde. The s/l variations of thus imply s/l variations in the average spacing of bubbles, whose seeds have an occurrence probability pattern P seed. For discussion we assume P EFI = P inst P seed, where P inst is the pattern of F region instability. The P EFI 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 north–south asymmetry. We examine evidence behind this dominance, and find it unconvincing. 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 P seed variations influence the P EFI pattern and depend on thermospheric gravity waves from tropospheric convection near the dip equator, then the seasonal maxima (minima) of P EFI could be explained, since they all occur above relatively warm (cold) surface features, where convection is maximal (minimal). Also, the hypothesis of the dominance of the Pseed term could explain an unusual December/January P EFI maximum in the deep, wide, normal Pacific minimum in the one data set obtained in El Niño years. Based on the experiments we consider, we predict that the s/l variations of P seed will be found to be similar to those of P EFI, and largely to explain them. Finally, we find reasons, based on the similarity of the variations to s/l patterns of the average scintillation index, for not using, as is commonly done, such scintillation patterns as substitutes for P EFI or P inst patterns.