Generalization and critique of the wind-shear theory of sporadicE

Previous discussions of the hypothesis that sporadic E layers result from a redistribution of existing ionization caused by vertical ionization drift have rested on special and somewhat unrealistic assumptions. This paper formulates the hypothesis in a more general way and compares its implications with recent experimental results concerning the structure of E8 layers and the connection between ionization profiles and wind profiles in the E region. The main results of the paper are as follows, (a) The predicted ionization profiles have cusp-like peaks (rounded off by diffusion) of variable width, the width of a peak being inversely proportional to the square of its height. Theory also predicts a substantial reduction of the electron density on either side of a peak. By contrast, observed ionization profiles have nearly rectangular peaks of uniform width ( ≲1 km) and variable height (1 ≤ Nmax/N0 ≲ 102) superimposed on an apparently unperturbed background. (b) Theoretically, layers of enhanced ionization will form in regions where the wind shear is exceptionally high. Observation shows that the reverse is true. (c) The fractional changes that can be produced by vertical ionization drift appear to be considerably smaller than those observed to be associated with E8 layers. A related hypothesis, that layers of enhanced ionization form through the trapping of vertically drifting ionization by narrow turbulent layers, also conflicts with experiment.