Some anomalies in the E-layer of the ionosphere

If in the course of our investigation some problems have been clarified it is evident from the foregoing discussion that many characteristics of the E -layer remain mysterious, either having several equally plausible explanations or none at all; and there is no dearth of theoretical problems awaiting solution. Let us however summarize briefly the salient features of the present position.

On the theoretical side we can now, with the aid of the formulae developed in Chapter 2, readily compute values of the maximum electron density Nᵐ throughout the day for any assigned variation of electron production q with solar zenith distance X, assuming a recombination law of electron loss. As a result, particularly with the extension of the calculation to the non -stationary conditions near sunrise and sunset, we may be certain that neither the anomalous variation of Δ fE nor the high exponent of cos X can be put down to inaccuracies of the theory. Hence we have been led to seek their origin elsewhere.

Undoubtedly the main step forward in this respect has been the identification of the pre-noon perturbations in Nᵐ and in the height of the layer as effects of the vertical drift of neutral ionization associated with the Sq current system. This not only accounts for several previously unexplained anomalies, including increased values of fE near the equator, but is also perhaps of somewhat broader scientific interest; for it provides independent evidence of the existence and the location of these ionospheric currents previously only inferred from the data of the magnetic variations at the ground.

This identification was first made on the basis of a statistical investigation of routine data; but the special measurements made at Slough have provided useful confirmation and have shown that the predicted dip in hᵐ does in fact occur. Our theoretical investigations of drift have also contributed by clarifying the nature of the drift effects and by enabling us to calculate the magnitudes of the drift velocity and of its gradient.

As to the high exponent of cos X fit, the September results have suggested as a possible explanation that stratification of the layer in the morning and evening may lead to incorrect readings of fE. It seems that if at such times the upper discontinuity is taken to indicate fE instead of the lower one selected in normal practice, then the anomalous variation with cos X would disappear. A decision on this question must however await further evidence.

Finally we have the low values of ΔfE at low cos X( leading to an apparent rapid increase of recombination coefficient with height; but so far no satisfactory explanation of this anomaly has been forthcoming, and there is only the somewhat nebulous suggestion that it may be related to the curious "ledging" phenomena which develop in the afternoon period.

Ledging may also be responsible for other unexplained features such as the unexpected pre-noon dip in yᵐ, and the anomalo!is variation of h'E in the September data. Indeed it is hardly possible to study a large number of E-region records without becoming convinced that stratification, including all the phenomena variously described as "Eˢ ", is a major problem for future investigation. It may be that, following clarification of the Sq effects, little progress will be made in understanding other peculiarities of the region until the problem of these various subsidiary layers, or "ledges", has been solved.