Structure of the Ionosphere

IN a recent paper, Appleton and Builder1 mention a striking fact that on certain occasions, when the ionosphere is being surveyed by the ‘pulse’ method, there is a reversal in the times of arrival of the two magneto-ionic components into which the upgoing ray is split. They attribute this to differential group retardation, and it occurred to me a short time ago that this effect might throw some further light on a suggestion I have made previously2, namely, that the whole intermediate space between the E and F layers is ionised to a value very little less than that of the top of the E layer.