In 1902 Kennelly (1902) and Heaviside (1902) almost simultaneously postulated the existence of a conducting layer in the upper regions of the earth’s atmosphere in attempts to provide a physical explanation for the feats of long distance radio communication which were then being achieved. Heaviside further made the suggestion that the conductivity of this region could be due to the presence of positive and negative ions produced, most probably, by the ionizing action of solar radiation. Direct evidence as the existence of this region however, was not forthcoming until 1925 when the experiments of Appleton and Barnett (1925) and Breit and Tuve (1926) proved the existence of downward coming radio waves seemingly reflected at heights of some hundreds of kilometres above their transmitting and receiving systems. Subsequent investigation (Mitra 1947) has shown that the ionization of the conducting layer commences at a height of about sixty kilometres and extends up to the highest limits of the atmosphere. The density of the ionization is not uniform through-out; there are ‘regions’ or ‘layers’ of maximum ionization. It is the highest of these regions, the so called F2 – layer, which provides the venue for the investigation which is reported in this thesis.
The radio wave, which provided the means whereby the ionosphere was discovered, has remained the principal tool for its investigation and the results reported here are derived from observations of radio waves which are reflected from or transmitted through the F2 – layer. The greater part of the information gained from reflection studies represents some of the results of an experimental programme carried out by the team of workers of the Radio Research Section of the Physics Department of the University of Queensland, Brisbane, using techniques to be described in the next section. The remainder of the data of this nature has been gained from the literature. Use will also be made of data obtained from investigations into the twinkling of radio stars, as reported in the literature, for it is believed that the results of these transmission experiments can help considerably in the interpretation of many of the effects observed locally by reflection methods.
In reflection studies, the observer initiates a radio wave, modulates it in some way so that it is recognisable, and the observes it after reflection by the ionosphere or notes its absence due to absorption in or penetration through the ionosphere. Particular attention is paid in this thesis to certain ‘abnormal’ effects which are at times associated with the reflection of radio waves whose frequencies are slightly less than those which are just able to penetrate the F2 – layer. These effects are abnormal not only in that they are observed less frequently than the effects which are regarded as ‘normal’, but also in that they defy explanation in terms of simple propagation theory based on the somewhat idealized piture of the ionosphere which is sufficient to explain the more common or normal effects.
In the next section the experimental techniques used will be outlined, while section III will be devoted to a discussion of the nature of the observations made. In section IV these observations will be interpreted in terms of diffraction from an F2 – layer which has horizontal irregularities of election density, and in conclusion section V will put forward several possible means where by these irregularities may be produced.