The magneto-ionic triple splitting of ionospheric echoes.

In 1933 T. L. Ackersley (1) observed F region ionospheric echoes -which appeared to be magneto-ionically split into three components instead of the usual two. as occurrence of such triple splitting was not reported again until 1936 when Toshniwal (2) in Allahabad and Harang (3) in Tromso both observed similar phenomena. Harang was the first to publish a P'f curve of the echo structure which showed beyond doubt that the third or z echo was of magneto-ionic origin and that it had the following characteristics: (i)The critical frequency was approximately... less than that of Us ordinary echo and fH less than that of the extraordinary echo. (ii)In the absence of an F1 layer, at a given frequency the heights of the three types of echoes were in the order X, 0, Z upwards. (iii)The 4 echo was weak compared with 0 and X echoes. These features made it probable that Z echoes were due to reflection at the third possible level predicted by the Appleton-Hartree magnetic-ionic theory. However, as we shall discuss in detail later, this theory which explains all the major features of ionospheric propagation, does not predict the simultaneous occurrence of three echoes at vertical incidence. The anomaly was recognised by many workers and several alternative summations and theories were advanced to account for it. None was conclusive, mainly because of insufficient experimental information.