Observations of Phase-Instabilities on a V.L.F. Transmission Path Passing Through the Auroral Zone

1. The phase-propagation of v.l.f radio waves over long transmission paths is characterized by random phase fluctuations superimposed on relatively large systematic variations caused by the diurnal changes in the altitude of the appropriate region of the ionosphere. These systematic variations can normally be reliably predicted from a knowledge of the geographical constants of the transmission path.
2. The present paper presents some representative results from a programme of continuous observations carried out at the Admiralty Surface Weapons Establishment, Portsdown, during the period May 1957, to June 1959, to determine whether the same predictability could be expected in the case of a transmission-path passing through the auroral zone.
3. Although the pattern of behaviour was generally in agreement with expectation, abnormalities were found sufficiently often to suggest that predictions could no longer be treated with the usual confidence.
4. Examples are given of the principal types of abnormality, together with an indication of the frequency of occurrence during the period ef the experiment. No clear-cut correlation has yet been found between these abnormalities and any of the phenomena which are usually associated with ionospheric disturbances, with the possible exception of Polar Cap Absorption.