SummaryAnomalous cusps which frequently appear at the high frequency end of ionosonde records of the F. region are explained as the result of modification of the ion distribution during the passage of typical travelling disturbances. They indicate the presence, not of vertical stratification but of horizontal gradients of ionization causing oblique reflection. It is suggested that other anomalous cusps are of similar origin.Anomalies on records of the F 1 region are also shown to be caused by travelling disturbances.It is demonstrated that these explanations provide useful guidance in the interpretation of ionospheric records and facilitate the further study of travelling disturbances.
SummaryTravelling disturbance manifestations on ionosonde records usually occur at different times on the "0" and "x" traces. It is shown that this is due to the divergence of the ordinary and the extraordinary rays in the Earth's magnetic meridian plane and that the sense of this time difference, therefore, gives a direct indication of the sense of the north-south component of movement of the disturbance. Furthermore, where the direction and speed of travel of a disturbance can also be determined from spaced station observations, the act,ual separation of reflection points of the 0-and the x-rays can be deduced. At Sydney, N.S.W. (33 0 52' S., 151" 11' E.) this is of the order of 30 km at the height of maximum ionization of the F 2 region. On h't records the corresponding time difference observed includes a component due to the vertical separation of reflection points if the front of the disturbance is not vertical. From records of this type taken at three spaced stations the horizontal component may be determined directly. Examination of some 430 such observations taken over a period of 4 years confirms the variation of the time difference with direction of travel predicted by theory, and also indicates the consistent presence of a forward tilt in the front of disturbances.The theoretical relation between the horizontal and vertical separation is then used to determine the slope of front of disturbances. It is found to have a mean value varying from 65 0 to the horizontal for disturbances travelling northward to a minimum of 51 0 for those travelling 120 0 E. of N.
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