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.

Anomalies which frequently appear on ionosonde records of the F region during the passage of travelling disturbances are classified into four main types; and the diurnal and seasonal distribution of their occurrence is discussed.

No abstract

No abstract