A study of auroral echoes at 19.4 megacycles per second

A study of observations of auroral returns at 19.4 Mc/s made with a low-powered transmitter from a geomagnetic latitude of 54°N (dip angle 73°) over a period of three years has quite consistently indicated the presence of field-aligned ionization. Shortly after ground sunset the reflections have been observed on 25 to 95 per cent of the days of each month during the period 1961–1963. Although the auroral echoes occur more frequently with increasing magnetic index, they are also recorded during quiet days. Frequency of occurrence shows a peak shortly after sunset throughout the year. The time of maximum activity, the presence of reflections without magnetic storms, and the direction of the echoes lead to the hypothesis that this field-aligned ionization occurs during periods of magnetic field changes. The altitude of the field-aligned variations tends to be 200–400 km during quiet periods and 100 km during disturbed magnetic conditions.