Sudden commencement and sudden impulse absorption events at high latitudes

Observations of sudden commencement absorption (sca) and sudden impulse absorption (sia) events at the south pole (74.2° invariant latitude) show that these events are concentrated on the dayside of the magnetosphere. Although the majority of the events were preceded by substorm activity in the midnight sector, sca events were observed after prolonged magnetic quiet. This finding indicates the presence of a source, less conspicuous than substorm processes in the magnetotail, that supplies electrons in the region of quasi-trapping.