On the relation of the ‘Slant E condition’ in polar cap ionograms to the solar wind sector structure

The ‘slant E condition’ (SEC) in polar cap ionograms has been shown previously to indicate excitation of the Farley-Buneman instability in the E region. The SEC at Godhavn (Greenland) is shown here to correlate with the solar wind sector structure, the occurrence rate of SEC being large a few days after passage of a sector boundary and declining in the trailing part of the sector. Analysis of SEC occurrence in terms of the solar wind velocity Vsw measured on board Heos 2 indicates that no SEC occurs when Vsw < 350–400 km/s (in apparent agreement with the interpretation of SEC in terms of the Farley-Buneman instability).