The belt of equatorial spread-F
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The morphology of the belt of equatorial spread-F at sunspot maximum is investigated, using I.G.Y. data, for magnetically quiet and magnetically disturbed conditions respectively. The belt is found to extend from about 30°S to 30°N in magnetic latitude and to have a region of very high incidence, exceeding 90 per cent for the early part of the night, about 20° wide in latitude and centred on the dip equator. In Africa and India there is little seasonal variation in quiet-day incidence, but a marked seasonal variation occurs in the American zone where there is a pronounced minimum in local winter. Throughout the whole belt from 30°S to 30°N there is a strong inverse correlation between spread-F incidence and magnetic activity. Moreover the post-sunset rise of the base of the layer, which usually precedes the onset of spread-F shows a marked positive correlation with spread-F incidence within the belt. No existing theory seems adequate to account for all of these facts but it seems likely that hydromagnetic disturbances are involved.