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.
CitationLyon, A. J., Skinner, N. J., & Wright, R. W. H. (1960). The belt of equatorial spread-F. Journal of Atmospheric and Terrestrial Physics, 19(3), 145-159.
University of Nairobi