The diurnal variation of the equatorial anomaly in the topside ionosphere at sunspot maximum

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From an analysis of topside ionograms taken during November 1969–January 1970 in the South East Asian sector (long. 105°E) the equatorial anomaly has been observed to appear by 1000 LT, reach maximum development by 1500 LT and the subsequent decay became arrested by 1700 LT. Around 1900 LT an enhancement, particularly of the southern hemisphere crest became apparent. Throughout the day an asymmetry existed of greater electron density (about 8% greater) at the southern crest and the position of this occurred at a greater magnetic dip location (about 8° dip greater) than that of the northern crest. It has been inferred that during the period under investigation there was a south— north wind, originating from a relatively high latitude thermal pressure bulge, blowing with a speed of about 40 ms −1 across the magnetic equator and acting together with a vertical upward wind of speed about 12 ms −1 at the magnetic equator. Corresponding to an increase of solar activity from R ̄ z = 30 to R ̄ z = 100 it appears that electron densities for all heights at 30°S dip increased by a factor 2.2 while over the magnetic equator the increases were greater, the corresponding factors being 2.6 at a height of 500 km and 3.0 at a height of 1000 km. The main effect of the increase of solar activity from sunspot minimum to maximum has been to produce an increase of the vertical upthrust of ionization above the magnetic equator, with probably little change in the meridional wind speed near the time of maximum development of the equatorial anomaly.