Correlation of ‘satellite estimates’ of the equatorial electrojet intensity with ground observations at Addis Ababa

Abstract

Of the 2000 OGO-4 and OGO-6 equatorial passes, 112 were within 10° either side of Addis Ababa. Twenty-five passes happened during magnetically quiet periods when the index a p ⩽ 4. Correlations were sought between the effect of the equatorial electrojet at satellite altitude and ground measurements. When the ground values ΔH (AA) were read at the LT corresponding to the local time at the longitude of the satellite, the correlation is very good for 80 per cent of the observations. The inverse slope of the ground effect versus satellite effect is about 3.8 for Addis Ababa. A ΔH (AA) threshold of about 40 gammas was found below which the satellite did not register any electrojet effect. Some observations with excessive residuals correspond to H (AA) traces of irregular shapes; others cannot be explained unless one postulates the existence of non-electrojet ionospheric currents which distort the magnetic field at satellite height much more than at the surface of the Earth.