GEOMAGNETIC SECULAR VARIATION AT ADDIS ABABA OVER THE LAST FOUR DECADES: COMPARISON WITH 1945-2000 IGRF MODELS

Abstract

Addis Ababa Observatory (aae) geomagnetic data analysed over the time-span 1958—1998 show that the annual mean values of the intensity have decreased since 1965 from 36186 nT to 35950 nT at a non-linear regression rate of 8—9 nT per year. Directional changes in the Earth's magnetic field that could be associated with southward and westward drifts of the dip equator are observed. On plotting the secular trends of the H, Z and D components, the variation in the H-component of Addis Ababa shows a near-sinusoidal cyclicity of about 40—50 years. The residual H, Z and D curves, obtained after removing polynomial fits, have been examined. The horizontal component H has been found to show pseudo-periodicity that would be considered to have parallelism with the 11-year sunspot cycle. Comparison of the observatory data with igrf models for the period 1958—2000 show that the igrf provides an excellent estimation of the geomagnetic Z and D components for the region, while it always underestimated the value of H. The mismatch in H may be due to the fact that the representative station being a dip equatorial station, it is under the influence of the equatorial electrojet and other solar activity effects. The comparison study in particular supports the existence of a relatively small magnetic anomaly in the region as implied by the igrf data.