Annual and semi-annual variations of the geomagnetic field components analysed by the multi-taper method

Abstract

Abstract Monthly means of three geomagnetic components (X-northern, Y-eastern and Z-vertical downward) from 154 observatories over the interval 1960-2002 have been analysed to determine the annual and semi-annual variations. The method applied is a multi-taper analysis, which gives a robust estimate of the power spectral density of the temporal variation of the field components. The main objective is to investigate the geographic dependence of the annual and semi-annual variations. Surprisingly, these variations in the vertical downward component are not homogeneously distributed all over the globe, having mainly a longitudinal dependency. The results of the analysis are briefly discussed with regard to possible mechanisms responsible for these variations and to mantle conductivity.