Geomagnetic Secular Variation during the Period from 1955 to 1960

Abstract

The isoporic charts of X, Y and Z of geomagnetic secular variation during the period from 1955 to 1960 are constructed based on data observed at 99 stations over the earth's surface including the Antarctic region, and illustrated in Figs. 1-9. Equivalent current system of the variation on the earth's core's surface is shown in Figs. 11-13.From the results of the spherical harmonic analyses of these isoporic charts, it is concluded that (a) the earth's magnetic dipole is decreasing in moment with the rate of -4.4×1022emu/yr, magnitude of the moment at 1960.0 being 8.046×1025emu; (b) the eccentric dipole is drifting westwards by 18′/yr, northwards by 12′/yr, and outwards by 3.4×10-4× the earth's radius/yr. It is shown that the other harmonics, are drifting mostly westwards also with the rate of several decimals of a degree per year.On the other hand, very intense foci of isopors are found in the Antarctic region, amounting to about 200γ/yr in Z. The current change on the earth's core's surface equivalent to the foci amount to about 0.1Amp/cm/yr. This value is strikingly large compared with intensity of the equivalent current of the earth's magnetic dipole, which is about 0.5Ampere/cm in this region.