Geomagnetic Secular Variation and Poloidal Magnetic Fields Produced by Convectional Motions in the Earth's Core

Abstract

A theory of electromagnetic induction by a convectional fluid motion within the earth's core is attempted in the hope of accounting for the localized intense secular variation in the geomagnetic field as has been found in the Antarctic area. If a toroidal magnetic field of 300 gauss at maximum is supposed to exist in the core, a steady convectional motion described by a spherical harmonic of order 5 and degree 5 can give rise to a poloidal magnetic field (degree 6, order 5) of which the Gaussian coefficient of the magnetic potential amounts to 0.045Γ at the earth's surface provided the radial velocity of the motion is taken as 0.01cm/sec.
A study of the growth of the field tells us that a secular change of the order of 100γ/yr is expected as long as a velocity of 0.1cm/sec is assumed. This order of velocity would not be impossible for such a localized motion as considered here.