Estimating conductivity anisotropy of lower mantle from geomagnetic observatory data

Anisotropy of electrical conductivity in the lower mantle, presumably caused by phase change of dielectric magnesiowustite, can be estimated from geomagnetic records of the globally distributed geomagnetic array. The components of the conductivity tensor corresponding to the tangential components of the geomagnetic field are obtained from the magnetic mode alone recorded on the surface. Inversion by the perturbation method is applied to monthly means of geomagnetic variations along the X, Y, and Z components for the period from 1920 to 2009, borrowed from the World Monthly Means Database. The anisotropy contribution is expressed as corrections to the conductivity tensor components. The contribution of anisotropy to induction responses becomes notable for geomagnetic variations with at least decadal periods and increases with the period length.