An Equivalent Source Model for the Geomagnetic Field

Abstract

A new model for the geomagnetic field is presented. It does not rely heavily on the present-day field, which has some ephemeral characteristics, but mostly on paleomagnetic data from lava flows, in particular the large Icelandic data set. The model consists of an axial dipole and vertical off centered dipoles representing the non-axial-dipole field. These off centered dipoles are drawn from a zero mean Gaussian distribution whose standard deviation is chosen to give model results agreeing with observations. These off centered dipoles are placed within the core so as to give the correct slope to the Lowes-Mauersberger function, and are concentrated toward the poles so as to produce the latitudinal variation of VGP angular standard deviation (ASD) seen in paleomagnetic observations. The central dipole varies around its mean value with a Gaussian distribution with a small standard deviation chosen to fit observations. The model is checked by comparing average field strengths as a function of VGP latitude with those obtained from intensities of magnetization in Icelandic lava flows. Because the off centered dipoles contribute to the first degree Gauss coefficients, the g 1 0 term can become small, sometimes resulting in low latitude VGPs. If the model is constrained to go through the Iceland ASD point, then it does a reasonable job of explaining the less well-defined lower latitude ASD results. To explain the results recently compiled by McElhinny and McFadden [1997] the standard deviation of the dipole intensities has to be reduced from 1060 nT to 740 nT.