Superdipole Geomagnetic Field in the Past

According to the model of the Earth’s cooling, before the solid core origin, there was a sub-adiabatic layer at the core-mantle boundary several hundred kilometers thick. In this layer magnetic field generation was suppressed. Removing the field generation region from the Earth’s surface led to an effective weakening of the small-scale component of the magnetic field. After the solid core appeared, convection extended throughout the whole bulk of the core with the possible exception of the F-layer at the core-mantle boundary, which is no more than 100 km thick. As a result the non-dipole counterpart of the magnetic field should be stronger at the Earth’s surface. The decrease of the ratio of the dipole to non-dipole field with the origin of the solid core was an order of magnitude or more.