Untersuchung von Feldumkehrungen an einem numerischen Modell des Geodynamos

Abstract

One of the most interesting aspects of the Earth's magnetic field is that it changes its polarity on geological time scales. In the last 160 million years, such field reversals occurred several times per million years, but with a very variable frequency. After a reversal, the magnetic north pole has more or less exchanged its place with the magnetic south pole. The internal magnetic field is generated within the liquid iron core of the Earth in a self-sustained dynamo process. The physical equations that govern this process can nowadays be solved numerically on high performance computers. The simulations reproduce the principal characteristics of the Earth's field quite nicely, e. g. the dominance of the dipole field, the magnetic flux concentrations at the core-mantle-boundary (CMB) and the westward drift of magnetic structures. However, reversals of the field are not a common feature of the models and it is unclear what conditions are required for obtaining them.We use a three-dimensional numerical model that solves the equations for convection and magnetic field generation in a rotating spherical shell. In an extensive study both the physical control parameters and the way the convection is driven are varied. The Rayleigh number Ra turns out to be an important control parameter regarding field reversals. It measures the forces that drive convection against the forces that hinder convection. Chaotic reversals of the dipole field arise beyond a certain value of Ra. For a suitable value of Ra, the mean dipole moment and the temporal evolution of the magnetic field resemble the known properties of the Earth's field from paleomagnetic data.In paleomagnetism, the declination D and inclination I of the magnetic field vector recorded in a rock sample are determined. On the assumption that the magnetic field is a purely dipolar field, one can calculate the longitude and latitude of the magnetic poles of this dipole field out of the (D, I) data. The south magnetic pole of this virtual field is called the virtual geomagnetic pole (VGP). Magnetic samples from deep sea sediments are used to reconstruct the paths that these VGP's take during a reversal. Surprisingly, the paths for the reversals of the last 10 million years have a stong tendency to fall along American and East Asian longitudes.Thermal core-mantle coupling could be a possible cause for the clustering of the VGP's: Due to temperature differences of the lowermost mantle the heat flux from the core into the mantle is inhomogeneous. These heat flux heterogeneities have an influence on the convection of the core and thus on the magnetic field. With the numerical model it is shown that thermal core-mantle-coupling indeed causes the observed clustering of the VGP's. Furthermore it can be shown that the heat flux pattern at the CMB crucially influences the reversal frequency of the dynamo.