The Geomagnetic Core Field

To fully describe the geomagnetic field it is necessary to either measure the intensity and two angles of direction or three orthogonal components. The angles are declination (the deviation of the local geomagnetic field lines from geographic north) and inclination (the angle of intersection with the Earth's surface). Orthogonal components are commonly chosen to be X, Y and Z for the directions towards geographic north, east and vertically down, respectively. The unit used to describe the geomagnetic field is the nanoTesla (nT), with the Tesla in fact being the unit for magnetic flux density.
When a measurement of the geomagnetic field is taken at any given point and time, the resulting value contains the superposition of fields having different origins and varying in magnitude: the core field, the lithospheric field, the external fields and the electromagnetically induced field. In 1838 Gauss, using spherical harmonic functions, developed a method to describe the geomagnetic field globally, providing a rough separation between internal and external contributions to the geomagnetic field. Geomagnetic field models based on spherical harmonics are still widely used, but due to the multitude of sources, a strict separation of all contributions is not feasible. The figure below shows such a model of the Earth's surface field, calculated using data from the CHAMP magnetic satellite and magnetic observatories. An interesting feature of the core field is the so-called South Atlantic Anomaly. This is a large area of very low field intensity (less than 20000 nT) over South America, the southern Atlantic and southern Africa. Moreover, from MAGSAT (1980) and CHAMP (2005) data we observed that the field there has been decreasing by some 10 % during the past 25 years.



Magnetic field intensity at the Earth's surface, as predicted by GRIMM model for epoch 2005.0 (top) and the percentage change of the geomagnetic field intensity from 1980 to 2005, as determined by the MAGSAT and CHAMP satellites (bottom).

The field component used to probe the structure and dynamics of the Earth's core is the radial component (sign-changed vertical component). Considering the mantle as an electrical insulator, the vertical component can be extrapolated at the core-mantle boundary, where its structure is more complicated than at the Earth's surface (see figure below). Distinct patches of reversed magnetic flux at the poles and below Africa can be identified, which could be related to the present day field decrease. The most prominent feature in this respect is the growing patch of reverse magnetic polarity beneath South Africa.



Vertical component of the magnetic field at the Earth's surface (top) and core-mantle boundary (bottom).

The core field is also subject to temporal variations, known as secular variation. Modeling the secular variation on characteristic timescales of the order of a few decades, can be significantly improved if we take advantage of all the available magnetic satellite data. It is obvious that the magnetic field does not change uniformly over the Earth. While the overall strength of the dipole field is decreasing, there exist a few regions where the field strength is increasing. An extremely strong decrease is seen in two areas, in the South Atlantic and in the Meso-American region. Again, the secular variation model can be extrapolated to the core-mantle boundary, where small-scale features can be observed.



Secular variation of the vertical component of the magnetic field at the Earth's surface (top) and core-mantle boundary (bottom).