Geomagnetism


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Main Magnetic Field


The Main Magnetic Field originates from a dynamo process in the fluid outer core of the Earth. It strongly dominates over the various other contributions to the geomagnetic field, accounting for over 95% of the field strength observed at the Earth's surface.


Fig. 1: Strength of the magnetic field at the Earth's surface in 2006, as given by the main field model POMME-3.0.

The main field changes slowly with a time scale of years. Accurate measurements of the magnetic field, provided by satellites and magnetic observatories, can be used to estimate the present changes in the field. The first time derivative is called the secular variation (Fig. 2). It shows that the field strength is decreasing in most parts of the World. The strongest decrease is seen the Caribbean. But there are also areas of increasing field strength, such as in the Indian Ocean.


Fig. 2: Secular variation of the strength of the magnetic field in 2006

The 2nd time derivative is called the secular acceleration (Fig. 3). Comparing the maps in Figures 2 and 3 reveals some interesting behavior of the magnetic field. For instance, the increasing field strength in the Indian Ocean (in Fig. 2) is decreasing in the East and increasing in the West (Fig. 3). Thus, this region of increasing field is moving westward. Similarly, the decreasing strength over the Americas (Fig. 2) is increasing in the East and decreasing in the West (Fig. 3). Hence, this feature is also moving westward. The general tendency of magnetic field features to move westward is called the westward drift. However, this is not a strict rule, as there are features in the Pacific which are moving eastward.


Fig. 3: Secular acceleration of the magnetic field in 2006

Using these time derivatives, the core field can be predicted for the upcoming couple of years. For example, the World Magnetic Model (WMM) provides a predicted value of the magnetic field vector at any desired location, up to 5 years into the future, based on the estimated mean secular variation. The International Geomagnetic Reference Field (IGRF) provides the historical core field, starting in 1900, including a similar prediction 5 years into the future. Finally, scientific geomagnetic field models, like POMME-4 provide the core field, including the secular acceleration, together with contributions from the lithospheric magnetization and magnetospheric currents.

Stefan Maus


Last modified: Friday, 20-Aug-2010 08:54:55 MDT