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The Earth’s Main Field

Any measure of the geomagnetic field at the surface of the Earth provides a value that is the sum of various contributions, each with different origin. These contributions can be considered separately, each of them corresponds to a different field:

  • Main field, produced in the fluid core by means of a mechanism that goes under the name of geodynamo;
  • Crustal field, produced by the magnetised rocks of the terrestrial crust;
  • External field, produced by the electric currents flowing in the ionosphere and in the magnetosphere as a consequence of the interaction between the solar wind and the geomagnetic field;
  • Electromagnetic induction field, produced by the currents induced in the crust and in the mantle from the time-varying external field.


Figure 1 Schematic representation of the different current systems responsible for the Earth’s magnetic field

The main field represents about 99% of the whole geomagnetic field observed on the surface of the Earth. A simple study of the geometry of this field shows its high similarity (for about 95% of the whole field) to the magnetic field produced by a dipole located at the centre of the Earth and whose axis forms an angle of around 11.5° with respect to the rotation axis of our planet.

If the Earth’s magnetic field were perfectly dipolar, geomagnetic poles (i.e. the intersection between the dipole axis and the Earth’ surface) would coincide with the magnetic poles (i.e. points where the direction of the geomagnetic field is perpendicular to the Earth’s surface). On the contrary, a significant portion of the geomagnetic field is non dipolar and is responsible for the differences between magnetic and geomagnetic poles.

It’s worth underlining that the main geomagnetic field, produced by a dynamo mechanism in the fluid outer core, it’s the most important contribution to the Earth’s magnetic field if it’s measured on the Earth’s surface. However, not far above the Earth’s surface, the effect of the other sources becomes stronger and stronger becoming of the same order of magnitude of the main field.

Figure 2 The geometry of the geomagnetic field is characterised by incoming field lines in the northern hemisphere and outcoming in the southern hemisphere. Therefore, a magnetic needle free to rotate around an axis will point its north magnetic polarity to the Earth south magnetic polarity (i.e. geographical North). However, it is custom to call North magnetic pole the magnetic pole located around the geographic North, and similarly South magnetic pole the one located around the geographic South. However, this has not always been the case. Many times in the history of the Earth the direction of the equivalent magnet has pointed in the opposite direction (see Reversals of the main field).

The Earth’s magnetic field varies on different time scales. Indeed, each of the sources of the geomagnetic field undergoes changes that produce transient variations or disturbances. The most important time variations involving the main magnetic field are secular variation and field polarity reversals.

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