97 -99% of the Magnetic Field at the Earth's surface
is produced by electric currents driven by a self-sustaining
dynamo process in the Earth's conducting liquid outer core. That
portion of the geomagnetic field generated in the Earth's core is referred
to as the Core (Main) magnetic field simply because it is the largest contributor
to the total geomagnetic field. It ranges in magnitude between 20,000 nanoTeslas
(nT) at the geomagnetic
equator and about 65,000 nT at the poles.
