Abstract
The magnetic field at the surface of the Earth, when averaged over a sufficient interval of time to remove transient variations (nominally 1 year), consists principally of a contribution originating from the Earth's core, called the 'main field', and a lesser contribution arising from permanent and induced magnetization of the crust. The International Geomagnetic Reference Field (IGRF) is the internationally adopted set of spherical harmonic models which are intended to represent the main (core) field and its secular variation. Regional models of the geomagnetic field represent a combination of the main field and the broad-scale crustal field. They are used as aids for navigation, surveying, certain military applications, geophysical exploration, determination of sea-floor ages and spreading history, and in studies of geomagnetic phenomena.
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