International geomagnetic reference field
From AMS Glossary
International Geomagnetic Reference Field
(Abbreviated IGRF.) A geomagnetic field defined by a mathematical formula and believed to be the best analytical description of the complete geomagnetic field.
This formula includes a series of spherical harmonic terms, each of which has a coefficient that determines the strength of that term. Since the geomagnetic field changes with time, these coefficients are upgraded every five years by a committee of the International Association of Geomagnetism and Aeronomy. The IGRF is thought to represent the geomagnetic field due to deep earth processes. It does not include wavelengths shorter than a few thousand kilometers since these are thought to be due to crustal magnetization rather than deep earth processes. Accordingly the value of the geomagnetic field measured at a location may differ somewhat from that given by the IGRF.