The International Geomagnetic Reference Field (IGRF) is a series of mathematical models of the main geomagnetic field and its secular variation. The models themselves consist of sets of spherical harmonic (or Gauss) coefficients. The IGRF has become a widely used means of deriving values of the geomagnetic field components in, for example, studies of magnetic anomalies a n d investigations of charged particle motions in the ionosphere and the magnetosphere.
Since the adoption of t h e first IGRF (IGRF 1965) by the International Association of Geomagnetism and Aeronomy (IAGA) in 1968 [IAGA Commission 2 Working Group 4, 1969], the IGRF has been revised four times, giving the IGRF 1975 [IAGA Division I Study Group on Geomagnetic Reference Fields, 1975], the third-generation IGRF [IAGA Division I Working Group 1, 1981], the fourth-generation IGRF [IAGA Division I Working Group 1, 1985], and the fifth-generation IGRF which is the subject of this note. Details of the derivation of the original IGRF have been given by Zmuda ; interested readers should see Peddie  and Barraclough  for descriptions of its development up to 1985.