The International Association of Geomagnetism and Aeronomy (IAGA), Working Group V-8 has recently released the year 2000 revision of the International Geomagnetic Reference Field—the standard mathematical description of the Earth’s main field and its secular variation. This eighth generation of the IGRF introduces a new main field model for 2000 (IGRF 2000), which includes secular variation terms for extending the 2000 field to 2005. Working Group V-8 examines various mathematical representations (models) of the Earth’s main field and its secular variation, and adopts new models at five-year intervals to keep the IGRF up to date. When first adopted, new epoch models are provisional and are given names such as IGRF 1995 or IGRF 2000. Each provisional ‘IGRF’-designated model is re-examined as new data become available. When no significant improvement in modelling accuracy is likely to be achieved, each provisional epoch model is replaced by a ‘Definitive Geomagnetic Reference Field’(DGRF) epoch model. The term IGRF refers collectively to all the epoch models. If a particular epoch model is intended, the reference must be specific, e.g. IGRF 2000 or DGRF 1990, rather than simply IGRF or DGRF. Each epoch model of the IGRF comprises a set of spherical harmonic coefficients (called Gauss coefficients in recognition of Gauss’ development of this technique for geomagnetism), gmn and hmn, in a series expansion of a geomagnetic potential function of internal origin, where a is the mean radius of the Earth (6371.2 km) and r, ø, ø are the geocentric coordinates (r is the distance from the centre of the Earth, ø is the longitude eastwards from Greenwich and ө is the colatitude, i.e. 90° minus the latitude). When converting between geodetic and geocentric coordinate systems, use of the IAU ellipsoid (International Astronomical Union et al. 1966) is recommended—an equatorial radius of 6378.16 km and a flattening of 1/298.25. The Pmn(cos ө) are Schmidt quasinormalized associated Legendre functions of degree n and order m (n ≥ 1 and m ≤ n). The maximum spherical harmonic degree of the expansion is N. For more details on main-field modelling, the reader is referred to Chapman & Bartels et al. (1940), Langel et al. (1987), Merrill et al. (1996) and Campbell et al. (1997). Models for the main field are truncated at N = 10 (120 coefficients), which represents a compromise adopted to produce well-determined main-field models whilst avoiding most of the contribution from crustal sources. Coefficients of the main field are rounded to the nearest nanotesla (nT) to reflect the limit of the resolution of the available data. Models for the secular variation are truncated at N = 8 (80 coefficients) and rounded to the nearest 0.1 nT yr− 1. Coefficients for dates between the 5-year epochs are obtained by linear interpolation of the corresponding coefficients for the neighbouring epochs. The IGRF coefficients and computer programs for synthesizing the magnetic field components are available from the World Data Centers (listed below), from the IAGA web page (http://www.ngdc.noaa.gov/IAGA/wg8/) and from national geomagnetic observatory agencies and geological surveys throughout the world. The coefficients are listed in Table 1. Details of the derivation of the IGRF 2000 main-field and secular variation model will appear in a special issue of Earth, Planets and Space. Spherical harmonic (Gauss) coefficients of the IGRF/DGRF models for the th century During the XXII General Assembly of the International Union of Geodesy and Geophysics held in Birmingham, UK, in July 1999, due to the poor quality of the candidate models proposed for the main field, a task force1 was created to prepare a better IGRF 2000 model using data from the Ørsted satellite. The new main-field model adopted for the 2000.0 epoch is the model provided by Nils Olsen and Terry Sabaka2. The new secular variation model for 2000–2005 is based on candidate models provided by the British Geological Survey, Edinburgh, and the US Geological Survey, Denver (Susan Macmillan and John Quinn), Institut de Physique du Globe de Paris (Benoit Langlais and Mioara Mandea) and Institute of Terrestrial Magnetism, Ionospheric and Radio Wave Propagation, Moscow (Vadim Golovkov). We thank the Ørsted team and the staff of magnetic observatories and survey organizations worldwide for providing the data on which the IGRF depends, and the World Data Centers for data services. For further information about the IGRF, contact IAGA Working Group V-8: Analysis of the Global and Regional Geomagnetic Field and its Secular Variation (Chair: Mioara Mandea, Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris, France. Fax: +33 238339 504, e-mail: mioara@ipgp.jussieu.fr).
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A 2000 study studied this question.