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International Geomagnetic Reference Field - 8th Generation
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Key words: IGRF, geomagnetic reference field, geomagnetic field model, secular variation A group of geomagnetic field modelers associated with the International Association of Geomagnetism and Aeronomy (IAGA) Division V, Working Group 8 periodically examines various geomagnetic field representations from which the Earth’s main field and its secular variation can be computed. They produce a set of coefficients to represent the main field at a particular epoch, usually every five years, and name it the International Geomagnetic Reference Field (IGRF). Also, if a previous IGRF is modified using new data not available at the time of its production, it is called a Definitive Geomagnetic Reference Field (DGRF). Note that, when referring to these models, the designation "IGRF" refers to all available models, viewed collectively. If a particular model is intended, the reference must be specific, i.e. IGRF2000 or DGRF1990, rather than simply IGRF or DGRF. The IGRF is a series of mathematical models describing the Earth’s main field
and its secular variation. Each model comprises a set of spherical harmonic coefficients (called
Gauss coefficients in recognition of Gauss’s development of this technique for geomagnetism),
where The IGRF models for the main field are truncated at
The IGRF coefficients and the computer programs for synthesizing the field components are available from the World Data Centers listed below, from the IAGA home page and from national geomagnetic observatory agencies and geological surveys throughout the world. Details about the derivation of the IGRF2000, that include secular-variation terms for forward-continuation of the field at epoch 2000.0 to 2005.0, appear in a special issue of Earth, Planets and Sciences (Vol.52, No. 12, 2000) dedicated to the IGRF2000, regional and global modeling. The IGRF is produced by the IAGA Working Group V-MOD, Geomagnetic Field Modelling. 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 force was created and was assigned the role of preparing a better IGRF 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 Sabaka. The new secular-variation model for the 2000.0-2005.0 time-span is based on candidate models provided by British Geological Survey, Edinburgh (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 would like to thank the Ørsted team, the staff of magnetic observatories and survey organizations worldwide for providing data on which the IGRF depends and the World Data Centers for data services. For further information about the IGRF models contactIAGA Working Group V-MOD (Chair: Susan Macmillan, British Geological Survey +331 288 1233, email: smac@bgs.ac.uk. |
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REFERENCES Campbell, W., Introduction to geomagnetic fields, Cambridge University Press, Cambridge, 1997. Chapman, S. and J. Bartels, Geomagnetism, 2 vols., pp 1049, Oxford University Press, London, 1940. Langel R.A., Main Field, in Geomagnetism, 249-512, vol. I, ed. J.A. Jacobs, Academic Press, London, 1987. Merrill, R., M.W. McElhinny, P.L. McFadden, The magnetic field of the Earth, Paleomagnetism, the core, and the deep mantle, Academic Press, San Diego, 1996. IAGA Division V, Working group 8, International Geomagnetic Reference Field, 1995 revision (chair Charlie Barton).
World Data Center for Rockets and Satellites Tel: +1 301 286 6695
World Data Center for Solid Earth Geophysics, Moscow Tel +7 095 930 0546
World Data Center for Geomagnetism, Edinburgh Tel +44 131 650 0234
World Data Center for Geomagnetism, Kyoto Tel +81 75 753 3929
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, in a truncated series expansion of a geomagnetic
potential function of internal origin:
is the mean radius of the Earth
(6371.2 km) and 
are the geocentric spherical coordinates
(
is the distance from the centre of the Earth,
is the longitude eastward from Greenwich and
is the colatitude (90°)
minus the latitude). When converting from geodetic to geocentric system, the
use of the IAU ellipsoid (International Astronomical Union, 1966) is recommended (equatorial
radius of 6378.16km and flattening 1/298.25). The 
are Schmidt quasi-normalized associated Legendre functions of degree
and order 
(
and 
).
The maximum spherical harmonic degree of the expansion is 
For more details on main-field modeling the reader is referred to Chapman and
Bartels (1940), Langel (1987), Merrill et al. (1996), Campbell
(1997).
(120 coefficients) which represents a compromise
adopted to produce well-determined main-field models while avoiding most of the contamination
resulting from crustal sources. The coefficients of the main field are rounded to the
nearest nanoTesla (nT) to reflect the limit of the resolution of the available data.
The IGRF models for the secular variation are truncated at 
(80 coefficients). This time the coefficients are rounded at the nearest 0.1
nT/year, this time to reduce the effect of accumulated rounding error. Coefficients
for dates between the 5-year epochs are obtained by linear interpolation of the
corresponding coefficients for the neighboring epochs. As with all such facilities,
users of the IGRF need to have some knowledge of its limitations if they are to apply
it correctly. Before using the IGRF please look at the '|
For page content questions please contact: Susan Macmillan
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URL: http://www.ngdc.noaa.gov/IAGA/vmod/igrf.html
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