Charles E. Barton
A 1995 revision of the International Geomagnetic Reference Field (IGRF) was adopted by the International Association of Geomagnetism and Aeronomy (IAGA) during the XXI General Assembly of the International Union of Geodesy and Geophysics held in Boulder, Colo., in July 1995.
The IGRF is a series of mathematical models of the Earth's main magnetic field and its secular variation. Each model comprises a set of spherical harmonic (or Gauss) coefficients, gmn and hmn , in a series expansion of the geomagnetic potential,
where a is the mean radius of the Earth (6371.2 km), 
is longitude eastward
from Greenwich, 
is geocentric colatitude, and
Pmn (cos ) is the
associated Legendre function of degree n and order m, normalized according to
the
convention of Schmidt [e.g., Chapman and Bartels,
1940; Langel, 1987]. N is the maximum
spherical harmonic degree of the expansion. The main field models are defined for
successive 5-year epochs. At the final epoch, presently 1995.0, a secular variation model is
included for forward continuation of the field for 5 years.
Spherical harmonic coefficients for the complete IGRF are listed in Table 1. The IGRF now consists of a new set of "IGRF" models at 5-year epochs from 1900.0 to 1940.0, the existing "DGRF" models at 5-year epochs from 1945.0 to 1985.0 [IAGA, 1991; Langel, 1992], a new DGRF 1990 model that replaces IGRF 1990, and a new IGRF 1995 model that includes secular variation (SV) terms for forward continuation of the 1995 field to the year 2000.0. Coefficients for dates between the 5-year epochs are obtained by linear interpolation between the corresponding coefficients for neighboring 5-year epochs.
The main field models are truncated at N = 10 (120 coefficients), which is the practical compromise adopted for producing well-determined main field models, while avoiding most of the contamination from crustal fields. Main field coefficients are rounded to the nearest nanotesla to reflect the limit of resolution of the observational data. The prospective secular variation model is truncated at N = 8 (80 coefficients) with coefficients rounded to the nearest 0.1 nT/yr, again reflecting the resolution of the available data. When converting between geodetic and geocentric coordinates, use of the IAU ellipsoid [International Astronomical Union, 1966] is recommended; it has an equatorial radius of 6378.160 km and flattening 1/298.25.
The IGRF 1995 model will be superseded once a definitive model of the main field at 1995.0 is adopted. Similarly, the IGRF models for 1900 through 1940 may be revised later. Details about the derivation of the 1995 revision of the IGRF will appear in a special issue of the Journal of Geomagnetism and Geoelectricity in 1996. The name "IGRF" refers collectively to the entire series of spherical harmonic models; if a particular epoch model is intended, the reference must be specific, for example, IGRF 1995 or DGRF 1980.
ASCII files of the IGRF coefficients and computer programs for synthesizing field components are available from the World Data Centers listed below and also from cooperating national geomagnetic observatory agencies and geological surveys throughout the world.
World Data Center-A: Solid Earth Geophysics National Geophysical Data Center NOAA, Code E/GCI 325 Broadway Boulder, CO 80303-3328 USA tel. +303-497-6521 fax +303-497-6513 e-mail info@ngdc.noaa.gov World Data Center-B2 Russian Geophysical Committee Academy of Sciences of Russia Molodezhnaya 3 Moscow 117 296 Russia tel. +7-930-0546 fax +7-930-5509 e-mail sgc@adonis.iasnet.com World Data Centre-C1: Geomagnetism British Geological Survey Murchison House, West Mains Road Edinburgh, EH9 3LA UK tel. +44-131-667-1000 fax +44-131-668-4368 e-mail drbar@wpo.nerc.ac.uk World Data Center-C2 for Geomagnetism Faculty of Science Kyoto University 606 Kyoto Japan tel. +81-75-753-3929/3949 fax +81-75-722-7884 e-mail request@kugi.kyoto-u.acijp World Data Center-A: Rockets and Satellites NASA/Goddard Space Flight Center Code 633 Greenbelt, MD 20771 USA tel. +301-286-6695 fax +301-286-1771 e-mail request@nssdca.gsfc.nasa.govThe IGRF is produced by IAGA Working Group V-8: Analysis of the Global and Regional Geomagnetic Field and Its Secular Variation. The new models adopted for the 1995 revision of the IGRF are based on candidate models provided by NASA's Goddard Space Flight Center, The Institute of Terrestrial Magnetism, Ionospheric and Radio Wave Propagation IZMIRAN, and jointly by the U.S. Navy and the British Geological Survey. We thank the staffs of magnetic observatories and survey organizations worldwide for providing the data on which the IGRF depends. For further information about the IGRF, contact IAGA Working Group V-8 (Chairman: C. E. Barton, Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601, Australia; fax +61-6-249-9986, e-mail cbarton@agso.gov.au).--IAGA Division V, Working Group 8
IAGA Division V, Working Group 8 participating members: C. E. Barton (Chairman), R. T. Baldwin, D. R. Barraclough, S. Bushati, M. Chiappini, Y. Cohen, R. Coleman, G. Hulot, P. Kotz‚, V. P. Golovkov, A. Jackson, R. A. Langel, F. J. Lowes, D. J. McKnight, S. Macmillan, L. R. Newitt, N. W. Peddie, J. M. Quinn, and T. J. Sabaka
C. E. Barton, Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601, Australia; fax +61-6-249-9986, e-mail cbarton@agso.gov.au
IAGA Division V Working Group 8, R. A. Langel, Chairman, International Geomagnetic Reference Field, 1991 revision, J. Geomagn. Geoelectr., 43, 1007, 1991.
International Astronomical Union, Proceedings of the 12th General Assembly, Hamburg, 12B, 594, 1966.
Langel, R. A., Main Field, in Geomagnetism, edited by J. A. Jacobs, Academic Press, San Diego, Calif., 249, 1987.
Langel, R. A., IGRF, 1991 Revision, Eos Trans. AGU, 73, 182, 1992.
Table 1. Spherical Harmonic (Gauss) Coefficients of the 1995 Revision of the IGRF
[Table archived at AGU's Anonymous FTP data archive: ftp://ftp.agu.org/data/95242d/95242d.htm]
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