The eighth generation of the International Geomagnetic Reference Field (IGRF) was adopted in 1999 by the International Association of Geomagnetism and Aeronomy (IAGA) Division V, Working Group 8. This differs from the previous generation by the addition of the IGRF 2000 which comprises a main-field model for the epoch 2000.0 and a predictive secular-variation model for 2000.0-2005.0. This paper lists the IGRF coefficients and includes contour maps computed using IGRF 2000.

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A new generation of the IZMST series (STM-space-time models based on data from observatories, POGS and Ørsted) was developed as a model of the main geomagnetic field from 1980.0 up to 2000.0. A set of the natural orthogonal components (NOCs) was used as the basic time functions. The NOCs were derived from data series from 23 observatories widely distributed on the globe. These series were extrapolated by hand from epochs 1997.0 or 1998.0 to 2000.0. The data set for the spherical harmonic analyse included observed vector values from the worldwide network of observatories, synthesized F values at 700 km, computed from WMM92.5, and X, Y, Z values computed from IGRF 2000. Depending on the data used, this gave a series of models called STM-XXX. These models were then compared with WMM92.5 (based partly on POGS data) and with IGRF 2000.0 (based entirely on Ørsted data). This comparison shows a rather good agreement over the globe except for the vector components of the field in the Southeast Pacific and Indian Oceans. Adding the Ørsted data to the database decreased this disagreement. The observatory biases, derived from STM OPE (Observatories, POGS, Ørsted) are stable over the whole time interval. Also reported in this paper is the derivation of a candidate IGRF secular-variation model for the 2000.0-2005.0 period.

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A candidate main geomagnetic field model for epoch 2000, and a secular variation model for the period 2000-2005, are proposed. The main field model is to degree and order 10, the secular variation one to degree and order 8. These models are derived using the method of least squares. A 1997.5 main field model was derived from annual mean values provided by geomagnetic observatories for the 1997.5 epoch, repeat station measurements made in 1997 and reduced to 1997.5, and scalar data since 1995 adjusted to 1997.5. A weighting scheme based on both geographical distribution and data quality was applied. This model was then extrapolated to the 2000.0 epoch, using previously derived secular variation models. To derive these secular variation models, twenty six main field models were firstly computed for epochs 1975.5 through 2000.5, using annual mean values of the X, Y, Z components of the magnetic field from observatories, with the same geographical distribution every year. When missing, annual mean values for 1998, 1999 and 2000 were estimated from extrapolated monthly means, using exponential smoothing and taking account of the seasonal variation. From these twenty six models, twenty five annual secular variation models were extracted, by taking the differences between consecutive main field models. Finally, to produce the IGRF candidate secular variation model, each Gauss coefficient of this set of secular variation models was extrapolated to give values for each year to 2005, using exponential smoothing. So, a mean secular variation model was obtained for the period 2000-2005 and this is proposed for adoption.

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The method of derivation of the joint UK/US spherical harmonic geomagnetic main-field and secular-variation models is presented. Early versions of these models, with the main field truncated at degree 10, are the UK/US candidates for the IGRF 2000 model. The main-field model describes the Earth's magnetic field at the 2000.0 epoch, while the secular-variation model predicts the evolution of this field between 2000.0 and 2005.0. A revised 1995.0 main-field model was also generated. Regional models for the continental US, Alaska and Hawaii were also produced as a by-product of the UK/US global modelling effort.

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A global evaluation is made of the four candidate models for IGRF 2000 (main field and secular variation) that were submitted to the IAGA Working Group V-8 by the deadline in February 1999. A description of the data used and the method of modelling is summarised for each candidate model. The models are then compared with one another and with recent data from observatories and magnetic surveys. It is concluded that none of the candidate main-field models is sufficiently good for an IGRF and that a new model should be derived using Ørsted data.

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For describing the main field model at the 2000.0 epoch and the secular variation over the 2000-2005 timespan, three candidate models for the International Geomagnetic Reference Field (IGRF 2000) were proposed at the beginning of 1999, called in alphabetical order IPGP00 (proposed by IPGP), IZMI00 (proposed by IZMIRAN) and USUK00 (proposed by USGS/BGS). A fourth model, IGRF95 (the updated IGRF 1995), was suggested by the Working Group chairman. The modelling methods and the data used are presented by each team elsewhere in this special issue. This study is an attempt to test these models using the total field intensity provided by the Ørsted satellite, the only data available from that satellite at the time when the two tests describing here were done. The first test consists of evaluating the differences between the real and the synthetic data computed from the candidate models. The second test compares the capability of the candidate models to reduce the Backus effect, using a predictive dip-equator position and Ørsted data. Both tests show that the quality of the candidate models is far from being acceptable, and, therefore, a new candidate model for the main field, using vectorial Ørsted data, is required.

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At its meeting in July 1999, the IAGA Working Group for Analysis of the Global and Regional Geomagnetic Field and Secular Variation (WG V-8) felt unable to decide on a main-field model to use for IGRF 2000. It therefore set up a small Task Force with the remit to produce a model by the end of the year. This paper is the Chairman's report of the working of that Task Force, outlining the various stages involved, and giving the background to the various decisions. He also makes some retrospective personal comments.

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The IGRF 2000 has been estimated from magnetic measurements taken by the Ørsted sattelite in summer 1999. For this purpose, three models have been derived: The first two models were estimated using a few geomagnetic quiet days in May and September 1999, respectively. The third model, called Oersted(10c/99), was derived from scalar data spanning six months and vector data spanning four months. In order to get a model for epoch 2000.0, the IGRF 95 secular variaion model has been applied to the data. The IGRF 2000 model was taken to be the internal degree/order 10 portion of Oersted(10c/99). We describe the data selection, model parameterization, parameter estimation and an evaluation of the three models.

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On this occasion the selection of the IGRF for 2000 was left to a small Task Force. Before it was accepted by the Task Force as IGRF 2000, the final candidate model (a truncated version of Ørsted(10c/99)) was compared with a comprehensive set of independent surface and satellite data. The method, data selection, and results of this comparison are described.

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Analysis of data from the Ørsted satellite indicates that the largest source of error arises from an anisotropic attitude uncertainty, related to poorly determined rotation about the axis of the star imager. For two data sets from May and December 1999, I quantify this error, and review and apply a formalism designed to allow for this problem. I argue that, when modelled correctly, this attitude uncertainty should not significantly damage the main field models obtained from Ørsted data.

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Olsen's method (Olsen, 1996) is applied to Magsat magnetic field data to deduce average quiet-time and activetime ionospheric current systems at middle to high North and South latitudes. Spot magnetic field values at ground level are calculated from the average models and are ‘ground-truthed’ against observatory data. The results suggest that synoptic ionospheric models from Magsat-quality satellite data may be helpful in reducing the contamination of the main-field by non-field-aligned ionospheric sources. We discuss how the technique may be applied to data from the Ørsted mission and the extent to which these ionospheric field models may therefore be useful in improved modelling of the geomagnetic main field.

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The IGRF coefficients inevitably differ from the true values. Estimates are made of the their uncertainties by comparing IGRF and DGRF models with ones produced later. For simplicity, the uncertainties are summarized in terms of the corresponding root-mean-square vector uncertainty of the field at the Earth's surface; these rms uncertainties vary from a few hundred to a few nanotesla. (It is assumed that the IGRF is meant to model the long-wavelength long-period field of internal origin, with no attempt to separate the long-wavelength fields of core and crustal origin; the models are meant for users interested in the field near and outside the Earth's surface, not for core-field theoreticians.) So far we have rounded the main-field coefficients to 1 nT; this contributes an rms vector error of about 10 nT. If we do in fact get a succession of vector magnetic field satellites then we should reconsider this rounding level. Similarly, for future DGRF models we would probably be justified in extending the truncation from n = 10 to n = 12. On the other hand, the rounding of the secular variation coefficients to 0.1 nT could give a false impression of accuracy.

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The scalar anomaly field determined from available Ørsted data is compared with the upward continued scalar anomaly field derived from Magsat data. Two techniques were used to remove the core field from the Ørsted satellite data. In the first method, monthly spherical harmonic core field models of degree and order 13 derived from scalar and vector data were subtracted, and in the second method, along-track high-pass filtering of scalar data only was used. In both methods, the binned residuals were interpolated to a sphere, and subsequently filtered. Monthly degree and order 13 spherical harmonic core field models were removed from Magsat vector data. The binned Magsat vector residuals were interpolated to a sphere, filtered, and upward continued by high degree spherical harmonic analysis. The corresponding Magsat scalar anomaly field at Ørsted altitude was then determined. For latitudes below 50 degrees, removal of the core field by signal processing techniques from presently available Ørsted data led to a scalar anomaly field in better agreement with that determined from Magsat data, than removal by spherical harmonic analysis.

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The International Geomagnetic Reference Field (IGRF) for 1900-2000 is tested in this paper. A peculiar behaviour is found in the time variations of higher-degree coefficients of the IGRF models. During the 1945-1955 period the spherical harmonic coefficients g^m_n and h^m_n with n > 7 show unusual jumps, while their variations before and after this period are quite smooth. These irregular variations have little effect on the main features of the surface magnetic pattern. However, when we extrapolate the field pattern downward through the insulating mantle to the core-mantle boundary (CMB), the contributions of the higher-degree coefficients become more important and are likely to affect the shape of the geomagnetic energy spectrum, distribution of magnetic flux, and magnetic determination of the conducting core radius. It seems necessary to re-examine and revise these coefficients in the IGRF models.

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