An IGRF candidate main geomagnetic field model for epoch 2000 and a secular variation model for 2000–2005

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.