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 vari ation 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. T hese models are derived using the method of least squares. A 1997.5 main fi eld model was derived from annual mean values provided by geomagnetic obser vatories 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 weight ing scheme based on both geographical distribution and data quality was app lied. This model was then extrapolated to the 2000.0 epoch, using previousl y derived secular variation models. To derive these secular variation model s, twenty six main field models were firstly computed for epochs 1975.5 thr ough 2000.5, using annual mean values of the X, Y, Z components of the magn etic field from observatories, with the same geographical distribution ever y year. When missing, annual mean values for 1998, 1999 and 2000 were estim ated from extrapolated monthly means, using exponential smoothing and takin g account of the seasonal variation. From these twenty six models, twenty f ive annual secular variation models were extracted, by taking the differenc es between consecutive main field models. Finally, to produce the ICRF cand idate secular variation model, each Gauss coefficient of this set of secula r variation models was extrapolated to give values for each year to 2005, u sing exponential smoothing. So, a mean secular variation model was obtained for the period 2000-2005 and this is proposed for adoption.