On predicting changes in the geomagnetic field

The present method of using constant secular variation rates to forecast magnetic components at a given site or to forecast spherical harmonic coefficients is known to be inaccurate. A new predictive method using trend and trigonometric functions fitted to known past values is used to extrapolate for a few years into the future. This provides an improvement over the usual linear extrapolation method. This method applied to the spherical harmonic coefficients (SHC) in the series of International Geomagnetic Reference Fields (IGRF) provides an analytic function for each SHC so that charts can be compiled for a few years in the future without using the idea of a constant secular variation. In this case, the trend and trigonometric functions utilize the same number of Fourier and trend coefficients as are now present in the IGRF models on which they are based so that they can completely replace the IGRF information.