Spatial Power Spectrum of the Main Geomagnetic Field, and Extrapolation to the Core

The spatial ‘power’ spectrum of the main geomagnetic field has been estimated for harmonics up to n= 500. It is shown to consist of two components, long wavelengths being dominated by fields originating in the core, and short wavelengths by fields originating in the crust; the cross-over occurs at n≥ 11, a wavelength ⩽ 3600 km. The core field is often approximated by a set of spherical harmonic coefficients. It is shown that at present main field coefficients for n≿ 9, and secular variation coefficients for n≿ 6, are not known with significant accuracy. Estimates are made of the standard deviations of the IGRF coefficients, and the standard deviation of the IGRF field deduced. This field is known to about 0.5 per cent at the surface but only to about 10 per cent at the core. Its time variation is known only to about 20 per cent at the surface, and is very uncertain at the core.