A geomagnetic field spectrum

Selected MAGSAT data are used to derive a spherical harmonic model of the earth’s internal magnetic field of degree and order 23. The power spectrum, with terms of the form (Mauersberger, 1956; Lowes, 1974): urn:x-wiley:00948276:media:grl1890:grl1890-math-0001 is computed. It exhibits a distinct change of slope at about n = 14 which is interpreted to mean that the field from the core dominates for n ⩽ 13 and the field from the crust for n ⩾ 15. The two portions of the spectrum are well represented by: Rn = 1.349 × 109 (0.270)n (nT)² for the core and Rn = 37.1 (0.974)n (nT)² for the crust. These representations can be used to establish order of magnitude inaccuracies in core field models due to crustal fields and due to inability to “observe” the core field wavelengths beyond n = 13 which are totally obscured by the crustal field.