Sensitivity of selected geomagnetic properties to truncation level of spherical harmonic expansions

Abstract

A two day selection of MAGSAT data is fit by spherical harmonic series truncated at increasing levels N F in the range 2<N F <14 to determine the extent to which the geomagnetic Gauss coefficients depend upon truncation level of the fit. The dependence is found to be concentrated in the range n<N F ≲n+4 decreasing rapidly in magnitude to a few nanotesla as n increases to about 6. Several geomagnetic properties are then examined by evaluating them at various truncation levels N with N F fixed and N<N F . The spectrum of magnetic energy (outside the core and outside the earth) as determined by the N F =23 MAGSAT model MGST 1/81 is evaluated. It suggests that the crustal anomaly contribution to magnetic energy outside the core will not exceed 0.1%, when the expansion is truncated at N=8. The positions of certain contours of the geomagnetic elements on the core-mantle boundary and their intersections needed for magnetic determination of the core motions, are examined and found to be relatively stable when N changes from 7 to 8, but to exhibit greater sensitivity for both larger and smaller values of N. This article contains supplementary material.