Geomagnetic spherical harmonic analysis

Summary An attempt is made to determine the optimum representation of the geomagnetic field in terms of spherical harmonics with regard to the number of parameters required for an adequate accuracy of fit, and considering the effect of allowing for the oblateness of the earth. A representative set of geomagnetic data was subjected to a set of analyses with from 8 to 120 coefficients for both spherical and oblate models of the earth. It is concluded that a sixth order analysis (48 coefficients) with no allowance for oblateness is the most compact representation of the field compatible with tolerable accuracy.