An evaluation of the main geomagnetic field, 1940-1962

A set of 21,695 selected survey and observatory annual mean data available for the interval 1940–1963 is used to make a new determination of the geomagnetic field and its secular change. The field is given by a series of 63 spherical harmonics (gnm and hnm to n = m = 7) and 35 time derivatives (ġnm and to n = m = 5). Although it is inferred from these results that a better fit could be made with existing data if more coefficients were used, the improvement is substantial enough for us to recommend that the coefficients be used as a replacement for previously derived coefficients to evaluate the field at the current epoch. One refinement in the present determination is the use of an oblate earth in place of the spherical approximation. It is illustrated that this modification is necessary to obtain meaningful extrapolations above the earth's surface. The standard errors and confidence levels are given for the coefficients to show that almost all are well determined. It is shown that, when the field components computed using this new series are compared with several weighted selections of survey data, the rms deviations average only 210 γ (1955–1963), a value smaller than that obtained by other spherical harmonic series known to us.