An evaluation of the main geomagnetic field, 1940-1962

Abstract

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 b.y a series of 63 spherical harmonics (g•m and h• m to n-m-7) and 35 time derivatives (•m and h• m 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 coefScients 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 • (1955-1963), a value smaller than that obtained by other spherical harmonic series known to us. Introduction. The history of the determination of the main geomagnetic field goes back more than a century; excellent bibliographies can be found in such works as those by Chapman and Barrels [1940], Vestine [1947], Mauersberger [1952], Fanselau [1956], and Kautzleben [1963]. Some of the more recent publications on the subject include those by Vestine [1960], Euler [1963], Fougere [1963, 1965], Heppner [1963], Leaton [1963], Vestine et al. [1963], and Fanselau et al.