The main magnetic field at epoch 1975 is defined by 168 spherical harmonic coefficients (those representing the internal part to degree and order 12). A further 80 coefficients (degree and order 8) define the secular variation and, in addition, 26 secular acceleration coefficients (those most significantly determined) are included. Details are given of the data used and the methods of reduction.
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Barraclough et al. (1975) studied this question.
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