The geomagnetic eccentric dipole: facts and fallacies

Abstract

The geomagnetic field is sometimes approximated as that of a single dipole displaced from the geocentre, i.e. by an eccentric dipole. By investigating how the field can be represented exactly by a spherical harmonic (multipole) series at a displaced origin, it is shown how this eccentric dipole depends on the choice of criterion of best fit. In particular it is explained how, in some approximations, the ‘best’ eccentric dipole need not be parallel to, or have the same magnitude as, the geocentric dipole, despite the physical invariance of the true dipole moment to change of origin. Finally, the reader is warned about attempts to give physical significance to the position of the eccentric dipole.