Multipole expansions in the representation of current sources

After obtaining the measured magnetic field or its components in the form of an isofield map one has to decide which approximation to use in solving the inverse problem. A single current dipole as an equivalent current source is often used. It will be shown when this approximation fails and one possible way to improve the equivalent source description. The expansion of current multipoles is discussed up to the second order. The localization of an equivalent current source in this case is considered. The application of this type of expansion is analysed and discussed.