Much of our current understanding of magnetospheric electrodynamics is based on the assumption that magnetic field lines often behave as electrostatic equipotentials. This assumption has allowed hemispheric patterns of ionospheric convection to be interpreted in terms of the large-scale circulation of plasma in the magnetosphere. However, the extent to which the equipotential field-line assumption is justified for different regions of the magnetosphere has not been adequately explored, largely because of the sparseness of magnetospheric measurements. In this paper, a mathematical formalism is developed that allows conjugate magnetospheric and ionospheric measurements to be compared with each other using a model magnetic field. The technique is demonstrated during an event interval in which Super Dual Auroral Radar Network measurements of ionospheric plasma drift are mapped to the magnetosphere using the Tsyganenko T01 magnetic field model and compared with conjugate measurements from the Cluster spacecraft electron drift instrument. The degree of consistency between the conjugate measurements is discussed in terms of (1) the accuracy of the magnetic field model, (2) the validity of the assumption of equipotential magnetic field lines, and (3) the presence of inductive electric fields in the magnetosphere. It is also shown how conjugate plasma drift measurements can be used to identify small inaccuracies in the location of ionospheric foot points specified by the magnetic model.