The modelling of the toroidal magnetic field induced by tidal ocean circulation

SUMMARY Observations of the ocean-induced magnetic field by the CHAMP magnetic space mission have the potential to be used as a constraint when examining ocean dynamics. This has initiated theoretical studies on the prediction of the poloidal magnetic field induced by the horizontal ocean-circulation flow. This study deals with the computation and analysis of the toroidal magnetic field induced by the tidal ocean-circulation flow on the background of the main Earth’s geomagnetic field. Since the induced toroidal magnetic field cannot be modelled by the single-layer approximation model used to predict the poloidal magnetic field, we treat the ocean as a spherical layer of a finite thickness and compute the toroidal magnetic field by a matrix-propagator technique with a source of electric currents in the ocean layer. Our numerical simulations based on the OMCT tidal ocean velocities show that the induced toroidal magnetic field is extremely sensitive to the vertical gradient of horizontal ocean flow. The larger this gradient, the stronger the induced toroidal magnetic field. Specifically, its magnitudes vary from 10−2 nT for barotropic flow to several nT for baroclinic flow. We show that the induced toroidal magnetic field generated by M2 tidal forcing is comparable in amplitude to the induced poloidal part of the field. The induced toroidal and poloidal magnetic fields differ, however, in their spatial behaviour.