There is, at present, strong evidence that on a decadal time scale the excess length of day variations are caused by the exchange of angular momentum between the liquid core and the solid mantle. Nevertheless the mechanism which facilitates the momentum exchange is still not understood. In the present work a mechanism by which the magnetospheric equatorial ring current system controls the momentum exchange is introduced, some empirical insights about that mechanism are obtained from relevant data and the magnitudes of the involved magnetic fields are evaluated from these data. ¿ American Geophysical Union 1995