The feasibility of using decadal changes in the geoelectric field to probe Earth’s core

Abstract

Electric field observations by using 1000 km scale submarine cables have been performed since early 1990s. One of the main purposes of the observations is to obtain observational constraints on the dynamics of Earth’s core such as the strength and the distribution of the toroidal magnetic field and its variation at the core mantle boundary. Several constraints have been obtained until present, but the electromagnetic plausibility of them have not been examined. In this paper, electromagnetic field variations generated by a simple spherical mean-field kinematic dynamo within an electrically conducting mantle are discussed. The field variations are assumed to be generated by perturbing a steady α 2 dynamo with torsional oscillation type zonal flows having period of 30 years. It is confirmed that the kinematic dynamo can generate the observed amplitude of electric voltage variation (∼100 mV) naturally. The amplitude of voltage variation is controlled mainly by the energy state of the dynamo, i.e., the magnetic Reynolds numbers, and the strength of the toroidal field variation at the CMB is determined by the magnetic Reynolds numbers and the conductance of the D″ layer. Potential obstacles for the detection of the 100 mV signal in 1000 km scale submarine cable voltages are the electric voltages induced by external magnetic field variations (magnetotelluric induction) and that induced by the ocean flow (motional induction). Although the magnetotelluric current with decadal time scales seems negligibly small, the motionally induced electric field variation can be as much as 100 mV for 1000 km scale. It is necessary to know the time variation of ocean flux in order to discuss the electric voltages generated in the deep interior of Earth correctly.