Re-evaluation of linear trend of submarine cable voltages for the study of the toroidal field variation at the CMB

Trans-oceanic submarine cable of several thousands kilometers in scale can be a useful tool of geophysical measurement. One of the geophysical information expected be contained in very long baseline cable voltages is that originated due to the toroidal magnetic field at the core-mantle boundary (CMB). It is well-known that the toroidal field cannot be observed by magnetic field measurements at the surface of the Earth. Probably, indirect detection of the magnetic field via long baseline electric voltage measurements is the only way to constrain on the toroidal field observationally. In this paper, we present recent estimate of long-term variation of the electric potential observed using submarine cables. A trial to remove the motionally induced electric potentical by ocean current is also presented.