Geomagnetic observatory data and ocean circulation

Abstract

In a systematic study of short-term variations in the geomagnetic field, one of us (DEW) has shown via spherical harmonic analysis that the annual variation alone has an internal part of which only a small component is due to induction from the external part. Electric potential measurements in the Pacific using disused telegraph cables had shown the existence of an annual variation when monthly means over many years had been determined, see Fig. 1. This annual term was attributed to electromagnetic induction from the seasonal variation in ocean currents: both vary in amplitude and phase from year to year. So it was postulated that the annual variation in the geomagnetic field has a part produced by electric currents induced in the ocean. This hypothesis was tested in two ways. From the spherical harmonic coefficients of the internal part, the electric current function, supposing the current flowed over the Earth's surface, was calculated. The extrema of the electric current function were found to show a marked correlation with the oceans. The electric current density in the oceans, so obtained, corresponds to the observed electric potential annual variation and was found, to an order of magnitude, to be equal to 0.1 A km?2.