LARGE-SCALE ELECTRIC-FIELD MEASUREMENTS ON THE EARTHS SURFACE - A REVIEW

There exist only a few reported measurements of quasi-stationary (near dc) electric potentials over very large spatial scales (hundreds of k ilometers or more) on the Earth's surface. Such measurements have typi cally been made using unpowered submarine telecommunications cables. T he measurements pose unique experimental challenges and require carefu l procedures to avoid data contamination by electrode contact potentia ls and local ground currents. In addition, there are possible interpre tational problems from pervasive, poorly understood, low-frequency ele ctric fields induced by ocean water motion through the Earth's station ary magnetic field. Nevertheless, estimates of the magnitude of the el ectric field computed from large-scale potential difference measuremen ts, made principally to date in the Pacific Ocean, can be used to plac e a limit on the size of the toroidal magnetic field at the core-mantl e boundary under certain conditions on the Earth's electrical conducti vity profile. Thus, large-scale electric potential measurements can se rve as an adjunct probe of the Earth's dynamo process in addition to m easurements of the poloidal magnetic field and its secular changes mad e at and above the surface of the Earth. A review of all of these data suggests that the toroidal and poloidal magnetic fields at the top of the core are comparable in magnitude.