Cable tests for current leaks from the core.

Scientists have few clues with which to understand the complex swirlings of conducting fluids in the earth's core. Most clues are embedded in the behavior of the magnetic field generated by the core currents, which act as a giant dynamo. But 20 years ago English geophysicist Keith Runcorn suggested another way to get at the workings of the geomagnetic dynamo humming in the core. He proposed that the core currents might leak up through the mantle and crust and could be measured on the surface of the earth. Now, in the July 5 SCIENCE, a group of scientists has taken him up on his idea and carefully tested it.

"We thought Runcorn's proposal was rather reasonable," says Louis Lanzerotti of AT&T Bell Laboratories in Murray Hill, N.J. "Given what we know about the core and the mantle, one could concoct reasons why we might or might not see something at the surface of the earth."

To search for possible leakage currents, Lanzerotti and his colleagues, along with two Italian researchers, used a telecommunications cable that AT&T was laying across the Atlantic. Runcorn himself and, independently, a group of Canadian scientists had performed similar experiments years ago with an abandoned telegraph cable in the Pacific Ocean. but Lanzerotti's group felt that the condition of the cable and its grounding were not known well enough in these past studies. The AT&T cable would enable them to do a much more careful and controlled experiment.

After laying 4,476 kilometers of cable, AT&T workers returned to shore for more, allowing the scientists to get 19 days of data with the unpowered cable grounded to the ocean floor. Using some fancy statistical footwork, the reserachers separated out the direct-current (DC) component due to core currents from the tangle of alternating-current (AC) signals arising from a variety of sources, such as the erratic flow of ions in the ionosphere, that induce currents in the earth. They obtained a DC potential voltage drop (which is proportional to current) of 0.072 [plus-or-minus]0.050 millivolts per kilometer.

"The bottom line is that the potential drop over the surface of the earth is very small, almost zero," says Lanzerotti. According to calculations by theorists, this implies one of two possibilities. Having essentially no current at the surface might mean that the two main components of the earth's magnetic field -- the toroidal part, which looks like a doughnut around the equator, and the poloidal or dipole part -- have equal magnitudes at the core-mantle boundary. This supports one model of the dynamo put forth by Friedrich Busse at the University of California at Los Angeles. But even if that model were correct, the field deep inside the core might still be different from that at the core-mantle boundary.

The other alternative provides information about the mantle. It is possible, in theory, that the core currents are prevented from reaching the surface by a layer in the mantle that has a conductivity much different from that of its neighbors.

Now the ball is back to the theorists, who must try to reconcile these results with the other existing data.

In addition to using the cable to study the electrical properties inside the earth, the researchers have found a number of other applications for the cable and these will be reported in future papers. One of the most exciting prospects is to use cables for very sensitive monitoring of ocean tides on a large scale. Because seawater is a conductor, it can induce a current in a cable as the water moves through the earth's magnetic field. The cable measures the integrated effect of ocean flow, which can then be used to test oceanographic models that have relied primarily on data from isolated sea-level gauges on the coasts.