Transatlantic Earth Potential Variations During the March 1989 Magnetic Storms

Measurements of the cable power supply voltage at the North American end of the fiber optic transatlantic telecommunications cable TAT‐8 during the March 1989 magnetic storm has provided a measure of the large scale changes in the total Earth potential across the Atlantic during the storm interval. East‐west potential changes as large as 700 volts (∼0.12 volts/km) peak‐to‐peak were observed, with many smaller amplitude variations also seen. The largest variations in Earth potential occurred during an extended interval of a very intense eastward electrojet as measured by a magnetometer at the North American terminus of the cable. The eastward electrojet current probably exceeded 10 6 amps. The design of the TAT‐8 cable power feed equipment is sufficiently conservative that even such unusually large Earth potentials as those measured during this storm were not a threat to the integrity of the communications system.