Abstract

The phenomenon of geomagnetically-induced currents in long conductors has been recognized since the mid-nineteenth century, when the use of the telegraph became prevalent in North America and Europe¹. Powering systems of telegraph and telephone cables are now engineered to eliminate large current surges, although the occasional significant geomagnetic disturbance, such as those produced by the February 1958², August 1972³ flare events, can still be a problem. However, induced voltages can be used, with complementary geophysical and geological data, to deduce features of the Earth's crust and upper mantle^13,14. Improved scientific understanding of mag-netospheric phenomena means that the geomagnetic induction process in long conductors, and thus in the Earth are better understood. For example, the cable outage problem associated with the August 1972 geomagnetic storm interval was qualitatively attributed not to the `line' currents associated with normal auroral current systems but rather to the significant enhanced currents flowing on the magnetosphere boundary at the time of an extreme compression of this boundary in greatly enhanced solar wind conditions³. We have measured the geomagnetic induction phenomenon on modern-day long cables to get better assessments of the induced currents in various types of geomagnetic variations. Our initial work was concerned with induction in the transatlantic telecommunications cable no. 6 (TAT-6) which is laid between Green Hill, Rhode Island (41.4° N, 71.7° W geographic) and St Hilaire de Riez, France (46.7° N, 1.9° W)⁴. We concentrate quantitatively here on induction by the solar-induced quiet day currents (S_q) in the ionosphere and find that calculated values agree well with observed values of S_q.