Outage of the l4 system and the geomagnetic disturbances of 4 august 1972

An outage of the Plano, Illinois, to Cascade, Iowa, link of the L4 coaxial cable occurred at about 2240 UT on 4 August 1972 during a large geomagnetic storm. The available geomagnetic data measured in North America, as well as data received from two satellite instruments, are analyzed. These data show that, at the time of the L4 outage, the boundary of the magnetosphere was pushed to unusually low altitudes by a greatly enhanced solar wind. As a result, large, rapid changes of the earth's magnetic field strength were observed over North America. It is demonstrated that the field changes at about 2241 to 2242 UT were of such magnitude as to induce earth currents of sufficient strength to produce the L4 outage by causing a high-current shutdown of the system link. The geomagnetic disturbances that produced the shutdown were not of the auroral-electrojet type normally associated with disruptions of power systems.