Outage of the L4 System and the Geomagnetic Disturbances of 4 August 1972

The solar and geomagnetic disturbances resulting from solar active region 11976 were truly outstanding in many regards. The principal solar region of the activity was in the highest solar activity class on an absolute scale; this major solar activity occurred during the declining phase of the 11-year solar cycle.1 The solar disturbances, propagating outward into interplanetary space, produced the largest galactic cosmic ray decrease on record. 2 The geomagnetic storms (with accompanying ionospheric and auroral disturbances) resulting from the interaction of the propagating solar disturbances with the earth's magnetosphere were the most severe recorded in well over a decade. The interaction of the greatly enhanced solar wind 3 with the earth's magnetic field on 4 August 1972 produced extreme compressions and distortions of the magnetosphere. It was during the period of the most 1817 severe magnetospheric distortion that an outage of the L4 coaxial cable carrier system occurred over the link from Piano, Illinois, to Cascade, Iowa. This paper, utilizing most of the available North American geomagnetic data, describes the geophysical occurrences and conditions at the time of the L4 outage. These data, plus magnetic field data from two satellites, demonstrate that the L4 outage was associated with the extreme compressions and distortions of the magnetosphere and not simply with greatly enhanced auroral currents, as are often assumed in discussions and models of magnetic storminduced power-system disruptions.