Ground-based data on magnetospheric wave activity in the American longitude sector are studied for a 13-day period that includes a major magnetic storm and some isolated substorm activity. The wave intensity in the 0.5- to 10- kHz range shows clear association with geomagnetic activity. A detailed examination of VLF spectra shows that the strongest waves emerging from the middle magnetosphere during the storm recovery period and during isolated substorm activity are often emissions stimulated by radiation from the electrical power distribution system. Several different types of power line radiation effects are illustrated by using broadband spectral data from stations in Antarctica and North America. It appears that man-made VLF noise has a strong influence on the energetic particle population in the magnetosphere.