Very-low-frequency (VLF) chorus activity detected by the Ogo 3 satellite has the highest probability of occurrence in regions threaded by geomagnetic field lines that intersect industrialized areas. Interpretation of these results is based on radiated power line harmonics that leak into the magnetosphere and stimulate the recorded emissions through cyclotron interaction with trapped energetic electrons. These results emphasize the need for a careful evaluation of the effects of man-made VLF noise on the ionosphere and magnetosphere.