THE NATURAL BACKGROUND LEVELS OF 50/60 HZ RADIO NOISE

This communication reports measurements of the natural background levels of 50 and 60 Hz radio noise by Stanford University ELF/VLF radiometers located at various sites around the world. The measurements are made on the magnetic fields of the noise using loop antennas, and they cover the first three harmonics of the power line frequencies (frequencies in the range 50-180 Hz). Due to the likelihood of contamination from local electrical power systems, or even from nonlocal systems, the measurements at each power line frequency were made at adjacent frequencies and the desired noise amplitudes obtained by interpolation. We show that the magnetic spectral densities of the natural noise typically lie in the range 20-600 fT/square-root Hz or, taking the 50/60 Hz frequencies specifically, in the range 150-600 fT/square-root Hz. These ranges should be typical for most locations on the earth's surface and for most seasons. Although we cannot rule out a solar cycle variation, our measurements and those of others suggest that it is likely to be small. In general, power line and electric appliance magnetic fields are much greater than these natural background levels. For example, a typical amplitude for the 60 Hz magnetic field fluctuations near an operating household appliance is 10 muT, which is over 10(7) times greater than our largest projected natural magnetic field amplitude for a 1 Hz bandwidth centered on 60 Hz.