ULF MAGNETIC-FIELDS GENERATED BY ELECTRICAL STORMS AND THEIR SIGNIFICANCE TO GEOMAGNETIC PULSATION GENERATION

The STAR Laboratory has been operating two independent electromagnetic noise monitoring systems in the vicinity of Stanford University for several years, one covering the ULF range (0.01-10 Hz) and the other the ELF/VLF range (10 Hz - 32 kHz). These two systems were operating during the relatively rare occurrence of a large local electrical storm on 23 September 1990 and their measurements (which cover 25 narrow frequency bands in the more than five-decade range 0.01 Hz - 32 kHz) provide a detailed picture of the ULF and ELF/VLF magnetic field fluctuations that accompanied the storm. As would be expected, considering the important role played by lightning in its generation, ELF/VLF radio noise is enhanced. However, the ELF/VLF enhancements are smaller than those that occur at ULF: the amplitude of the magnetic field fluctuations in a 100 Hz band centered on 2 kHz was approximately doubled, whereas in the range 0.2-0.5 Hz the amplitude was increased roughly 20 times, and the increase was about 50 times for the range 0.01-0.02 Hz. Some years ago I proposed that repetitive lightning in electrical storms could trigger Pc 1 geomagnetic pulsation events, but these new results suggest that there may exist a broader link between electrical storms and geomagnetic pulsations. Considering that there are perhaps 2000 electrical storms in progress around the world at any time, and that large ULF electrical current systems below the ionosphere can stimulate ULF hydromagnetic waves in the ionosphere, the new measurements suggest that the electrical currents associated with thunderstorms, taken as a whole, may be a source of ULF energy in the magnetosphere and thus influence the generation and amplification of some classes of geomagnetic pulsations.