Extremely Low Frequency (ELF) Atmospheric Noise Level Statistics for Project Sanguine

Statistics of narrow-band extremely low frequency (ELF) noise for use in Sanguine systems analysis were obtained at locations in Malta and Guam. The results of four seasons of ELF atmospheric noise measurements are discussed. In particular the contributions of local thunderstorms to high noise levels, i.e., the 1 percent exceedance levels, are shown. Spectra of the 1 percent and 50 percent exceedance levels of the magnetic-field intensity component, obtained from the sensor aligned for maximum reception from northern Wisconsin are plotted. At both locations, the noise levels were highest during the fall season. The diurnal variation of the median noise levels correlated with worldwide thunderstorm activity, while the diurnal variation of the mean square levels correlated with local thunderstorm activity. When few local thunderstorms occur during a season, the cumulative distributions of the 250 s samples for the magnetic-field intensity can be approximated by a log-normal distribution almost to the 1 percent exceedance level.