Seasonal variations of globally measured ELF/VLF radio noise

Our laboratory has been conducting a global survey of extremely low frequency (ELF) and very low frequency (VLF) radio noise since February 1985. Eight measurement stations around the world record the instantaneous noise amplitude in each of sixteen narrow-frequency bands in the 10-Hz to 32-kHz frequency range, and we have calculated the monthly averages of these amplitudes for the four stations with the longest times of operation. The period, amplitude, and phase of temporal variations in the averages are important indicators of the sources and propagation characteristics of the noise in the various frequency bands. Furthermore, since the principal source of ELF/VLF radio noise is lightning, long-term variations of the noise must relate to changes in global thunderstorm activity. We find that the noise amplitudes vary seasonally by up to a factor of 4 in some of the sixteen frequency bands; in addition, many of the variations correlate quite well with global lightning flash rates.