A comparison of sferics as observed in the very low frequency and extremely low frequency bands

Abstract

A large number of sferics were photographically recorded in the very low frequency (VLF) and extremely low frequency (ELF) bands at a UCLA field station in Hawaii. From the characteristic VLF waveforms it was clear that the VLF signals were generated from lightning discharges. It was found that an observable ELF component (slow tail) followed the VLF component in almost all cases. It was also found that about one third of the sferics observed were ELF signals, similar in appearance to slow tails but not preceded by observable VLF oscillations. Peak amplitudes were measured for both the VLF and ELF components of almost 3000 sferics. The results were tabulated in groups according to (1) whether the sferics were recorded during the day or during the night, (2) whether the polarity of the initial excursion of the ELF signal was positive or negative, and (3) whether the VLF and ELF components appeared together or separately. Amplitude distribution histograms were plotted for all cases. For those sferics possessing both VLF and ELF components, the VLF to ELF peak amplitude ratios were also tabulated separately as in (1) and (2) above, and ratio-distribution histograms were plotted. The more important results obtained from the histograms were as follows. 1. No significant differences were found between the amplitude distributions for the ELF waveforms that were preceded by VLF oscillations and those that were not. Hence, it is probable that both groups were generated by lightning discharges. 2. For both daytime and nighttime sferics the median value of the ELF amplitude was greater for ELF waveforms of positive polarity than for waveforms of negative polarity. 3. For both daytime and nighttime sferics the median value of the VLF/ELF peak-amplitude ratio was greater for ELF waveforms of negative polarity than for waveforms of positive polarity. 4. The polarity of the ELF waveform was predominantly negative at night and positive during the day (verified by a count of the polarities of almost 6000 additional ELF waveforms). An attempt is made to explain the experimental results in terms of known properties of lightning discharges, and some of the difficulties in making such an interpretation are indicated.