The Electric Field Spectra of Lightning Return-Strokes in the Interval from 2 to 500 kHz

Abstract

Several thousand time-domain electric field waveforms of return-strokes were recorded using digital techniques. 218 first return-stroke waveforms selected from 21 effective distances over a range of 2 to 600 km were Fourier analysed with a sampling interval of 1 μs and number of samples 512 or 1024. Mean spectral intensities and standard deviations were obtained for frequencies from 2 to 500 kHz, with an interval of 1 or 2 kHz. For distant events, a low frequency peak was observed in the 1- to 10- kHz band, consistent with previous studies; but found to move systematically from low to high frequency within the band as the distance increased. The average fall-off between 10 kHz and 100 kHz was as 1/f in agreement with previous studies.

The degradation due to propagation over a finite-conducting earth was clear from a “knee” observed around 200 kHz, which was found to move to 100 kHz as the distance increased. From the frequency attenuation, the RF ground conductivity of south-eastern Botswana is estimated above 0.005 mho m⁻¹.

Indexing terms:

• Lightning electric fields
• Frequency spectra