Electromagnetic radiation from erratic paths of lightning strokes

A possible source of low-frequency electromagnetic radiation from lightning is the tortuous path of the return-stroke current. A number of photographs of lightning channels have been measured, and their tortuous paths have been Fourier-analyzed. It has been found that the amplitudes of the component harmonic oscillations of the paths diminish approximately as the inverse 3/2 power of the oscillation frequency. A theoretical analysis of the electromagnetic field strengths radiated from typical tortuous paths indicates maximum values in the low-frequency band of the order of 10–20 μv sec/m, at ranges of approximately 100 km.