The RF spectra of first and subsequent lightning return strokes in the 1‐ to 200‐km range

We present the first experimental characterization of the frequency spectra of first and subsequent stroke electric fields over a distance range from about 1 km, where the fields are primarily electrostatic, to 200 km, where they are primarily radiation. Spectra are presented to about 700 kHz for lightning within 12 km and to about 300 kHz for lightning at 50 and 200 km. We show that the return stroke ground wave spectrum beyond 50 km has a peak near 4 kHz but that within 10 km the spectrum shows a steady increase with decreasing frequency to 1 kHz. Subsequent strokes beyond 50 km contain more energy above 100 kHz relative to the peak spectral value than do first strokes. Frequency spectra at all ranges fall off roughly as 1/ƒ for frequencies between 5 and 100 kHz, whereas the falloff above 100 kHz is faster as the distance to the stroke increases. From this high-frequency attenuation we determine an RF conductivity for central Florida of between 0.002 and 0.005 Ω−1/m.