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Characteristic average power spectra in intervals from 25 to 100MHz are presented for electromagnetic radiation fields from some specific lightning events: preliminary breakdown processes, downward negative stepped- and dart- leaders, return strokes, K changes in cloud-to-ground lightning (CG) and in intra-cloud lightning (IC) flashes, and upward positive leaders in artificially triggered lightning flashes. The spectra are obtained by means of the Fourier transformation of wideband records of electric fields from lightning. Results show that the spectral patterns for these events are obviously dissimilar in the frequency band studied. The spectrum for the IC, the preliminary breakdown process and the K change usually peaks once around 60MHz, while that for the downward negative leader peaks initially around 60-70MHz then around 26MHz as the leader propagates down out the clouds. The spectrum for the upward positive leader in artificially triggered lightning, however, peaks at about 26MHz, whether the leader is propagating below or within the clouds. The dissimilarities in the spectral pattern in the range of 25-100MHz between different lightning events may imply that these events have dissimilar discharge mechanisms and provide a basis for identifying these different discharge events.Department of Building Services Engineerin

Spectral Patterns of Lightning Radiations in Intervals of 25 to 100 MHz