Fine structure in RF spectra of lightning return stroke wave forms.

The power spectra of the wide-band (10 Hz-100-kHz) magnetic field signals for a number of lightning return strokes measured during a thunder-storm which occurred in Lindau in August, 1984, have been calculated. The RF magnetic field data are obtained with the engineering unit of the Galileo Jupiter Probe lightning experiment. Each return stroke data stream is passed through an adaptive filter designed to whiten its spectrum. The largest sample in the filtered output is set to zero, and the power spectrum of the filtered data stream is computed using five prolate spheroidal data windows in the time domain and a fast Fourier transform algorithm. The spectra of the magnetic field data definitely show fine structure, with two or three distinct peaks in the spectra of many of the waveforms. A peak at f~60-70kHz is often seen in the power spectra of the waveform time segments preceding and following the rise- to-peak amplitude of the return stroke.