A High-Speed Time-Resolved Spectroscopic Study of the Lightning Return Stroke: Part I. A Qualitative Analysis

Abstract The first time-resolved spectra of return strokes between the cloud and ground have been obtained. During the summers of 1965 and 1966 twenty-two spectra were obtained at the Institute of Atmospheric Physics, Tuscon, Ariz. The spectra were recorded with two high-speed streaking cameras converted to slitless spectrographs. The conversion was accomplished by mounting Bausch and Lomb replica transmission gratings in front of the cameras' objective lenses. The gratings are blazed for 5500 A and have 600 lines mm−1. Inverse dispersions from 70'140 A mm−1 were used. Most of the data were obtained with a Beckman and Whitley high-speed camera. A 200-mm objective lens was used to focus the return stroke on a 0.5-mm horizontal slit. Thus, a 10-m section of the lightning channel was isolated for a discharge occurring at a distance of 4 km. Data have been obtained with a time resolution of 2-5 μsec. All spectra have been recorded on film calibrated for intensity and wavelength with a xenon source of known re...