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 Å and have 600 lines mm?1. Inverse dispersions from 70'140 Å 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 ?sec. All spectra have been recorded on film calibrated for intensity and wavelength with a xenon source of known relative spectral emittance. The following data have been obtained. Spectral emissions from 4000?6600 Å have been recorded with 10 Å wavelength resolution. All emissions have been attributed to neutral hydrogen or to neutral or singly ionized atoms of nitrogen and oxygen. No molecular or doubly ionized emissions have been identified in these spectra. The time for luminosity to rise from zero to its peak in a section of the channel is 10 ?sec or less. Several faint lines due to neutral nitrogen and oxygen atoms persist for 150 ?sec. The H-alpha line is present in these spectra. The recorded time sequence of spectral emissions from a section of the lightning channel is 1) line radiation from singly ionized atoms, 2) continuum, and 3) line radiation from neutral atoms. A flash has been recorded composed of at least 5 strokes. Two types of strokes are observed in this flash. The first type is characterized by intense short-lived emissions from singly ionized nitrogen atoms (NII) and a long lasting H-alpha emission. Continuum emission is relatively weak. In the second type, the singly ionized nitrogen emissions (NII) persist for a relatively long time and the H-alpha emission is very intense but short-lived. Continuum emission is relatively strong.