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

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  • 1 Westinghouse Research Laboratories, Pittsburgh, Pa.
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Abstract

A quantitative analysis has been completed of the first time-resolved spectra of return strokes. All values refer to approximately a 10-m section of the return-stroke channel. Ten return-stroke spectra, eight with 5-μsec resolution and two with 2-μsec resolution, have been analyzed to determine their temperature-time curves. The peak temperature in five of the ten spectra is in the 28,000–31,000 K range despite the use of different slitless spectrographs and different multiplet intensity ratios for the measurements. The highest peak temperature was calculated to be 36,000 K. Temperature errors are on the order of 10–25%. A temperature rise in two of the strokes has been calculated in the first 10 μsec from data having 5-μsec resolution. The two recorded strokes with 2-μsec resolution have monotonically decreasing temperature-time curves. It is shown that if the number density of a particular emitting species is known, the relative channel radius within which the particular radiators are contained can be calculated as a function of time. The NII radiation reaches peak intensity in 5–10 μsec, the continuum radiation attains maximum within 10–15 μsec, while the H-alpha emission is most intense in the 20–50 μsec period. The effective excitation potential of the continuum radiation lies between that of the ions and the neutrals and may therefore be due to radiative recombination or radiative attachment.

Two spectra with H-alpha emissions have been quantitatively analyzed. The first spectrum shows an increasing intensity to 50 μsec followed by a monotonic decrease. The second H-alpha spectrum attains maximum intensity in 20 μsec, decreases to a local minimum at 35 μsec, and then decreases monotonically after a small maximum at 45 μsec. The second maximum, or luminosity enhancement, is probably associated with a branch providing additional charge to the return-stroke channel. The Stark-broadened half-width of the H-alpha line has been measured as a function of time with 5-μsec resolution. From the half-width measurement an electron density on the order of 1018 cm−3 has been calculated in the first 5 μsec, decreasing to 1–1.5 × 1017 cm−3 in 25 μsec. Errors are on the order of 50%.

Abstract

A quantitative analysis has been completed of the first time-resolved spectra of return strokes. All values refer to approximately a 10-m section of the return-stroke channel. Ten return-stroke spectra, eight with 5-μsec resolution and two with 2-μsec resolution, have been analyzed to determine their temperature-time curves. The peak temperature in five of the ten spectra is in the 28,000–31,000 K range despite the use of different slitless spectrographs and different multiplet intensity ratios for the measurements. The highest peak temperature was calculated to be 36,000 K. Temperature errors are on the order of 10–25%. A temperature rise in two of the strokes has been calculated in the first 10 μsec from data having 5-μsec resolution. The two recorded strokes with 2-μsec resolution have monotonically decreasing temperature-time curves. It is shown that if the number density of a particular emitting species is known, the relative channel radius within which the particular radiators are contained can be calculated as a function of time. The NII radiation reaches peak intensity in 5–10 μsec, the continuum radiation attains maximum within 10–15 μsec, while the H-alpha emission is most intense in the 20–50 μsec period. The effective excitation potential of the continuum radiation lies between that of the ions and the neutrals and may therefore be due to radiative recombination or radiative attachment.

Two spectra with H-alpha emissions have been quantitatively analyzed. The first spectrum shows an increasing intensity to 50 μsec followed by a monotonic decrease. The second H-alpha spectrum attains maximum intensity in 20 μsec, decreases to a local minimum at 35 μsec, and then decreases monotonically after a small maximum at 45 μsec. The second maximum, or luminosity enhancement, is probably associated with a branch providing additional charge to the return-stroke channel. The Stark-broadened half-width of the H-alpha line has been measured as a function of time with 5-μsec resolution. From the half-width measurement an electron density on the order of 1018 cm−3 has been calculated in the first 5 μsec, decreasing to 1–1.5 × 1017 cm−3 in 25 μsec. Errors are on the order of 50%.

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