Opacity of N II ion radiation in the lightning discharge plasma channel

Spectral diagnosis is an important way to research the characteristics of lightning discharge plasma. It is crucial to the reliability of diagnostic results that the light source meets the optical conditions. This paper explored and developed a method for determining whether a light source is optically thin from experimental spectra. Based on the Boltzmann distribution of excited state population and the definition of spectral lines’ intensity, the theoretical intensity ratio and the experimental intensity ratio of two N II ion characteristic spectral lines (480.33 nm and 594.16 nm) with equal upper excitation energy were analyzed by using the spectra of nine lightning return strokes. The calculation result shows that the intensity ratios of these two spectral lines in the spectra of return strokes with different temperatures agree with the theoretical values. Combined with the calculated absorption coefficient and optical thickness of the N II 594.16 nm spectral line, it is further confirmed that the N II ionic lines in the lightning discharge plasma channel meet the optically thin condition. Furthermore, the variation of the temperature along the channel height under optically thin conditions also verified the rationality of the result. This research provides a theoretical basis for researching the characteristic parameters of lightning discharge plasma.