Characteristic Analysis of Continuous Radiation Spectrum of Lightning Plasma
DONG Xiang-cheng1, CHEN Jian-hong2*, LIU Guang-qiao1
1. BaiLie School of Petroleum Engineering, Lanzhou City University, Lanzhou 730070, China
2. School of Electronic and Information Engineering, Lanzhou City University, Lanzhou 730070, China
Abstract：The spectrum of lightning plasma is characterized as a rich linear spectrum of NⅡ, NⅠ, OⅠ and HⅠ superimposed on strong continuous radiation background, of which the temperature of the lightning return channel can be higher than 10 000 Kelvin and in the channel nitrogen and oxygen molecules are nearly completely dissociated. When the continuum spectrum is analyzed, the influence of each component’s molecular band spectrum on the continuum spectrum is not taken into account. The cloud-to-ground lightning discharge spectrum was recorded by a slit-free grating spectrograph with a spectral range of 400～1 000 nm, and a large number of univalent nitrogen ion spectra were observed in the low-frequency range of visible region of the spectrum, and no other evident ion spectra were observed. It is believed that the continuous radiation is mainly produced by the interaction between nitrogen ions and free electrons, including bremsstrahlung radiation and recombination radiation. In terms of bremsstrahlung radiation, when the plasma temperature is below 1×104 K, the continuous spectrum is characterized as a flat spectrum, the radiation intensity is weak, and the intensity increases in the ultraviolet section with the increase of the plasma temperature, which has no significant effect on the profile feature of a continuous spectrum of lightning plasma invisible sections. In terms of recombination radiation, assuming that the lightning plasma features the local thermodynamic equilibrium and is optically thin, and based on the classical radiation theory of hydrogen-like ions, the Gaunt Factor is introduced for quantum mechanical modification. Besides, considering that the continuous spectrum may occur in a highly excited state with large probability as ions bind free electrons during recombination, the approximate calculation method for non-hydrogen-like complexions is employed to analyze the recombination radiation of nitrogen ions, and then the functional relation between the recombination radiation coefficient and the wavelength of the continuous spectrum is derived to obtain the characteristic curve of continuous radiation spectrum of nitrogen plasma under specific parameter and to compare with profile observations of continuous lightning spectrum. Thus, it is found that the plasma electron temperature is closely related to the position of continuous radiation spectrum peak, therefore concluding that the surface electron temperature of lightning discharge channel can be reliably diagnosed by the profile fitting of the continuous lightning spectrum. Furthermore, the Z* value of the real effective nuclear charge number of nitrogen ions also has a significant effect on the continuous spectrum characteristic. In other words, if the Z* value is small, the jumping feature of the continuous spectrum will be enhanced, and if the Z* value is large, the broadening feature of the continuous spectrum will be enhanced, and the deviation from the bottom of the measured spectrum profile will be increased. By comparison, it is found that, when the Z* value is taken from 2 to 4, the theoretical curve is in good consistency with the measured spectrum profile. The range of Z* value is determined by the ion species, and the introduction of effective nuclear charge number of ions, the Z* value, may fully explain the jumping feature of the continuous spectrum of lightning plasma at a particular wavelength.
Key words：Lightning spectrum; Continuous radiation; Spectral profile; Electron temperature
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