Formation mechanism of the continuous spectral profile of lightning plasma

The cloud-to-ground lightning discharge spectrum was recorded using a slitless grating spectrograph with a spectral range of 400~1000 nm. Abundant monovalent nitrogen ion lines were observed in the lower-frequency range of the visible spectra, whereas other important ion lines were not clearly observed. Under the action of the cloud-to-ground electric field, a large number of electrons in the lightning plasma channel poured onto the ground and quickly heated the channel, such that the temperature decreased along the radial direction of the channel. This process enhanced the interaction between the nitrogen ions and the electrons near the channel surface, producing continuous radiation. The continuous radiation mechanism of lightning mainly includes bremsstrahlung and recombination radiation, which correspond to the Coulomb collision between the nitrogen ions and free electrons and the capture of free electrons. When the plasma temperature is lower than 10,000 K, the continuous bremsstrahlung spectrum is a flat spectrum, which has no obvious influence on the profile characteristics of the continuous spectrum in the visible light range. For recombination radiation, an approximate calculation method for non-hydrogen-like complex ions was introduced on the basis of the classic hydrogen-like ion radiation theory. The Gaunt factor was used for quantum mechanic correction to analyze the recombination radiation process of nitrogen ions, on the basis of above, the functional relationship between the recombination radiation coefficient of the continuous spectrum and the wavelength was derived. Finally, a characteristic curve was drawn for the continuous radiation spectrum of nitrogen plasma. The curve was compared with the observed profile of the continuous lightning spectrum, revealing that the temperature of the electrons on the plasma surface is closely related to the position of the continuous radiation spectrum peak; the effective nuclear charge number Z^* of the introduced nitrogen ions has a significant effect on step feature and broadening characteristics of the continuum spectrum. By comparison, when Z^* was set to 3, the theoretical curve was highly consistent with the profile characteristics of the continuous spectrum. The range of Z^* was determined by the type of ions. The introduction of Z^* could help to effectively explain the step feature of the continuous spectrum of the lightning plasma at a given wavelength.