Formation mechanism of the continuous spectral profile of lightning plasma
doi: 10.37188/CO.2021-0018
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摘要: 利用光谱范围为400~1000 nm的无狭缝光栅光谱仪记录了云对地闪电放电光谱，在可见光谱的低频段观测到丰富的一价氮离子谱线，没有明显观测到其他重要的离子谱线。闪电通道内大量电子在电场作用下向地面倾泻使通道快速加热，沿通道径向温度降低，通道表面附近氮离子与电子的相互作用增强从而产生连续辐射。闪电的连续辐射机制主要包括轫致辐射和复合辐射，对应于氮离子与自由电子的库仑碰撞和对自由电子的捕获。当等离子体温度低于10000 K时，轫致连续辐射谱为平坦谱，其对连续谱在可见光范围内的轮廓特征没有明显影响。复合辐射方面，以类氢离子经典辐射理论为基础，引入非类氢的复杂离子近似计算方法，用Gaunt因子进行量子力学修正，分析氮离子的复合辐射过程。据此导出连续光谱复合辐射系数与波长的函数关系，由关系式绘制氮等离子体连续辐射光谱的特征曲线，与闪电连续光谱观测结果进行比较，发现等离子体表面电子温度与连续辐射光谱谱峰的位置密切相关；引入氮离子的有效核电荷数Z*对连续谱的阶跃特征和谱翼展宽特性有显著影响。对比发现，当Z*为3时，理论曲线与连续光谱的轮廓特征高度一致。Z*的取值范围由离子种类决定，有效荷电荷数Z*能很好地解释闪电等离子体在给定波长下连续光谱的阶跃特征。Abstract: 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.
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