闪电等离子体连续辐射谱特征分析

闪电等离子体光谱特征是在强连续辐射背景上叠加丰富的NⅡ，NⅠ，OⅠ，HⅠ线状谱，闪电回击通道温度可达万开以上，通道内氮分子和氧分子接近完全离解，分析连续谱时，不考虑各组分分子带状谱对连续谱的影响。使用摄谱范围在400~1 000 nm的无狭缝光栅摄谱仪记录云对地闪电放电光谱，在光谱可见区低频段观测到大量一价氮离子谱线，未观测到明显的其他离子谱，认为连续辐射机制主要由氮离子与自由电子相互作用产生，包括轫致辐射和复合辐射。等离子体温度在1×104 K以下时轫致辐射连续谱呈现出平谱特征，辐射强度较弱，随着等离子体温度升高在紫外段辐射强度增加，对可见段连续谱轮廓特征没有明显影响。对于复合辐射，设闪电等离子体符合局域热力学平衡和光学薄条件，以类氢离子经典辐射理论为基础，以冈特因子作量子力学修正，考虑到复合过程中自由电子被离子俘获，大概率出现在高激发状态，引入非类氢的复杂离子近似计算方法分析氮离子复合辐射过程，导出连续谱复合辐射系数与波长的函数关系，获得特定参数条件下氮等离子连续辐射谱特征曲线，与闪电连续谱轮廓观测结果比较，发现等离子体电子温度与连续辐射谱谱峰位置密切相关，认为通过对闪电连续谱轮廓拟合，能够可靠诊断闪电放电通道表面电子温度。研究认为氮离子实有效核电荷数Z*取值对连续谱特征也有显著影响，Z*取值小，连续谱跃变特征增强；Z*取值大，连续谱展宽特征增强，从而与实测谱轮廓底部背离增大。通过多次对比发现Z*取为2~4时，理论曲线与连续谱观测轮廓具有较好的一致性，Z*的取值范围由离子种类决定，引入离子有效核电荷数Z*，能够很好地解释闪电等离子体连续谱在特定波长出现跃变的特征。

Characteristic Analysis of Continuous Radiation Spectrum of Lightning Plasma

The spectrum of lightning plasma is characterized as a rich linear spectrum of NⅡ,NⅠ,OⅠand HⅠsuperimposed on strong continuous radiation background,of which the temperature of the lightning return channel can be higher than 10000 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 400~1000 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.