广州塔闪电光谱特性分析

Analysis on Lightning Spectral Characteristics of Canton Tower

利用广州高建筑物雷电观测站获得的600 m高广州塔上一次闪电3个回击放电过程的光谱资料,详细分析了广州塔上闪电光谱随时间的演化和随高度的变化特性,并通过对比实测的一组氮原子(NI)[856.8 nm, 859.4 nm, 862.9 nm]多重态的谱线强度比和理论计算值之比,验证了闪电近红外光辐射满足光学薄条件。结果表明:3个回击放电通道约在200 m以下发光较强;在回击放电初期,当向上传输的电流波还未到达通道顶部时,底部通道径向辐射光谱由较强的离子线和较弱的中性原子线组成,而通道顶部径向辐射光谱主要取决于下行先导,由较弱的离子线和较强的中性原子线组成;当回击电流波向上传输到通道顶部后,整个通道径向辐射出很强的离子线和很强的中性原子线,且离子线总强度和原子线总强度均随通道高度的增加而减小;在回击放电70μs以后,200 m以上通道离子线总强度和原子线总强度随通道高度的增加基本保持不变。此观测结果也直接证实了闪电放电通道由一个辐射离子线的高温核心和一个辐射中性原子线温度相对较低的外围电晕组成。

Objective The lightning locations of tall buildings are relatively predictable with high occurrence probability,and the lightning of tall buildings does not need a larger cost compared with artificially triggered lightning.Therefore,tall buildings can provide a good observation platform for lightning research.Additionally,with the rapid development of urbanization,the probability of lightning striking tall buildings is increasing.Thus,the study on the lightning of tall buildings can provide practical references for the lightning protection design of tall buildings.With deepening research on the physical characteristics of lightning discharge,the spectral diagnosis of lightning plasma has become an important tool for measuring lightning properties.At present,the observations and research of lightning spectra mainly focus on natural lightning and artificially triggered lightning,but there are few studies on lightning spectral observations of tall buildings.In addition,the optical thickness of the lightning channel is an important prerequisite for quantitative analysis of the lightning spectrum.Due to the lack of spectral resolution in previous experimental systems,the experimental verification of the optical thickness of lightning NI and OI radiation in the nearinfrared spectrum is rare.This paper employs the spectra of one lightning with three return strokes on the 600-meterhigh Canton Tower obtained at the Tall Object Lightning Observatory in Guangzhou(TOLOG)to analyze the evolution and variation characteristics of the spectra with the time and the channel height in detail.Experimental verification of the optical thickness of lightning nearinfrared radiation is also presented by comparing the measured intensities of spectral lines of NI[856.8 nm,859.4 nm,862.9 nm]multiplet with the theoretical values.This study hopes to deepen the scientific understanding of the microcosmic physical process of lightning discharge and provide an experimental basis for the quantitative analysis of the nearinfrared lightning spectrum.Methods The TOLOG with six stations is established by Chinese Academy of Meteorological Sciences and Guangdong Meteorological Service.Spectral observations are set up at Station 1 and recorded by a slitless spectrograph with a highspeed camera.The splitting system of the spectrograph is a plane transmission grating,which is placed tightly in front of the objective lens of the camera.Based on the spectra of one lightning with three return strokes on the 600-meterhigh Canton Tower,the evolution and variation characteristics of the spectra with the time and the channel height are analyzed.In addition,the influence of opacity on the spectral line intensity of lightning plasma can be determined with the intensity ratio of the spectral lines,and one way to determine the optical thickness is to compare the intensities of several lines with the same upper energy level within the same multiplet.Thus,after comparing the measured intensities of spectral lines of NI[856.8 nm,859.4 nm,862.9 nm]multiplet with the theoretical values,this study presents the experimental verification of the optical thickness of lightning nearinfrared radiation.Results and Discussions The results show that the discharge channels of three return strokes on the Canton Tower have stronger luminescence below 200 m(Fig.6).In the initial discharge stage of the return stroke,when the upward current wave does not reach the top of the channel,the radial spectral radiation at the bottom of the channel is composed of stronger ionized lines and weaker neutral lines.Meanwhile,the radial spectral radiation at the top of the channel mainly depends on the downward leader and is composed of weaker ionized lines and stronger neutral lines(Figs.4-5).When the current wave is transmitted to the top of the channel,the whole channel radially radiates strong ionized lines and strong neutral lines,and the total intensities of ionized lines and neutral lines all decrease with the increasing channel height(Figs.4-5).After 70μs discharge,the total intensities of ionized lines and neutral lines remain basically unchanged with the channel height above 200 m(Figs.5-6).This observation directly confirms that the lightning channel consists of a hot core radiating ionized lines and a cold peripheral corona radiating neutral lines.Additionally,intensity ratios of the spectral lines and the theoretical optically thin limit within the NI[856.8 nm,859.4 nm,862.9 nm]multiplet show that the measured ratios of NI lines within this multiplet are basically unchanged with the time(Fig.7),which means that the nearinfrared spectrum of the lightning channel meets the optically thin condition.Conclusions Based on the spectra of one lightning with three return strokes on the 600-meterhigh Canton Tower obtained at the TOLOG,the evolution and variation characteristics of the spectra with the time and the channel height are analyzed first in detail.Experimental verification of the optical thickness of lightning nearinfrared radiation is also presented by comparing the measured intensities of spectral lines of NI[856.8 nm,859.4 nm,862.9 nm]multiplet with the theoretical values.The results show that the discharge channels of three return strokes on the Canton Tower have stronger luminescence below 200 m.In the initial discharge stage of the return stroke,when the upward current wave does not reach the top of the channel,weak neutral lines in the nearinfrared band are radiated by the channel when the ionized lines in the visible band just appear in spectrum at the bottom of the channel.When the intensity of ionized lines in the visible band peaks,the intensity of neutral lines in the nearinfrared band also peaks.This is different from previously reported observations,which directly confirms that the lightning channel consists of a hot core radiating ionized lines and a cold peripheral corona radiating neutral lines.In the initial discharge stage of the return stroke,the total intensities of ionized lines and neutral lines all decrease with the increase in the channel height.After 70μs discharge,the total intensities of ionized lines and neutral lines remain basically unchanged with the channel height above 200 m.