Spatial Evolution of Temperature and Electron Density in First Return Stroke Channel of Cloud-to-Ground Lightning
Objective Lightning is a discharge phenomenon that involves high currents and strong electromagnetic radiation.The peak temperature of the discharge channel can reach up to 30000 K due to the high current of the return stroke process.The channel's air becomes ionized,generating free electrons and ions and creating a plasma channel.Lightning activity in the Qinghai-Tibet Plateau has increased due to global warming.The strong currents in the lightning discharge channel can cause significant damage to buildings,humans,and livestock on the ground.Therefore,lightning hazards and protection have been of key concern.Spectroscopic diagnosis of lightning plasma has become an essential tool for measuring the lightning discharge process due to the further study of its physical properties.The quantitative analysis of the spectra enables the calculation of the basic parameters of the lightning channel's physical characteristics.Currently,there are studies available on the evolution of temperature and electron density along the lightning return stroke channel,but fewer studies on the spatial evolution characteristics of the temperatures and electron densities of the core channel and the peripheral corona sheath channel.The spatial evolutions of temperatures and electron densities in the core and peripheral corona sheath channels are closely related to the distribution of lightning discharge energy and the transmission characteristics of the discharge current.We use a slitless spectrograph to obtain the spectral information from the first return strokes of four lightning.The temperatures and electron densities of the core channel and the peripheral corona sheath channel are calculated and investigated to characterize the variation of the temperature and electron density along the discharge channel.Furthermore,we explore the relationship between the core channel temperature and the relative intensity ratio of the nitrogen ion line.The study is expected to provide some clues and implications for the exploration of the energy and current transport characteristics of lightning discharge plasma.Methods We report on the use of a slitless spectrograph,consisting of a transmission grating and an M310 high-speed camera,to obtain spectral data of the first return strokes of four cloud-to-ground lightning during a field experiment in the Qinghai-Tibet Plateau.The structural characteristics of the return stroke spectra were analyzed.The temperatures of the core channel and the peripheral corona sheath channel were calculated using the multiple-line method combined with plasma theory.Additionally,the electron densities of the core channel and the peripheral channel were obtained using the Stark broadening method for the NⅡ ion line and the OI atomic line,respectively.The study investigates the evolution characteristics of temperatures and electron densities of the core channel and peripheral corona sheath channel.Moreover,the relationship between the core channel temperature and the relative intensity ratio of the nitrogen ion line was investigated.Results and Discussions The results indicated that during the first return strokes of the four lightning flashes,the temperature and electron density of the core channel exhibited three trends along the channel height:decrease,slight increase,and little change(Fig.6).The lightning with stronger discharge showed a faster decrease of current along the return stroke channel,and the values of temperature and electron density in the core channel were higher than those in the peripheral corona sheath channel(Fig.6).The temperature of the corona sheath channel remained relatively constant,at around 20000 K,as the channel height increased.The electron density exhibited trends of slight decrease and little change along the channel,with a slower decrease rate than that of the core channel(Fig.7).Furthermore,a significant correlation existed between the relative intensity ratio of the NⅡ 500.5 nm and 568.0 nm spectral lines in the return stroke spectra and the core-channel temperature(Fig.5).Generally,a higher temperature of the core channel during the lightning return stroke corresponded to a larger ratio of the relative intensities of the 500.5 and 568.0 nm spectral lines on the return stroke spectrum.Conclusions The study analyses the spatial evolution characteristics of temperatures and electron densities of the core channel and peripheral corona sheath channel during the return strokes of lightning,based on the four first return stroke spectra obtained by a slitless spectrograph.Additionally,the study explores the relationship between the core channel temperature and the relative intensity ratio of the nitrogen ion line.The results indicated a strong correlation between the relative intensity ratio of the NⅡ 500.5 nm and 568.0 nm spectral lines on the return stroke spectra and the core-channel temperature.This provides a simpler analytical method for further study of the channel temperature change.The temperature and electron density of the core channel exhibited similar variations along the channel,with trends of decrease,slight increase,and little change.The temperature and electron density values in the core channel were higher than those in the peripheral corona sheath channel,suggesting that most of the ionic and neutral atomic lines originate from radiation in different regions of the channel.The temperature of the peripheral corona sheath channel did not change significantly and the electron density showed trends of decrease and little change along the channel.The changes in temperature and electron density in the corona sheath channel were smoother than those in the core channel.
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