Study On The Spectrum And Physical Characteristics Of Bipolar Lightning

The occurrence probability of the natural bipolar lightning is small.The corresponding the formation mechanism,the discharge characteristics and charge structure of thundercloud of the bipolar lightning are different from the ordinary negative or positive lightning.Study on the bipolar lightning is always concerned by the researchers.The lightning discharge forms a plasma channel with temperature for tens of thousands of degrees,accompanied by light,thunder and electromagnetic radiation.Lightning spectroscopy is one of the effective ways diagnosing the physical properties inside the lightning channel.At present,the spectrum of the bipolar lightning has not been reported.In this paper,using the slitless spectroscopy and fast antenna systems,a bipolar lightning produced at summer thunderstorm in Datong,Qinghai is recorded.We analyze the spectral structure of the two return strokes with opposite polarity and M-components during continuing current,and compare their spectral characteristics in different channel sections.Meanwhile,according to the spectral data,the channel physical parameters such as the channel temperature,electron density and conductivity are calculated and the change of these physical parameters with the channel height and time are analyzed,the correlation with discharge process is discussed.And according to the thunder of the lightning,its frequency spectra are analyzed.In addition,the spectrum of a cloud-to-ground lightning is recorded by a multi-channel slit spectrograph,and the channel temperature and electron density are calculated,which will lay a foundation for more accurate quantitative analysis of the physical characteristics of the discharge channel.The main contents include:1.The spectrum of the return stroke channel of a cloud-to-ground lightning is recorded by a multi-channel spectrometer,which has a high wavelength resolution with wavelength range of 600 nm~1000 nm.The electron density is calculated by Stark broadening method using the spectral lines of OI,NI and H_?emission.Meanwhile,the channel temperature is calculated by the multispectral lines of the OI emission.2.Using a slitless spectrograph,the spectra of a natural bipolar cloud-to-ground lightning with two return strokes,with a spectral range from 400 to 1000 nm,has been recorded,the subsequent positive return stroke occurred retracing the preexisting channel created by the first negative stroke.They both were followed by a long continuing current going with M-components pulse discharge.The spectral analysis suggests that the polarity of the discharge and the directions of the current have little effect on the structure of the spectrum,while the discharge strength,including current magnitude and duration,is the main factor determining the structure of the spectrum.The spectra of the positive and the negative return strokes are characterized by strong NII lines within the visible range.The spectra of the M-components were mainly from the contribution of OI and NI emissions in the infrared range.In addition,the spectral characteristics in different channel sections show that background intensities for both the return strokes and all the M-components have an increasing trend with increasing height along the channel.3.According to the spectral data and plasma theory,the physical parameters such as the channel temperature,electron density and conductivity are calculated,and the change of these physical parameters with channel height and time are analyzed.The results show that the temperature and conductivity of the two return strokes are obviously increasing with the rise of height.In contrast,the ones of the M-components during the continuing current after the first return stroke do not show obvious monotonous variation along the channel,but the total spectral intensity and luminous radius increase clearly with the rise of height along the channel.Time evolution characteristics of the above parameters show that the channel temperature and electron density after the first negative return stroke have the same variation which are decreasing with increasing time.In addition,at initial stage of the continuing current,the channel temperature decreases slowly with time and even that of M1 is higher than the one of the return stroke due to the short time interval between M-component and the return stroke or M-component.It confirms that the temperature is not only related to the discharge intensity,but related to the duration of the current and the time interval of the current pulses.For the subsequent positive return stroke and M-components,the amplitude of the electric field change is the main reason of affecting the channel temperature,and the channel temperature is positive correlated to the electric field change.4.Based on a thunder signal of a natural bipolar cloud-to-ground lightning,the time domain waveform and the frequency spectra of the thunder in observation location have been analyzed.According to the propagation theory of acoustic waves,the thunder frequency spectra near generation location of the lightning are deduced.In addition,the thunder frequency spectra characteristics of the bipolar and common negative cloud-ground lightning are analyzed.Using the main peak frequency,the energy stored in unit length channel is estimated.