【作者】 薛思敏；

【导师】 袁萍；

【作者基本信息】 西北师范大学 ， 原子与分子物理， 2016， 博士

【摘要】 自然界中,双极性闪电发生的概率极小。双极性闪电的形成机制、放电特性以及相应的雷云电荷结构等都迥异于普通的正、负闪电。因此,人们对双极性闪电的研究始终极为关注。闪电放电形成一个温度高达数万度的等离子体通道,并伴随发光、雷声和电磁辐射。闪电光谱是诊断闪电通道内部物理性质的有效手段之一。目前,还没有双极性闪电光谱的研究报道。本论文利用无狭缝光谱仪、配合快天线雷电电场变化仪记录了青海省大通县夏季雷暴中一次双极性云对地闪电过程,分析了其相反极性的两次回击和继后连续电流过程中M分量的光谱结构,并比较了沿通道、距地面不同高度处的光谱特点。同时,根据光谱资料,计算了双极性地闪的两次回击和M分量的通道温度、电子密度等物理特性参数,分析、探讨了这些参数沿通道、随时间的演化特性及其与放电过程的相关性。又根据该闪电的雷声信号,讨论了其频谱特性。此外,本论文利用多通道狭缝光谱仪记录了一次具有较高波长分辨率的地闪回击通道光谱,并计算了通道的温度和电子密度,为更准确地定量分析放电通道的物理特性奠定了基础。具体的研究内容包括:1.利用多通道光谱仪记录了一次地闪回击通道具有较高波长分辨率的光谱,波长范围600 nm~1000 nm。分别利用OI、NI和H_?谱线的Stark加宽计算了通道的电子密度。同时,采用OI的多条谱线计算了回击通道的温度。2.利用无狭缝光谱仪记录了一次具有两个回击的双极性云对地闪400~1000 nm波长范围的光谱。这个闪电的继后正回击和首次负回击共享同一个放电通道,两次回击后都有长连续电流并伴随M分量脉冲放电。光谱分析表明:放电极性和电流传输方向对光谱结构的影响较小,而放电强度包括电流大小和持续时间是决定光谱特性的主要因素。正、负回击的光谱特征在可见光区域主要为强的NII离子线,M分量的光谱主要是近红外区NI和OI原子辐射的贡献。此外,不同通道截面的光谱特征表明,回击和M分量光谱的背景强度随通道高度的增加呈增大的趋势。3.利用双极性闪电光谱资料、结合等离子体理论计算了回击和M分量的通道温度、电子密度、电导率等物理参数,并分析了其随通道高度的变化关系以及其时间演化特性。结果表明:正负两个回击通道的温度、电导率随通道高度的增加呈增大的趋势。首次回击后连续电流初期M分量的通道温度和电导率随高度没有表现出明显的单调变化,但通道的光谱强度和光学观测半径随通道高度增加有明显的增强现象。上述参量的时间演化特性表明:首次负回击后通道温度和电子密度具有相同的变化规律,随时间的延续呈衰减趋势。回击后的连续电流初期,由于M分量与回击以及几个M分量之间的时间间隔较小,通道温度降低较慢,甚至M1通道温度高于回击通道温度。这也证实:通道温度不仅与放电强度相关,还与电流的持续时间和电流脉冲的时间间隔密切相关。对于继后正回击与其后M分量过程,由于其电流脉冲时间间隔较大,电场变化的幅值是影响通道温度的主要原因,电场变化与通道温度具有正相关性。4.依据双极性地闪的雷声信号,分析了观测点雷声的时域和频谱特性,并利用声波在大气中的传播理论将其频谱还原到声源附近,对比分析了该双极性闪电与普通负地闪的雷声信号时域和频谱特性,利用其主峰值频率估算了单位长度通道所储存的能量。更多还原

【Abstract】 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.更多还原