闪电双向先导理论及观测:极性不对称、不稳定及间歇性

闪电双向先导原理的提出及观测验证是闪电物理研究近几十年最重要的进展之一,而正、负先导及流光的极性不对称性及传输的持续、间歇性,是理解闪电各种过程物理机制的关键.本文对闪电双向先导的概念及进展进行了总结和讨论,重点强调了正、负先导流光传输机制特别是门限电场的不对称性,阐述了正先导传输的连续性及其在闪电始发、负先导的空间先导形成、不稳定先导通道中的反冲先导建立等过程中独特的启动作用.     

基于VHF辐射源短基线定位系统对闪电放电过程的研究

利用闪电VHF辐射源短基线定位系统2009年东北大兴安岭地区的观测资料,对正、负地闪和云闪放电过程中的VHF辐射源进行了定位研究.根据闪电VHF辐射源的二维位置(方位角和仰角)随时间的演变特征,结合同步观测的快、慢电场变化资料分析发现,持续时间较长的地闪预击穿过程在云中的放电通道呈双层结构,预击穿过程结束阶段的云内放电表现为反冲流光,预击穿过程的平均速度在10⁴m/s量级.预击穿过程为梯级先导的发展提供了必要的条件,梯级先导从预击穿起始位置开始并向下发展,产生较强的辐射,平均速度在10⁵m/s量级.K过程主要是流光沿之前已电离通道的传输.正、负地闪回击阶段前后的放电特征有明显不同,正地闪回击之后,连续电流期间的较长时间的云内放电产生较强的VHF辐射.对闪电在125---200MHz频段范围的VHF辐射频谱特征的统计分析发现,辐射能量呈现出幅值随频率增加而减小的趋势,在通带范围上基本遵循f^-2.9的衰减率递减.     

一次对地转移电荷极性两次反转的人工引发雷电特征及反转机制分析

利用闪电甚高频干涉仪动态成像结果,并结合通道底部电流、电场和光学观测资料对一次罕见的、对地转移电荷极性反转两次的双极性人工引发雷电放电特征进行了详细研究,首次观测到直接导致极性反转的云内击穿放电过程,探讨了对地转移电荷极性两次反转的物理机理.结果发现,此次闪电无回击过程、初始连续电流阶段对地转移电荷极性经历了负-正-负的两次反转,转移电荷量分别约–40.0 C,+13.3 C和–1.0 C.负极性初始连续电流衰减过程中,一支击穿空气的负先导起始于已电离的正先导分支通道上并形成悬浮通道,水平发展28.816 ms后,接地主通道上发生负极性击穿连接到了悬浮通道末端正电荷聚集处或双向发展的悬浮通道的正极性端,随后对地转移电荷极性快速反转为正.负先导熄灭后,对地转移电荷极性缓慢反转为负.击穿空气的负先导连接到接地主通道和持续发展的正先导是此次双极性闪电对地转移电荷极性发生两次反转的重要原因.而负先导的起始,与正先导多分支同时发展引起的先导通道和环境电势分布差异密切相关.     

闪电引起的天电和红闪现象的模拟研究

用二维时变的电磁场模型，研究了闪电放电产生的天电和红闪现象.模拟结果表明：闪电放电产生的电场包括辐射场和准静电场.辐射场在地面-电离层波导中传播形成大气中的天电.由准静电场引起的N₂分子第一个正波段的光辐射形成红闪现象.用模拟的方法表明，在红闪的空间结构发生变化的过程中，有明显“V”形结构; 在红闪的整体机构中，中心部分是一个光强度相对较强的核，周围包着光强度相对较弱的晕. 关键词： 红闪 天电 电磁场模型     

闪电光谱研究进展

简要回顾了早期闪电光谱的研究,分两个阶段综述了近二十年来国内外闪电光谱研究的最新进展。在20世纪60年代和70年代初,利用胶片相机获取的闪电光谱虽有诸多不足之处,但仍然取得了很多非常重要的研究成果,既为后期闪电光谱的研究奠定了坚实的基础,也指明了发展方向。2001年以来,国内首先利用普通数码摄像机组装成无狭缝光栅摄谱仪开始开展闪电光谱研究。普通数码摄像机虽然解决了胶片相机存在的问题,但拍摄速度较慢(50帧·s^-1),只能研究闪电回击通道在该时间范围内的整体性质。尽管如此,这些工作又一次推动了闪电光谱研究的发展,国际范围开始关注闪电光谱研究。Warner等于2011年用高速摄像机组装的无狭缝光谱仪记录到了云-地闪电梯阶先导的光谱,大幅度提高了拍摄速度(10 000帧·s^-1)。2012年以来,国内也开始利用以高速摄像机作为记录系统集成的无狭缝光栅摄谱仪捕获闪电光谱,用更高时间分辨率对闪电通道不同阶段的辐射光谱开展了大量研究,并取得了多项引人瞩目的成果,主要包括自然闪电的梯级先导、直窜先导和回击以及球状闪电和闪电通道核心的光谱研究。2017年,W...     

基于蒙特卡罗方法的卫星光学波段闪电辐射观测模拟

我国下一代静止气象卫星风云四号将首次搭载光学闪电成像仪上天,对闪电和与之相联系的强对流天气进行实时、连续观测。对未来风云四号卫星闪电观测资料的特性进行预先研究,一个重要的途径就是建立光学波段的闪电辐射传输模拟算法,对闪电辐射的物理过程及卫星观测到的闪电辐射光学特征进行模拟研究。结合风云四号卫星闪电成像仪观测的几何和波长参数,在构建简化的闪电光源模型和雷暴云模型的基础上,利用蒙特卡罗随机模拟和光子追踪方法,模拟得到了风云四号卫星闪电成像仪观测到的云顶闪电信号,并对其辐射特征进行了初步研究。结果表明,基于蒙特卡罗方法的闪电辐射传输模拟,可以从理论上揭示卫星光学波段闪电辐射观测与最重要影响参数之间的联系,这将为未来星载闪电资料的应用提供非常有价值的信息。     

球状闪电之谜

介绍了球状闪电的一般性质和研究价值，用等离子体物理理论描述了从普通闪电到球状闪电的物理过程，讨论了球状闪电的孤子模型和化学反应模型．     

自然闪电条件下敷地线缆耦合试验

研制了一套能够同时测量线缆皮线和芯线感应电流的试验系统,研究自然闪电条件下敷地线缆的耦合问题。结果表明:不同位置处的线缆皮线感应电流波形基本一致,但幅值存在一定差异,说明感应电流空间分布不均匀; 所测线缆芯线和皮线感应电流的时域波形和频谱比较相似,能量集中在1 kHz~1 MHz之间; 正负地闪芯线感应电压波形均为单一脉冲型,波形持续时间10~49 μs; 正地闪线缆感应电压的幅值范围及幅值平均值都大于负地闪相应结果,说明正地闪产生的线缆耦合作用大于负地闪; 不同雷电过程的线缆感应电压幅值与磁感应强度有较小的相关性,而同一雷电过程则基本成线性关系; 正地闪线缆感应电压波形持续时间、半峰值宽度、10%~90%上升时间的范围及均值都比负地闪的要大。     

飑线系统中的闪电辐射源分布特征及云内电荷结构讨论

本文利用SAFIR3000闪电定位资料和多普勒天气雷达资 料分析了2010年6月13日发生于北京的飑线 系统中的闪电活动特征并讨论了电荷结构, 发现闪电都集中分布在飑线的前部线状强回波区域内, 仅在消散阶段在层云降水回波内发生的闪电数目明显增加. 通过闪电分布与降水的关系研究发现, 总闪电与对流降水整体相关系数达到了0.82, 云闪与对流降水的整体相关系数为0.76, 表明闪电的发生与雷暴云内动力和微物理过程密切相关. 基于闪电的辐射源分布特征, 讨论了飑线内电荷分布特征. 研究结果表明在飑线成熟阶段, 闪电辐射源密度的分布呈现出双层结构的分布特征, 下部的闪电辐射源中心位于4 km高度处, 上部的辐射源中心位于11 km高度处, 根据闪电双向先导的传输方式, 闪电辐射源密度高值区对应于负先导在正电荷区的传播, 飑线对流云区内总体呈现出中间为负电荷区, 上部和下部分别为正电荷区的三极性的电荷结构：上部正电荷区位于10–12 km高度处, 中部负电荷区位于8–10 km高度处, 下部正电荷区位于4–7 km高度处. 关键词： 飑线 闪电辐射源 电荷结构 SAFIR3000     

闪电连续电流的光谱分析及通道温度特性研究

连续电流是闪电放电过程中的一个重要子物理过程,它是指雷暴云局部电荷中心在回击之后沿原通道对地的持续放电过程。在连续电流阶段,原本发光微弱的通道其亮度有时会突然增强,这种现象被称为叠加了M分量,自20世纪连续电流被发现以来,国内外学者进行了许多观测研究。目前主要是利用电磁学和光学的观测手段揭示其放电和发光的宏观特征,利用光谱观测对其通道内部微观的发光信息和物理特性等的研究还很缺乏。如关于连续电流阶段放电通道内的温度特性参数目前鲜有报道,而温度是研究闪电连续电流放电通道物理特性所必需的基本参量,也是预防连续电流引起的雷电灾害事故所关心的参数。依据由无狭缝高速光谱仪观测的一次云对地闪电首次回击后叠加三个M分量的连续电流过程的光谱资料,分析了整个放电过程中光谱的演化特征,计算了连续电流放电过程电流核心通道和外围电晕通道的温度,研究了两者随通道高度的变化特性。结果表明,在初始回击阶段,通道的光辐射主要是激发能较高的一次电离的氮离子辐射,在之后连续电流阶段,通道的光辐射则主要是激发能较低的中性氮、氧原子辐射。离子线辐射在回击初期时最强,氢Hα线和红外波段的中性原子线在M1时最强,连续谱在M2时最强。近红外波段的四条线OⅠ 777.4, NⅠ 746.8, 821.6和868.3 nm在整个放电过程都可以被观测到。在连续电流阶段,电流核心通道温度为42 060～43 940 K,比相应回击核心通道温度高6 020～7 900 K;外围电晕通道温度为16 170～20 500 K;通道核心温度和电晕温度均随时间变化不大;通道核心温度随通道上升呈减小趋势,而外围电晕温度随通道上升呈增大趋势。     

Modeling the development of the stepped leader of a lightning discharge

A stochastic-deterministic model is presented for the propagation of a downward-moving leader. Lightning formation is described by a stochastic growth of branching discharge channels which is determined by the electrostatic field. The dynamics of the electric field and of the charge distribution over the lightning structure are calculated deterministically. The model includes the initiation of lightning, a preliminary discharge in a cloud, the propagation of a downwardmoving stepped leader toward the earth, and the initiation and upward motion of a return stroke from the earth’s surface. Numerical execution of the model yields a dynamic picture of the development of the downward-moving leader and of the intracloud discharge structure. The effect of the charge density in the cloud and the parameters of the developing channels on the spatial-temporal, current, and charge characteristics of the stepped leader’s propagation are studied. The effect of free-standing structures on the distribution of points on the earth’s surface where lightning strikes is examined. Zh. Tekh. Fiz. 69, 48–53 (April 1999)     

Numerical simulations of thunderstorm-induced corona processes near lightning rods installed on grounded structures

For the purpose of estimating the efficacy of a lightning rod, the current of a corona was numerically calculated from a rod placed (i) centrally on the roof of a vertical grounded cylindrical structure, a model of a man-made object, and (ii) at the top of a grounded hemispherical structure of large radius, such as a hill or mountain. The calculations were carried out for a slowly varying thundercloud electric field and when this field was enhanced by the charge of an approaching downward leader. In case (i), variation of the ratio between the height and radius of the cylindrical structure leads to a variation in the (a) corona current from the tip of the rod over a wide range and (b) distance from which lightning is attached to the rod. In case (ii), it is shown that, contrary to the case of rods installed on the ground surface, a slowly rising thundercloud electric field can be sufficient to initiate streamers and upward leaders from rods tens of meters in height installed on the top of tall, grounded, hemispherical structures. When the thundercloud electric field is enhanced by the charge of an approaching downward leader, the discharge processes near the rod depend on its length and the height of the tip of the downward leader relative to the top of the hemispherical structure, but are almost independent of the hemisphere radius.     

Statistical model of the lightning leader attraction to ground objects

A statistical model of the lightning channel attraction to ground objects is proposed based on the analysis of the available experimental data on the breakdown of long air gaps and the propagation of the lightning leader channel. The model allows one to estimate the probability of lightning interception by a lightning rod and of lightning damage of the protected object. Examples of calculating the probabilities of lightning strike to the lightning rod and neighboring areas are presented.     

Mechanism of generation of runaway electrons in a lightning leader

A mechanism is analyzed of the electric field enhancement in a lightning leader up to the level permitting runaway of low-energy electrons. The ionization wave propagation in the preionized domain in front of the leader makes it possible to overcome the limitation imposed on the field intensity by transversal expansion of the leader front. By means of numerical simulations, it is demonstrated that, at the final stage of formation of a new leader step, generation of an electric field is possible in the channels of the streamer zone ahead of the new step with intensity sufficient for electron runaway and, consequently, for producing the X-ray and γ-ray pulses observed in correlation with the lightning leader steps.     

Fundamental processes in long air gap discharges

The development of atmospheric lightning is initiated and sustained by the formation in virgin air of ‘streamer corona’ and ‘leader’ discharges, very similar to those observed in laboratory long sparks. Therefore, the experimental and theoretical investigations of these laboratory discharges have become of large interest to improve the physical knowledge of the lightning process and to develop self-consistent models that could be applied to new protection concepts.In the present paper the fundamental processes of the subsequent phases of long air gap discharges are analyzed, from the first corona inception and development to the leader channel formation and propagation. For all these processes simulations models are discussed that have been essentially derived and simplified by the authors, in order to develop sequential time-dependent simulation of the laboratory breakdown, with both positive and negative voltages. The possibility of extending these models to the case of natural lightning is discussed in the companion paper, presented in this same volume. To cite this article: I. Gallimberti et al., C. R. Physique 3 (2002) 1335–1359.     

雷云电场作用下长地线表面正极性辉光电晕放电的仿真研究

在雷云电场的缓慢作用下, 一种无流注的正极性辉光电晕在接地物体表面起始, 向周围空间注入大量正极性空间电荷, 从而改变雷电先导对雷击目的物的选择. 本文对雷云电场作用下起始于长地线表面的正极性辉光电晕放电进行了仿真研究; 考虑了正极性离子与其他离子的附着与碰撞作用, 建立了一种精确的二维正极性辉光电晕模型; 并通过在实验室内开展高压电晕放电试验, 测量了不同背景电场下的电晕电流; 与本文所建模型的仿真结果进行对比, 对模型的正确性进行了验证. 基于上述模型, 对正极性辉光电晕在雷云感应作用下的起始发展过程与电晕特性进行了仿真模拟, 得到了该电晕的电晕电流、正离子密度分布规律以及正离子迁移规律. 发现在雷云电场作用下, 电晕放电产生的正离子在迁移初期于垂直于地线的平面内基本呈圆对称状均匀分布, 但随着离子逐渐远离地线其分布不再均匀, 呈拉长的椭圆形分布, 多数离子最终分布于地线上方区域并逐渐向雷云方向迁移; 由于正离子在地线上方迁移区聚集形成的正空间电荷背景对行进电子束具有衰减和消耗作用, 抑制了电子崩的形成, 并降低了电子崩转化为流注的概率, 阻止了新的电子崩对流注的不断注入, 同时正空间电荷背景使气体的碰撞面增大, 增加了与电子的复合概率, 引起大量电子的消耗, 最终抑制了电子崩的形成与流注的发展, 地线表面的上行先导得到抑制.     

Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse. I. Experimental results

Using a plasma channel produced by an ultrashort laser pulse, we have studied the laser triggering and guiding of a positive leader from the tip of a 2-m vertical rod standing on the bottom plane of a 7-m plane-plane gap. The purpose of this setup was to reproduce in the laboratory the electric field conditions leading to the onset of a positive upward leader from a ground rod as a downward negative leader is approaching during a thunderstorm, in order to demonstrate the working principle of a possible future laser lightning rod. The leader triggering properties of the laser-created plasma channel have been studied as a function of the synchronization of the laser pulse with the voltage impulse applied to the gap. We show that the laser pulse reduces the inception voltage of the leader compared to its normal value and that the laser plasma channel guides the propagation of the upward leader at a velocity ten times higher than that of an ordinary leader, with a significantly lower charge per unit length. We show that laser guiding of the leader significantly reduces the breakdown voltage of the gap and that the effect of the laser channel at the end of a lightning rod can be compared quite favorably with the effect of an additional metal rod of the same length.     

Electrodynamics of a convective cloud

An attempt has been made to describe analytically the electrodynamics of a convective cloud on the basis of a one-dimensional convective cell with solid gas rotation. The cloud electrification is due to the interaction between heavy (large raindrops and particles of hail) and light (microparticles of water and ice) particles. As a result, the particles acquire unlike electric charges. The large-scale electric field in the cloud is stipulated by boundary effects and influences considerably the motion of the heavy fraction of aerosol particles. A scenario is proposed for the development of an intracloud charge, by which the large-scale electric field does not reach the breakdown value, staying at the level of the corona discharge field, while an increase in the irregular component of the electric field is continued and achieves the breakdown value in the small-scale electric cells induced by analogues of plasma beam instabilities. The basic electric discharge occurs against the background of multiple discharges inside the cell which provide for the leader lightning channel. Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, Nos. 1–2, pp. 123–137, January–February, 1997.