Characterization of Upward Leaders and the Attachment Process in Downward Cloud-to-Ground Lightning

闪电附着在地面上的物体上时和之后所产生的能量可以引发火灾，损坏/摧毁设备，伤害/杀死人和动物，引起爆炸，并扰乱制造过程。这些风险对电力传输和分配、电信、天然气管道、公共安全和保护、户外活动管理、林业、航天发射以及公共/私人住宅和基础设施产生严重影响。然而，潜在的“依恋过程”是非常复杂和多变的，我们不可能对它有一个相对粗略的理解。因此，这个项目的广泛的科学动机是解决闪电附着的问题，最近被表达为闪电研究中十大悬而未决的问题之一：“闪电附着在地面上的高架物体和平地上的物理机制是什么？”具体来说，研究人员将测量和检查与闪电附着过程相关的电流、电磁场和高速视频。目标是详细描述与闪电附着的不同阶段相关的物理过程。这项研究将提供直接测量地面和地面结构在闪电附着之前的向上电流，这被认为是雷击伤害的主要来源。此外，研究结果将直接影响雷击距离计算和领导者模型，这些模型对设计/增强防雷技术至关重要。此外，在这个项目中，研究人员将利用、增强和维护一个独特的（至少在美国）现有的当前测量设备。除了上面提到的直接重要性之外，这种基础设施在未来的基础科学光谱分析研究中是需要的，这些研究将有助于理解闪电通道中温度分布的时间演变。最后，该项目将伴随一项以科学、技术、工程和数学（STEM）为重点的教育计划，对不同背景的学生产生广泛影响，并培养电磁学、地球科学和大气电学领域的下一代研究人员。作为该项目的一部分，本科生和研究生（包括STEM的少数民族）将接受理论和观察方面的培训，并有机会参与一个涉及不同背景和专业知识的合作者的研究项目。在本项目中，研究人员将解决与地面物体闪电附着相关的一些关键科学问题，包括（未连接的和连接的）向上引线在其开始时和向下引线附着之前的特性，闪电附着过程的宽带电磁场特性，慢锋和快速过渡特征如何与闪电附着的突破阶段相关，以及自然闪电回击特征（如峰值电流和电荷转移的时间演变）如何与向上先导特征（如速度、持续时间、亮度和垂直程度）相关。电流测量将在肯尼迪航天中心（KSC） 91.5米高的塔顶现有的雷击电流测量设备上进行，该塔顶位于美国雷击发生率最高的地区之一。这是目前在美国运行的唯一的自然闪电电流测量设备。与目前的测量相结合，研究人员将部署并执行作为本研究的一部分，电磁场和超高速摄像机分别在距离塔约1.5公里和700米的距离进行测量。现有的直流测量系统允许研究人员记录与向上引线相关的电流，时间和振幅分辨率分别为40 ns和约1 A，在200 kA饱和。新购买的高速摄像机测量系统将提供闪电附着的光学记录，时间和空间分辨率分别为几微秒和1米左右。此外，将在距离塔约200米的地方测量下行自然闪电中向上引线的宽带电场导数和300 MHz的窄带甚高频发射。所有的测量都将有gps时间戳，以便它们的时间相关。该项目的研究成果预计将引起科学界的强烈兴趣，并将通过会议和同行评议的期刊进行分发。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The energy delivered during and following lightning attachment to objects on the ground can start fires, damage/destroy equipment, injure/kill people and animals, cause explosions, and disrupt manufacturing processes. These risks have serious impacts on electric power transmission and distribution, telecommunications, gas pipelines, public safety and protection, management of outdoor activities, forestry, space launches, and public/private homes and infrastructure. However, the underlying 'attachment process' is sufficiently complex and variable that it has not been possible to gain more than a relatively crude understanding of it. Therefore, the broad scientific motivation of this project is to address the question of lightning attachment, recently expressed as one of the top ten open questions in lightning research: "What is the physical mechanism via which lightning attaches to elevated objects on the ground and to the flat ground?" Specifically, the researchers will measure and examine correlated electric currents, electromagnetic fields, and high-speed videos associated with the lightning attachment processes. The goal is to characterize in detail the physical processes associated with the different stages of lightning attachment. The study will provide direct measurements of the upward current from the ground and structures on the ground just prior to lightning attachment, which is thought to be a primary source of lightning injury. Further, the findings will directly impact lightning striking distance calculation and leader models that are essential for designing/augmenting lightning protection techniques. Also, in this project the researchers will utilize, augment, and maintain a unique (at least in the United States) existing current measurement facility. Beyond the direct importance noted above, this infrastructure is needed for future basic-science spectral-analyses studies of optical emissions from upward leaders and during early stages of lightning attachment that will help understand the time-evolution of the temperature profile in lightning channels. Finally, this project will be accompanied by an education program focused on science, technology, engineering, and mathematics (STEM) to have a broad impact on students with diverse backgrounds and to train the next generation of researchers in the areas of electromagnetism, geosciences, and atmospheric electricity. Undergraduate students and a graduate student (including minorities in STEM) will be trained in theory and observations as a part of this project and will have opportunities to participate in a research program involving collaborators with diverse backgrounds and expertise. In this project, the researchers will address some key scientific questions related to lightning attachment to objects on the ground including those regarding the characteristics of (unconnected and connecting) upward leaders at the time of their inception and before their attachment to downward leaders, the broadband electromagnetic field characteristics of the lightning attachment process, how the slow front and fast transition signatures are related to the breakthrough phase of lightning attachment, and how natural-lightning return stroke characteristics such as peak current and time evolution of charge transfer are related to upward leader characteristics such as speed, duration, brightness, and vertical extent. The electric current measurements will be performed at an existing lightning-current measurement facility on top of a 91.5-m tall tower at the Kennedy Space Center (KSC) in one of the highest lightning-incidence regions in the United States. This is presently the only natural-lightning electric current measurement facility operating in the United States. In conjunction with the current measurements, the researchers will deploy and perform as part of this study, electromagnetic field and extremely high-speed video camera measurements at distances of about 1.5 km and 700 m from the tower, respectively. The existing direct current measurement system allows the researchers to record currents associated with upward leaders with time and amplitude resolutions of 40 ns and about 1 A, respectively, saturating at 200 kA. The newly-purchased high-speed video camera measurement system will provide optical records of lightning attachment with temporal and spatial resolutions of a few microseconds and about 1 meter, respectively. Additionally, broadband electric field derivative and narrowband VHF emissions at 300 MHz from upward leaders in downward natural lightning will be measured at about 200 m from the tower. All measurements will be GPS-time-stamped allowing their time-correlation. Research findings from this project are expected to be of intense interest to the scientific community and will be distributed via conference and peer-reviewed journals.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.