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

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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.