University Study Examines Critical Lightning Attachment Processes
Key atmospheric electricity studies that may give researchers a better understanding of the implications for lightning safety and protection of missions and assets were recently conducted at Kennedy Space Center (KSC).
Aerospace, Physics and Space Sciences associate professor Amitabh Nag, along with research professor Kenneth Cummins, graduate student Mathieu Plaisir, Distinguished University Professor Hamid Rassoul and researchers from KSC, recently published the paper, “Inferences on Upward Leader Characteristics from Measured Currents” in Atmospheric Research.
Lightning coming down from a stormy sky and striking ground or grounded objects can be a common sight in places like Florida. Just before lightning attaches to a grounded object, a set of “attachment processes” occur. These include the creation of one or more upward moving lightning leaders or plasma channels (usually carrying positive charge) from ground or grounded objects near the downward progressing lightning leader (usually carrying negative charge). A lightning strike to ground occurs when one of these positively charged plasma channels collides with the downward moving negative plasma channel via a process called the “breakthrough phase.” Improved understanding these processes and the detailed nature of natural lightning currents and electromagnetic fields are key goals of this project, which is funded by the National Science Foundation and the U.S Air Force.
For this project, Nag and his team instrumented the KSC Industrial Area Tower (IAT) to measure electric currents flowing at the base of the lightning channel when it strikes the tower. At present, the IAT is the only such natural lightning current measurement facility in the United States. This tower is of modest height (about 300 feet), allowing for the study of downward lightning, which is the most common variety of cloud-to-ground lightning. Also, the IAT is located in a region with a lightning flash density of 8 to 12 flashes/sq. km/year, which gave the team a good place to study the charges. The team placed research equipment on the top as well as near the structure to study the behavior of downward lightning.
To date, two downward lightning flashes that have attached to this instrumented tower have been recorded, providing unique information about the time evolution and magnitude of current during natural lightning. The team has also determined that even when lightning does not hit the tower it can produce upward unconnected leaders (UUL), in response to lightning striking within a few hundred meters (about 500 feet).
“As we expected, we found that even if lightning was striking the ground nearby there were upward leaders produced from the tower, and they remained unconnected to the main discharge from the cloud,” Nag said. “These have significant amounts of electric charge and currents in excess of 100 amps associated with them and can pose a threat to ground-based objects when it comes to damage from heating.”
Nag and his team analyzed currents associated with these UULs initiated from the tower. Current measurements of UULs in natural lightning are relatively rare and provide important insights into the development and propagation of upward leaders. They reported that both positive and negative charges are transferred sequentially to ground by these UULs, making them a “bipolar” lightning process. This type of lightning process is relatively uncommon.
Eight UULs were initiated from the KSC-IAT between August 1, 2018, and Nov. 15, 2019. All UULs occurred in response to downward negative lightning leaders that attached to ground near the tower.
Apart from ground-based objects, UULs also pose a threat to people whose bodies can become conductors for electric currents from such upward leaders. More research is needed to better understand these unique discharges, which could help lead to improvement of lightning protection standards for both people and structures.
“We learned about the characteristics of these upward leaders, the inherent nature of the pulsing of the current as these upward leaders are attracted to the downward leader, as well as the collapse and rapid switching of polarity of the current,” Nag said. “Such studies not only help us to properly identify the attributes of these discharges but also helps us quantify the threat that these discharges pose to human activity on ground.”