Lightning and Thunderstorm Studies

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This research focuses on furthering our understanding of how lightning is produced inside thunderstorms, and how some lightning flashes occasionally produce strong bursts of gamma rays. Gamma rays are similar to but much more powerful than x-rays, and studying how they are produced helps us better understand how lightning begins and develops. The bursts, called terrestrial gamma-ray flashes (TGFs), were first detected by satellites, and are upward-directed events caused by in-cloud (IC) lightning. Although upward TGFs continue to be detected by satellites, the detection rate at any given location is very low, and there have been no close observations of IC flashes that produced satellite-detected TGFs. More recently, studies at the large-area cosmic ray-detecting Telescope Array facility in west-central Utah have shown that cloud-to-ground (CG) lightning can produce downward TGFs. The studies, which are being conducted in collaboration with colleagues at the University of Utah, have provided the first detailed observations of how TGFs are produced by lightning, and are to be expanded upon in the studies going forward. At the same time, similar studies are to be conducted at the Kennedy Space Center in Florida, where there is a greater chance of obtaining observations of IC flashes that produce satellite-detected TGFs. The studies are important because they advance our understanding not only about how TGFs are produced, but also provide valuable information about the lightning discharge processes themselves. They are also cross-disciplinary, in this case between traditional lightning investigations and high-energy particle physics, and provide excellent learning opportunities and experiences for graduate and undergraduate students. The studies utilize observational data obtained from a comprehensive set of lightning detection techniques and systems, including lightning mapping arrays (LMAs) that use multi-station time-of-arrival measurements of the VHF radio signals produced by lightning to determine the 3-D structure and development of flashes, and dual broadband VHF interferometer systems that show the detailed development of flashes continuously in time, with sub-microsecond resolution. Combined with high-speed measurements of the electric field change produced by the different discharge processes, the observations provide complete pictures of the development of lightning flashes both in time and space. The Telescope Array detects Compton electrons produced by the gamma-ray bursts, not only with sub-microsecond resolution and continuously over time, allowing the TGFs to be accurately correlated with the lightning data, but also with 1.2 km grid spacing over tens of kilometers extent – a major factor in detection efficiency. The comparisons show the TGFs occur during energetic initial breakdown pulses (IBPs) of CG flashes, and are playing a crucial role in understanding how IBPs are produced and their effect on how lightning is initiated and continues to develop. The combination of large areal extent and continuous time coverage of the Utah study allows several to ten or more TGFs to be detected in a given summer monsoon season. Similarly continuous coverage of the lightning activity will be obtained in the east central Florida area around Kennedy Space Center, which has a much higher incidence of lightning and thunderstorms, and is within the latitudinal coverage range of the orbiting TGF-detecting satellites.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.