The Microphysics of Lightning: Observing the Optical Emission from Downward Terrestrial Gamma-ray Flashes

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This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).Terrestrial Gamma-ray Flashes (TGFs) are bursts of high energy photons of sub millisecond duration that are produced by lightning. The study of TGFs, both their initiation and propagation, is of major interest to lightning researchers. The Telescope Array Surface Detector (TASD), a 700 km2 cosmic ray detector located in the western desert of Utah, U.S.A. is currently the world’s leading detector in the study of downward-directed TGFs. This project will allow further investigation of the initiation phase of downward TGFs by measuring their optical component using a high-speed video camera and a photometer at the Telescope Array site. The results of this project will improve our understanding of lightning, which in turn will improve the ability to mitigate its negative effects and associated radiation hazards. The outreach component of this work will allow scientists to describe the microphysics of lightning to the public at a powerful and impactful level through various communication venues. While it is known that TGFs are produced inside thunderstorms and in correlation with lightning, both the mechanism responsible for producing TGFs and the relation of intra-cloud discharges to TGFs are still unknown. There are several leading possible mechanisms that are believed to produce TGFs. Multiple studies conclude that each mechanism’s optical signature is distinctly different. The intensities at which the optical emission at different wavelengths is detected would provide key information about the lightning development sequence. This award will allow the observation of the relationship between downward-directed TGFs and their optical emissions using a high-speed video camera and a photometer in conjunction with Lightning Mapping Array (LMA) stations, electric field-change detectors, and a high-speed broadband VHF interferometer. Results from these observations will be directly compared to the optical emissions observed simultaneously with the ASIM satellite’s upward-directed TGFs. Most importantly, it will provide answers to critical questions about TGFs and even lightning initiation processes, and will further the understanding regarding one of the top ten questions in lightning research: What are the mechanisms responsible for producing and propagating gamma-ray events in our atmosphere?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.