Collaborative Research: Strategic Observations of Terrestrial Gamma-Ray Flashes and Related Phenomena

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This award supports investigation of understanding terrestrial gamma-ray flashes (TGFs) generated among a small fraction of lightning flashes generated inside thunderstorms. TGFs are extraordinarily powerful bursts of gamma radiation; gamma-rays are particles of invisible light, similar to x-rays but in this case with energies up to about 100 times higher than the x-rays used by doctors and dentists. Although TGFs are rare, the consequences of a direct hit by one of these flashes to a plane full of people are significant. The crew and passengers could receive a very high, sudden dose of dangerous radiation, possibly enough to cause immediate radiation sickness and a significant risk of cancer later in life. An unsolved and interesting problem in atmospheric physics regarding to TGFs is how, why, and when TGFs are produced. The research team seeks understanding of how the gamma-rays get as bright as they do and why (fortunately) they only happen in maybe one out of a thousand lightning flashes based on their current understanding of the ``building blocks'' of a TGF and how high-energy radiation might naturally happen during lightning. The investigation will potentially enable the team to predict involvement of TGFs in lightning flashes that interact with aircraft and provide valuable information on aviation risks for public safety. In this project, the research team will deploy a number of gamma-ray detectors (previously constructed under funding from NSF, NASA, and the Air Force) to ground-based sites, aircraft, and high-altitude balloons in order to gather new data on TGFs in places they have been seen before, such as the western coast of Japan in winter, summer thunderstorms in Florida, and the eyewalls of hurricanes. Each site and the method of observation have been chosen for measurements of TGFs close to their production locations. One of the key observations will be how bright TGFs can get and how faint they get. Currently temporal variations of TGF radiation intensity predicted by leading models have significant uncertainty. To address this uncertainty with consideration of a possible series of rapid bursts of radiation intensity with time, the fast response detector of plastic scintillators for observing each gamma-ray interaction will be used to determine the correct model physics. Combining measurements of lightning characteristics (mostly via radio emission) with complementary meteorological data, the team would be able to understand not only the detailed behavior of lightning that produces TGFs, but also lightning data of similar high quality for situations where a TGF did not get produced, which is equally important to understand.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.