Collaborative Research: Observations of High-energy Radiation Associated with Thunderstorms

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The project will conduct a series of field campaigns to observe phenomena producing high-energy radiation in thunderstorms. These include terrestrial gamma-ray flashes (TGFs), long-lived gamma-ray glows observed in many parts of the thunderstorm charge structure, and x-rays associated with the stepping of lightning leaders. The overall strategy is to locate probing sensors in or near the high-field regions where these emissions are primarily generated. The sensors to be used include two gamma-ray instruments developed at the University of California, Santa Cruz, under previous NSF awards, as well as sensors provided by our collaborators that sense lightning remotely via static and dynamic electric and magnetic fields. Intellectual Merit:All the phenomena under study were discovered within the last 30 years or so, with research only becoming widespread in the last decade. Some of the many open questions that will be addressed by this project include: whether downward TGFs occur in natural lightning, whether glows (an avalanche of relativistic electrons) contribute significantly to the discharging of thunderstorms, and whether positive lightning leaders emit x-rays and low-energy gamma rays the way negative leaders do. The primary advantage the field measurements have over others are 1) the proximity of the detection sites to the radiation sources to be studied (thundercloud charge centers and lightning leaders); and 2) the high time resolution and throughput capabilities of the detectors.Broader Impacts:The selected field sites (an urban tower in Canada, a region of very low altitude storms in Japan, and a very high altitude site in Mexico) represent the three situations in which it is at least feasible that downward TGFs might produce high enough radiation fields at the ground to represent a public safety issue (upward TGFs seen from space produce dangerous levels of radioactivity within thunderclouds at 10--15~km altitudes). Thus this program should allay (or justify) any concern about radiation at ground level nearly comprehensively. Significant participation from undergraduate and graduate students, including first-generation and otherwise underserved undergraduates, is part of the program.