Downward Gamma Ray Showers from Natural Lightning

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Lightning is known to produce Terrestrial Gamma Flashes (TGFs) which are bursts of high-energy gamma rays that last up to a few milliseconds. How the gamma rays are produced and whether they can tell us more about lightning has been an active research area for a couple of decades. Recently, a research team from the particle astrophysics community teamed with lightning researchers to obtain new observations of gamma rays using the Telescope Array Surface Detector (TASD) in Utah, which is a 700 square kilometer cosmic ray observatory. This award will continue these investigations into the generation of TGFs. The main societal impact from this award will be through the increased understanding of TGFs, which may provide additional information on lightning, a well-known natural hazard. The project will also help to provide public outreach opportunities to enhance the scientific literacy of the population and provide training opportunities for the next generation of scientists.The research team will follow up on their prior award which demonstrated that downward gamma ray showers are produced at the beginning of negative cloud-to-ground lightning. This observation was made in southwestern Utah using a combination of the TASD and Lightning Mapping Array (LMA) stations. Now that this phenomenon has been discovered, the research team plans to address specific questions about the mechanisms responsible for gamma ray production and characterization of the types of lightning events in which TGFs occur. New observations will be conducted with the addition of a high-speed broadband VHF interferometer and fast electric field change sensor. Concurrently, the TASD is expanding in size, leading to more events that can potentially be observed. The specific objectives of the proposal include: 1) Simultaneous observation of downward gamma flash events with TASD, LMA, Slow- and Fast Antennas and broadband interferometer to significantly increase the sample size of downward TGFs, 2) Determine conclusively via fast-E spheric measurements whether the observed gamma flashes have the expected correlation with Initial Breakdown Pulses (IBP) and Energetic Intracloud Pulses (EIP), and 3) Continue advances in modeling of gamma showers and the TASD response, and use these models to further characterize shower source size, altitude, and gamma ray angular distribution.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.