Global Exploration of the Conditions of Downward Terrestrial Gamma-ray Flash (TGF) Production

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This program seeks to understand when, where, and why terrestrial gamma-ray flashes (TGFs) accompany lightning. TGFs are intense bursts of high-energy radiation that were first discovered by satellites orbiting Earth when thunderstorms were taking place below. They emit enough radiation to pose a health risk to humans aboard an aircraft within a few hundred meters of the region of energy release. Perhaps most mysteriously of all, the vast majority of lightning flashes don't produce a TGF, and we don't yet understand what's special about the ones that do. Most of the TGFs that have been observed have been directed upward toward space, but there are also downward TGFs, which have had far fewer observations and appear to be associated with very different types of lightning. Radiation sensors will be deployed to environments around the world that are all rich in lightning but otherwise diverse -- high and low altitude sites, and sites with strong summer lightning and strong winter lightning. TGF detections and non-detections will be compared with data on radio waves that show what kind of lightning occurred. This will help sort out when TGFs are most likely to be expected, lending insight as well into how they happen.Deployment sites will include Mt. Santis in Switzerland and Mt. Fuji in Japan, where it has very recently been shown that summertime lightning produces TGFs, and the west coast of Japan, where it has been known for several years that winter thunderstorms produce TGFs close to the ground. TGFs in these environments have been produced by positive and negative intracloud lightning, by natural negative cloud-to-ground lightning, and by negative cloud-to-ground lightning initiated by an upward positive leader from a tower. Observations will be made with newly developed detector arrays that have accurate (microsecond) timing and detectors of a range of sizes to dramatically improve dynamic range (so that faint/distant and bright/nearby TGFs can be easily characterized). A new type of detector will be deployed alongside the existing ones; this is a digital dosimeter with sub-second time resolution that will quantify the radiation dose from TGFs so close that all the conventional detectors go into saturation or shut down. Characteristics of the TGFs and associated sferics will be searched for systematic differences between TGFs associated with cloud-to-ground and intra-cloud lightning; TGFs associated with uniquely powerful sferics and those with only weak sferics; and, from the other perspective, characteristics of lightning that produces TGFs versus lightning that doesn't, all with the goal of clarifying the production mechanisms (e.g. relativistic feedback in different geometries versus production of seed electrons by leaders).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.