Investigating the Correlation Between Lightning and High-Energy Particles in the Earth's Atmosphere

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This project supports for the operation and analysis of data from a Lightning Mapping Array (LMA), collocated with the Telescope Array (TA), the largest astroparticle physicsobservatory in the Northern Hemisphere. The merger of these arrays allows (1) study of the known phenomenon of gamma ray emission by lightning with a wide aperture detector capable of reconstructing gamma showers generated by lightning strikes, and (2) a search for the long-speculated "seeding" of lightning strikes by cosmic ray air showers. Intellectual Merit:It has been known for over a decade that x-ray and gamma radiation (~100 keV to 10 MeV) is emitted by lightning strikes. This phenomenon has been observed by ground-based detectors in association with both natural and induced lightning, and by space-based detectors. Recently, physicists studying data collected by the 700 square kilometer TA Surface Detector (SD) in Western Utah, U.S.A. have observed energetic elementary particles in coincidence with lightning strikes. In a subset of these events, "showers" have been reconstructed which point back to the particles' origin in the Earth's atmosphere. This has not been seen before, and it implies that the energetic radiation may be traced to particular times and features within the lightning strike.The LMA is the ideal instrument to couple with the TA SD in order to perform these studies. A version of this instrument used for site testing purposes has been temporarily located at the TA site. The LMA consists of nine VHF detectors, spread over hundreds of square kilometers, which detect impulsive radiation from lightning. These impulses may be reconstructed with 10 microsecond timing accuracy and better than 100 meter position accuracy, and used to create a 3-dimensional GPS-timed reconstruction of a lightning strike.The merger of TA/LMA is also the ideal instrument to search for evidence of a more speculative - but more profound - connection between particle astrophysics and climate: The seeding of lightning strikes by cosmic ray air showers. Since the electric fields present in thunderstorms are typically an order-of-magnitude less than the 30 kV/cm needed for dielectric breakdown of air, it is evident that something else must be assisting the process. The TA/LMA synergy will have the unique ability to search for cosmic ray coincidences with lightning, adding a tantalizing piece of speculative research to its capabilities.Broader Impacts:The synergy will create a rare opportunity for particle astrophysics research to have such a direct impact on the national priority of meteorological research. Electrical storms in the atmosphere have had a profound impact on the development of life on Earth, and continue to shape individual human lives and society. Yet mysteries remain as to the causes and nature of lightning. A large cosmic ray observatory is the ideal pairing with a lightning detector to probe two of these mysteries: (1) From where within a lightning strike do high-energy particles arise? (2) Can cosmic ray air showers provide the "seed" of atmospheric dielectric breakdown? For the relatively modest cost of maintaining a lightning mapping array, the TA/LMA observatory will become an important showpiece illustrating the benefits of NSF Particle Astrophysics to society as a whole.