UC Santa Cruz

Terrestrial Gamma-ray Flashes (TGFs) are extremely bright, short bursts of gamma radiation associated with lightning flashes. The story of their discovery spans nearly a century and involves contributions from researchers in many different subdisciplines of physics - lightning physicists, astrophysicists, and meteorologists to name a few. This work summarizes those contributions and the evolving theories on how thunderstorms are able to generate the enormous ”avalanches” of relativistic electrons responsible for the gamma rays.This work also summarizes 3 important contributions from the Santa Cruz High Energy At- mospheric Physics group over the past 5 years: 1. Development of a new ground based detector array with μs absolute timing accuracy, large dynamic range, high resolution data during triggered events, and EMI noise sealing. 2. Quantification of the expected flux from the reverse positron beam of a TGF under the so- called ”feedback model”, and the detectability of such an event from various altitudes on the ground. 3. Observation of two TGFs detected from an unprecedented angle of more than 70◦ off the vertical axis, and Monte Carlo simulations of deviations in source parameters that could lead to the observable beam of gamma-rays reaching farther than existing theory says it should.