Optical Emissions Associated with Terrestrial Gamma-ray and Lightning Flashes at the Telescope Array Detector.

Terrestrial Gamma-ray Flashes (TGFs) are intense bursts of gamma rays originating in Earth's atmosphere, often associated with lightning activity during thunderstorms. These flashes are believed to result from relativistic runaway electron avalanches triggered by strong electric fields. In this study, we analyze multiple TGFs observed at the Telescope Array Surface Detectors site using a suite of instruments, including a fast antenna, a Lightning Mapping Array, a high-speed camera, and a spectroscopic system. Our focus is on the time-resolved leader spectra of the optical component linked to downward TGFs, covering the 400–900 nm range during the 2022 and 2023 lightning seasons. The spectra reveal that emissions from singly ionized nitrogen and oxygen appear before and after TGF detection, while neutral emissions coincide with the TGF detection. Observations from the spectroscopic system provide valuable insights into the mechanisms driving lightning and TGF initiation and propagation. These findings highlight key optical signatures associated with TGFs, offering new perspectives on their behavior during thunderstorm activity.