First Time‐Resolved Leader Spectra Associated With a Downward Terrestrial Gamma‐Ray Flash Detected at the Telescope Array Surface Detector

Optical emissions associated with Terrestrial Gamma ray Flashes (TGFs) have recently become important subjects in space‐based and ground‐based observations as they can help us understand how TGFs are produced during thunderstorms. In this paper, we present the first time‐resolved leader spectra of the optical component associated with a downward TGF. The TGF was observed by the Telescope Array Surface Detector (TASD) simultaneously with other lightning detectors, including a Lightning Mapping Array (LMA), an INTerFerometer (INTF), a Fast Antenna (FA), and a spectroscopic system. The spectroscopic system recorded leader spectra at 29,900 frames per second (33.44 μ ${\upmu }$s time resolution), covering a spectral range from 400 to 900 nm, with 2.1 nm per pixel. The recordings of the leader spectra began 11.7 ms before the −18 ${-}18$ kA return stroke and at a height of 2.37 km above the ground. These spectra reveal that optical emissions of singly ionized nitrogen and oxygen occur between 167 μ ${\upmu }$s before and 267 μ ${\upmu }$s after the TGF detection, while optical emissions of neutrals (H I, 656 nm; N I, 744 nm, and O I, 777 nm) occur right at the moment of the detection. The time‐dependent spectra reveal differences in the optical emissions of lightning leaders with and without downward TGFs. Plain Language Summary Spectroscopy is a powerful tool in lightning research, as it enables the identification of chemical components within the lightning channel. In this study, we present time‐resolved leader spectra of the optical components associated with a downward TGF detection. The chemical makeup of these optical emissions includes nitrogen, oxygen, and hydrogen, along with measurements of their intensities and lifetimes. This marks the first time that spectroscopic optical components of lightning leaders associated with downward TGFs have been recorded. Optical emissions of ions appear before and after the detection of the TGF while optical emissions of neutrals appear at the moment of the TGF detection. Key Points The first time‐resolved leader spectra (400–900 nm) linked with downward Terrestrial Gamma ray Flash (TGF) were measured Ion optical emissions start 167 μs before detecting the downward TGF and end 267 μs after it Neutral optical emissions occur right at the moment of the downward TGF detection