First Time-Resolved Leader Spectra associated with a Downward
Terrestrial Gamma-Ray Flash detected at the Telescope Array Surface
Detector
Abstract
Optical emissions associated with Terrestrial Gamma-ray Flashes (TGFs)
have recently become important subjects in space-based and ground-based
observations since these emissions play crucial roles in understanding
the generation of TGFs 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 at 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 $\mu$s time resolution), covering a spectral
range from 400 nm to 900 nm, with 2.1 nm per pixel. The recordings of
the leader spectra began 11.7 ms before the $-18$ 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 $\mu$s before and 267 $\mu$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.