Time-Resolved Leader Spectra of Downward Terrestrial Gamma-Ray Flashes Observed at the Telescope Array Surface Detector

Abstract

The first time-resolved leader spectra associated with a downward Terrestrial Gamma-ray Flash (TGF) observed by the Telescope Array Surface Detector (TASD) were recently reported by Kieu et al. (2024, https://doi.org/10.1029/2024JD041720). Building on that study, this paper presents the time-resolved leader spectra of two additional downward TGFs detected by the TASDs. The first TGF, triggered once, occurred during the Initial Breakdown Pulses (IBPs) of a negative return stroke with a low peak current ( 18.5 kA). The second TGF, triggered twice, was detected during the initial leader stage of a high-peak-current ( 133.4 kA) return stroke. The electric field change (E-change) waveform caused by the leader preceding this return stroke had a brief duration (1.2 ms) and lacked strong IBPs. These unusual characteristics suggest that this lightning flash may represent a distinct type of event associated with TGF detection not previously observed at the TASD. Most importantly, the optical emissions of both TGF events align with previous findings and show ion emission lines before and after TGF detections while only neutral emission lines were recorded at the moment of TGF detections.,We continue our previous studies on optical emissions from lightning leaders associated with downward Terrestrial Gamma-ray Flashes (TGFs) observed at the Telescope Array Surface Detector (TASD). In this study, we present time-resolved leader spectra of the optical components associated with two downward TGFs. The first TGF was classified as a single-trigger event associated with a low peak current return stroke, similar to the TGF reported in our previous work (similar to -18 kA). The second event was classified as a multi-trigger event associated with a high peak current (133.4 kA) return stroke. The second TGF was recorded with an unusual electric field change (E-change) waveform and without strong Initial Breakdown Pulses. This type of E-change waveform, associated with TGF detection, had not been observed at the TASD before. The spectra of the two TGFs are consistent with our previous findings: optical emissions from ions appear before and after TGF detection while neutral emissions occur at the moment of TGF detection.,