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

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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.

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