Ground-Based Observation of a TGF Occurring Between Opposite Polarity Strokes of a Bipolar Cloud-To-Ground Lightning Flash

We present the ground-based observation of a terrestrial gamma-ray flash (TGF) that occurred between positive (second) and negative (third) strokes of a five-stroke bipolar cloud-to-ground lightning flash. Those two strokes shared the same channel to ground at a distance of 200 m or so from the Lightning Observatory in Gainesville (LOG), Florida. Earlier TGF observations at ground level in Florida (a total of four) were either associated with the initial continuous current (ICC) of rocket-triggered lightning flashes or occurred during the relatively steady current following the return-stroke current peak in natural lightning flashes; that is, in the presence of current-carrying channel to ground. The TGF presented here occurred in a different context: at the early (in-cloud) stage of negative leader entering the remnants of the channel previously created by the positive stroke. The TGF had a duration of 35 mu s and consisted of 18 pulses with amplitudes ranging from 114 to 912 keV. The overall flash context in which the TGF occurred was as follows. The first (143-kA) stroke was negative and terminated on ground 1.4 km from LOG, the second (12-kA) stroke was positive and forged a new path to ground about 200 m from LOG, and the third (28-kA) stroke (TGF producer) was negative and followed the path of the second (positive) stroke. The fourth (43-kA) stroke was negative and created a new termination on ground about 900 m from LOG, and the fifth (4.5-kA) stroke was also negative and followed the fourth-stroke channel. Plain Language Summary Thunderstorms are known to produce bursts of energetic radiation called Terrestrial Gamma-ray Flashes, but they are usually observed from satellites and very rarely from the ground. Here we present one of these rare observations, which was made during a bipolar lightning flash. This is a new context for TGF production that hasn't been reported before. The TGF was recorded at the Lightning Observatory in Gainesville (LOG), Florida, on 2 November 2018 at the early (in-cloud) stage of negative leader entering the remnants of the channel previously created by the positive stroke. The TGF had a duration of 35 mu s and consisted of 18 pulses with amplitudes ranging from 114 to 912 keV.