Discerning TGF and Leader Current Pulse in ASIM Observation.

Terrestrial gamma ray flash (TGF) observations made by the Atmosphere‐Space Interaction Monitor (ASIM) have demonstrated that these TGFs are accompanied by a prominent optical pulse from a hot leader channel. It is hard to confidently resolve the true sequence of the events in the source region due to temporal proximity of the involved processes. Here we report a bright long duration TGF together with its associated optical recordings showing clear temporal separation between the TGF and the optical pulse. In this observation the optical pulse is clearly distinct and subsequent relative to the TGF. The corresponding lightning discharge occurred at the very end of the TGF. We conclude that the current surge inside the lightning leader channel cannot be responsible for generation of this TGF. The current surge that produced the associated optical pulse can itself be conditioned by the TGF and may be responsible for the TGF termination. Plain Language Summary: TGFs observed from space are found to be associated with current surges in lightning leader channels. These current surges emit radio waves and can be detected with lightning detection networks. They also produce optical pulses which can be observed by the optical sensors on board of the space satellites. The fact that TGFs have usually short duration does not allow to define the real sequence of events in the source region due to timing uncertainties. In this paper we report a unique observation of a rare coincidence of a long duration TGF accompanied by an optical pulse and a high peak current lightning detection. Duration of the TGF is one order of magnitude larger than the overall observational uncertainty, which allows us to reliably discern the TGF and the accompanying current pulse in the leader channel. We could confidently conclude that the TGF was generated first, in the very end of the TGF the current surge in the leader channel occurred, and the optical pulse was produced. The appearance of the current surge close to the end of the TGF can indicate that the current surge is conditioned by the TGF, and, reciprocally, it could condition the TGF termination. Key Points: Accompanying optical pulse is subsequent to TGFTGF precedes the current surge in the leader channel and cannot be generated by this current surgeTGF may be terminated by the current surge in the leader channel [ABSTRACT FROM AUTHOR]