Discerning TGF and Leader Current Pulse in ASIM Observation

TGFs being the bursts of high energy photons shot from Earth’s atmosphere to space, are known to be produced during the initial upward propagation of the +IC lightning leader. VLF and LF radio sferics can often be found in association with the short duration TGFs. The Atmosphere-Space Interactions Monitor (ASIM) instrument provides synchronous X- and gamma-ray measurements with optical recordings in 180-240 nm, 337 nm and 777.4 nm wavelength. This allows for simultaneous detection for TGFs and the lightning processes associated with them.

ASIM TGF observations have shown that TGFs within the FOV of the optical instruments are always accompanied by the prominent optical pulse which starts the lightning flash. TGFs have a clear tendency to slightly precede the optical pulse, but the short duration of TGFs together with the optical delay of the lightning light propagating through the cloud do not allow to confidently resolve the true sequence of these events.

The same problem is present in radio measurements: radio signature from TGF current is usually mixed with lightning current in the recordings due to temporal proximity of the processes involved.

Here we report a remarkable, high fluence and long duration TGF, together with its associated optical recordings. This observation shows clear distinction between the TGF and the associated optical pulse: the optical pulse is subsequent to the TGF, as it starts after the TGF is terminated. This allows to conclude that strong current surges inside the leader channel are not responsible for the TGF generation, and, in turn, the current surge producing the optical pulse can be conditioned by the generated TGF, or even be responsible for TGF termination.