Observing the Origin of Downward Terrestrial Gamma-Ray Flashes at the Telescope Array

Abstract:

We report the first close, high-resolution observations of downward-directed TGFs detected by the large-area Telescope Array cosmic ray observatory in central Utah, consisting of 512 3 m² thin scintillator detector units on a grid covering 700 km². A broadband VHF interferometer, lightning mapping array, and fast electric field change sensor overlook the array, allowing for high resolution measurements of lightning activity alongside particle data at ground level. Three -CG flashes of August and one low-altitude IC in October of 2018 were included in this study, all occurring in the first couple milliseconds of their respective flashes during strong IBPs produced by streamer-based fast negative breakdown (FNB). Three of the TGFs were further associated with strong sub-pulses. Gamma-ray showers detected at ground level consisted of multiple 3-8 microsecond bursts; in three cases the overall gamma arrival duration lasted over 100 microseconds. Basic GEANT simulations show that the individual burst durations are not substantially extended due to atmospheric scattering effects. Read out by FADC with 20 ns timesteps, the surface detectors can resolve individual incident particles and are used to place lower limits on their energies. We find unambiguous evidence for photons with energies of at least 6.4 MeV, and possibly several times higher given the well-understood physics of Compton conversion. This unique combination of data constitutes the most robust system for observation of downward TGFs and sheds light on the mechanism of TGF production in general.
The results suggest that the RREA responsible for producing TGFs is initiated by embedded transient conducting events (TCEs) within the fast streamer system or by fast streamers themselves. The enhanced electric field ahead of the advancing FNB then facilitates further avalanche development into the multi-MeV range.