New Observations of Intermediate-Duration Downward Terrestrial Gamma-ray Flashes

We present the most recent observations of the lightning research program at the Telescope Array cosmic ray observatory in Utah, U.S.A. This program focuses on understanding the initial breakdown stage of lightning through observations of downward Terrestrial Gamma-ray Flashes (TGFs).

The Telescope Array Surface Detector (TASD) is a 700 square kilometer array of plastic scintillator detectors located in Utah's western desert. It consists of 507 three-square-meter detectors on a 1.2 km grid. The TASD was designed to detect particle showers generated by the interaction of ultra-high energy cosmic rays with the Earth's atmosphere.

Recent observations have demonstrated that Terrestrial Gamma-ray Flashes (TGFs) detected by satellites are associated with high-current energetic in-cloud pulses during ascending negative leaders. Meanwhile, a relatively small number of observations have been made of TGFs on the ground, owing to the greater atmospheric attenuation of gamma rays and the relatively rare occurrence of TGFs sufficiently close to detectors.

Previously (Abbasi et al 2018, Belz et al 2020) we reported joint observations by TASD, Lightning Mapping Array (LMA), sferic sensor and broadband interferometer of particle showers coincident with lightning. These consisted of energetic showers of approximately 5 microsecond duration with footprints on the ground of order 10 square kilometers, originating in the first one to two milliseconds of downward lightning leaders and coincident with high-current processes within the leaders. Scintillator waveform and simulation studies confirmed that these showers must consist primarily of gamma radiation, thus the observations were identified TGFs near their initiation threshold. The TASD-observed downward TGFs were in general of shorter duration and lower fluence than their satellite-detected upward counterparts.

Here, we report the new observation and study via the Monte Carlo technique of several events of significantly longer duration and higher fluence than previously observed. These events bridge the gap between the prior TASD and satellite based detections, and thus further demonstrate the similarity between the upward and downward varieties of TGF.