Ground Level observation of Gamma-ray Showers in Coincidence With Downward Lightning Leaders

Terrestrial gamma-ray flashes (TGFs) are bursts of gamma-rays initiated in the Earth’s atmosphere. TGFs were serendipitously first observed over twenty years ago by the BATSE gamma ray satellite experiment. Since then, several satellite experiments have shown that TGFs are produced in the upward negative breakdown stage at the beginning of intracloud lightning discharges. In this proceeding, we present the ground-based first observation that TGFs are also produced by the downward negative breakdown occurring at the beginning of negative cloud-to-ground flashes. The TASD detector is a 700 km detector in the southwestern desert of Utah, an area hundreds of times larger than other ground-based detectors of lightning-associated events, making it the largest such detector to date. It is comprised of 507 (3 m) plastic scintillator detectors on a 1.2 km square grid. The LMA stations are colocated within and around the array, and the slow electric field antenna is located close to the center of the TASD detector. The data discussed in this work was collected by this suite of instruments between 2014 and 2016. Gamma ray showers were observed in the first 1-2 ms of downward negative breakdown prior to cloud-to-ground lightning strikes. The shower sources were observed by the LMA detector at an altitude of a few kilometers or less above ground level. The detected energetic burst showers have a footprint on the ground typically ∼ 3–5 km in diameter. The duration of the bursts are of the order of several hundred microseconds. GEANT simulation studies indicate that the showers are consistent with a forward-beamed primary gamma rays of 10 − 10 primary photons. This result may provide a new insight into the understanding of the TGF phenomenon.