# Gamma-ray Showers Observed at Ground Level in Coincidence With Downward Lightning Leaders

## TL;DR

This study reports ground-based detection of gamma-ray showers coinciding with downward lightning leaders, revealing detailed temporal and spatial characteristics of terrestrial gamma-ray flashes (TGFs) during lightning events.

## Contribution

It provides the first detailed ground-based observations of TGFs associated with lightning, including structure, timing, and origin altitude, supported by simulations and multi-instrument data.

## Key findings

- Gamma-ray showers occur at 3-5 km altitude during initial lightning breakdown.
- Showers consist mainly of downward-beamed gamma radiation.
- Ground-based detection is more sensitive than satellite observations.

## Abstract

Bursts of gamma ray showers have been observed in coincidence with downward propagating negative leaders in lightning flashes by the Telescope Array Surface Detector (TASD). The TASD is a 700~square kilometer cosmic ray observatory located in southwestern Utah, U.S.A. In data collected between 2014 and 2016, correlated observations showing the structure and temporal development of three shower-producing flashes were obtained with a 3D lightning mapping array, and electric field change measurements were obtained for an additional seven flashes, in both cases co-located with the TASD. National Lightning Detection Network (NLDN) information was also used throughout. The showers arrived in a sequence of 2--5 short-duration ($\le$10~$\mu$s) bursts over time intervals of several hundred microseconds, and originated at an altitude of $\simeq$3--5 kilometers above ground level during the first 1--2 ms of downward negative leader breakdown at the beginning of cloud-to-ground lightning flashes. The shower footprints, associated waveforms and the effect of atmospheric propagation indicate that the showers consist primarily of downward-beamed gamma radiation. This has been supported by GEANT simulation studies, which indicate primary source fluxes of $\simeq$$10^{12}$--$10^{14}$ photons for $16^{\circ}$ half-angle beams. We conclude that the showers are terrestrial gamma-ray flashes (TGFs), similar to those observed by satellites, but that the ground-based observations are more representative of the temporal source activity and are also more sensitive than satellite observations, which detect only the most powerful TGFs.