Modeling Downward-Directed Terrestrial Gamma-Ray Flashes

International audienceTerrestrial gamma-ray flashes (TGFs) are bursts of high-energy photons originating in the electric fields of thunderclouds. They have been routinely observed by spaceborne instruments since their discovery by the Compton Gamma-ray Observatory in 1994. These TGFs have been shown to be associated with the propagation of upward intra-cloud lightning. Until recently, ground-based observations of TGFs were few and far between. The detections that were made happened in association with rocket triggered or cloud-to-ground lightning. The number of observations of downward-directed TGFs has now greatly increased, as cosmic ray observatories in Utah and Argentina have reported detections of high-energy particles correlated to lightning activity (Abbasi et al., 2017, 2018; Mussa & Colalillo, 2017). The goal of the present work is to demonstrate how observations of TGFs on the ground pose a valuable addition to spaceborne detectors. The proximity to the event, as well as the possibility of observing the same event with several detectors, may reveal new information about the still elusive production mechanisms of TGFs. We present a study of downward-directed TGFs using a Monte Carlo model to simulate photon transport through the atmosphere. In this way, we quantify how differences in timing, geometry, and spectrum of the initial burst would be reflected in ground-based observations. We also compare how time structures seen at ground level translate to observations made at satellite altitude. We show that simulations based on physical models can improve the interpretation of ground-based observational data, leading to a better understanding of TGFs and their production mechanisms. References: Abbasi, R. U., et al. (2017). The bursts of high energy events observed by the telescope array surface detector. Phys. Lett. A, 381(32), 2565-2572. Abbasi, R. U. et al. (2018). Gamma ray showers observed at ground level in coincidence with downward lightning leaders. J. Geophys. Res.: Atmospheres, 123, in press. Mussa, R., & Colalillo, R. (2017), Observation of high energy radiation in the Surface Detectors of the Pierre Auger Observatory in correspondance with lightning strikes. Abstract [AE31A-06] presented at 2017 Fall Meeting, AGU, New Orleans, LA, 11-15 Dec