Numerical Parameter‐Space Studies of Various Types of Thundercloud Gamma‐Ray Emissions

Until recently, mainly two types of hard radiation were observed in thunderstorms: intensive, short‐lived terrestrial gamma‐ray flashes (TGFs) and weak, long‐lasting gamma‐ray glows (GRGs). The Airborne Lightning Observatory for Fly's Eye GLM Simulator and TGFs (ALOFT, GLM: Geostationary Lightning Mapper) flight campaign revealed additional gamma‐ray phenomena of intermediate characteristics, including flickering gamma‐ray flashes (FGFs) consisting of multiple pulses with moderate intensity and duration. Here, we propose a model for reproducing hard radiation signals from thunderclouds, applying relativistic feedback discharge (RFD) theory and using a set of ordinary differential equations resembling a Lotka‐Volterra predator‐prey system. The model reproduces signals similar to the above‐mentioned emission types, and a deeper parameter‐space exploration reveals the following. Strong GRGs mainly result from an electric field building up slowly while remaining below the self‐sustained feedback threshold, whereas TGFs are caused by a rapid jump in the electric field well beyond that threshold. FGFs serve as a middle ground resulting from the field increasing moderately until self‐sustained feedback is barely attained. Compared with TGFs and GRGs, ALOFT's FGFs require a relatively large charge‐layer separation distance to be produced, in agreement with their current status of being exclusively observed in large tropical thunderclouds. The simulations also indicate that the minimum charge layer distance needed to reproduce those FGFs can be reduced by allowing the electric field to increase slowly toward the minimum feedback threshold before rising at moderate pace. Our results indicate that RFD could play a significant role in several hard radiation phenomena produced in thunderstorms.