As thunderclouds charge, the large‐scale fields may approach the relativistic feedback threshold, above which the production of relativistic runaway electron avalanches becomes self‐sustaining through the generation of backward propagating runaway positrons and backscattered X‐rays. Positive intracloud (IC) lightning may force the large‐scale electric fields inside thunderclouds above the relativistic feedback threshold, causing the number of runaway electrons, and the resulting X‐ray and gamma ray emission, to grow exponentially, producing very large fluxes of energetic radiation. As the flux of runaway electrons increases, ionization eventually causes the electric field to discharge, bringing the field below the relativistic feedback threshold again and reducing the flux of runaway electrons. These processes are investigated with a new model that includes the production, propagation, diffusion, and avalanche multiplication of runaway electrons; the production and propagation of X‐rays and gamma rays; and the production, propagation, and annihilation of runaway positrons. In this model, referred to as the relativistic feedback discharge model, the large‐scale electric fields are calculated self‐consistently from the charge motion of the drifting low‐energy electrons and ions, produced from the ionization of air by the runaway electrons, including two‐ and three‐body attachment and recombination. Simulation results show that when relativistic feedback is considered, bright gamma ray flashes are a natural consequence of upward +IC lightning propagating in large‐scale thundercloud fields. Furthermore, these flashes have the same time structures, including both single and multiple pulses, intensities, angular distributions, current moments, and energy spectra as terrestrial gamma ray flashes, and produce large current moments that should be observable in radio waves.
