Plain Language Summary

We provide explanation of spectacular naturally occurring bursts of X-rays and accompanying radio emissions that are observed in association with lightning activity in the Earth's atmosphere. These burst events are commonly referred to as terrestrial gamma ray flashes (TGFs). The core physical process leading to these events is avalanche multiplication of relativistic electrons accelerated by force exerted by electric field. In geophysical context, the electric field of sufficient magnitude and spatial extent for supporting these avalanches is generated either as an ambient field due to thundercloud electrification or produced in streamer zones of lightning leaders. These electrons radiate energetic photons (X-rays) as they scatter by nuclei of nitrogen and oxygen atoms in air. These X-rays are radiated in all directions and some fraction is radiated in opposite direction of electron motion. These particular X-rays lead to seeding of new relativistic seed electrons due to the photoelectric effect and thus strong amplification of the original avalanche. It has been established recently that these feedback effects occur on very small spatial scales, and in this work, we report the first fully time-dependent simulations of related dynamics, similarity analysis allowing better understanding of characteristics of events observed at different altitudes, and quantitative comparisons with observations.