Abstract | Terrestrial gamma-ray flashes (TGFs) are bursts ($\sim$100 \mu s) of high-energy photons originating from the Earth's atmosphere in association with thunderstorm activity [e.g., Briggs et al., JGR, 118, 3805, 2013]. The TGF photon spectrum (typically extending from $\sim$10 keV to $>$40 MeV) is characteristic of bremsstrahlung from high-energy electrons propagating in the atmosphere as they collide with atomic nuclei of air molecules. Although TGFs are believed to be produced inside thunderclouds (below 15 km altitude), the underlying physical mechanisms are still debated. Large-scale relativistic runaway electron avalanches (RREAs) along with relativistic feedback caused by positrons and photons have been proposed to occur in thunderclouds and to produce TGFs [e.g., Dwyer et al., Space Sci. Rev., 173, 133, 2012]. It has also been found that the production of thermal runaway electrons by stepping lightning leaders and their further acceleration could explain the TGF spectra and fluences for intracloud (IC) lightning electric potentials above $\sim$100 MV [Xu et al., GRL, 39, L08801, 2012; Celestin et al., JGR, 120, 2015]. In both scenarios, runaway electron avalanches take place and the related bremsstrahlung produces the TGF.
In addition, secondary processes accompanying the transport of TGF gamma rays through the atmosphere lead to the production of high-energy electrons and positrons capable of escaping into space [Dwyer et al., GRL, 35, L02815, 2008]. These events are often referred to as Terrestrial Electron Beams (TEBs) and represent an unforeseen and not yet quantified source of high-energy electrons and positrons ($>$1 MeV) in the Earth's radiation belts [Dwyer et al., 2008; Briggs et al., GRL, 38, L02808, 2011]. In this work, we will review the most recent advances in the field of TGFs and TEBs.
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