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The generation of x- and gamma-rays in atmospheric dischargeshas been studied intensively since the discovery of terrestrialgamma-ray flashes (TGFs) by the Compton gamma-ray Observatoryin 1991. Emissions are bremsstrahlung from high energyparticles accelerated in large scale atmospheric electricfields associated with thunderstorms. Whereas observations noware many, both from lightning and the laboratory, the phases ofthe discharge where emissions are generated are still debatedand several processes for electron acceleration have been putforward by theorists. This paper address the electronacceleration in streamer region of lightning. We present thefirst 'beam-bulk' model of self-consistent streamer dynamicsand electron acceleration. The model combines a Monte CarloCollision code that simulates the high-energy electrons (> 100eV) and a fluid code that simulates the bulk of the low-energyelectrons and ions. For a negative streamer discharge, we showhow electrons are accelerated in the large electric field inthe tip of the streamer and travel ahead of the streamer wherethey ionize the gas. In comparison to the results obtained witha classical fluid model for a negative streamer, the beam-bulkmodel predicts a decrease of the magnitude of the peak electricfield and an increase of the streamer velocity. Furthermore, weshow that a significant number of runaway electrons is lost bydiffusion outside of the streamer tip. The results presentedhere do not yet include extra amplification nor accelerationfar away from the streamer to explain the electron energiesseen in TGFs. Still, in the light of those results, weemphasize that the production of runaway electrons fromstreamers needs to be simulated including the self-consistentfeedback of runaways on the streamer. Simulations with abeam-bulk model may not only help to understand the fundamentalatmospheric processes behind TGFs, but also pave the way forthe interpretation of remote sensing of the most energeticdischarges in the Earth's atmosphere and thus help to addresstheir environmental impact.

Runaway electrons from a ‘beam-bulk’ model of streamer: application to TGFs