Abstract
The generation of x- and gamma-rays in atmospheric discharges has been studied intensively since the discovery of terrestrial gamma-ray flashes (TGFs) by the Compton gamma-ray Observatory in 1991. Emissions are bremsstrahlung from high energy particles accelerated in large scale atmospheric electric fields associated with thunderstorms. Whereas observations now are many, both from lightning and the laboratory, the phases of the discharge where emissions are generated are still debated and several processes for electron acceleration have been put forward by theorists. This paper address the electron acceleration in streamer region of lightning. We present the first ‘beam-bulk’ model of self-consistent streamer dynamics and electron acceleration. The model combines a Monte Carlo Collision code that simulates the high-energy electrons (100 eV) and a fluid code that simulates the bulk of the low-energy electrons and ions. For a negative streamer discharge, we show how electrons are accelerated in the large electric field in the tip of the streamer and travel ahead of the streamer where they ionize the gas. In comparison to the results obtained with a classical fluid model for a negative streamer, the beam-bulk model predicts a decrease of the magnitude of the peak electric field and an increase of the streamer velocity. Furthermore, we show that a significant number of runaway electrons is lost by diffusion outside of the streamer tip. The results presented here do not yet include extra amplification nor acceleration far away from the streamer to explain the electron energies seen in TGFs. Still, in the light of those results, we emphasize that the production of runaway electrons from streamers needs to be simulated including the self-consistent feedback of runaways on the streamer. Simulations with a beam-bulk model may not only help to understand the fundamental atmospheric processes behind TGFs, but also pave the way for the interpretation of remote sensing of the most energetic discharges in the Earthʼs atmosphere and thus help to address their environmental impact.
Highlights
IntroductionThe theoretical models, put forward since the discovery of terrestrial gamma-ray flashes (TGFs), are built on Wilsons proposal that the electrons accelerated in thunderstorm fields may reach the runaway regime [9]
The lower rate we found can be explained by several factors: the electric field is of lower magnitude inside the streamer tip due to the production of high energy electrons itself; the electric field outside the streamer tip is below Ek in our work; more runaway electrons are expected to be lost by diffusion and our streamer radius is 1 order of magnitude smaller
In this paper we have applied for the first time a ‘beam-bulk’ model to simulate the runaway production from cold electrons accelerated in streamer tips present in lightning leader region
Summary
The theoretical models, put forward since the discovery of TGFs, are built on Wilsons proposal that the electrons accelerated in thunderstorm fields may reach the runaway regime [9]. With particle models, if cold electrons can be accelerated into the runaway regime in the small-scale fields of streamers feeding a leader, and if the number of those runaway electrons is high enough to explain observations [12,13,14,15,16]. The high energy particles (photon, electron positron and neutron) associated with TGF modify the radiation environment at low altitudes. We discuss the production mechanism and the production rate of runaway electrons
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