Modelling microscopic features of streamer encounters, electric fields, electron beams and X-ray bursts

Abstract:

Thunderstorms emit terrestrial gamma-ray flashes (TGFs), beams of photons with quantum energies of
up to 40 MeV. Likewise electric discharges in the laboratory, mimicing lightning on a small spatial and
energetic scale, emit X-rays whose energies are limited by the available potential difference between
the two electrodes. For a maximal available difference of 1 MV and a gap distance of 1 m between the two
electrodes, we will present the energy and spatial distribution of generated X-rays.
For that we have followed the motion of preaccelerated, monoenergetic and monodirectional electron
beams with energies between 100 keV and the maximal available energy of 1 MeV for different electric
field configurations using a particle Monte Carlo code. Omitting any field, we present the subsequent energy and spatial distribution of X-rays
and analyse how the photon number depends on the initial electron energy. Fig. 1 shows the position and energy of photons generated by Bremsstrahlung after 0.3 ns by beams of 500 000 electrons with initial energies of 1 MeV moving in the z
direction in STP air. The electrons have generated electron avalanches and all have cooled
down and attached to oxygen after 0.3 ns. Every cross represents one photon projected onto the xz plane; the photon energies E_γ are color coded. We see that photons with energies of approx. 1 MeV can be produced and that the high-energy tail of X-rays is beamed
towards the direction of the initial electron beam whereas low-energy photons show a more isotropic
behaviour. Analysing the cross sections of photons interacting with air we conclude that photons travel
several meters in air and can reach detectors several meters from the position of the discharge. By
estimating the electric field ahead of the discharge corona and by simulating the motion of electron
beams in these fields, we exclude that electrons travel as far as photons and disturb the measured X-ray
signal.