395 research outputs found
The emission of energetic electrons from the complex streamer corona adjacent to leader stepping
We here propose a model to capture the complexity of the streamer corona
adjacent to leader stepping and relate it to the production of energetic
electrons serving as a source of X-rays and -rays, manifesting in
terrestrial gamma-ray flashes (TGFs). During its stepping, the leader tip is
accompanied by a corona consisting of multitudinous streamers perturbing the
air in its vicinity and leaving residual charge behind. We explore the relative
importance of air perturbations and preionization on the production of
energetic run-away electrons by 2.5D cylindrical Monte Carlo particle
simulations of streamers in ambient fields of 16 kV cm and 50 kV
cm at ground pressure. We explore preionization levels between
m and m, channel widths between 0.5 and 1.5 times the
original streamer widths and air perturbation levels between 0\% and 50\% of
ambient air. We observe that streamers in preionized and perturbed air
accelerate more efficiently than in non-ionized and uniform air with air
perturbation dominating the streamer acceleration. We find that in unperturbed
air preionization levels of m are sufficient to explain
run-away electron rates measured in conjunction with terrestrial gamma-ray
flashes. In perturbed air, the production rate of runaway electrons varies from
s to s with maximum electron energies from
some hundreds of eV up to some hundreds of keV in fields above and below the
breakdown strength. In the presented simulations the number of runaway
electrons matches with the number of energetic electrons measured in alignment
with the observations of terrestrial gamma-ray flashes. Conclusively, the
complexity of the streamer zone ahead of leader tips allows explaining the
emission of energetic electrons and photons from streamer discharges.Comment: 29 pages, 11 figures, 2 table
Influence of the angular scattering of electrons on the runaway threshold in air
International audienceThe runaway electron mechanism is of great importance for the understanding of the generation of x- and gamma rays in atmospheric discharges. In 1991, terrestrial gamma-ray flashes (TGFs) were discovered by the Compton Gamma-Ray Observatory. Those emissions are bremsstrahlung from high energy electrons that run away in electric fields associated with thunderstorms. In this paper, we discuss the runaway threshold definition with a particular interest in the influence of the angular scattering for electron energy close to the threshold. In order to understand the mechanism of runaway, we compare the outcome of different FokkerPlanck and Monte Carlo models with increasing complexity in the description of the scattering. The results show that the inclusion of the stochastic nature of collisions smooths the probability to run away around the threshold. Furthermore, we observe that a significant number of electrons diffuse out of the runaway regime when we take into account the diffusion in angle due to the scattering. Those results suggest using a runaway threshold energy based on the FokkerPlanck model assuming the angular equilibrium that is 1.6 to 1.8 times higher than the one proposed by [1, 2], depending on the magnitude of the ambient electric field. The threshold also is found to be 5 to 26 times higher than the one assuming forward scattering. We give a fitted formula for the threshold field valid over a large range of electric fields. Furthermore, we have shown that the assumption of forward scattering is not valid below 1 MeV where the runaway threshold usually is defined. These results are important for the thermal runaway and the runaway electron avalanche discharge mechanisms suggested to participate in the TGF generation
Streamer propagation in the atmosphere of Titan and other N2:CH4 mixtures compared to N2:O2 mixtures
Streamers, thin, ionized plasma channels, form the early stages of lightning
discharges. Here we approach the study of extraterrestrial lightning by
studying the formation and propagation of streamer discharges in various
nitrogen-methane and nitrogen-oxygen mixtures with levels of nitrogen from 20%
to 98.4%. We present the friction force and breakdown fields Ek in various
N2:O2 (Earth-like) and N2:CH4 (Titan-like) mixtures. The strength of the
friction force is larger in N2:CH4 mixtures whereas the breakdown field in
mixtures with methane is half as large as in mixtures with oxygen. We use a 2.5
dimensional Monte Carlo particle-in-cell code with cylindrical symmetry to
simulate the development of electron avalanches from an initial electron-ion
patch in ambient electric fields between 1.5Ek and 3Ek. We compare the electron
density, the electric field, the front velocities as well as the occurrence of
avalanche-to-streamer transition between mixtures with methane and with oxygen.
Whereas we observe the formation of streamers in oxygen in all considered
cases, we observe streamer inceptions in methane for small percentages of
nitrogen or for large electric fields only. For large percentages of nitrogen
or for small fields, ionization is not efficient enough to form a streamer
channel within the length of the simulation domain. In oxygen, positive and
negative streamers move faster for small percentages of nitrogen. In mixtures
with methane, electron or streamer fronts move 10-100 times slower than in
mixtures with oxygen; the higher the percentage of methane, the faster the
fronts move.Comment: 34 pages, 11 figures, 1 tabl
Runaway electrons from a ‘beam-bulk’ model of streamer: application to TGFs
International audienc
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