Modeling of X‐ray emissions produced by stepping lightning leaders

Abstract

Intense and brief bursts of X-ray emissions have been measured during the stepping process of both natural cloud-to-ground (CG) and rocket-triggered lightning flashes. In this paper, we investigate theoretically the energy spectra of X-rays produced by the bremsstrahlung emission of thermal runaway electrons accelerated in the inhomogeneous electric field produced around lightning leader tips. The X-ray energy spectrum depends on the physical properties of the associated lightning leaders. Consequently, X-ray measurements can be used for diagnostics of the electrical properties of lightning stepped leaders. We report simulation results of the photon energy spectra produced by 5 and 10 MV negative CG lightning discharges that would be measured from the ground using ideal detectors. We also quantify theoretically the radial dependence of X-ray energy spectra received at ground level during the leader stepping process. Moreover, it is found that the ground radiation generated in this process is harmless to humans.

Variability in fluence and spectrum of high‐energy photon bursts produced by lightning leaders

“…The proportionality coefficient in equation is obtained from the assumption that 10 11 photons with energies greater than 10 keV are produced in the case of a potential drop of 5 MV (see section 4). Following this procedure, we find that ξ ≃3.4 × 10 6 , which is consistent with the upper limit of 10 10 found by considering that all the streamers eventually constituting the streamer zone emit runaway electrons when they reached their limit of growth [ Celestin and Pasko , ; Xu et al , ]. The total number of photons corresponding to other potential drops is obtained through the assumption that this coefficient does not depend on the potential drop.…”

“…This model simulates the dynamics of electrons propagating in air with energies from sub‐eV to GeV. The model has been used in recent studies to quantify thermal runaway electron emissions from streamer discharges [ Celestin and Pasko , ], properties of RREAs in homogeneous fields, and the acceleration of electrons in inhomogeneous fields produced by lightning leaders [ Celestin et al , ; Xu et al , , ]. The initial energy of the thermal runaway electrons is defined as 65 keV because those are believed to be produced by expanded streamers developing in the vicinity of the leader tip during the negative corona flash process [ Celestin and Pasko , ].…”

“…X‐ray bursts have been observed to be produced during the formation of new leader steps in negative cloud‐to‐ground lightning [ Dwyer et al , ]. Although the energy of these X‐ray bursts was found to be insufficient for the RREA mechanism to be involved [ Dwyer et al , ], they can be explained by the production and acceleration of thermal runaway electrons in the vicinity of the leader tip during negative corona flash stages of stepping lightning leaders and the subsequent production of bremsstrahlung [ Celestin and Pasko , ; Xu et al , ]. The same phenomenon could be at the origin of the production of TGFs under the assumption that high‐potential intracloud leaders are involved [ Celestin and Pasko , ].…”

Observations of corona in triggered dart‐stepped leaders

“…[], using an X‐ray pin‐hole‐type camera to image triggered lightning leaders near ground, suggested that the X‐ray source region for subsequent leaders that occurred in a preexisting channel, inferred to be below the leader tip, had a radius of 2 to 3 m. Xu et al . [] found that X‐ray bursts produced by 5 MV to 10 MV leaders would theoretically produce X‐ray burst spectra consistent with the experimental results reported by Schaal et al . [].…”

VHF and UHF Electromagnetic Radiation Produced by Streamers in Lightning

“…Similarly, with the same assumption for cases II, III and IV, the required numbers of streamers are ∼ 3.5 × 10 5 , ∼ 5 × 10 4 and ∼ 2.5 × 10 6 , respectively, and thus the UHF radiation energy would be ∼3 ×10 −4 J, ∼1 × 10 −5 J, and ∼2 × 10 −5 J, respectively. from the electrostatic consideration of the streamer zone (e.g., Celestin et al, 2015), 10 5 -10 7 streamers are required to give a potential drop of 2-20 MV near the lightning leader tip, which is commonly accepted for typical lightning leaders and also consistent with analysis of ground-based observation of X-rays produced by lightning (e.g., Xu et al, 2014). Specifically, the blue curve corresponds to the parameters: = 1.7 × 10 9 s −1 , = 7.5 × 10 7 s −1 , I CM0 = 0.44 A⋅m, and T 0 = 6 ns.…”