A model is proposed capable of accounting for the local electric field increase in front of the lightning stepped leader up to magnitudes allowing front electrons to overcome the runaway energy threshold and thus to initiate relativistic runaway electron avalanches capable of generating X-ray and γ ray bursts observed in negative lightning leader. The model is based on an idea that an ionization wave, propagating in a preionized channel, is being focused, such that its front remains narrow and the front electric field is being enhanced. It is proposed that when a space leader segment, formed ahead of a negative lightning leader, connects to the leader, the electric potential of the leader is transferred through the space leader in an ionizing wave that continues into the partly ionized channels of preexisting streamers of the space leader. It is shown with numerical simulations that the ionization channels of streamers limit the lateral expansion of the ionization wave, thereby enhancing the peak electric field to values allowing an acceleration of low-energy electrons into the runaway regime where electrons efficiently generate bremsstrahlung. The results suggest that the inhomogeneous ionization environment at the new leader tip amplifies the production rate of energetic electrons relative to a homogeneous environment considered in the past studies.
The threshold field for the electric gas discharge in air is ≈26 kVcm−1atm−1, yet the maximum field measured (from balloons) is ≈3 kVcm−1atm−1. The question of how lightning is stimulated is therefore one of the outstanding problems in atmospheric electricity. According to the popular idea first suggested by Loeb and developed further by Phelps, lightning can be initiated from streamers developed in the enhanced electric field around hydrometeors. In our paper, we prove by numerical simulations that positive streamers are initiated, specifically, around charged water drops. The simulation model includes the kinetics of free electrons, and positive and negative ions, the electron impact ionization and photon ionization of the neutral atmospheric constituents, and the formation of space charge electric fields. Simulations were conducted at air pressure 0.4 atm, typical at thundercloud altitudes, and at different background electric fields, drop sizes, and charges. We show that the avalanche‐to‐streamer transition is possible near drops carrying 63–485 pC in thundercloud fields with intensity of 10 kVcm−1atm−1 and 15 kVcm−1atm−1 for drops sizes of 1 mm and 0.5 mm, respectively. Thus, the electric field required for the streamer formation is larger than the measured thunderstorm fields. Therefore, the results of simulations suggest that second mechanisms must operate to amplify the local field. Such mechanisms could be electric field space variations via collective effects of many hydrometeors or runaway breakdown.
[1] To localize an altitude of the neutron source responsible for the neutron flux enhancements observed on the ground, numerical simulations of photonuclear production and transport to on-ground detector locations were carried out. The neutron fluence calculated for the volumetric source located at the altitudes 8-12 km is consistent with that estimated from neutron numbers measured on the ground. This altitude range is consistent with the idea that the burst of hard g rays detected recently by Tsuchiya et al. (2007Tsuchiya et al. ( , 2009 originate from a volumetric intracloud g ray source. Most likely, the photonuclear reactions caused by bremsstruhlung of descending relativistic runaway electron avalanches account for the neutron flux increases observed in the thunderstorm atmosphere.Citation: Babich, L. P., E. I. Bochkov, I. M. Kutsyk, and R. A. Roussel-Dupré (2010), Localization of the source of terrestrial neutron bursts detected in thunderstorm atmosphere,
The hypothetical mechanism of electric field amplification at contact of positive and negative streamers in a streamer corona up to magnitudes required for the generation of runaway electrons and secondary Bremsstrahlung in the x-ray range, observed in long spark discharges in the open atmosphere, is analyzed. The development of two streamers, moving towards each other in interelectrode gaps of the centimetre range, is numerically simulated at applied voltages from 73 to 250 kV. It is shown that the size of the domain with strong electric field, with intensity sufficient for the thermal electron runaway, is of 1-2 mm. The mean field intensity in this domain increases up to magnitudes of ≈250-280 kV cm −1 . The maximum energy, to which electrons are capable of energizing in such field, is in the range of 20-70 keV. However, the electron energy is limited by an extremely small life-time of the strong field domain (less than 20 ps).
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