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Abstract Sharp point electrodes generate significant electric field enhancements where electron impact ionization leads to the formation of electron avalanches that are seeded by photoionization. Photoionization of molecular oxygen due to extreme ultraviolet emissions from molecular nitrogen is a fundamental process in the inception of a positive corona in air. In a positive corona system, the avalanche of electrons in the bulk of the discharge volume is initiated by a specific distribution of photoionization far away from the region of maximum electron density near the electrode where these photons are emitted. Here, we present a new approach to finding the inception conditions for a positive corona, which is based on a differential formulation of the photoionization problem. The proposed iterative solution considers the same inception problem that has been solved in the existing literature by using either an integral approach to photoionization or a differential formulation of photoionization and considering the inception problem as a boundary-value eigenvalue problem. The results are validated by comparisons with previous integral formulations and time dynamic plasma fluid solutions in planar and spherical geometries. The results illustrate ideas advanced in Kaptzov (1950Elektricheskiye Yavleniya v Gazakh i Vacuumep 610) providing a physically transparent connection between an effective secondary electron emission coefficient due to volume photoionization in a positive corona system and the secondary electron emission in conventional Townsend discharge theory. The results also demonstrate the significance of boundary conditions for accurate corona solutions that are based on a differential formulation of photoionization. 

Abstract Terrestrial gamma‐ray flashes are linked to growth of long bidirectional lightning leader system consisting of positive and stepping negative leaders. The spatial extent of streamer zones of a typical lightning leader with tip potential exceeding several tens of megavolts is on the order of 10–100 m. The photoelectric absorption of bremsstrahlung radiation generated by avalanching relativistic runaway electrons occurs efficiently on the same spatial scales. The intense multiplication of these electrons is triggered when the size of the negative leader streamer zone crosses a threshold of approximately 100 m (for sea‐level air pressure conditions) allowing self‐replication of these avalanches due to the upstream relativistic electron seeds generated by the photoelectric absorption. The model results also highlight importance of electrode effects in interpretation of X‐ray emissions from centimeter to meter long laboratory discharges, in particular, a similar feedback effect produced by generation of runaway electrons from the cathode material. 

Abstract First‐principles plasma fluid modeling is used for investigation of electrical gas discharges ignited by a configuration of two approaching conducting hydrometeors with typical radii on the order of several millimeters under thunderstorm conditions (i.e., at an elevated location in the Earth's atmosphere corresponding to half of air density at ground level and at applied electric field approximately half of that required for avalanche multiplication of electrons in air). It is demonstrated that ultraviolet photons produced by the electrical discharges developing due to the electric field enhancement in the gap between two hydrometeors and resultant photoionization in the discharge volume lead to much less stringent conditions for conversion of these discharges to a filamentary streamer form than in the case not accounting for the effects of photoionization. It is also demonstrated that this photoionization feedback is critical for understanding and correct description of the subsequent streamer discharges developing on the outer periphery of two hydrometeors whose potential is equalized due to the electrical connection established by the initial streamer discharge between them. The initial streamer ignition between the hydrometeors can be preceded by the corona development, which can have detrimental effect on the ignition. However, it is demonstrated that for hydrometeors approaching with a speed of10 m/s the effect of this onset corona is small. 

Abstract Terrestrial gamma‐ray flashes (TGFs) are high‐energy photon bursts originating from the Earth's atmosphere. In this study, using first‐principles Monte Carlo simulations, we quantify the effects of Compton scattering on the temporal and spectral properties of TGFs induced by a tilted source geometry. Modeling results indicate that the source orientation is a critical parameter in TGF analysis but has been significantly underestimated in previous studies. Offset distance between the lightning source and satellite location cannot be used as a single parameter characterizing Compton scattering effects. In the tilted geometry, Compton scattering effects are more pronounced in the falling part of TGF pulses and can lead to an increase of the falling part of TGF pulses by several tens of microseconds. Moreover, by performing curve‐fitting analysis on simulated TGF light curves, we explain how the symmetric and asymmetric pulses measured by the Gamma‐Ray Burst Monitor on Fermi satellite are consistent with the Compton scattering effects. Fermi‐measured TGF pulses can be fully explained using Gaussian‐distributed TGF sources with an average duration of ∼206 μs. 

In order to initiate streamers and leaders under thunderstorm conditions the electric field should reach values higher than the critical breakdown field Ek (i.e., similar to 30 kV/cm/atm. However, the maximum electric field in thunderstorms measured by balloons is similar to 6-9 kV/cm/atm. In present work, to achieve the electric field amplification required for streamer initiation, a system of two approaching spherical hydrometeors is investigated. Streamer initiation is determined from a Meek number, describing electron multiplication in fields above Ek. We have found the relationships between radii of particles for successful streamer initiation in the gap between these two particles and also on the outside periphery of the two-particle system when the particles are connected by a discharge channel. Furthermore, we estimated the frequency of streamer initiation using three realistic hydrometeor size model distributions available in the literature and found that the scenario of streamer initiation on the outside periphery is only possible for relatively high electric fields >= 0.5Ek at altitudes of 3 and 6 km.