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Abstract The strong very high frequency (VHF) radiation from compact intra‐cloud discharges (CIDs) is attributed to streamers. An analytical model, taking altitude and applied electric field as input, is developed for effective representation of current for a double‐headed exponentially growing streamer. The decay of streamer current is attributed to two‐ and three‐body attachment of electrons to molecular oxygen. The model predicts streamers of growing strength and spatial scales at altitudes where electron losses due to three‐body attachment are suppressed with reducing air pressure. We show that CIDs at higher altitudes develop during a longer period such that the spectral content of recorded sferics shifts toward lower frequencies. The model is used to interpret the recorded sferics of two CIDs originating from km altitude in terms of radio signals emanating from an ensemble of streamers. The driving thundercloud electric fields are found to be , where is conventional breakdown threshold field. 

Abstract The time dynamics of positive corona ignition from an initial seed electron to the appearance of space charge effects is studied at atmospheric pressure in a spherically symmetrical geometry. The time-dependent model incorporates a recent photoionization model and boundary conditions to properly treat the corona discharge in the proximity of the anode surface. The applied voltage satisfying the self-sustainability condition of the discharge is first calculated. Then, the time dynamics of the discharge is studied for higher and lower applied voltages. The exponential growth in electron density is identified before space charge effects significantly shield the applied electric field. The related characteristic time of exponential growth in electron density is calculated as a function of applied voltage. The time scale can be tens of nanoseconds when applied voltage is close to the threshold or less than one nanosecond when increasing voltage 10% above the threshold for atmospheric pressure. Analogous to the equality of ionization timescale and dielectric relaxation time in stable streamers, a characteristic timescale is used to predict the behavior of the discharge. When the characteristic time is about one-tenth of the dielectric relaxation time, a one-tenth reduction in the applied field on the anode is observed. As the dielectric relaxation time drops below ten times the characteristic time, the discharge behavior diverges for different applied voltages, and the characteristic time is not a good predictor of further discharge dynamics. 

Abstract We revisit the problem of photoionization of small admixtures of nitrogen and oxygen molecules in atmospheric pressure helium plasma originally formulated in the pioneering work of Naidis (2010J. Phys. D: Appl. Phys.43402001). The radiation trapping of resonance emission lines in atomic helium is quantified, and it is demonstrated that photoionization occurs due to radiative decay of the electronicAstate of helium molecules. The collisions and atomic precursors that populate the excitedAstate of the helium molecule are clearly identified. The Einstein probabilities for the transition from bound and quasi-bound rovibrational levels of theAstate to the continuum of the groundXstate are provided. A kinetic scheme for the production of the fast component of ultraviolet emissions in atmospheric pressure helium plasma is proposed. The photoionization of molecular oxygen and molecular nitrogen as impurities in 99.9% and 99.99% purity helium is studied. 

Abstract We present a theory based on the conventional two-term (i.e. Lorentzian) approximation to the exact solution of the Boltzmann equation in non-magnetized weakly ionized plasma to efficiently obtain the electron rate and transport coefficients in a magnetized plasma for an arbitrary magnitude and direction of applied electric field $\vec{E}$and magnetic field $\vec{B}$. The proposed transcendental method does not require the two-term solution of the Boltzmann equation in magnetized plasma, based on which the transport parameters vary as a function of the reduced electric field $E/N$, reduced electron cyclotron frequency ${\omega }_{\mathrm{c}\mathrm{e}}/N$, and angle $\mathrm{\angle }\vec{E},\vec{B}$between $\vec{E}$and $\vec{B}$vectors, whereNis the density of neutrals. Comparisons between the coefficients derived from BOLSIG+’s solution (obtained via the two-term expansion when $\vec{B}\ne 0$) and coefficients of the presented method are illustrated for air, a mixture of molecular hydrogen (H₂) and helium (He) representing the giant gas planets of the Solar System, and pure carbon dioxide (CO₂). The new approach may be used in the modeling of magnetized plasma encountered in the context of transient luminous events, e.g. sprite streamers in the atmosphere of Earth and Jupiter, in modeling the propagation of lightning’s electromagnetic pulses in Earth’s ionosphere, and in various laboratory and industrial applications of nonthermal plasmas. 

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 The first observation of possible transient luminous events (TLEs) on Jupiter was reported recently. Whereas initiation of elves on Jupiter has been theoretically studied before, the possibility of sprite streamer inception on Jupiter has not been investigated yet. Here we critically review the literature concerned with TLEs on Jupiter. Subsequently, we report on development of a numerical model for modeling of magnetized streamers in presence of Jupiter's strong magnetic field. The model utilizes the knowledge provided by recent observations of Jupiter's lightning and magnetic field by the Juno spacecraft. It is demonstrated that sprite streamer inception under realistic atmospheric conditions on Jupiter is possible. 

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. 

Abstract Electron detachment from O⁻is important for understanding of lightning‐induced upper atmospheric discharges. Contrary to previous studies, Rayment and Moruzzi (1978) (RM78) argue that the associative detachment reaction of O⁻with N₂proceeds with N₂in its ground state. Here, we analyze the experimental setup in RM78 and demonstrate that vibrationally excited N₂may have in fact contaminated the results, the theoretical approach in RM78 requires corrections, and the rate calculations provided in RM78 are inconsistent. As the vibrational temperature of N₂remains relatively low in the initial stages of gas discharges in air, i.e., streamer formation, we conclude that if in fact vibrationally excited N₂is required for the O⁻ + N₂→ N₂O + e reaction to proceed, the process will happen only in later stages of the discharge, e.g., during streamer to leader transition. Controlled experiments are required to reconcile the literature on the reaction of O⁻with ground state N₂.