[1] We propose a nighttime source of metastable oxygen atoms O( 1 D) in the mesosphere. Here O( 1 D) atoms are generated by electron impact dissociation of O 2 molecules in sprite halo events, where electrons are accelerated in strong electric fields associated with lightning discharges. The peak production rate of O( 1 D) in a single sprite halo event is estimated to be as large as 10 4 cm À3 for a duration of 1 ms, which is comparable to, or even larger than, the steady rate by solar-UV radiation. We also discuss the altitude dependence of O( 1 D) production in sprite halos and the dependence on day and night.
A randomly stepped leader propagation model is developed to study gigantic jets, a new type of lightning, connecting thunderclouds to the ionosphere. The thundercloud is considered as one electrode igniting gigantic jets and the ionosphere is assumed as the other. The propagation of stepped leader is considered as a field controlled random growth process. The electric field is produced due to the thundercloud charges and the selfconsistently propagating leader. A leader propagation probability is proposed to determine whether the leader grows at the next step and what the step direction of the leader is in case of growth. The results show that leader propagation spans ∼72 km from igniting position to the ionosphere. The simulation of leader propagation appears to be in agreement with the structure of observed gigantic jets.
Discharge structures and properties of dual-frequency capacitively-coupled plasma (CCP) discharges in SF6 are studied by using a one-dimensional Particle-in-Cell/Monte Carlo (PIC/MC) method. The simulations are carried out at a gas pressure of 25 mTorr for a fixed low-frequency (LF) source of 3.2 MHz and a high-frequency (HF) source of 27-50 MHz. Results show that in dual-frequency discharges both sources nonlinearly interact even if the source frequencies are significantly different from each other (e.g., 3.2 and 50 MHz). In the presence of the LF source, the electron density in the bulk plasma reduces in comparison with the density for the case without LF source. The non-equilibrium behavior between the electron-SF6 collisional ionization or attachment and density diffusion is found for a HF source frequency higher than 49 MHz. The electron temperature in the bulk plasma is higher than 5 eV, which sustains the discharge by compensating the loss of electrons due to strong attachment.
Abstract:The effect of gas flow in low pressure inductively coupled Ar/N 2 plasmas operating at the rf frequency of 13.56 MHz and the total gas pressure of 20 mTorr is studied at the gas flows of 5-700 sccm by coupling the plasma simulation with the calculation of flow dynamics. The gas temperature is 300 K and input power is 300 W. The Ar fractions are varied from 0% to 95%. The species taken into account include electrons, Ar atoms and their excited levels, N 2 molecules and their seven different excited levels, N atoms, and Ar + , N + , N 2 + , N 4 + ions. 51 chemical reactions are considered. It is found that the electron densities increase and electron temperatures decrease with a rise in gas flow rate for the different Ar fractions. The densities of all the plasma species for the different Ar fractions and gas flow rates are obtained. The collisional power losses in plasma discharges are presented and the effect of gas flow is investigated.
The initiation of giant electrical discharges called as "gigantic jets" connecting thunderclouds to the ionosphere is investigated by numerical simulation method in this paper. Using similarity relations, the triggering conditions of streamer formation in laboratory situations are extended to form a criterion of initiation of gigantic jets. The energy source causing a gigantic jet is considered due to the quasi-electrostatic field generated by thunderclouds. The quasi-electrostatic field is assumed to be axisymmetrical. We calculate the electric fields for different thundercloud charges. The electron dynamics from ionization threshold to streamer initiation are simulated by the Monte Carlo technique. It is found that gigantic jets are initiated at a height of ∼18-24 km. This is in agreement with the observations. The distributions of electron positions and electron energies at different initiation heights are presented. The method presented in this paper could be also applied to the analysis of the initiation of other discharges such as blue jets and red sprites.
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