Numerical Simulation Of Negative Streamers At Different Pressures

How lightning discharges initiate in thunderclouds is still an unsolved problem in lightning community. Negative streamers are thought to play an important role in lightning initiation, and researches concerning negative streamers at different pressure are sure to better the understanding of initiation of lightning discharges at different heights in thunderstorms. Such studies are quite few in the literature. In this paper we present numerical studies of negative streamers at different pressures, in an attempt to present some insights into how intra-cloud lightning discharges and cloud-to-ground lightning discharges are initiated at different heights.In this paper, a cylindrically symmetrical two-dimensional model is used to simulate negative streamers between planar parallel plates in nitrogen. Continuity equations of electrons and positive ions are used to describe the streamers, and the electric field produced by space charge is settled by coupling the Poisson equation to the continuity equations. In the model, a flux-corrected transport (FCT) technique is used to solve the continuity equations and an adaptive grid refinement strategy is used to increase the efficiency of computing. Effects of initial situation and background electric field on the characteristics of streamers at standard pressure, as well as at different pressures are studied.The simulation results indicate that, after the initial stage of adjustment, negative streamers will propagate in a steady-state mode, which are governed by the speed of ionization termed background ionization frequency which is a function of ambient pressure and background electric field together. Streamers initiated with larger background ionization frequency will propagate faster and have larger current amplitude. Initial ionization conditions will also influence the characteristics of streamers, in a manner that larger total ionization density on cross section will speed up the evolution of streamers into steady-state mode and propagate faster.Pressure(and/or background electric field) influences the characteristics of negative streamers via background ionization frequency:under the same background electric field, background ionization frequency increases with the decreasing pressure, that is to say, negative streamers will develop faster and have larger current at low pressure, implying that streamers are more likely to initiate in the region at relatively lower pressure; while at the same pressure, the background ionization frequency increases with the intensity of the background electric field, negative streamers will develop faster and have larger current, streamers tend to initiate in the region with stronger background electric field. Our results indicate that in thunderstorms negative streamers are more likely to initiate at low pressure (higher level in height) than at high pressure, as that intra-cloud lightning discharges are observed to occur more frequently than cloud-to-ground lightning discharges which initiated at a relative low height than intra-cloud discharges.