Numerical simulation of electric field distribution inside streamer zones of positive and negative lightning leaders

Abstract

It is widely known that there is an asymmetry in development modes of positive and negative leaders. Negative leaders in air always propagate in a step-wise manner, while positive ones can grow either continuously (at least optically) or via steps (restrikes). The step-formation mechanism in negative and especially positive leaders is poorly understood. In particular, it is still unknown how space stems, precursors of space leaders, form inside and/or at the boundary of the streamer corona, where an expected electric field is sufficiently smaller than the dielectric strength of air. The study presents numerical simulation of electrostatic conditions inside streamer zones of positive and negative lightning leaders. It is shown that, regardless of polarity, streamer coronas are characterized by highly nonuniform electric field distribution. Moreover, it appears that they always contain the areas of locally increased electric fields sometimes exceeding the breakdown level that can be the potential places of space stems origin. Due to the well-known asymmetry between propagation threshold fields of positive and negative streamers, the number of such zones and the magnitudes of electric fields inside them are sufficiently bigger for negative leaders. A significant factor influencing the level of electric field amplification is found to be the share of streamer zone channels that deviate from an expected direction of growth. The role of the streamer corona fractal dimension and the leader channel sheath line charge density is also analyzed. The model predicts that the space stem formation conditions worsen with increasing altitude above the mean sea level and get better with rising humidity which is in line with observations. The study is helpful for further development of the theory of propagation mechanisms of positive and negative leaders.