Mechanisms of sprite initiation, morphology, and lightning polarity asymmetry

Sprites are spectacular optical emissions in the mesosphere induced by transient lightning electric fields above thunderstorms. Recently, significant efforts have been devoted to the understanding of the inception mechanism of sprite streamers, the origin of different sprite morphologies, and the lightning polarity asymmetry in producing sprites. In this paper, we present a combination of observational and modeling results explaining the physical parameters and processes that are important for the resolution of these outstanding issues. We first emphasize the presence of plasma inhomogeneities in the lower ionosphere as a necessary condition for the initiation of sprite streamers. Then we explain the relation between sprite morphology and the characteristics of the causative lightning discharges using plasma fluid modeling results in comparison with optical and radio observations. Finally, we discuss the critical factors that account for the lightning polarity asymmetry in producing sprites.