Kostinskiy et al. (2015a), using a high-speed infrared (2.5-5.5 μm) camera, discovered the so-called unusual plasma formations (UPFs) in artificial clouds of charged water droplets. UPFs had complex morphology including both streamer-like regions and hot channel segments. They were observed both in the presence and in the absence of hot leader channels developing from the grounded plane toward the cloud. In this paper, which is aimed at revealing the genesis of UPFs, we present two UPFs that occurred inside the initial corona streamer burst of positive polarity emitted from the grounded plane, prior to the formation (or in the absence) of associated hot leader channel. These streamer bursts developed at speeds of 5 to 7 x 10^5 m/s over 1 to 1.5 m in apparently clear air before entering the negatively-charged cloud and producing UPFs at its periphery. Hot channel segments within UPFs were formed in very short times of the order of 1 μs or less. It is not clear if the UPFs were caused solely by the enhanced electric field near the charged cloud boundary or other factors also played a role. Occurrence of UPFs may be a necessary component of any lightning initiation mechanism (Kostinskiy et al., 2020; Iudin et al., 2021).

Based on experimental results of recent years, this article presents a qualitative description of a possible mechanism (termed the Mechanism) covering the main stages of lightning initiation, starting before and including the initiating event, followed by the initial electric field change (IEC), followed by the first few initial breakdown pulses (IBPs). The Mechanism assumes initiation occurs in a region of ≈1 km with average electric field E≥0.3 MV/(m∙atm), which contains, because of turbulence, numerous small ‘E-volumes’ of 10^-4-10^-3 m with E≥3 MV/(m∙atm). The Mechanism allows for lightning initiation by either of two observed types of events: a high power VHF event such as a Narrow Bipolar Event, or a weak VHF event. According to the Mechanism, both types of initiating events are caused by a group of relativistic runaway electron avalanche particles (where the initial electrons are secondary particles of an extensive air shower) passing through many ‘E-volumes’, thereby causing the nearly simultaneous launching of many positive streamer flashes. Due to ionization-heating instability, unusual plasma formations (UPFs) appear along the streamers’ trajectories of the streamers. These UPFs combine into three-dimensional (3D) networks of hot plasma channels during the IEC, resulting in its observed weak current flow. The subsequent development and combination of two (or more) of these 3D networks of hot plasma channels then causes the first IBP. Each subsequent IBP is caused when another 3D network of hot plasma channels combines with the chain of networks caused by earlier IBPs.