  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 km3 with average electric field E>0.3 MV/(m
atm), which contains, because of turbulence, numerous small 'Eth-volumes' of
10^-4-10^-3 m3 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
Eth-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.
