  In this article we present the following hypothesis: thunder-cell ejects
highly energetic protons, each of which creates a tree-structure of weakly
ionized trajectories that can develop into a lightning channel. The
tree-structure and the channel have the same geometry so the mean free path of
a proton corresponds to the average length of the channel between two
successive nodes (branching points). We show this length is around 660 m in
lower Earth atmosphere. Effects of Coulomb interaction and various outcomes of
proton-nucleus reaction are taken into account. A prediction of CG/CC ratio
that follows agrees well with the available data, but only measurements of
lightning geometry can reveal whether the hypothesis is any closer to the
correct explanation of the phenomenon.
