Toward a better understanding of negative lightning stepped leaders

Negative lightning leaders observed at very close (within a few kilometers) distances often exhibit heavy branching with many active tips forming a network-like structure with a descending multi-tip "ionization front " whose transverse dimensions are of the order of hundreds of meters. The presence of such front makes the lightning attachment process more complex than usually assumed. Negative leader branches extend in a step-like manner, with each step necessarily generating a traveling wave moving positive charge from the branch tip up along the channel, like a mini return stroke. The leader stepping process has a number of generally unrecognized consequences. Specifically, a stepped-leader branch tip can collide with the lateral surface of an adjacent branch (usually at an angle of about 90). Further, the step-related positive-charge waves propagating from the tip up along the channel can reactivate decayed negative branches at higher altitudes. Overall, a heavily-branched negative stepped leader creates a highly-structured and rapidly-changing electric field pattern inside and in the vicinity of the volume it occupies, which causes complex interactions between the branches.