Channels of Charge
Obviously, the main channel of electric current here is the one through which the return stroke rushes upward, brilliantly illuminating the downward zig-zag of the successful step leader. [The step leaders are invisible paths of charge which step rapidly downward from the cloud before the visible return stroke flashes up through the first one to reach ground.] But what about the downward step leaders which failed? These channels are ionized enough to glow when the main one, to which they are all connected, lights up in the skyward surge of current from the ground. But because the step leaders all point down, their channels give the illusion of a downward flash in the main channel. Very deceptive!
Norman OK (1 Jun 99) Looking SE