A wave guide model of lightning currents

In this paper, a lightning channel is simulated by a vertical wire of finite length and of moderate electric conductivity. At low frequencies, such a wire behaves like a resonant wave guide cavity in which only discrete resonant wave modes can be generated. The theory of resonant waves in a wire of finite diameter is outlined and applied to the return stroke with electric contact to the earth and to intracloud K-strokes without electric contact to the earth. Two types of current wave forms exist: aperiodic waves of the Bruce-Golde type and damped oscillations. The configurations of the electric charge density and the electric current in the wire are presented for the various wave modes as functions of height and time. From the observations of lightning current and of sferic wave forms, it is apparent that the first mode dominates, and that both types of wave forms can be identified. From these wave forms, the channel parameters-length and diameter can be derived. For the higher order modes, the resonance model loses its validity and freely propagating waves are expected during the whole lightning flash. These waves contribute to the observed continuous spectrum. The skin effect becomes important increasing the effective resistance of the wire with frequency. Since these waves interfere destructively, they do not contribute significantly to an effective charge transport.