Known properties of the lightning leader channels point towards a return stroke moving with the speed of light

Without loss of generality, the return stroke can be treated as a current pulse propagating along the leader channel while undergoing attenuation and dispersion. According to the measurements, the return stroke travels along the leader channel with a speed less than the speed of light. However, if the return stroke speed were to be lower than the speed of light, the propagating return stroke would generate an electric field traveling ahead of the return stroke front with the speed of light. Since the leader channel is conducting and finite in thickness, the electric field that extends ahead of the return stroke front would interact with the leader channel generating currents that effectively push the return stroke front speed to the speed of light. In this paper we will make a case for the fact that the return stroke speed is actually equal to the speed of light. The slower return stroke speeds obtained in experimental investigations are shown to be caused by the rapid dispersion of the return stroke current as it propagates along the leader channel. The analysis presented in the paper is restricted to the lightning subsequent return strokes but the conclusions are also valid for lightning first return strokes.