Return stroke transmission line model for stroke speed near and equal thatof light

Assuming that the lightning return stroke transmission-line model is applic able, we derive an expression for the return-stroke magnetic field for an a rbitrary return stroke speed and from that expression show that for a retur n stroke speed equal to the speed of light c the electric and magnetic fiel d waveforms at all points in space and the current waveform are identical. Recent measurements indicate that the electric field and current waveforms are similar for about 100 ns for triggered lightning return strokes, potent ially implying that the initial return stroke speed is actually near c for that time, that is, for the bottom 30 m or so of the triggered lightning ch annel. While for the transmission-line model the current waveform and the t otal electric field waveform are identical for a return stroke speed of c, we show that each of the three individual components (electrostatic, induct ion, and radiation) that comprise the total field varies significantly with distance.