The pilot streamer in lightning and the long spark

Abstract The absence of significant pulsations on records of the electrostatic field of the stepped leader further establishes the existence of the pilot streamer, which must carry approximately the same current (320 A) and charge per unit length as the leader behind it. Pulsation theories of the stepped leader process are therefore inadmissible. The average radius of pilot and leader streamers in lightning is found to be 2⋅5 m , thus excluding single-avalanche theories of the step process. A new explanation of the step process is given, according to which the charge on the pilot progressively reduces the field at its starting-point in front of the arrested leader until current ceases, owing to the disappearance of free electrons by capture. A positive ‘electrode’ space charge then develops at this point and in 3μs creates a field sufficiently strong for the development of a fast step streamer which overtakes the whole pilot in less than 1μs and converts a new section of the channel into a conductor with are characteristics. The pilot streamer processes in lightning and the long spark have nearly identical velocities, current densities, channel field strengths and ratios of length to radius; it is concluded that they are produced by the same mechanism. It is suggested that the preliminaries to spark breakdown between 1 and 3 MV involve a transition from the narrow electron avalanche to the broad pilot process.