High-speed video and electric field observation of a negative upward leader connecting a downward positive leader in a positive cloud-to-ground flash

•The leader of a positive lightning flash moves in a stepped fashion or not?•E-field pulses before the flash could be due to the stepping of the leader or not.•We show that the pulses are due solely to the upward negative connecting leader.

It has been discussed in past papers if a positive leader propagates in a stepped fashion or not. Some studies state that, in some positive cloud-to-ground flashes, the electric field pulses observed just before the connection of the positive leader to ground could be a manifestation of the stepping of the leader. Some authors say, however, that it may be due to the stepping of the negative upward connecting leader during its ascension toward the downward positive leader.In this work we present a well-documented case of a positive cloud-to-ground flash that shows that the pulses observed in the electric field preceding the return stroke are due solely to the upward propagation of a negative connecting leader. This flash was recorded simultaneously by a fast electric field sensor and a high-speed camera in July 2012 in the Black Hills area of South Dakota.