Luminosity progression in dart-stepped leader step formation

Abstract

Using a high-speed optical imaging system, we have observed the step formation bright pulse discharges occurring at the tip of dart-stepped leaders of rocket triggered lightning with a time resolution of 0.1 µs and a spatial resolution of about 1.4 m. Each of the step formation pulse discharges appeared to initiate at a location immediately below the bottom of its previous pulse discharge and to propagate in bidirectional (upward and downward) waves with a speed on the order of 107 m/s. The downward waves of the pulse discharges tended to slow down significantly after they propagated a distance of about 2m. Based on the results observed in this study and those published in literatures, we propose a conceptual view of leader step formation.