High-speed video observations of a lightning stepped leader

High-speed video images of eight branches of a lightning stepped leader recorded at a frame rate of 300 kiloframes per second (kfps) (3.33 mu s exposure) are analyzed, representing the fastest published frame rate measurements of stepped leader stepping by about 1 order of magnitude and the first observations of space stems/leaders associated with stepped leaders. Sixteen occurrences of space stems/leaders were imaged in 14 different frames at various distances in front of the descending leader tip. A total of 225 frames (about 0.75 ms) involving 82 steps of the downward moving, negatively charged stepped leader were captured, followed by 45 frames of leader channel illumination by the return stroke after the ground attachment of the primary leader channel. The stepped leader exhibited characteristics similar to those observed in both dart-stepped leaders in triggered lightning and in long laboratory sparks. In most cases, the space stem/leader in one frame connects to the leader tip above in the subsequent frame, extending the leader channel. Most connections are associated with significant isolated brightening of the space stem/leader and the connection region, followed by frames of upward propagating reillumination of the existing leader channel. Assuming the leader to be 1 km distant, we measure the 16 space stems/leaders to be on average 3.9 m in length and separated from the previous leader channel tip by an average of 2.1 m. For the 82 steps, interstep intervals are on average 16.4 ms and step lengths are on average 5.2 m.