High-speed video observations of a lightning stepped leader

Abstract

[1] 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 ms 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.

High‐speed video observations of a natural negative stepped leader and subsequent dart‐stepped leader

“…These observations are consistent with laboratory observations of negative stepped leaders as described in section 1, where the weakly luminous filaments correspond to corona streamers and the strongly luminous segments correspond to space leaders. These results are also consistent with recent observations made by Hill et al [2011] where lower resolution imagery obtained using a high-speed camera shows luminous segments appearing a few meters ahead of the main leader tip before being incorporated into the leader channel. The impulsive optical and microwave radiations are also consistent with the interpretation that the luminous segments are space leaders, as the attachment of these with the main leader channel would result in a current surge capable of generating a pulse of optical and microwave radiations.…”

“…However, due to the difficulty in capturing natural lightning flashes at close range, very few studies exist to date that offer a close-up view of the dielectric breakdown process of a natural lightning negative stepped leader. Hill et al [2011] have presented perhaps the earliest such observation, showing visible but poorly resolved features of the breakdown process at a distance of approximately 1 km and at a camera frame rate of 300,000 frames per second. Triggered lightning has been better studied using high-speed video, including observations by Biagi et al [2009Biagi et al [ , 2010 that show highly resolved features of the breakdown process in front of a dart-stepped leader near ground at frame rates of 50,000 and 240,000 frames per second.…”

Dynamics of streamer‐to‐leader transition at reduced air densities and its implications for propagation of lightning leaders and gigantic jets

“…The negative space leader extremity becomes the new leader head and the process is repeated [Bazelyan and Raizer, 1998, p. 255]. The formation of a space leader ahead of the negative leader is a fundamental stage in the leader progression, as evidenced in laboratory [e.g., Ortega et al, 1994;Reess et al, 1995;Bazelyan and Raizer, 1998;Gallimberti et al, 2002] and lightning [e.g., Biagi et al, 2009Biagi et al, , 2010Hill et al, 2011] observations. Although positive and negative leaders exhibit different dynamical features, it can be seen, from the above discussion, that the streamer-to-leader transition (or in other words, the air heating) is a fundamental process that defines leader propagation in both cases.…”

The Similarity of the Action of Franklin and ESE Lightning Rods under Natural Conditions

“…The charge deposited on a stepped leader channel is transported both by the streamers propagating ahead of the leader tip and by the step current associated with the rapid elongation of the leader channel when the space leader connects with the existing leader channel [6,12]. In the analysis, a simplifying assumption that the bulk of the charge deposited on the stepped leader channel is transported by the step current is made.…”

“…It is an established fact that the leaders of negative first return strokes of lightning ground flashes (i.e., stepped leaders) transport negative charge towards ground in a stepped manner [6]. Research work conducted by Krider et al [7] and Cooray and Lundquist [8] show that the bulk of the leader charge is transported by the current associated with the rapid elongations of the stepped leader during the formation of steps (i.e., step current).…”