Using high‐speed video cameras operating with framing rates of 20 and 525 kfps, we imaged the attachment process of a natural negative cloud‐to‐ground flash, occurring at a distance of 490 m. Nine upward leaders were observed. A total of 12 space stems/leaders in 47 steps of the downward negative stepped leader were captured. The two‐dimensional length of them was between 2.0 and 5.9 m, with an average of 3.0 m. The average interstep interval, step length, and two‐dimensional speed of the downward negative leader and that of upward positive leader were statistically analyzed. The last step of the downward negative leader making contact with the upward connecting leader was recorded. The two‐dimensional length of the final imaged gap between the tips of opposite‐polarity leaders was estimated to be about 13 m.
We present time‐correlated ultra‐high‐speed video camera and electromagnetic field measurements of the attachment processes in a natural negative cloud‐to‐ground stroke. The video camera frame exposure time and pixel resolution were 740 ns and 0.91 m/pixel, respectively. The common streamer zone (CSZ) was first observed 2.52 µs preceding the first frame showing the return stroke (RS) in progress, when the upward and downward leader‐tips were 9.8 m apart. In the next frame, the two leaders were observed to have propagated toward each other within the CSZ, with their tips being 0.91 m apart. Our observations show with unprecedented precision/clarity that (a) the slow front in the field waveform is associated with the CSZ, and (b) the “proper” start of the RS is marked by the onset of the fast transition in the field waveform which occurs at the completion of the attachment processes (when the upward and downward leaders have merged).
more » « less- Award ID(s):
- 1934066
- NSF-PAR ID:
- 10449707
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 50
- Issue:
- 16
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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