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Article

Abrupt Elongation (Stepping) of Negative and Positive Leaders Culminating in an Intense Corona Streamer Burst: Observations in Long Sparks and Implications for Lightning

JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES. 2018. Vol. 123. No. 10. P. 5360-5375.
Kostinskiy A., Rakov V.

Properties of positive and negative leaders developing in air gaps ranging from 4 to 10 m that
were subjected to 100/7,500-μs voltage impulses were examined using a two-frame, high-speed video
camera with image enhancement. Abrupt extension (stepping) that culminated in a bright and structured
corona streamer burst was observed for both negative (expected for the “classical” stepping process) and
positive (expected for the so-called restrike process) leaders. Selected high-quality images of five negative
and four positive leaders with pronounced corona streamer bursts are presented here. The morphology of
corona streamer bursts was essentially independent of polarity. Streamer bursts exhibiting nearly spherical
symmetry were observed. For the four positive leaders, the newly added channel sections (steps) were
almost straight and had lengths ranging from about 50 to over 120 cm. For the five negative leaders, most of
the steps were curved and their 2-D lengths were some tens of centimeters. It is generally thought that
positive leaders in both long sparks and lightning extend continuously or exhibit optically unresolvable steps
whose length is comparable to the leader tip size (1 cm or less) and that for sparks only when the
absolute humidity is relatively high (>10 g/m3 or so) or voltage rise time is relatively long (around 1 ms or
more) can larger steps occur. In this study, both modes of propagation for different branches of the same
positive leader were observed.