Optical and Electric Field Signatures of Space-Leader Evolution in Negative Cloud-to-Ground Lightning Stepped Leaders

In this study, we examine space leaders in 15 negative natural cloud-to-ground lightning strokes, eight of which occurred near the Melbourne Lightning Observatory and seven of which attached to or occurred near the NASA Kennedy Space Center Industrial Area Tower. All strokes were captured on ultra-high-speed video cameras operating at frame rates ranging from about 400k to 780k frames per second. Additionally, broadband electromagnetic field measurements were available for a subset of these strokes. All strokes were geolocated by the U.S. National Lightning Detection Network (NLDN). The NLDN-reported peak currents for these strokes ranged from -13 to -228 kA. The upper and lower limits of the fields of view of our cameras were different for different strokes; the maximum and minimum altitudes of observation were about 978 and 18 m above ground level, respectively. The median space leader lengths and retrograde propagation speeds were 8.6 m and 3.7 x 10⁶ m/s, respectively. Space leader length appeared to be primarily dependent upon the return-stroke peak current and was weakly related to the inception altitude.