High-speed video observations of space stems and leaders in natural negative cloud-to-ground lightning

Abstract:

A space stem is a luminous plasma segment that has been observed to form ahead of the leader tip during the negative leader stepping process. A space stem can extend in length to develop into a space leader which may connect to the main (pre-existing) leader channel.

In this study, we examine in detail two negative stepped leaders observed in July 2020 using a high-speed video camera operating at 400 and 575 kilo-frames per second. The frame exposure times were 1.8 µs and 1 µs, respectively. The camera is a part of the Melbourne Lightning Observatory (MLO), located on the campus of Florida Institute of Technology in Melbourne, Florida. These two negative first strokes were geolocated by the U.S. National Lightning Detection Network (NLDN); their ground strike points were at distances of 2.7 km and 7.9 km from the MLO, respectively, and their NLDN-estimated return-stroke peak currents were 17 and 104 kA, respectively. We estimated the two-dimensional distances between pre-existing leader channel (PELC) and newly-formed space stems as well as the length of space leaders. For the 17-kA stroke, the pixel resolution was 1.5 m. Sixteen space stems, 11 of which later evolved into space leaders, were recorded. Fifteen additional space leaders were also observed. All but one of the 26 space leaders connected to the PELC. The median space-stem-to-PELC distance was 5.2 m and the median space-leader length was 4.5 m. These observations are generally consistent with results from previous studies that examined space stems and leaders in natural and rocket-triggered lightning. For the 104-kA stroke, the pixel resolution was 4.4 m; 5 space stems were recorded of which 4 evolved into space leaders and connected to the PELC. Seven additional space leaders were also captured on video. Of these 11 space leaders, 8 connected to the PELC. The median space-stem-to-PELC distance was 18 m and the median length of space leaders was 22 m. These values are 3.4 and 4.8 times larger than those for the 17-kA stroke. These discrepancies can perhaps be attributed to the differences in the camera pixel-resolution and/or the return stroke peak current for the two strokes, but more data are needed to examine these factors.