Circuitous attachment process in altitude-triggered lightning striking a 30-m-high tower

Abstract

The altitude rocket-and-wire technique was used to trigger lightning to strike a 30-m-high communication tower. A comprehensive dataset was obtained, including discharge current and electromagnetic-field changes, as well as high-speed video images. An attachment process with an abnormal circuitous S-shaped connection channel was observed between the lower extremity of the triggering wire and the tower top, with a total channel length of 25.3 m, which was much longer than the direct connection channel of 13.0 m. The downward negative leader from the triggering wire and the upward connecting positive leader from the tower top “missed” each other by a vertical distance of 4.1 m before turning to the horizontal direction and eventually completing the breakthrough phase. The attachment process produced a sharp current pulse with a duration of 19 μs and a peak of 11.6 kA, transiently promoting the upward positive leader development from the upper extremity of the triggering wire. The simulation of the background electric field indicated that the electric field surrounding the tips of the initial leaders did not involve a dominant direction with pronounced larger intensity. The leader streamer/corona zones with a divergent feature resulted in directional uncertainty for the initial steps. As the leaders developed and the associated streamer/corona zones expanded, the attraction effect was enhanced and the leaders turned to the horizontal direction. The common streamer zone (CSZ) developed when the gap between the leaders was about 8 to 9 m, resulting in the occurrence of the breakthrough phase and accomplishment of the attachment. For the last return stroke, the long interstroke interval facilitated channel cooling and a decrease in conductivity. This promoted the occurrence of a dart-stepped leader instead of dart leader, and led to the alteration of the attachment route from an S-shaped channel to a direct connection channel.