Simultaneous observations of optical and electrical signals in altitude‐triggered negative lightning flashes

Abstract

This paper presents an analysis of the experimental data on five negative lightning flashes initiated using the altitude‐triggering technique in China. The data include highly time‐resolved optical images and electric fields measured 60 m and 1300 m from the lightning channel. The triggering technique involves the launching upward of a small rocket trailing a wire electrically floating. The data show that these 5 flashes have a similar chronological sequence of events, including a bidirectional leader system followed by a mini‐return stroke and a bidirectional discharge process. The bidirectional leader system consists of an upward positive leader initiated from the top of the wire and a downward negative stepped leader from the bottom, with the onset of the former prior to the latter by 3 to 8.3 ms. The downward negative stepped leader, having a step interval of 12–30 μs, appears to pause and resume several times while the upward positive leader extends forward continuously. With the downward negative stepped leader close to ground, a mini‐return stroke occurs between the ground and the bottom of the wire. The mini‐return stroke propagates upward with a speed of 1–2 × 108 m/s and emits intense light signals similar to a normal return stroke below the bottom of the wire. It becomes invisible after entering the bottom of the wire and appears again as a bright upward discharge from the top of the wire several microseconds later. This upper bright discharge ceases after propagating forward several hundred meters at a speed of 1.5–5.4 × 107 m/s. The cessation of the upper bright discharge is obviously associated with the disintegration of the wire at that moment. Right after the cessation of the upper bright discharge, a bidirectional discharge process starts from the bottom of the wire with its positively charged part having an upward speed of 3–10 × 105 m/s and its negatively charged part a downward speed of 2–2.6 × 105 m/s. Reflection of current waves at the bottom of the wire due to the explosion of the wire at that moment may be a major reason for the occurrence of this lower bidirectional discharge.