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

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 × 1O8m/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 × 107m/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 × 105m/s and its negatively charged part a downward speed of 2-2.6 × 105m/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.