High-speed video observation of stepwise propagation of a natural upward positive leader

Abstract

Using a high-speed video camera operated at 150,000 frames per second, we have documented the upward propagation of an initial positive leader from a 325m meteorological tower in Beijing. The stepwise development of the upward positive leader was clearly revealed during its self-sustained propagation toward the cloud base, providing the first optical evidence for the stepping processes of a positive leader. The leader developed with definitive intermittent pauses and re-establishment with abrupt jump of the leader top. Obvious brush-like corona zone emitting outward from the leader top was identified in the frame of stepping, and the luminosity waves propagated downward along the already formed channel from the leader top immediately following the stepping, indicating that a current wave was generated at the leader top and subsequently traveled down the channel. The channel luminosity decreased during the leader pause stage, with the corona zone dimmed correspondingly. The positive leader experienced an average interstep interval of 61.7 µs (ranging between 30 µs and 120 µs) and an average 2-D speed of 8.1 × 104 m/s. The transient speed of the step jump was estimated to be larger than 7.3 × 105 m/s with an average step length of 4.9 m. The need of sufficient positive charge accumulation at the leader top could be the main cause for the stepping characteristics.