Propagating properties of a upward positive leader in a negative triggered lightning

Abstract

Propagating properties of a upward positive leader are analyzed by using the data from the artificially triggered lightning experiment in Shandong 2009, including the high-speed camera records and simultaneous surface electric field fast change measurements at 30 m, 60 m and 480 m from the channel. The tip of the leader is brighter, and the leader's 2-D partial propagating speed along its channel irregularly changes in a wide range. This indicates that the upward positive leader exhibits obviously stepped characteristics. The average speed is 9.8×104 m/s from 340 m to 705 m at the beginning stage of the leader propagating. The upward positive leader is initiated at a speed of 3.8×104 m/s. The partial speed shows an increasing tendency with height increasing. The electric field change waveform of upward positive leaders has a regularly stepped shape. The step interval of 28 steps recorded ranges from 14 μs to 39 μs with a geometrical mean value of 25.1 μs. The induced step length varies from 0.9 m to 3.7 m with a geometrical mean value of 1.7m. The electric field change of upward positive leaders includes the slowly stepped change and pulsed change. The high-speed camera records and electric field measurements show that the channel of the leader tip bending can eliminate or weaken the stepped property of the electric field change. The positive step leader has a similar propagating mechanism to that the negative leader has, that is a step of leader originates from a bi-directional streamer front end of the leader tip.