Evolution of the Leader Discharge in Bi-Directional Propagation System in Altitude-Triggered Lightning

Abstract

On 18 June 2023, comprehensive observations were conducted to an altitude-triggered lightning flash. Upward positive leader (UPL) and downward negative leader (DNL) in a bi-directional development system were detected simultaneously by a high-speed camera, together with the coordinated measurements of magnetic field and very-high-frequency (VHF) emissions. High-speed images reveal, for the first time, the enhancement of the UPL's propagation speed by DNL in the bi-directional leader system. Concretely, the upward positive leader initially originates from a suspended wire tip and propagates at a two dimensional (2D) speed of 5.32 × 10⁴ m/s, and after about 6.3 ms, its propagation speed was enhanced to 1.12 × 10⁵ m/s when the stepped DNL started advancing at an average speed of 1.44 × 10⁵ m/s. Additionally, based on the evolution of channel luminosity and the variations of magnetic radiation, it is found that there is a consistency in luminosity variation between the ascending channel and the descending channel in the bi-directional leader system, and the amplitude of the magnetic field increases when the negative discharges start at the bottom wire end with intensive VHF emissions. Those facts indicate that the DNL has an effect, may be a positive one, on the UPL's development in the early stage of altitude-triggered lightning.