Characteristics of the close leader/return stroke electric field change and its response to the corresponding charge density along the lightning channel.

Generally speaking, at very close distances, any return stroke electric field change value that is smaller than that of the preceding leader is indicative of the presence of the leader charge unneutralized by the return stroke. In order to reproduce the V shape structure characteristics of close dart leader/return stroke field change, we employ two existing models, one for the 'source charge' leader model and the other for MTLL return stroke model, both based on the assumption of uniform leader charge distribution along the channel and the complete neutralization of the leader charges by the following return stroke process. The simulated results show that the return stroke electric field is inversely related with the return stroke speed at early times (within few tens of microseconds from the beginning of the return stroke), while at later times the field is dominated by the deposited charge density component, the close electric field is independent of speed and the return stroke electric field change equals to that of the preceding leader. Therefore, at early times it might cause some uncertainty to judge whether the charges deposited by the dart leader are completely neutralized by the following return stroke process based on the difference between the return stroke and the leader field on the ground.