Evolution of line charge density of steadily-developing upward positive leaders in triggered lightning

We present an approach for retrieving the temporal and spatial evolution of the charge density and the current of a steadily-developing upward leader based on ground observations of electrical fields and high speed camera images of two upward positive leaders initiated by a classical rocket-triggered lightning before the wire disintegration. For a period of a few milliseconds, for the upward leader segments developing above altitude about 200m, we obtained the leader speed, calculated the leader charge density and the leader current. For the two leaders, both the leader speed and the charge density, hence the leader current, showed a trend of increase as the leader propagated upward. There was a good consistency in the variation trend between the calculated leader current and the leader channel brightness. The leader current calculated agreed well with the current measured at the channel base, indicating that our proposed model for retrieving the leader charge density is effective and reliable.