Figure 5. Leader propagation 2D speeds.
4.2 DC background current
Some distinct patterns were observed in the current plots of leaders A and B. Some preliminary luminosity and current pulses at the tip of the lightning rods were observed (the first four current pulses appearing in images 1 to 4 in Figure 2) a few tens of microseconds before the upward leaders start. Once the leaders start a continuous propagation (images 5 to 23 in Figure 2 after t = −350 µs), several current pulses occur superimposed on a constant DC current. The DC current of leader B (UCL) is much higher than the DC current of leader A (UUL) and both increase exponentially (Figure 6). A similar growth in current was modelled by Becerra and Cooray (2009), however no current pulses were present in their simulation and there was no statement about the exponential character of the current growth. A similar pattern was also observed by Visacro et al. (2017), although it was not reported to have an exponential increase.