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.