Figure 6. Exponential growth of the DC current measured for
leader B (UCL, upper plot) and leader A (UUL, bottom).
4.3 Leader pulses alternation and synchronism
One interesting feature that can be observed in Figures 2 and 4 is that
although leaders A and B have their luminosity and current pulses
increasing in time as the downward stepped leader approaches, they are
not synchronized at the beginning (up to image 18 in Figure 2 and up to
t = −114 µs in Figure 4).
It is also possible to observe from the high-speed video (in
Supplementary Information – videos 1 and 2) that the downward leader is
highly branched (Figure 1b) and the branches alternate luminosity during
their downward propagation. The upward leaders A and B respond to
different downward propagating branches and, as the branches alternate
in propagation and intensity, so do leaders A and B accordingly.
However, during the last 100 µs, the alternation ceases, all downward
leader branches intensify and consequently leaders A and B synchronize
and pulse together. This period coincides with the rapid exponential
growth of the DC base current (Figure 6, upper plot). It seems that the
proximity of the downward leader branches is such that differences in
leader branches propagation and intensities are not driving the upward
leaders differently anymore.
4.4 Leader charge density and charge transfer
From the current and 2D leader length measurements it was possible to
estimate how the transferred charge and the linear charge density of the
leaders vary with time. In Figure 7 the continuous lines show the charge
transferred by the upward leaders A and B and the triangles their linear
charge density during propagation. The charge and the charge density
during the return stroke is not plotted for the UCL leader but its
effects are seen in the charge and charge density plots of the UUL
leader and will be addressed in the next section.
The charge densities of the leaders do not stay constant but increase
rapidly with time during the propagation of the upward leaders (a
similar result was obtained by Chen et al. 2013 for triggered
lightning). This means that for both UUL and UCL the charge in the
leader increases much faster than the length of the leader, which has a
constant speed as shown in Figure 5. Note also that the step like
character of the charge variation is due to the current pulses during
the leader propagation.