Figure 3. Electric field change caused by the approach of the
negative downward leader, the initiation of the positive upward leaders,
and the return stroke (t = 0 ms).
Leaders A (UUL) and B (UCL) had their current measured by the current
sensors installed on structures A and B in Figure 1a (lightning rod
masts of buildings P1 and P2 respectively). Figure 4 shows the current
measurements of upward connecting leaders and the electric field during
550 µs before the return stroke. In the plot, the numbers 1 to 24
indicate the correspondent video images (shown in Figure 2) associated
with the current pulses. 18 unipolar current pulses of some tens of amps
(10’s A) from both vertical masts are observed during this interval. The
positive polarity of the pulses indicates an upward-directed transfer of
positive charge. The highest current pulse (413 A; t = -180 μs) appears
in leader B. Close to the occurrence of the return stroke, the pulses
are superimposed on a DC current that was not considered when measuring
the pulse current peak. The blue curve in Figure 4 traces the electric
field changes during this time interval. The field starts saturated
before pulse number 3 and saturates again with the occurrence of the
return stroke.