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.