* Does not include the charge transfer due to the return stroke. ** Median value of 8 measurements.*** On top of a 280-m height mountain.
4.5 Time interval and amplitude of electric field changes and current pulses
The electric field measurements show abrupt negative changes that are synchronized with the occurrence of current pulses in upward leaders. The changes in the electric field produced by the positive upward leaders have the same orientation of the change produced by the negative return stroke. Positive charges are carried by the leaders over the electric field antenna in both cases. However, between the abrupt negative changes caused by the positive leaders, it is possible to see a slower positive change in the electric field due to the approach of the negative leader. As the positive charges displaced by the upward leaders are much closer to the electric field sensor than the negative charges of the downward leader, the overall change of the field prior to the return stroke is negative. Also, due to the larger distance from leader A to the field sensor, current pulses of leader A (red curve in Figure 4) produce smaller negative electric field changes than leader B pulses (black curve in Figure 4).
The average values of time interval between current pulses present in leader A and B are very similar, 24.2 and 22.8 μs respectively (see Table 3). If all intervals are considered, the average time interval between pulses is 23.4 μs with a standard deviation of 4.7 μs (see histogram of the distribution in Figure 8a). The average time interval between current pulses in leaders A and B (23.4 μs) is close to the interstep time interval found by optical or electric field sensors for negative cloud-to-ground stepped leaders (Hill et al., 2011; Krider et al., 1977). This strongly suggests that the current pulses present in upward leaders are induced by the electric field change produced by the steps in the propagation of the negative downward leader. Similar time intervals between the current pulses for UUL from a 91.5 m tall tower (Nag et al., 2021) are also shown in Table 3 for comparison. The time intervals between the current pulses from a 7 m tall structure observed by Schoene et al. (2008) (12 to 21 μs), and from a 60 m tall tower on top of a hill observed by Arcanjo et al. (2019) (30 to 50 μs) are also close to the average interstep time found in this work.
Table 3 also shows the current amplitude for leader pulses. In both leaders the amplitude of the pulses gets higher during the approach of the downward leader to the upward leaders (Figure 8b). The amplitude of the pulses present in leader B (UCL) are much more intense than those in leader A (UUL). The values obtained by Nag et al. (2021) for an instrumented 91.5 m tower are also presented for comparison. Note that the amplitudes observed by Nag et al. (2021) in seven UULs are lower than the ones obtained in the present study. This can be explained by the fact that in our study the downward leader that induced the upward leaders occurred overhead the instrumented lightning rods and produced a -73 kA peak current return stroke. In Nag et al. (2021), the downward leaders produced return strokes with peak currents ranging from -13 to -69.3 kA but at larger distances (185 to 783 m) from the instrumented tower.
Table 3 - Characteristics of the interval between pulses and current amplitude for leaders A (UUL) and B (UCL)*.