* 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)*.