Figure 7. Charge density and charge transferred by leaders A
(UUL) and B (UCL).
4.5 Backflow current
Approximately 500 µs after the attachment of the downward negative
leader with leader B (UCL), when the intense luminosity of the return
stroke subsides, no upward leaders are visible, meaning that they have
all collapsed. The electric field that was driving the propagation of
the UULs from several nearby structures collapses with the occurrence of
the return stroke in B. Therefore, the charges contained in these
leaders flow back to their origin in a very short time, creating an
intense current in the opposite direction.
In fact, this can be observed in the current and charge transfer plots
of leader A (UUL) in Figure 4 and in Figure 7 respectively. Note that
once the return stroke starts, there is a current polarity reversal and
charge transfer decrease. The reversed current of leader A (UUL) reaches
a minimum of -701 A. This backflow current was modelled by Becerra and
Cooray (2009) and measured by Schoene et al. (2008), Visacro et al.
(2010), and Nag et al. (2021) for structures with different heights
(Table 2).
Table 2 also shows the height of the structure that starts the upward
leader, the leader length, the total charge transferred, and linear
charge density for leader A (UUL) and B (UCL). The length values of
leaders A and B were estimated through the analysis of the frames
acquired for the high-speed camera v12. Total charge was calculated
through the integration of the current measurements up to the moment
when the UUL starts to collapse (current reversion). In this table, we
compare our values of positive leader linear charge density with
estimated values by models or measured in some triggered lightning
flashes and in laboratory discharge studies (no values were found in
natural flashes studies).
Table 2 - Height of structure, leader length, total transferred
charge, and linear charge density of leaders A and B.