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.