Figure 8. a) Distribution of time intervals between pulses in leader A (UUL) and leader B (UCL); b) peak current of pulses occurring during the initiation and progression of the leaders.
Summary and Conclusion
In this study, for the first time, the current was measured simultaneously in two upward and competing leaders during the attachment process of a downward negative leader. The connection took place with the lightning rod of a residential building and was simultaneously recorded with high-speed cameras, electric field sensor and current sensors giving the opportunity to compare the time interval between current pulses, the amplitude of the pulses and the behavior of the DC currents of an UCL and an UUL leader.
The approach of the negative downward leader induced 6 upward leaders prior to the return stroke. The brightest and longest ones were initiated from instrumented lighting rods and had their current intensities measured. One of them (from structure B, building P2) connected to the downward leader.
The downward leader and upward leaders A and B propagated at a constant speed, being the downward leader 2.0 to 5.5 times faster than the upward leaders and the UCL 17% faster than the UUL. The final jump speed (upward and downward leaders combined) is estimated to be at least 45 times faster than the average speed of the UCL prior to the jump. Although the peak current of the return stroke in this case was much higher than the previous ones analyzed by Saba et al. (2017) (Table S1 in Supplementary Information), the speed ratios (Vd/Vucl and Vd/Vuul) were similar. However, the charge of the downward leader, which is related to the intensity of the return stroke, may have influenced the speed of the upward leaders (all of them faster than previously reported cases). The faster speeds of the downward leader and the upward connecting leader reduced the time interval between the UCL leader inception and return stroke. The distance between the down-coming negative leader tip and the tip of the vertical rod at the inception of a stable upward positive leader (102 m) was not significantly different from previous cases (82, 120 and 62 m).
DC currents were measured during the propagation of both leaders (A and B). Both increased exponentially with a similar growth predicted by Becerra and Cooray (2009). The DC current of leader B (UCL) was much more intense than the DC current of leader A (UUL).
Pulses superimposed on the DC current of the upward leader were reported before. Although the amplitudes of the pulses are different in the case of taller structures (Visacro et al., 2010, tower height of 60 m over a 300 m tall mountain), the pattern of the pulses observed in this case is very similar to current measurements of UUL reported from tall towers (e.g. Figure 8 in Visacro et al., 2010), and from small structures (Figure 6 in Schoene et al., 2008, grounded vertical conductor of 7 m height over a flat terrain). Multiple current pulses occur from the lightning rods during the approach of the negative downward leader and the amplitude of the current pulses tend to increase with time; the current peaks of the superimposed leader pulses range from 18 A to 413 A; the pulses superimposed on UCL leaders are larger than the pulses superimposed on UUL leaders.
The average time interval between current pulses in upward leaders is close to the interstep time interval found by optical or electric field sensors for negative cloud-to-ground stepped leaders, which 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
The upward leaders A and B respond to different downward propagating branches and, as the branches alternate in propagation and intensity, so do leaders A and B accordingly. During the last 100 µs there is a rapid exponential growth of the DC base current, the alternation of the leaders ceases, all downward leader branches intensify and consequently leaders A and B synchronize and pulse together.
Although the leaders have a constant speed, their linear charge density increases rapidly during their propagation. The total charge of the UCL is 5 times larger than the total charge of the UUL. The average linear densities for upward leaders (49 and 82 µC/m) were obtained for the first time for natural lightning. Previous measurements and estimations (from triggered lightning, laboratory measurements and theoretical studies) show values that range from half to 4 times higher.
The identification and characterization of the UCL and UUL reported here can help not only the understanding of the attachment process when several upward leaders are induced, but also the impact of these upward leaders in vulnerable equipment, in the ignition of flammable vapors and in injures caused to humans (Schoene et al., 2008; Becerra and Cooray, 2009).
Acknowledgements
The authors would like to thank Lie Bie (Benny), Raphael B. G. Silva, Marco A. S. Ferro, Hugh Hunt, Guilherme Aminger, and Eliah Fernanda S. Sabbas for all support in equipment installation and data acquisition. We also thank Dr. Alexandre Piantini, Dr. Acácio Silva Neto, Dr. Celso Pereira Braz and staff from the high-voltage facility at the Institute of Energy and Environment (IEE) at the University of S. Paulo, Brazil for the calibration tests of the current transformer sensor. This work was supported by research grants from FAPESP (Project 2012/15375-7); CNPq (Projects 141450/2021-5, 153799/2022-6).
Data Availability Statement
The high-speed videos analyzed in this work are available at: https://doi.org/10.5281/zenodo.7244891
References
Arcanjo, M., Guimarães, M., & Visacro, S. (2019). On the interpeak interval of unipolar pulses of current preceding the return stroke in negative CG lightning. Electric Power Systems Research ,173 (March), 13–17. https://doi.org/10.1016/j.epsr.2019.03.028
Becerra, M., & Cooray, V. (2009). On the interaction of lightning upward connecting positive leaders with humans. IEEE Transactions on Electromagnetic Compatibility , 51 (4), 1001–1008. https://doi.org/10.1109/TEMC.2009.2033265
Chen, M., Zheng, D., Du, Y., & Zhang, Y. (2013). Evolution of line charge density of steadily-developing upward positive leaders in triggered lightning. Journal of Geophysical Research Atmospheres ,118 (10), 4670–4678. https://doi.org/10.1002/jgrd.50446
Hill, J. D., Uman, M. A., & Jordan, D. M. (2011). High-speed video observations of a lightning stepped leader. Journal of Geophysical Research Atmospheres , 116 (16), 1–8. https://doi.org/10.1029/2011JD015818
Krider, E. P., Weidman, C. D., & Noggle, R. C. (1977). The electric field produced by lightning stepped leaders. Journal of Geophysical Research . https://doi.org/10.1029/jc082i006p00951
Lu, W., Chen, L., Ma, Y., Rakov, V. A., Gao, Y., Zhang, Y., et al. (2013). Lightning attachment process involving connection of the downward negative leader to the lateral surface of the upward connecting leader. Geophysical Research Letters , 40 , 5531–5535. https://doi.org/10.1002/2013GL058060
Lu, W., Gao, Y., Chen, L., Qi, Q., Ma, Y., Zhang, Y., et al. (2015). Three-dimensional propagation characteristics of the leaders in the attachment process of a downward negative lightning flash. Journal of Atmospheric and Solar-Terrestrial Physics , 136 , 23–30. https://doi.org/10.1016/j.jastp.2015.07.011
Naccarato, K. P., Saraiva, A. C. V., Saba, M. M. F., & Schumann, C. (2012). First Performance Analisys of BrasilDAT Total Lightning Network in Southeastern Brazil. In International Conference on Grounding and Earthing & 5th International Conference on Lightning Physics and Effects (p. 6). Bonito.
Naccarato, K. P., De Paiva, A. R., Saba, M. M. F., Schumann, C., Silva, J. C. O., & Ferro, M. A. S. (2017). Preliminary comparison of direct electric current measurements in lightning rods and peak current estimates from lightning location systems. 2017 International Symposium on Lightning Protection, XIV SIPDA 2017 , (October), 319–323. https://doi.org/10.1109/SIPDA.2017.8116944
Nag, A., Cummins, K. L., Plaisir, M. N., Wilson, J. G., Crawford, D. E., Brown, R. G., et al. (2021). Inferences on upward leader characteristics from measured currents. Atmospheric Research , 251 (November 2020), 105420. https://doi.org/10.1016/j.atmosres.2020.105420
Les Renardières, G. (1977). Positive discharges in long air gaps at Les Renardières - 1975. Electra , 53 .
Rizk, F. A. M. (1990). MODELING OF TRANSMISSION LINE EXPOSURE TO DIRECT LIGBTNING STROKES . IEEE Transactions on Power Delivery(Vol. 5).
Rizk, F. A. M. (1994). Modeling of Lightning Incidence to Tall Structures Part I: Theory. IEEE Transactions on Power Delivery ,9 (1).
Saba, M. M. F., Schumann, C., Warner, T. A., Helsdon, J. H., & Orville, R. E. (2015). High-speed video and electric field observation of a negative upward leader connecting a downward positive leader in a positive cloud-to-ground flash. Electric Power Systems Research ,118 , 89–92. https://doi.org/10.1016/j.epsr.2014.06.002
Saba, M. M. F., Paiva, A. R., Naccarato, K. P., Siqueira, F. V. C., Sabbas, F. S., Silva, J. C. O., et al. (2017). Current measurements of upward leaders from buildings. 2017 International Symposium on Lightning Protection (XIV SIPDA) , (October), 2–5.
Saba, M. M. F., Ferro, M. A. S., Cuadros, E. T., Custódio, D. M., Nag, A., Schumann, C., et al. (2019). High-Speed Video Observation of a Dart Leader Producing X-rays. Journal of Geophysical Research: Space Physics , 124 (12), 10564–10570. https://doi.org/10.1029/2019JA027247
Schoene, J., Uman, M. A., Rakov, V. A., Jerauld, J., Hanley, B. D., Rambo, K. J., et al. (2008). Experimental Study of Lightning-Induced Currents in a Buried Loop Conductor and a Grounded Vertical Conductor.IEEE Transactions on Electromagnetic Compatibility , 50 , 110–117. https://doi.org/10.1109/TEMC.2007.911927
Visacro, S., Murta Vale, M. H., Correa, G., & Teixeira, A. (2010). Early phase of lightning currents measured in a short tower associated with direct and nearby lightning strikes. Journal of Geophysical Research , 115 (D16104), 1–11. https://doi.org/10.1029/2010JD014097
Visacro, S., Guimaraes, M., & Murta Vale, M. H. (2017). Features of Upward Positive Leaders Initiated From Towers in Natural Cloud-to-Ground Lightning Based on Simultaneous High-Speed Videos, Measured Currents, and Electric Fields. Journal of Geophysical Research: Atmospheres , 122 , 12,786-12,800. https://doi.org/10.1002/2017JD027016