Previous Theory of Leader Growth
For many years it was believed that lightning initiates as a leader of charge that grows out of a main charge region of the same polarity. If you visualize a water balloon filled with water representing negative charge, poking a hole in the water balloon would allow a narrow stream of water (a negative leader) to flow. This leader of negative charge would flow to the ground, and upon contact with the oppositely charged ground, drain this charge downward in the form of a return stroke. Following the return stroke, there would be less negative charge in the main charge region. The figures below visually show this concept.
Current Understanding of Leader Growth
In 1960, Heinz Kasemir proposed that lightning develops as a bidirectional leader with a positive and negative end and zero net charge along the leader [Kasemir, 1960]. This revolutionary theory was highly opposed until the late 1980s when analysis of data from aircraft lightning strikes showed that lightning did in fact initiate as a bidirectional/bipolar leader from the aircraft [Fischer et al., 1985; Mazur, 1989a, 1989b; Laroche et al., 1991]. Development of 3-dimensional lightning mapping technology in the early 2000s [Krehbiel et al., 2000] showed that lightning initiates between thunderstorm charge regions and bidirectional leaders propagate into charge regions of opposite polarity. Specifically, lightning initiates and grows as a bidirectional leader with the positive end propagating toward and into a negative charge region and the negative end propagating toward and into a positive charge region [Krehbiel et al., 2008]. More recently high-speed cameras have recorded the initiation and development of bidirectional lightning leaders outside of storms near previously formed lightning channels [Montanyà et al., 2015 and Warner et al., 2016]. Analysis of these videos clearly show the initiation, bidirectional development and identifiably different behavior exhibited by the positive and negative ends of the leaders.
The high-speed video below shows a rare example of bidirectional leader development near a previously formed positive leader channel. The negative end of the new bipolar leader connects with the positive channel forming a new positive leader branch.
Even though scientists now know that naturally-occurring lightning is bidirectional leader with opposite polarity ends, one of the fundamental questions of how these leaders start is still not fully understood. The question of how lightning initiates is still unclear because the measured electric fields inside thunderstorms are not high enough to cause spontaneous ionization (breakdown) of the ambient air. Therefore, some other mechanism is likely contributing to the initiation. Two of the leading theories are:
- Enhancement of the electric field locally due to the shape and/or possibly concentration and arrangement of hydrometeors in a strong electric field. The local enhancement of the electric field may allow for spontaneous breakdown of the ambient air thus initiating the leader.
- Initial ionization of the air due to cosmic rays, which starts the breakdown and leader formation.
Measuring the electric field, specifically at the initiation point of a lightning flash is very challenging and visually observing it is nearly impossible since it usually occurs inside the storm cloud. What we do know now is shown graphically below.
With the knowledge that lightning is bidirectional, we will next explain the different types of lightning flashes that take place due to this development.
Fischer, B. D., P. W. Brown and J. A. Plumer (1985) Research in lightning swept stroke attachment patterns and flight conditions with the NASA F-106B airplane, paper presented at the International Aerospeace and Ground Conference Lightning and Static Electricity, Paris, France.
Krehbiel, P. R., R. J. Thomas, W. Rison, T. Hamlin, J. Harlin, and M. Davis, Lightning mapping observations in central Oklahoma (2000), Eos Trans. AGU, 81, 21-25.
Krehbiel, P. R., J. A. Riousset, V. P. Pasko, R. J. Thomas, W. Rison, M. A. Stanley and H. E. Edens (2008), Upward electrical discharges from thunderstorms, Nature Geoscience, 1(4), 233-237, doi:10.1038/ngeo162
Laroche, P., V. Idone, A. Eybert-Berard, and L. Barret (1991) Observations of bidirectional leader development in a triggered lightning flash. NASA Conf. Publ. 3106, I, 35.
Mazur, V. (1989a), Triggered lightning strikes to aircraft and natural intracloud discharges, J. Geophys. Res., 94(D3), 3311-3325.
Mazur, V. (1989b), A physical model of lightning initiation on aircraft in thunderstorms, J. Geophys. Res., 94(D3), 3326–3340.
Montanyà, J., O. van der Velde and E. R. Williams (2015) The start of lightning: Evidence of bidirectional lightning initiation, Nature, Scientific Reports, 5:15180, doi: 10.1038/srep15180.
Warner, T. A., M. M. F. Saba, C. Schumann, J. H. Helsdon Jr., and R. E. Orville (2016), Observations of bidirectional lightning leader initiation and development near positive leader channels, J. Geophys. Res. Atmos., 121, doi:10.1002/2016JD025365.