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. This revolutionary theory was highly opposed until the late 1980s when analysis of data from lightning strikes to instrumented research aircraft showed that lightning did in fact initiate as a bidirectional and bipolar leader from two spatial separate parts of the aircraft. Development of 3-dimensional lightning mapping technology in the early 2000s 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. More recently high-speed cameras have recorded the initiation and development of bidirectional lightning leaders outside of storms near previously formed lightning channels. 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. From the initiation point the negative end of the new leader branches multiple times and exhibits clear stepping behavior, whereas the positive end does not branch and propagates in a more continuous fashion. One of the negative branches of the newly formed bipolar leader connects with the positive channel forming a new positive leader branch.
Even though scientists now know that naturally-occurring lightning is a bidirectional leader with opposite polarity ends, one of the fundamental questions of how these leaders start is still unclear. 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. Therefore, this fundamental aspect of lightning (i.e., lightning initiation) has yet to be fully understood.
What we do know now, however, is that lightning develops bidirectionally as a channel of extremely hot, ionized air known as plasma that contains a surplus of electrons and negative ions at one end (the negative end shown in blue below) and with positive ions and a deficit of electrons on the other end (the positive end shown in red below).
With the knowledge that lightning is bidirectional, we will next explain the different behavior exhibited by each polarity and the types of lightning flashes that take place due to this development.