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 negatively charged 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 due to the ionization of neutral air 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 (with a deficit of electrons) propagating toward and into a negative charge region and the negative end (with a surplus of electrons) propagating toward and into a positive charge region. An illustration of this process is shown below. A bidirectional leader forms in between two charge regions. The positive end of the leader, shown in red propagates toward the negative charge region, and the negative end of the leader, shown in blue, propagates toward the positive charge region. The leader is a highly thermalized conductive plasma containing positive and negative ions and free electrons.
It is very important to note that it is neutral air that is ionized due to the strong electric field created by the charge regions. The ions and free electrons that are created from the neutral air then move under the influence of the electric field to counter the charge imbalance caused by the charge regions. The charge on the ice crystals and graupel does not “flow” from the charge regions during lightning as this charge is attached to hydrometeors and immobile. This charge only serves as the source of force (found in the electric field) that results in neutral air becoming ionized.
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.
With this understanding, we now know that lightning initiates between thunderstorm charge regions with the bidirectional leader propagating toward and into the charge regions.
The Initiation Mystery
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 neutral 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.
However, researchers continue to discover new aspects of this process that may someday lead to a clear answer as to how lightning starts. Recent research findings suggest that the very first initiation process of a lightning flash may involve the development of very fast (10,000s of km/s) positive streamer activity, which sets the stage for bipolar leader development. Streamers are not thermalized like leaders and therefore are not hot, however, ionization occurs as well as current flow and streamers can develop into thermalized plasma leaders. Stay tuned.
Next section: Positive and Negative Leaders Behave Differently
Previous section: Electrification of Clouds