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One the interesting aspects of lightning that has been well documented by high-speed video recordings is the difference in behavior between the positive and negative ends of the bidirectional lightning leader. This difference in behavior is being studied by researchers.
I find that positive leaders are the most fascinating to observe. A bright non-branched positive leader will meander for long distances in a seemingly curious explorative state. Below is an example of a non-branched positive leader that connects with ground causing a return stroke. It was filmed at 100,000 images per second.
Compare this with a positive leader that branches as shown below.
When positive leaders branch, they tend to produce many branches and these branches struggle to remain ionized. Therefore, they tend to be faint and frequently decay. Which then brings us to one of the most interesting unique behaviors exhibited by positive leaders. For reasons that are still not understood, a bright, fast bidirectional leader frequently forms on decayed positive leader branches which attempts to reionize the decayed branch. These are currently referred to as recoil leaders, but there continues to be discussion on the most appropriate name given that the process is still not well defined. What is clear is that this reionization initiates back from the tip of the decayed positive leader tip and attempts to reestablish the branch in a fully ionized state. Because the decayed branches are still somewhat conductive, the reionization process and therefore the tips of the recoil leaders travel rapidly (1,000 – 10,000 km/s) which is at least one order of magnitude faster than the original leader speed which is typically in the 100s km/s. However, recoil leaders most often fail to reestablish the branch in a fully ionized state and simply decay after forming. Those that do have their negative ends connect with luminous positive main channels will experience a return stroke like process due to the negative end connecting with the positive channel and experiencing a reduction in resistance and electrical potential difference that causes rapid electron acceleration toward the branch point that traverses the recoil leader path toward the decayed branch tip. The video below shows three recoil leaders that form along a decayed upward positive leader branch.
To summarize the unique characteristics of positive leaders:
- Non-branched positive leaders tend to be bright and meander.
- Highly-branched positive leaders tend to be weakly luminous and decay easily.
- Recoil leaders (bidirectional reionization) form on decayed positive leader branches back from the tip of the decayed leader and attempt to reestablish the branch in a fully ionized state.
- The negative end of the recoil leader can connect with a main positive channel and exhibit a “return stroke like” reillumination of the recoil leader.
Negative leaders, or the end of the bidirectional leader network with a surplus of electrons, propagate quite differently than positive leaders. They exhibit a much more pronounced stepping behavior as they add new leader segments through the additional of short bidirectional leader stems of approximately 50 m in length. The video below shows branched negative leaders recorded at 100,000 images per second.
Notice how the negative leader tips are bright compared to the trailing leader. This is due to the stepping breakdown (ionization) of the air at the tips of the leaders. Bright negative leaders tend to branch and the branches do not decay as frequently seen in positive leaders. When a branch does decay, occasionally the branch will exhibit reionization, but curiously, this reionization initiates at the branch point and not along the decayed leader branch near the tip like that seen in the development of a recoil leader on a decayed positive leader branch. The negative end of the reionization travels from the branch point outward along the decayed negative leader branch, and if the main channel is weakly luminous, an increase in luminosity likely associated with positive breakdown travels simultaneously from the branch point back along the main negative channel. An example of this redevelopment is shown below. It is important to point out that this reionization is rarely seen compared to the prolific recoil leader development that occurs on branched positive leaders.
The animation below summarizes the difference in the branch decay and reionization of positive (red) and negative (blue) leaders.