One of the interesting aspects of lightning that has been well documented by the analysis of high-speed video recordings is the difference in behavior between the positive and negative ends of the bidirectional lightning leader during its development and propagation. Before we explain the different type of lightning flashes, we will first explore these leader behavioral differences as they result in identifiably unique aspects of flashes.
I personally 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. The speed of a bright non-branched positive leader tends to be similar to that of negative leaders, about 300 km/s. Below is an example of a non-branched positive leader that connects with ground causing a return stroke. The first video shows the leader filmed at 100,000 images per second (ips) and the second shows the same leader filmed at 10,000 ips.
When positive leaders branch, they tend to slow down (10 – 100 km/s) and produce many branches. Frequently, these branches struggle to remain luminous or never reach a brightly luminous state before decaying. What is unclear is whether the branching is a result of the reduction in speed or whether the reduction in speed is due to the branching. It may be that both are the coincidentally the result of a yet to be understood physical process. We frequently observe upward propagating positive leaders from tall objects (to be discussed later) remain unbranched and brightly luminous until reaching cloud base; the presumed location of a negatively charged screening layer. Upon reaching this layer, which acts as a negative potential well favorable for positive leader propagation, the upward positive leader often transitions to horizontal propagation and branches while slowing down. Other times, multiple positive leaders visible below cloud base will have individual branches that behave quite differently. Some branches will remain bright and not branch, while other spatially separate branches, which are still associated with the same flash, branch prolifically.
Which then brings us to one of the most interestingly unique behaviors exhibited by positive leaders. For reasons that are still not well 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 thermally ionized plasma 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. 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 conductive, current carrying positive channel. Upon connection, the negative end of the recoil leader will experiencing a reduction in resistance (short circuit) that causes rapid electron acceleration toward the branch point and this “zone of accelerating electrons” causes increased collisional thermalization (temperature and brightness increase). This zone of reionization travels away from the branch point along the recoil leader original path toward the decayed branch tip. The video below shows three recoil leaders that form along a decayed upward positive leader branch from an upward flash. The recoil leaders unsuccessfully attempt to reestablish the decayed branch in a sustained reionized state.
Now that we have defined recoil leaders, let’s look at some examples of positive leaders that branch and observe the recoil leader development that results.
The following is a positive ground flash from the vault region of a severe thunderstorm. The video recorded at 7,200 ips shows the highly branched and weakly visible positive leaders as the proceed downward toward the ground. You will notice that just prior to the connection with ground, the positive leader brightens and no longer produces recoil leaders. A time-integrated video “still image” of the flash is shown after the video.
The following video shows positive leader associated with a cloud flash recorded at 1,000 ips. Three separate main positive leader branches develop and propagate from the right to left. The upper positive leader is highly branched, weakly luminous and produces many recoil leaders. The middle branch is brighter, branches less and only produces a few recoil leaders. The lower branch is bright and does not branch nor produce recoil leaders. A time-integrated still image follows the video.
Upward flashes, which will be discussed in a separate section, typically produce upward propagating positive leaders. Below are high-speed videos of highly branched upward propagating positive leader development and associated recoil leader development. You can see that the recoil leaders develop back from the tip of the decaying 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. Furthermore, the tips of negative leaders tend to change direction erratically compared to positive leaders. 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.
Below is a high-speed video of negative leader development recorded at 7,200 ips. Notice the erratic directional change of the leader tips compared to the positive leaders shown previously. The arrows point out reionization that occurs on decayed negative leader branches. As will be shown, this redevelopment occurs very differently than that seen with decayed positive leaders. The video is followed by a time-integrated still image.
Bright negative leaders tend to branch and the branches do not decay as frequently as positive leaders. When a negative branch does decay, occasionally the branch will exhibit reionization, but interestingly, this reionization process initiates at the branch point and not along the decayed leader branch near the tip like that seen with 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 bidirectional positive breakdown travels simultaneously from the branch point back along the main negative channel. Two examples 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 process of positive (red) and negative (blue) leaders.
To summarize the unique characteristics of negative leaders:
- Exhibit clear stepping leader growth behavior and erratic directional change compared to positive leaders.
- Frequently branch, but branches remain bright (especially branch tips) and tend not to decay.
- Redevelopment on decayed branches occurs much less frequently and typically initiates at decayed branch point with a forward propagating reionization front that travels the length of the decayed branch.
- Frequently, continued and sustained growth of the decayed negative branch follows reionization unlike positive leaders.
As stated previously, the reason for the difference in decayed lightning leader branch reionization behavior is unknown. One theory suggests that a difference in branching density between positive and negative leaders leads to differential electric field shielding creating a more favorable electric field environment for recoil leader formation on decayed positive branches. In the next sections, we classify the different types of lightning flashes and look at how the different leader behavior leads to unique flash components.
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