A ground flash is simply a cloud flash in which one end of the bidirectional leader network connects with the ground. As with a cloud flash, the ground flash initiates between two in cloud charge regions. However, one end of the leader propagates beyond the initial attractive charge region and onward toward the inductively charged ground. As the leader approaches the ground, typically within 150 m, the electric field between the leader and ground strengthens to the point that an oppositely charged leader initiates from the taller objects and rises upward toward the descending leader. More than one upward leader may initiate, however, the first to connect with the downward leader will initiate the return stroke.
The return stroke results from the sudden decrease in resistance along the leader channel path and the difference in electrical potential between the two leaders. Upon connection, electrons rapidly accelerate causing intense heating and brightening of the connected channel. The zone of acceleration travels away from the connection point at approximately 1/3rd the speed of light in both directions along the joined leader paths. During the time that the return stroke traverses the previously formed leader network (typically 10s of microseconds), the channel temperature can rise to 30,000ºC and peak current achieve 10s to even 100s of kiloamps. Once the return stroke reaches the end of the leader network, the channel, if still connected to ground, achieves the electrical potential of the ground. The upper extent of the leader network may continue to grow following the return stroke if there is a large enough potential difference between the channel and the in cloud charge region. If this occurs, current continues along the entire channel path.
It is important to note that only a small portion of the bidirectional leader network is visible during a cloud-to-ground flash. In fact, typically only a portion of one end of the leader is visible.
Negative Cloud-to-Ground Flash (-CG)
A -CG flash has the negative end of the bidirectional leader connect with ground, and electrons accelerate downward during the resulting return stroke. On average, 90% of all CG flashes are negative. The importance of a smaller lower positive charge region that underlays the main negative charge rgion is apparent as it serves as a small positive potential well for the negative end of the leader to propagate toward and through before continuing toward the positively charged ground.
Once the return stroke reaches the upper extent of the leader network, the channel may fade, ending the flash. If the channel grows following the return stroke, the channel will remain luminous as current flows through the channel. In many cases, the channel path will decay, but then after a short delay, a decayed upper positive leader branch will initiate a fast bidirectional leader that attempts to reionize the decayed channel path. This type of leader is currently referred to as a recoil leader, and it is unclear how and why they initiate on decayed positive leader branches as similar behavior is not seen on decayed negative leader branches. Most often, recoil leaders fade shortly after forming, however, in some cases, the negative end of the recoil leader will continue down the entire decayed return stroke channel path all the way to ground and cause another return stroke. This return stroke / channel decay / recoil leader / return stroke process can be repeated multiple times during -CG flashes, and defines a multistroke CG flash. The flickering one sees when witnessing a -CG flash in real time is due to this repeated process.
The following are high-speed videos of -CG flashes.
Revisiting the Bolt From The Blue
Another type of -CG flash that is frequently misidentified as a +CG flash initiates between the two main charge regions of a storm. However, if the upper positive charge region is not as big as the lower main negative charge region, the negative end of the leader can fill the upper main positive potential well first. The negative end of the leader may then propagate outside the main updraft of the storm and and travel toward and connect with the ground well outside the storm. This type of flash has been referred to as a “Bolt From The Blue.”. Due to the initially incorrect concept that a leader emerges from a charge region with the same charge as that region, it was believed that the channels that extend outside the upper portion of a storm must be positive having emerged from the upper positive charge region. However, relatively recent research using 3-dimensional lightning mapping has shown this not to be the case.
However, as will be explained in the next section, there are situations in which channels that emerge from the upper part of the storm anvil region frequently result in +CG flashes.
Positive Cloud-to-Ground Flash
A +CG flash has the positive end of the bidirectional leader network descend toward and connect with the ground. Upon connection, electrons accelerate up the channel away from ground as opposed to down the channel toward ground in the -CG return stroke case. Since the typical storm charge tripole has a main positive over main negative over smaller positive region. The two charge regions the form the basis for the bidirectional leader development must be inverted from that required for a -CG flash. In some cases, storms due develop an inverted tripole charge arrangement, and a bidirectional leader develops between a main positive charge region overlaying a small negative charge region. In this case the process would be the same as a -CG flash, but with opposite polarities as shown in the figures below.
Figures – inverted storm +CG
However, if the storm has a normal polarity charge arrangement, +CG flashes can still occur, and this is sometimes seen in a storms vault region which is the area underlying the storm anvil downwind of the updraft. In this case the bidirectional leader will develop between the upper positive charge region and a smaller negative screening layer charge region. Due to smaller potential negative well presented by the screening layer, the positive end of the leader will continue past the screening layer and continue to the negatively charged ground. The following figures illustrate this process.
Highly branched positive leaders tend to be unstable and decay followed by repeated recoil leader development on decayed branches attempting to reionize the branches. The following is a +CG flash where highly branched positive leader descend from the vault region of a thunderstorm.
As in the case of the -CG, the return stroke channel may continue to grow following the return stroke, but in this case it will grow as an upward propagating negative leader. Unlike positive leader growth, negative leader growth tends to be more stable and long lived resulting in longer continuing current in the return stroke channels compared with the -CG flashes. In addition, decayed negative leader branches do not exhibit recoil leader development like decayed positive leader branches, and therefore, multiple return strokes are rarely observed in +CG flashes.
Below is a high-speed video of a +CG flash in which portions of both ends of the leader network are visible before and after the return stroke. After the return stroke the channel network continues to grow as an upward propagating negative leader.