On the evening of July 17th, 2023, I witnessed the most spectacular upward lightning flash of my research career. What made it the most spectacular was not only the behavior of the upward flash but how I was able to observe it and analyze it.
Title of a Book – a single digital still image
The digital still camera that recorded the flash was from a 10 second exposure, so the entire flash occurred in that 10 second span. You see the total accumulation of light emitted by the flash and captured by the camera sensor. Bright lightning leaders extend from three towers up towards the overlying clouds and turn horizontally and branch prolifically.
Table of Contents – standard-speed video recording
An Ultra-High Definition (3840×2160 pixels) video camera recorded the flash at 30 images per second. Viewing the recording shows the rapid development of multiple leaders that appear attached to towers on the ground. These leaders spread horizontally near the top of the camera frame exhibiting intense brightness, branching and flickering. It lasts just over half a second, and it is difficult to tell that the leaders travel upward from the towers.
Text of a Novel – high-speed video recording
A Vision Research Phantom v2012 high-speed camera recorded the flash at 50,000 images at 768×368 pixels. Playing the recorded video back at 30 images per second, the 0.6 second flash takes nearly 9 minutes to watch. In those 9 minutes you see a novel play out before you. There are multiple chapters…
- Triggering Flash and +CG Return Stroke
- Upward Leader Initiation and Initial Growth
- Recoil Leader Development
- Recoil Leader Connections
- Establishment of Continuing Current
- Flash Decay
…and within each chapter are storylines, different characters and plot twists.
The story begins with a triggering lightning flash that has one end of its bipolar leader development (the positive end) reach down from the cloud and connect with the ground forming a powerful return stroke that briefly sends an estimated +77.9 kiloamps of peak current surging through the channel to ground. Although this happens behind the camera and out of the field of view. You see the foreground light up with blinding intensity. The electric field changed caused by this return stroke causes the initiation of upward positive leaders from three communications towers that due to their shape, enhance the electric field locally. As a result, leaders form from their tips and propagate upward towards the negative charge that has been increased overhead.
As the saturating bright light of the return stroke fades, the electric charge arrangement has now been significantly changed and the upward positive leaders continue to grow in response to the electric field between their tips and the negative charge overhead introduced by the return stroke and continued negative leader growth that likely follows the return stroke.
As the positive upward leaders grow they begin to branch forming multiple branches and channel pathways. The branches remain relatively dim and soon bright, short segments start to form on the dim or invisible branches. These are recoil leaders. The are bipolar/bidirectional thermalized reionization along the decayed branches that attempts to reestablish the branches as hot, conductive, plasma channels.
Early on these bipolar recoil leaders form short luminous segments that die out. Some reach a branch point and travel out a different branch forming a check mark shape. As the flash progresses, the recoil leaders grow longer and their negative ends start to connect with either a luminous channel from which the decayed leader branched or they travel all the way back to the tower tip along a fully decayed channel and cause a bright return stroke that travels back up the recoil leader pathway. Upon reaching the tip of the decayed branch, the return stroke often extends the branch impulsively illuminating other decayed branches and promoting new positive breakdown at the tip. This process repeats seemingly endlessly as the branches extend further and further as a result of the recoil leader activity.
And then, one recoil leader travels back to a tower and causes a return stroke that does not fully decay. Instead, this channel remains illuminated in a process known as continuing current. The outer end of this illuminated channel is growing continuously as a positive leader causing current and therefore luminosity to continue along the entire length all the way to the tower tip. As long as the positive end of the leader continues to grow there will be current and luminosity along its entire length to ground. As with every story, however, and end must come and as the positive leader stops growing due to a decreasing electric field between its tip and the surrounding negative cloud and leader charge, the channel fades and dies. The upward lightning flash is over and the story concludes.
The orchestra of reillumination and decay that has played out on the branched upward leaders from the three different communications towers and resulting beautiful symphony of dynamic physical processes leave me in awe as I watch it play out over 9 minutes.
It was a novel definitely worth…”reading.” And my favorite to date.
Epilogue
Electrons freed from neutral nitrogen and oxygen molecules and atoms along the entire length of the plasma channel move in the direction away from the growing positive leader tip and toward the tower and ground. However, over the course of the entire flash they only travel 10s of centimeters in distance. Essentially, negative charge in the form of ionized electrons and negative ions shift along the entire channel toward the ground leaving the ground with more electrons than before the upward positive leaders developed. Hence, electric charge has been effectively lowered to ground due to the upward positive leaders. However, the preceding positive cloud-to-ground return stroke essentially did the opposite to cause the response seen from the towers. A +CG return stroke raises electrons and negative ions upward along the return stroke channel path and connected leader network formed prior to the return stroke. This shift of negative charge upward and over the towers caused an opposite response from the towers. The positive leaders from the towers acted to lower negative charge from the ionized neutral air in order to offset the negative charge raised by the triggering flash. Hence, these types of flashes are know as lightning-triggered upward lightning.
Lightning Data provided by Earth Networks, Inc., an AEM company.