1 February 2021
AGU press contact:
Liza Lester, +1 (202) 777-7494, [email protected] (UTC -5:00)
Contact information for the researchers:
Rubin Jiang, Chinese Academy of Sciences, Beijing, China, [email protected] (UTC +8:00)
Abhay Srivastava, North Eastern Space Applications Center, Meghalaya, India [email protected] (UTC +5:30)
Caught on high-speed video, lightning streamers of opposite polarity approach and connect in this sequence of video frames, slowed more than 10,000-fold. The common streamer zone appears in the last two frames before the whiteout of the lightning flash. The real time length of this gif is about 0.00003 seconds (30 microseconds).
Credit: Jiang et al/Geophysical Research Letters/AGU.
WASHINGTON—Scientists have used high-speed video to capture a fleeting moment of connection that sets the path of a lightning flash.
A new study reports two consecutive images, each representing 1/380-thousandth of a second, illuminating how downward-questing lightning fingers from a lightning cloud, called lightning leaders, and upward reaching leaders from the ground, make the final jump to meet in a lightning flash.
Many lightning leaders coming from thunderclouds seek the connection to the ground, but generally only one lightning channel will form, and its path is unpredictable. The new study reports the best resolution detection of the connection process to date.
“It is a very important issue because, in lightning development, there are a lot of branches. The target of the lightning strike is not determined at the beginning when it initiates from the cloud. The attachment process is the process that eventually determines the object that’s struck by the lightning flash,” said Rubin Jiang, an atmospheric physicist at the Chinese Academy of Sciences Laboratory for Middle Atmosphere and Global Environment Observation and a co-lead author of the new study published today in AGU’s Geophysical Research Letters, AGU’s journal for high-impact, short-format reports with immediate implications spanning all Earth and space sciences.
“Our study makes more specific the breakthrough phase. That means when two lightning leaders get close enough to each other and they have not completely connected. In lightning research history it is the so-called ‘final jump,’” Jiang said.
This moment of attachment happens so quickly scientists have not had a clear picture of whether several close lightning leader streamers coalesce and shrink into a single channel, like laced fingers clasping, or single positive and negative streamers meet, like Michelangelo’s famous fresco of the creation of Adam, and become the sole channel that heats up to the lightning flash.
Competing scenarios for the final jump that connects negative lightning leaders from a cloud (blue) and positive leaders from the ground (red). Branching streamers burst from the tips of the leaders. Left, nearby streamers coalesce in a common streamer zone. Right, single streamers meet and expand in a common streamer zone, while the losers fade away.
Credit: Jiang et al/Geophysical Research Letters/AGU.
The new study suggests the single connection of the second scenario is more likely.
“Until now, by the natural lightning observation only one frame is observed of this phase, because it is happening in microseconds,” said Abhay Srivastava, a research scientist at the North Eastern Space Applications Centre in Meghalaya, India, and co-lead author of the new study. “We observed more than one frame, so we can separate out what is happening over that final jump.
Connection in a flash
The visible flash of lightning is the culmination of a process that begins when negative charge builds in a thundercloud. The concentration of negative charge overhead induces a complementary positive charge on the ground. Cloud-to-ground lightning is the flow of electricity between these fields. It requires an intense build-up of charge because air is not a good conductor.
The build-up in the cloud charges nearby air molecules, creating a more conductive medium. The charged cloud sends out faint, low-current “stepped leaders” that can jump tens of meters in microseconds, branching frequently. Bursts of streamers from the leader tips initiate each step, and this streamer zone is believed to make first contact with counterparts from the ground.
As the negatively charged leaders near the ground, they call leaders upward from positively charged objects on the ground. Scientists call the moment of contact the breakthrough phase. At that moment, the path of the lightning flash is set. A low-current common streamer zone forms and quickly heats to a plasma channel that can carry a high current of charged particles and the bright return stroke from the ground lights the sky in a lightning flash. An open question is whether this contact is made by one streamer, or many converging on a single path.
High-speed video frames catch the moment of connection between a negatively charged lightning leader reaching down from a cloud and positively charged leader reaching up from the tip (blue triangle) of a 325-meter meteorology tower in Beijing, China. Each frame lasts 2.63 microseconds (0.0000263 seconds). Frames (b) and (c) show the common streamer zone that sets the path of the lightning strike in frames (d) and (e).
Credit: Jiang et al/Geophysical Research Letters/AGU.
The new study caught this breakthrough phase of a lightning strike on a 325-meter (1000-foot) meteorology tower in Beijing, China, on high-speed video. Two frames showed downward and upward leaders formed a common streamer zone when they came within 23 meters (75 feet) of each other. A single luminous thread in this zone suggested first contact between streamers of upward and downward leaders, according to the authors.
This thin connection grew into the lightning discharge channel, lending support to the idea that the early contact between streamers of upward and downward lightning leaders sets the route of the lightning flash, but the authors would need more observations to confirm it.
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Notes for Journalists
This research study will be free available for 30 days. Download a PDF copy of the paper here. Neither the paper nor this press release is under embargo. Full video and image files are available for download.
Paper title:
“Fine structure of the breakthrough phase of the attachment process in a natural lightning flash”
Authors:
Rubin Jiang, Shanfeng Yuan, Hongbo Zhang, Zhuling Sun, and Dongfang Wang, Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Science, 6 Beijing, China
Abhay Srivastava, Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Beijing, and North Eastern Space Applications Centre, Department of Space, Meghalaya, India
Xiushu Qie, Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO) and College of Earth and Planetary Science, University of Chinese Academy of Science, Beijing, China
Caixia Wang, College of Science, Beijing Information Science and Technology University, Beijing, China
Guanlin Lv and Zongxiang Li, College of Electronic Engineering, Chengdu University of Information 13 Technology, Chengdu, China