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Lightning is a common atmospheric event. Follow UK Met Office, there are three million lightning flashes per day on Earth, or about 44 flashes per second. Even so, scientists have yet to conclude exactly how lightning forms inside a thunderstorm cloud. That could change, however, with the help of new video recorded by a team of researchers.
In 2018, a very important lightning flashed through the Low Frequency Array (LOFAR) radio telescope array in the Netherlands. These telescopes have recorded details that have only recently been processed. A new one paper due to be published in the magazine Geophysical research letter, outlining what causes lightning.
‘What a shame. It’s the most energetic process on the planet, we have religions centered around this thing and we don’t know how it works,” Brian Hare, a lightning researcher at the University of Groningen and co-author of the new research paper. Joseph Dwyer, a physicist at the University of New Hampshire and co-author of the new paper said: ‘People have been sending balloons, rockets and planes into thunderstorms for decades and have never seen one. The electric field is anywhere large enough”. ‘It’s a real mystery how this happened.’ Dwyer has been studying lightning for 20 years.
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| LOFAR ‘superterp’. This is part of the core of an extended telescope located near Exloo, Netherlands. Photo by LOFAR / ASTRON, used under CC BY 3.0 |
For such a common phenomenon, and one that has been seriously studied for hundreds of years, how does it take so long to gather good data? The clouds are opaque, for starters, and even advanced cameras struggle to see through the clouds and witness the lightning forming. Scientists have ventured into the storm to try to find out. However, ‘For a long time, we really didn’t know the conditions inside a thunderstorm at the time and place where lightning started,’ Dwyer added.
Dwyer and the rest of the team used LOFAR. Network radio telescopes are commonly used to survey distant galaxies and stars, but are also well suited for observing lightning. When a thunderstorm is heard, LOFAR cannot observe deep space, so instead, LOFAR is tuned to detect radio pulses originating from lightning flashes.
This is not the first time radio detectors have been used to observe lightning, but LOFAR is a modern tool and can be much faster, about 200 times faster than devices used in the past. It can also map lightning in three dimensions. ‘LOFAR measurements are giving us the first really clear picture of what’s happening inside the thunderstorm,’ Dwyer said.
When a lightning bolt forms, it generates millions of radio pulses. To turn this huge data collection from thousands of LOFAR antennas into a 3D image requires complex algorithms. After analyzing data from August 2018, the team determined that the radio pulses supported one of two leading theories about how common types of lightning form. The new observations support the idea that lightning starts from ice crystals inside clouds. As Quanta magazine put it, ‘It starts with clusters of ice crystals inside the cloud. The chaotic collisions between the metallic crystals remove some of their electrons, leaving one end of each ice crystal positively charged and the other negatively charged. The anode tip attracts electrons from nearby air molecules. More electrons flow in from more distant air molecules, forming bands of ionized air extending from each end of the ice crystal. These are called streaming players. ‘
Each crystal head creates a ‘cluster of emitters’ and these emitters continue to branch out over and over again. The emitters heat the surrounding air, then pull electrons from the air molecules, and thus a larger current flows to the ice crystals. When a streamer becomes hot enough and conductive enough, it becomes the leader, by which the lightning can travel. Here is an animated GIF of the data recorded with the array:
“This is what we are seeing,” said Christopher Sterpka, first author of the new paper. ‘After the avalanche stopped, we saw a lightning leader nearby.’
Surprisingly, there is a bit of overlap between lightning and COVID research. Recent findings observed Lightning activity fell by more than 10% in the first few months of the ongoing pandemic. The researchers believe this is due to the lockdown because people travel less and there are fewer pollutants in the atmosphere, which reduces nucleation sites for ice crystals.
New research has failed to convince all scientists to believe in a rival theory that cosmic rays from outer space collide with electrons inside thunderstorms. Perhaps cosmic rays still have a role to play.
Ute Ebert, a physicist at Eindhoven University of Technology in the Netherlands, studies the onset of lightning but was not involved in the new study. She notes that despite the high-resolution data provided by LOFAR, the new paper does not show ice crystals ionizing the surrounding air. ‘Where did the first electron come from? How does an electrical discharge start near an ice particle? ‘ There’s also the issue of exactly how streamers become leaders. The emitters become hot and conductive, but the exact sequence of events is not resolved.
