Lightning is a widespread atmospheric event. Per the Met Office in the UK, there are three million flashes of lightning every day on Earth, or about 44 strikes per second. Despite this, scientists haven’t been able to conclude how exactly lightning forms inside a thundercloud. That could be changing though, with the help of new video recorded by a team of researchers.
In 2018, a very important lightning bolt flashed overhead the Low Frequency Array (LOFAR) radio telescope array in the Netherlands. These telescopes made detailed recordings that were just recently processed. A new paper due to be published in the journal Geophysical Research Letters, outlines what triggers lightning.
‘It’s kind of embarrassing. It’s the most energetic process on the planet, we have religions centered around this thing, and we have no idea how it works,’ said Brian Hare, a lightning researcher at the University of Groningen and a co-author of the new research paper. ‘People have been sending balloons, rockets and airplanes into thunderstorms for decades and never seen electric fields anywhere near large enough,’ said Joseph Dwyer, a physicist at the University of New Hampshire and a co-author on the new paper. ‘It’s been a real mystery how this gets going.’ Dwyer has been studying lightning for 20 years.
|The LOFAR ‘superterp’. This is part of the core of the 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 did it take so long to gather good data? Clouds are opaque, for starters, and even advanced cameras have struggled to see through the clouds and witness lightning forming. Scientists have ventured into the storm to try to figure out. However, ‘For a long time we really have not known what the conditions are inside a thunderstorm at the time and location that lightning initiates,’ added Dwyer.
Dwyer and the rest of the team employed LOFAR. The radio telescope network is typically used to investigate distant galaxies and stars, but it’s also well-suited to lightning observation. When there are thunderstorms overheard, LOFAR can’t observe deep space, so it’s instead tuned to detect the radio pulses that originate in lightning flashes.
It’s not the first time radio detectors have been used for observing lightning, but LOFAR is state-of-the-art and can is much faster, about 200 times faster, than previously-used instruments. It can also map lightning in three dimensions. ‘The LOFAR measurements are giving us the first really clear picture of what’s happening inside the thunderstorm,’ said Dwyer.
As a lightning bolt forms, it produces millions of radio pulses. To turn this huge collection of data from thousands of LOFAR antennas into a 3D image required sophisticated algorithms. After analyzing the 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 with ice crystals inside the cloud. As Quanta Magazine puts it, ‘It starts with clusters of ice crystals inside the cloud. Turbulent collisions between the needle-shaped crystals brush off some of their electrons, leaving one end of each ice crystal positively charged and the other negatively charged. The positive end draws electrons from nearby air molecules. More electrons flow in from air molecules that are farther away, forming ribbons of ionized air that extend from each ice crystal tip. These are called streamers.’
Each crystal tip produces ‘hordes of streamers,’ and these streamers continue to branch off repeatedly. The streamers heat up the surrounding air, which then pulls electrons from air molecules, and thus a larger current flows to the ice crystals. When a streamer becomes hot and conductive enough, it becomes a leader, along which lightning can travel. Below is an animated GIF of the data recorded with the array:
‘This is what we’re seeing,’ said Christopher Sterpka, the first author on the new paper. ‘After the avalanche stops, we see a lightning leader nearby.’
Surprisingly, there’s a bit of overlap between the lightning research and COVID. Recent findings observed that lightning activity dropped by more than 10% during the first few months of the ongoing pandemic. Researchers believe that this is due to lockdowns because people traveled less, and there were fewer pollutants in the atmosphere, which reduced possible nucleation sites for ice crystals.
The new research hasn’t dissuaded all scientists from believing a rival theory that cosmic rays from outer space collide with electrons inside thunderstorms. Perhaps cosmic rays still play a role.
Ute Ebert, a physicist at the Eindhoven University of Technology in the Netherlands, studies lightning initiation but wasn’t involved in the new study. She notes that despite the high-resolution data provided by LOFAR, the new paper doesn’t show ice crystals ionizing the surrounding air. ‘Where is the first electron coming from? How does the discharge start near to an ice particle?’ There’s also the matter of precisely how streamers turn into leaders. Streamers become hot and conductive, but the exact series of events is not settled.