So Dwyer and his team turned to the Low Frequency Array (LOFAR), a network of thousands of small radio telescopes mainly in the Netherlands. LOFAR often looks at distant galaxies and exploding stars. But according to Dwyer, “it also works very well for measuring lightning.”
When a thunderstorm hits, there is little useful astronomy that LOFAR can do. So instead, the telescope tunes its antennas to detect a stream of radio pulses of about a million or so emitted from each flash. Unlike visible light, radio pulses can pass through thick clouds.
Using radio detectors for lightning mapping is not new; purpose-built radio antenna has Long-observed hurricane in New Mexico. But those images are low resolution or only in two dimensions. LOFAR, a state-of-the-art astronomical telescope, can map light on a meter-by-meter scale in three dimensions and at a frame rate 200 times faster than previous instruments could achieve. “LOFAR measurements give us the first really clear picture of what’s going on inside a thunderstorm,” says Dwyer.
A real lightning bolt generates millions of radio pulses. To reconstruct a 3D image of lightning from the data, the researchers used an algorithm similar to the one used during the Apollo moon landings. The algorithm continuously updates what it knows about the object’s location. While a single radio antenna can only indicate the raw direction of the flash, adding data from the second antenna will update the position. By steadily iterating over LOFAR’s thousands of antennas, the algorithm creates a clear map.
When the researchers analyzed data from the August 2018 lightning, they found that the radio pulses were all coming from a 70-meter-wide area deep within the storm cloud. They quickly deduced that the pulse model supported one of the two leading theories about how the most common type of lightning started.
An idea suggested that cosmic rays — particles from outer space — collide with electrons inside thunderstorms, causing an electron avalanche that enhances the electric field.
New observations indicate rival theory. It starts with clusters of ice crystals inside the cloud. The chaotic collisions between 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, with individual emitters branching over and over. The emitters heat the surrounding air, ripping electrons from the air molecules so a larger current flows into the ice crystals. Ultimately, a streamer becomes hot and conductive enough to be a leader—a channel along which a complete lightning streak can suddenly move.
“This is what we are seeing,” says Christopher Sterpka, first author on the new paper. In a video showing the beginning of the flash that the researchers generated from the data, the radio pulses grew exponentially, possibly due to the overflow of transmitters. “After the avalanche stopped, we saw a lightning leader nearby,” he said. In recent months, Sterpka has compiled more films about lightning starts that look similar to the first.
