Therefore, Dwyer and his team turned to the Low Frequency Array (LOFAR), a network of thousands of small radio telescopes, mainly located in the Netherlands. LOFAR usually stares at distant galaxies and exploding stars. But according to Dwyer, “it happens to also be suitable for measuring lightning.”
When thunderstorms pass overhead, LOFAR has little useful astronomy. Therefore, the telescope adjusts its antenna to detect approximately one million radio pulses emitted by each lightning bolt. Unlike visible light, radio pulses can pass through thick clouds.
The use of radio detectors to map lightning is nothing new.Dedicated radio antenna has Long-observed storms in New MexicoBut these images have low resolution or only two-dimensional. LOFAR is the most advanced astronomical telescope that can draw illumination maps in three dimensions on a meter-to-meter scale, and the frame rate is 200 times faster than previous instruments. “LOFAR measurements provide us with the first truly clear picture of what is happening inside the thunderstorm,” Dwyer said.
A physical lightning can generate millions of radio pulses. In order to reconstruct the 3D lightning image from the chaotic data, the researchers used an algorithm similar to that used in the Apollo moon landing. The algorithm continuously updates the known information about the location of the object. Although a single radio antenna can only indicate the general direction of the flash, adding data from the second antenna will update the position. By stably looping through thousands of LOFAR antennas, the algorithm constructs a clear map.
When the researchers analyzed the lightning data in August 2018, they found that all radio pulses came from a 70-meter-wide area deep in the storm cloud. They quickly deduced that the pulse pattern supports one of the two main theories about how the most common types of lightning started.
An idea It is believed that cosmic rays-particles from outer space-collide with electrons in thunderstorms, triggering an avalanche of electrons and strengthening the electric field.
The new observations indicate Competition theory. It starts with clusters of ice crystals in the clouds. The turbulent collision between the needle-like crystals wiped away some of their electrons, making one end of each ice crystal positively charged and the other negatively charged. The positive electrode draws electrons from nearby air molecules. More electrons flow in from more distant air molecules, forming bands of ionized air that extend from the tip of each ice crystal. These are called streamers.
Each crystal tip produces clusters of streamers, and individual streamers branch again and again. The streamer heats the surrounding air, extracting electrons from the air molecules, so that a greater current flows to the ice crystals. Eventually, a streamer becomes hot enough and conductive enough to become a leader—a fully mature lightning can suddenly propagate along this channel.
“This is what we see,” said Christopher Stepka, The first author of the new paper. In the film showing the start of the flash light made by the researchers based on the data, the radio pulse increases exponentially, probably because of the large number of streamers. “After the avalanche stopped, we saw a lightning leader nearby,” he said. In recent months, Sterpka has been compiling more similar Lightning Enlightenment movies.