Mark Megahan
The formation of a serendipitous lightning bolt, forming inside a thundercloud parked right above their telescopes, revealed something no one has seen before. A network of radio telescopes in the Netherlands made detailed recordings during the 2018 summer storm. It took a while to process the data but the recently published results were worth the wait. The whole enchilada is due to be released soon in the journal “Geophysical Research Letters.”
Lightning debate settled
Well, that’s that, the researchers declare. Their observations “settle a long-standing debate about what triggers lightning.” That split instant when “bolts arise, grow and propagate to the ground” has been a mystery until now.
“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,” Brian Hare insists. He studies the spectacular discharges from the warm, cozy labs at the University of Groningen.
We teach kids in elementary school that lightning starts inside a thundercloud when ice crystals rise and fall back as hail.
“The hail rubs off the ice crystals’ negatively charged electrons, leading the top of the cloud to become positively charged while the bottom becomes negatively charged.” Like the two plates of a gigantic capacitor, this “creates an electric field that grows until a gigantic spark jumps across the sky.”
The thing they don’t tell the kiddies is that they really don’t know how. The electric fields inside clouds are about 10 times too weak to create sparks.” Lightning shouldn’t happen.
“People have been sending balloons, rockets and airplanes into thunderstorms for decades and never seen electric fields anywhere near large enough,” explains Joseph Dwyer, a physicist at the University of New Hampshire and a co-author with Brian Hare on the new paper.
LOFAR wasn’t loafing
The team found out by accident that the Low Frequency Array (LOFAR) “just so happens to work really well for measuring lightning, too.” Usually the “network of thousands of small radio telescopes,” mostly in the Netherlands, “usually gazes at distant galaxies and exploding stars.”
That means it’s hooked up to some amazing computing power. Astronomers can’t do a thing in a thunderstorm. So, while they went for a beer, Dwyer and Hare got to work.
They tuned “its antennas to detect a barrage of a million or so radio pulses that emanate from each lightning flash.” Radio antennas have been used for that purpose before but can only produce a 2-D map in low resolution.
LOFAR, on the other hand, “can map lighting on a meter-by-meter scale in three dimensions, and with a frame rate 200 times faster than previous instruments could achieve.” They were shocked. “The LOFAR measurements are giving us the first really clear picture of what’s happening inside the thunderstorm,” Dwyer beams.
Once the computers got done crunching the numbers for display, the team was awestruck. It’s all there in every detail. Lightning “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.”
The really cool part is that each “crystal tip gives rise to hordes of streamers, with individual streamers branching off again and again. The streamers heat the surrounding air, ripping electrons from air molecules en masse so that a larger current flows onto the ice crystals. Eventually a streamer becomes hot and conductive enough to turn into a leader — a channel along which a fully fledged streak of lightning can suddenly travel.”
