'Whistling' Volcanic Lightning Heard Halfway Around the World
Lightning strikes unleash intense bursts of visible light and very-low-frequency (VLF) radio waves, among other kinds of energy. With a VLF receiver, anyone can listen to the constant chatter of Earth's lightning, estimated at 8 million strikes every day. (Not every lightning bolt becomes a whistler.)
A worldwide listening network is tuned to one particular lightning sound, called whistlers. These eerie electronic signals supposedly got their name from soldiers, who compared the sound to falling grenades. Modern ears might liken whistlers to a video game's "pew-pew-pew" soundtrack.
Whistlers are pulses of VLF radio energy that have traveled into space, leaping from one side of Earth to the other along the planet's magnetic field lines. Scientists monitor whistlers because the beautiful noise tells them about the planet's protective bubble of charged particles, called the plasmasphere. Whistlers on Venus and Jupiter suggest lightning also crackles on other planets.
Now, however, researchers have also linked a flurry of whistlers detected in Dunedin, New Zealand, to processes deep inside the Earth. For the first time, scientists have connected whistlers to volcanic lightning, according to a study published July 2 in the journal Geophysical Research Letters.
"I think it's really cool," said Jason Bortnik, a researcher at the University of California, Los Angeles, who was not involved in the study. "We're establishing a new connection between deep Earth and space."
Sing the Earth electric
On a busy day for space music, whistlers may zap Dunedin, on the South Island, around 1,000 times. But the lightning's not local. Dunedin has stormy weather — it's far enough south (and cold enough) to host a penguin colony — but lightning there is rare, said lead study author Claire Antel, a graduate student in physics at the University of Cape Town in South Africa.
Instead, Dunedin's whistlers (which are radio waves from lightning) arrive from space along one of Earth's magnetic field lines.
Some of Dunedin's whistlers originate in thunderstorms offshore Central and North America, according to earlier studies by Andrew Collier, a physicist and scientist at Exegetic Analytics in Durban, South Africa, who is also a co-author on the new study. The energy pulse zooms outward, and then arcs back down on the far side of the planet, traveling thousands of miles in a matter of seconds. As it travels, the energy disperses, with high frequencies arriving faster than low frequencies. This spreads out the pulse so it arrives sounding like a whistle, with a descending tone.
A spectrogram of whistlers recorded in Dunedin, New Zealand.Credit: Claire Antel
But Dunedin's magnetic conjugate point — the spot where the magnetic field line arcing out of New Zealand bends back down into Earth — is located in Alaska's Aleutian Islands, Antel said. And that fact helped solve a whistler mystery.
On July 12, 2008, Collier and his colleagues detected an astounding peak in Dunedin's whistler activity, which initially defied explanation. The network picked up more than 15,000 whistlers that day, and the researchers found even more when they pored over the records.
Seeking a source for the spike in whistlers, Collier sent Antel searching through records of lightning strikes and volcanic eruptions. She found a match in the Aleutian Islands.
The Kiwi connection
Alaska's Mount Okmok erupted on July 12, 2008. Within 35 minutes, the whistler count at Dunedin started rising, Antel and her co-authors reported. The network recorded more than 21,000 whistlers within 10 hours of the eruption. After Okmok's ash plume collapsed, cutting off the lightning, the whistlers quieted.
"We've solved the Dunedin whistler mystery," Antel told Live Science.
Another whistler spike hit Dunedin starting March 23, 2009, when Mount Redoubt erupted several times before quieting on April 4. But not every volcanic blast in Alaska blew the whistle in Dunedin, Antel discovered. There were just a few whistlers from an eruption at Kasatochi Island on Aug. 7, 2008.
Mount Okmok erupting on Aug. 2, 2008.Credit: Janet Schaefer, Alaska Volcano Observatory
The differences could be due to eruption size, Antel said. Volcanic lightning isn't as well understood as lightning produced in storms, but small volcanic eruptions are less likely to generate the electric charges that lead to lightning. The biggest volcanic blasts bear enormous bolts in their towering clouds, similar to the massive thunderclouds that menace the U.S. Midwest in summer.
Antel thinks there's potential for probing volcanic lightning with whistlers, and perhaps even monitoring Earth and other planets for new eruptions. "Okmok and Redoubt might be very special, or maybe volcanoes are very good at generating whistlers," she said. "Maybe we can use whistlers to provide evidence of volcanoes on other planets. It's still unclear how far we can take this."
However, the worldwide lightning-tracking network already picks up volcanic lightning, so it's unlikely that listening to whistlers would boost eruption monitoring, said Robert Holzworth, a professor of earth and space sciences at the University of Washington in Seattle, who was not involved in the study.
"Right now, it's in the 'interesting phenomenon' category," Holzworth said. "The most important implication of studying whistlers is monitoring space weather and gaining good, up-to-the-minute knowledge of what's going on in space."
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