12 April 2016
By Lauren Lipuma
Hot ash, gas and lava spewed into the atmosphere by volcanoes can block out the sun, down planes and bury entire towns. As if that weren’t ferocious enough, the most violent volcanic eruptions also generate lightning.
Humans have observed volcanic lightning for thousands of years – Pliny the Younger even mentioned it when he described the eruption of Mt. Vesuvius that destroyed Pompeii in 79 AD. But scientists have had trouble studying this phenomenon because the flashes last only for milliseconds and thick clouds of ash can hide it from view.
Now, two new studies published in Geophysical Research Letters, a journal of the American Geophysical Union, are unraveling some of the mysteries of this violent process, and revealing the similarities – and differences – between volcanic lightning and the kind of lightning produced by thunderclouds.
The new studies show that both the eruption of the volcano and the presence of ice particles in the eruption’s plume play a role in generating volcanic lightning.
Understanding how this process works could enable scientists to use volcanic lightning to monitor and track the progress of powerful eruptions in real time, according to the studies’ authors. This method could help monitoring agencies detect eruptions at remote volcanoes more quickly than traditional methods, and help forecasters predict how the ejected ash and gas will travel through the atmosphere, they said.
“Anything that helps give us an earlier warning of explosive activity – particularly for remote volcanoes – is going to make our airways a little bit safer,” said Alexa Van Eaton, a volcanologist at the U.S. Geological Survey Cascades Volcano Observatory in Vancouver, Washington, and lead author of one of the studies. “Earlier warning means a faster ash dispersal forecast. We’re finding that it’s possible to detect eruptions earlier with volcanic lightning than is possible with satellite detection alone.”
How does lightning form?
In storm clouds, lightning forms when ice particles collide with each other and become electrified. But how lightning forms in volcanic plumes is less clear.
Scientists know that when volcanoes erupt, magma explodes from below the Earth’s surface and particles of ash split apart. The particles also scrape against each other and create friction. These processes – referred to as near-source charging processes – create a buildup of electric charge.
“There is no lightning in a regular storm without ice,” Van Eaton said. “But when it comes to volcanic plumes, there have been bipolar views that either it’s only the ash that’s causing the charge … or the ice is the only thing that’s causing the charge.”
The results of these two new studies help to resolve these opposing views.
In one of the new studies, a team of researchers recorded high-speed video observations of volcanic lightning at Sakurajima volcano on the island of Kyushu, Japan, which has been erupting persistently since 2009.
The researchers synchronized high-speed video footage of Sakurajima erupting with data from nearby acoustic sensors and electromagnetic field measurements, which allowed them to better understand how electrical discharges were happening within the plume. Based on these observations, they concluded that near-source electrification is common to all volcanic eruptions and that ice crystals aren’t needed to generate lightning.
The study shows that creation of electrical charge is a fundamental property of volcanic plumes, according to Corrado Cimarelli, a volcanologist at Ludwig Maximilian University in Munich, Germany and lead author of the study. “Where there is ash violently ejected in the atmosphere, there will be electrical discharges, independent of the magnitude of the eruption,” he said.
Lightning due to near-source charging is a better way to monitor volcanoes because it is linked to the presence of ash in the atmosphere and happens at early stages of plume formation, he added.
Footage of volcanic lightning at Sakurajima taken by Cimarelli’s research team.
The researchers mapped out lightning from Calbuco’s eruption while tracking the movements of the volcanic plume. Far downwind of the volcano, lightning strokes followed the movements of the very top of the ash plume where it was cold enough for ice crystals to develop, according to Van Eaton.
“The lightning basically decoupled from all the other ash particles that were falling to the ground,” she said. “Instead, they seem to follow the ice crystals that stayed high in the atmosphere.”
There was also a surge in lightning during the second phase of the eruption when the ash and gas traveled as a ground-hugging flow, rather than rising vertically into the atmosphere. According to Van Eaton, this means that ash particle collisions inside the flow created a layer of charge on the ground strong enough to generate lightning without the presence of ice.
How important is ice for generating lightning?
The Calbuco and Sakurajima studies show that both ice particles and the near-source charging processes are important for generating volcanic lightning, according to the researchers.
“What we can clearly say is that the electrification of the volcanic plume starts immediately,” she said. “However, the evolution of charge, in a very big eruption, tends to be enhanced by the development of ice.”
It would be interesting to apply both research team’s approaches to monitor one volcanic eruption, she added. “We just all need to be at a big eruption at the same time.”
— Lauren Lipuma is a public information specialist and writer at AGU. You can follow her on twitter at @lipumal.