Volcanoes! Lightning! And Radioactive Gas, Too

As plumes of ash rise from a volcano, volcanic lightning sometimes erupts within its clouds. As it pierces through the billowing darkness, resembling a supernatural rage, the lightning’s incandescent flashes warn volcanologists far away that a potentially dangerous tower of ash is skyward.

In order to get lightning, the plume must be electrified. Generally, it’s assumed that ash particles jostling within the volcanic tempest build up huge electrical imbalances that can only be neutralized by supercharged bolts. But something else may contribute to the electrification of volcanic plumes.

By sending balloon-lofted electricity-detecting instruments over Sicily’s hyperactive Stromboli volcano, scientists discovered that even volcanic clouds that lack ash contain substantial amounts of charge. The source of this phantom electrification could be the decay of volcanic radon, an odorless and colorless radioactive compound.

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A similar process may occur above radon-emitting volcanoes around the world, said Karen Aplin, a physicist at the University of Bristol’s Department of Aerospace Engineering and a co-author of the study published this month in Geophysical Research Letters. If radon-driven electrification influences the behavior and lifetime of volcanic ash plumes — the very same that can down planes, bury cities and suffocate farms — then understanding the gas’s role could help people downwind from an eruption be better prepared.

It’s well-known that the friction between volcanic ash generates huge amounts of electrical charge. This process, known as triboelectricity, is the same reason that you can rub a balloon on your head and marvel as it sticks there, hands-free.

Ice particles bumping into ash particles also aids electrification, as does lava tearing itself apart. The simultaneous interaction of these processes makes studying volcanic lightning and unraveling its enigmatic intricacies difficult.

To simplify things, the authors of this study tried something new.

They wanted to measure the charge in an ash-free plume, and Stromboli — which has spectacular lava fountains matched only by its extremely gassy outbursts — was the perfect place for the experiment.

According to their calculations, the charge the researchers detected within these volcanic belches could easily be generated by the decay of radon, a significant component of the gases emitted by the volcano’s activity. It’s well-established that as radon decays, it releases charged particles that collide with compounds in the air, ionizing them. This, the researchers suspect, was electrifying the ash-free plume.

“We can’t be sure that it is radon which is generating the charge in such plumes without many further measurements,” said Keri Nicoll, an atmospheric scientist at the Universities of Reading and Bath and the study’s lead author. But she added that it’s very likely that “there are no other known charge generation mechanisms that can explain our observations.”

While it contributes, radon is probably not the sole arbiter of volcanic lightning. It certainly generates charge, but as Dr. Aplin explained, “it’s unlikely that the radon mechanism alone could ever be strong enough to generate volcanic lightning without the help of ash.”

Cassandra Smith, an expert on volcanic lightning at the University of South Florida who was not involved in the study, agrees. She pointed out that no lightning was generated by that radon-rich, ash-free plume over Stromboli. She has also seen lightning appear during eruptions at Sakurajima volcano that feature extremely small amounts of ash.

With all this in mind, Dr. Smith speculated “that ash charging mechanisms are still dominant for lightning generation in volcanic plumes.” But there will be future opportunities to quantify radon’s contribution to plume electrification and volcanic lightning.

Oliver Lamb, a volcanologist at the University of North Carolina at Chapel Hill who was not involved in the study, said that radon escapes from volcanoes “in all corners of the world,” including Chile’s Villarrica, Guatemala’s Fuego, Japan’s Sakurajima and Nicaragua’s Masaya.

While more work remains to be done, Dr. Lamb said that the radon mechanism “does look very promising.”

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