Corrado Cimarelli
Much of the lightning that flickers around and within the ash plumes of erupting volcanoes is triggered by static electricity, which builds up when ash particles scrape against each other in flight. Now, a field study suggests Earth’s natural radioactivity may also help volcanic plumes get electrically charged—even when those clouds contain little or no ash.
Scientists have long known that radon, a radioactive gas, is a part of the plumes that spew from active volcanoes. When those radioactive atoms decay, they emit charged particles and create “daughter” elements that also decay and emit charged particles of their own. In the fall of 2017, using balloon-borne instruments (pictured above) lofted from the peak of Stromboli—an active volcano on an island near the toe of Italy’s “boot”—researchers measured how much electrical charge builds up in an eruption plume for the first time.
In some parts of the eruption cloud, the numbers of charged particles per cubic meter were at least 80 times the numbers found in a typical cloud on an overcast day, the team reports this month in Geophysical Research Letters. Data also reveal that positive and negative charges migrate to different parts of the eruption plume, setting up voltage differences. Those differences aren’t strong enough to trigger lightning by themselves, but in ash-filled plumes they may either slightly add to or diminish the charge differences generated by static electricity, the researchers note. It’s not yet clear, they add, how such changes would influence the strength, frequency, or brightness of volcanic lightning.