Muons reveal the whopping voltages inside a thunderstorm

Physicists used subatomic particles to probe the inner workings of a cloud


STORM SURGE  Subatomic particles called muons can expose a thunderstorm (like this one) storing up a huge electric potential — more than a billion volts.

Ian Froome/Unsplash

An invisible drizzle of subatomic particles has shown that thunderstorms may store up much higher electric voltages than we thought.

Using muons, heavier relatives of electrons that constantly rain down on Earth’s surface, scientists probed the insides of a storm in southern India in December 2014. The cloud’s electric potential — the amount of work necessary to move an electric charge from one part of the cloud to another — reached 1.3 billion volts, the researchers report in a study published March 15 in Physical Review Letters. That’s 10 times the largest voltage previously found by using balloons to make similar measurements.

High voltages within clouds spark lightning. But despite the fact that thunderstorms regularly rage over our heads, “we really don’t have a good handle on what’s going on inside them,” says physicist Joseph Dwyer of the University of New Hampshire in Durham who was not involved with the research.

Balloons and aircraft can monitor only part of a cloud at a time, making it difficult to get an accurate measurement of the whole thing. But muons zip right through, from top to bottom. “Muons that penetrate the thunderclouds are a perfect probe for measuring the electric potential,” says physicist Sunil Gupta of the Tata Institute of Fundamental Research in Mumbai, India.

BEARING FRUIT The GRAPES-3 experiment (shown) measures muons that rain down on Earth. The pitter-patter of the electrically charged subatomic particles drops off during thunderstorms, unmasking the electrical inner workings of clouds. The GRAPES-3 Experiment
Gupta and colleagues studied the muons’ behavior with the GRAPES-3 experiment in Ooty, India, which observes around 2.5 million muons every minute. During thunderstorms, that rate drops, as muons, which are electrically charged, tend to be slowed by a thunderstorm’s electric fields. That means fewer particles carry enough energy to register in the scientists’ detectors.

Using computer simulations of a thunderstorm, the researchers determined the electric potential necessary to explain the drop in muons spotted during the 2014 storm. The team also estimated the storm’s electric power: It was similar to the output of a large nuclear reactor, at around 2 billion watts.

The result is “potentially very important,” Dwyer says. But “with anything that’s new, you have to wait and see what happens with additional measurements.” And the researchers’ simulated thunderstorm was simplified, he says. It consisted of one region of positive charge, and another negatively charged region, whereas real thunderstorms are more complex.

If confirmed, though, such high voltages inside a thunderstorm could explain a puzzling observation: Some storms send bursts of high-energy light, called gamma rays, upward (SN: 5/30/15, p. 12). But scientists don’t fully understand the processes that could create such energetic light. If thunderstorms indeed reach the billion-volt level, that could account for the mysterious light.

Editor’s note: This story was updated March 25, 2019, to reflect that the paper is now published and on April 8, 2019, to correct the definition of the cloud’s electric potential. Electric potential is the amount of work needed to move an electric charge, not an electron.

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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