PULSE Magazine | April 2019 Issue

WHAT IS... IN A THUNDERCLOUD

A Single Thundercloud Carries 1 Billion Volts of Electricity

When Benjamin Franklin tied a key to a kite and flew it into a lightning storm, he briefly became an appliance plugged into the strongest power generator on Earth.

Franklin knew, as most people do, that thunderstorms are incredibly powerful. Researchers have tried to estimate precisely how powerful for more than a century, but have always come up short — even the most sophisticated airborne sensors are inadequate because thunderclouds are just too big and unpredictable to measure. Now, in a paper published Mar. 15 in the journal Physical Review Letters, researchers in Ooty, India, have come up with a shocking new answer — thanks to a little help from some cosmic rays. [Electric Earth: Stunning Images of Lightning] Using an array of sensors designed to measure electric fields and the intensity of muons — heavy particles that constantly rain down from Earth's upper atmosphere, decaying as they pass through matter — the team measured the voltage of a large thundercloud that rolled over Ooty for 18 minutes on Dec. 1, 2014. The researchers found that, on average, the cloud was charged with about 1.3 gigavolts of electricity, which is 1.3 times 10^9 volts — roughly 10 million times more voltage than is supplied by a typical power outlet in North America. "This explains why thunderclouds are so destructive," study co-author Sunil Gupta, a cosmic ray researcher at India's Tata Institute of Fundamental Research, told Live Science. "If you dissipate this massive amount of energy through anything, it is going to cause severe devastation."

It's raining muons

Gupta and his colleagues primarily study muons — electron-like particles that are created when cosmic rays bash into various atoms in Earth's atmosphere. These particles have about half the spin of electrons but 200 times the weight, and are very good at penetrating matter. A muon raining down from the atmosphere can travel deep into the ocean or miles underground in just a fraction of a second, as long as it has enough energy.

Muons lose their energy when something gets in their way — say, a pyramid, for example. In early 2018, scientists discovered two previously unknown chambers inside the Great Pyramid of Giza by setting up muon detectors around the structure and measuring where the particles lost (and didn't lose) energy.

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