Cosmic-Ray Muons Reveal a 1.3-Gigavolt Thunderstorm, Ten Times Any Direct Measurement
India's GRAPES-3 telescope recorded 487 storms in nine years. A new study explains why it sees far more of them to the east.
A particle detector built to study cosmic rays has turned out to be a thunderstorm probe. The GRAPES-3 muon telescope in Ooty, India, has measured an electrical potential of 1.3 gigavolts inside a thundercloud. Direct measurements by aircraft and balloons have only reached about 130 million volts, so the muon reading is roughly ten times higher.
The instrument has a 560-square-meter detector and records about 4 billion muons a day. Muons are heavy cousins of the electron that form when cosmic rays, mostly high-energy protons from space, strike the atmosphere. "They're highly penetrating particles," said Sunil Gupta of the Tata Institute of Fundamental Research. "Muons are actually an ideal gift for doing these kinds of studies." He compares them to "an electric current flowing through the atmosphere."
Inside a thundercloud, the strong electric field changes the muon flux. Positive muons slow down and negative ones speed up. Because more positive muons than negative ones reach the ground, the imbalance becomes a measurable signal. The idea that storms build up gigavolt-scale potentials goes back to Charles Thomson Rees Wilson, who won the 1927 Nobel Prize in physics, but direct measurements always fell short of that.
Between April 2011 and December 2020, GRAPES-3 identified 487 thunderstorm events. A puzzle emerged: 81.5% came from the detector's eastern field of view and only 13.7% from the west, nearly a sixfold difference. Real storms showed no such preference, so something in the detection method had to explain it.
A new study in the Journal of Cosmology and Astroparticle Physics says the answer is Earth's magnetic field. This century-old "east-west effect" deflects charged cosmic rays differently depending on direction. Simulations showed the muon charge ratio is about 1.37 looking east and about 1.14 looking west. A larger surplus of positive muons gives the detector more sensitivity to storms in the east, without those storms being any stronger.
The team tested the explanation both ways. When direction-dependent charge ratios went into the simulation, a strong east-west asymmetry appeared that matched the data. When the same ratio was applied everywhere, the asymmetry vanished. "If nature provided equal numbers of positive and negative muons throughout the field of view, we wouldn't be able to observe the thunderstorm phenomenon with the current setup," said first author Hari Haran Balakrishnan.
The finding strengthens the case for using muons as a natural, ground-based way to monitor storms. Direct electrical measurements require sending aircraft or balloons into dangerous weather, which limits how many storms can be sampled and for how long. Muons arrive continuously and can be recorded from the ground. Thunderstorms, as the researchers put it, are still surprisingly mysterious, and a detector designed for fundamental physics is now helping to explain one of the most familiar events in the sky.
