How Particles From Space Are Helping Indian Scientists Measure Thunderstorm Electricity
A detector built to study particles from space has turned out to be good at studying thunderstorms. A new study, reported on October 2, 2026, explains a puzzle in its data.
The detector
GRAPES-3 is an experiment in Ooty, India. It includes a 560-square-meter muon telescope that records around 4 billion muons every day, according to the Phys.org report on the study by SISSA Medialab. Muons are short-lived charged particles made when cosmic rays hit the atmosphere. Most primary cosmic rays are protons, and they strike air nuclei and create showers of secondary particles. At ground level, the detected particles are mostly muons.
How muons reveal a thunderstorm
Inside a thundercloud, electric charge is separated, which creates strong electric fields. When muons pass through, the field slows positive muons and speeds up negative ones. There are more positive than negative muons, since primary cosmic rays are mostly positive, so the two effects do not cancel out. The total muon flux shifts slightly, and scientists can infer the electric potential in the cloud.
An earlier GRAPES-3 result measured a potential of 1.3 gigavolts in a thunderstorm, said Sunil Gupta of the Tata Institute of Fundamental Research (TIFR) in Mumbai. Phys.org notes that earlier direct measurements had reached only about 130 million volts, though physicist C.T.R. Wilson suggested in the 1920s that clouds might reach such voltages.
The east-west puzzle
Between April 2011 and December 2020, GRAPES-3 detected 487 thunderstorm events. Of these, 81.5% appeared in the east of its field of view and 13.7% in the west. Independent instruments that monitor the atmospheric electric field showed real storms did not favor the east.
The new study, published in the Journal of Cosmology and Astroparticle Physics, explains why. Earth's magnetic field deflects charged cosmic rays, so positive particles reach the atmosphere more easily from some directions. Simulations showed the ratio of positive to negative muons is about 1.37 in the east and 1.14 in the west. A bigger imbalance makes the detector more sensitive to a storm's electric effect. "It's not that the voltage is different," Gupta says. Storms to the east are not necessarily more powerful.
Why it matters
The result strengthens the case for using cosmic-ray muons as a natural probe of thunderstorm electricity. It is also a reminder that a detector's blind spots can look like real patterns in the data.
Sources
- Phys.org (SISSA Medialab), "Cosmic-ray particles help probe powerful electric fields inside thunderstorms", October 2, 2026: https://phys.org/news/2026-10-cosmic-ray-particles-probe-powerful.html
Facts checked: October 3, 2026.