During a thunderstorm, the sky acts like a giant battery: charge builds up in the cloud, and a lightning bolt discharges it. That charge is invisible, but cosmic particles (muons, which constantly rain down on Earth from space) can sense it. As a muon flies through a charged cloud, it speeds up or slows down slightly, and these changes reveal the voltage inside the cloud.
The GRAPES-3 telescope in India recorded the muon flux during thunderstorms for ten years. It turned out that nearly six times more muons arrive from the eastern side of the sky than from the western side. The reason is not the clouds themselves but the fact that muons come in two varieties, positive and negative, and there are more positive ones in the flux. Earth's magnetic field bends positive particles more strongly, so more of them arrive from the east, and they are better at 'feeling' the charged top of a thundercloud.
The scientists tested this picture with computer simulations. If you remove the difference between positive and negative muons, the east–west asymmetry disappears. So it is precisely the charge imbalance that makes eastern storms more 'visible' to the telescope. This is the first time the muon charge has been directly linked to the direction of a thunderstorm.
Now muon telescopes can be used as sensitive detectors of electric fields in the atmosphere. A storm that is invisible to the eye still gives itself away through the particle flux.
as written by the authors arXiv:2603.26112
The electric fields inside thunderstorms can significantly modify the intensity of secondary cosmic ray muons at the ground level, producing measurable variations in their intensity ($$\Delta$$I$$_{\mu}$$). By utilizing the decade-long observations of thunderstorms (April 2011-December 2020) by the GRAPES-3 muon telescope (G3MT), a directional asymmetry in $$\Delta$$I$$_{\mu}$$ is observed, with nearly six times more events being detected from the east than the west directions. Using detailed CORSIKA Monte Carlo simulations, it is shown that this asymmetry is caused by the variations of the muon charge ratio R$$_\mu$$ (N$$_{\mu^+}$$/N$$_{\mu^-}$$). The anisotropic R$$_\mu$$ in turn, is caused by the systematic changes in geomagnetic cutoff rigidities, and subsequent selective filtering of predominantly positively charged primary cosmic rays. As a consequence, the R$$_\mu$$ increases systematically from west to east across the G3MT field of view, enhancing the sensitivity of east directions to positively charged thunderstorm top. Monte Carlo simulations with constant R$$_\mu$$ show that the directional asymmetry disappears, demonstrating the muon charge imbalance to be the dominant driver of the observed asymmetry. The dependence of R$$_\mu$$ on the hadronic interaction is also studied by comparing seven combinations high-, and low-energy hadronic interaction generators, which show a $$\lesssim$$7% spread in R$$_\mu$$, and $$\lesssim$$14% variation in the derived thunderstorm potentials. These results provide the first quantitative link between the muon charge asymmetry caused by the geomagnetic field, and the directional distribution of thunderstorms, reinforcing the role of muon observations as a probe of gigavolt potentials in atmospheric electrical structures.
Muons are born in the upper atmosphere when cosmic rays collide with air nuclei. In one second, about one muon passes through the palm of your hand.
In Stanislaw Lem's novel 'Solaris', the planet's ocean manipulates gravity to affect the station's orbit. Here, a thunderstorm controls the particle flux, a similar motif of invisible influence.