This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: An instrument built to study cosmic rays has turned out to be a powerful tool for investigating thunderstorms. Earlier observations with the GRAPES-3 muon telescope revealed that thunderclouds can develop electrical potential differences far greater than those ever measured directly.
But they also uncovered a puzzle: the instrument detected many more thunderstorm events in the eastern part of its field of view than in the western part. A new study by the same team, published in the Journal of Cosmology and Astroparticle Physics, has now explained why. The asymmetry does not mean that thunderstorms are actually more common to the east.
Instead, it arises from the way the geomagnetic field affects the cosmic rays traveling through Earth's magnetosphere, making GRAPES-3 more sensitive to thunderstorms from some directions than others. The result strengthens the case for using these cosmic-ray particles as a natural probe of thunderstorm electricity. Cosmic rays are a continuous flux of extremely high-energy particles arriving at Earth from outer space.
Most primary cosmic rays are protons. Because they are electrically charged, their trajectories are deflected by Earth's magnetic field. When they enter the atmosphere, they collide with atomic nuclei in the air, generating showers of secondary particles.
Among these are muons, short-lived charged particles produced at high energies and traveling at speeds close to that of light. "The cosmic rays we detect at ground level consist predominantly of muons," explains Sunil Gupta of the Tata Institute of Fundamental Research (TIFR) in Mumbai, India, one of the authors of the study. "They're highly penetrating particles." GRAPES-3, located in Ooty, India, includes a 560-square-meter muon telescope that records around 4 billion muons every day.
Positive and negative muons do not arrive at the detector in equal numbers. Since primary cosmic rays are mostly positively charged, positive muons are also more abundant in the atmosphere. Physicists describe this imbalance using the muon charge ratio—the ratio of positive to negative muons—which is normally greater than one.
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