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Researchers operating the GRAPES-3 muon telescope in Ooty, India, have successfully measured the internal electricity of storm clouds by tracking high-energy cosmic rays. This breakthrough, published in the Journal of Cosmology and Astroparticle Physics, confirms that subatomic particles can act as natural probes to audit atmospheric electrical fields, finally resolving a decade-long anomaly regarding storm detection patterns that had long eluded direct aircraft or balloon measurements.
Muons as Atmospheric Probes
Primary cosmic rays, largely composed of positively charged protons, collide with atomic nuclei in Earth’s atmosphere. This triggers cascades of secondary particles, including muons. These highly penetrative, charged particles travel at near-light speeds to reach surface-level detectors. When muons traverse a thundercloud, the cloud’s intense internal electric field alters their trajectory and velocity. Because the atmosphere naturally contains more positive than negative muons, the storm’s field creates a detectable variation in the flow of these particles reaching the ground, allowing scientists to calculate the cloud’s internal voltage.
Earth's Magnet Drives Asymmetry in GRAPES-3 Storm Detections
Between April 2011 and December 2020, the GRAPES-3 experiment recorded 487 storm events. Researchers observed an unexpected asymmetry: 81.5% of detections occurred in the eastern sector of the instrument’s field of view, while only 13.7% originated from the west.
Hari Haran Balakrishnan, a researcher at the Tata Institute of Fundamental Research (TIFR) in Mumbai, noted that if nature provided an equal number of positive and negative muons across the entire field of view, the current configuration would not be able to observe the phenomenon. The disparity is instead driven by the Earth’s magnetic field. This “east-west effect” acts as a magnetic filter, slowing low-energy positive particles from the east more effectively than those from the west. Computational simulations confirmed that the ratio of positive to negative muons is 1.37 in the east compared to 1.14 in the west, increasing the telescope’s sensitivity to storms in that direction.
Quantifying Gigavolt Potentials
By accounting for this magnetic bias, the GRAPES-3 telescope functions as a precise metric tool. The team measured internal storm potentials of up to 1.3 gigavolt (1.3 billion volts), providing a new perspective on theories first proposed by Nobel laureate Charles Thomson Rees Wilson in the 1920s. While direct measurements taken by aircraft instruments have historically struggled to surpass 130 megavolts, the use of muons—which Sunil Gupta of the TIFR describes as a “gift” that flows continuously through the atmosphere—offers a way to verify these massive electrical scales.
Limitations of Traditional Airborne Sensing
Direct measurements using aircraft or weather balloons are technically difficult and provide only isolated samples of a massive, dynamic phenomenon. These methods have historically struggled to capture the full electrical potential of a storm, often recording values an order of magnitude lower than theoretical predictions. The 1.3 gigavolt measurement recorded by the GRAPES-3 team remains significantly higher than the 130-megavolt limit previously established by traditional airborne instrumentation.
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