Science News

India's telescope detects monstrous thundercloud

K. S. Jayaraman

doi:10.1038/nindia.2019.37 Published online 26 March 2019

© G3MT experiment

A record-breaking thunderstorm electric potential of about 1.3 billion volts detected by a telescope in south India's hill resort Ooty has stunned scientists. The thunderstorm potential – 10 times greater than the largest value ever reported – has been recorded by the GRAPES-3 Muon Telescope (G3MT) operated by Mumbai-based Tata Institute of Fundamental Research (TIFR)1.

Lightning and thunderstorms, spectacular manifestations of sudden discharge of charged clouds have fascinated mankind. But they also cost thousands of human lives every year worldwide. Researchers have earlier flown airplanes and balloons into the centers of thunderstorms to study their electrical structure, but they could only probe the small region and not the entire cloud.

Physicist Sunil Gupta of TIFR devised a method that makes use of muons – heavier cousins of electrons – to measure the electric potential of thunderclouds. Muons are produced by cosmic rays and constantly rain down on the Earth’s surface penetrating the clouds.

The G3MT) in Ooty – an array of 400 muon detectors, spread over 6.2 acres – can observe around 2.5 million muons every minute. That rate drops during thunderstorms since the electrically charged muons are slowed down by a thunderstorm’s electric fields, resulting in fewer particles getting picked up by the detector. In other words, the number of muons (flux) detected by the telescope is related to the electrical voltage of a thundercloud.

From muon flux measurements, Gupta and his colleagues were able to estimate the electric potential of thunderstorms using computer simulations, treating a thundercloud as a giant capacitor made of two parallel plates 2-km apart that generates an upward-pointing electric field.

Between 2011 and 2014, the researchers gathered data on 184 thunderstorms, but what astonished them was the one on 1 December 2014 that was accompanied by a big drop in muon flux. Using computer simulations, the researchers calculated the electrical potential in the storm to be 1.3 billion volts.

The team also estimated the monster storm’s electric power to be around two billion watts – similar to the output of a large nuclear reactor. “That’s a massive amount of power." says Gupta, Unfortunately, it’s unlikely that we’ll ever be able to harness the remarkable power held in thunderstorms."

Scottish physicist C T R Wilson, who won the Nobel Prize in 1927 predicted that thunderclouds may generate giga-volt electric fields. "So it is pleasing that a modern cosmic ray detector has verified this experimentally,” Subir Sarkar, head of Particle Theory Group of the University of Oxford, told Nature India.

In 2015 the same detector had detected a gigantic breach in the Earth’s magnetic field, triggered by a solar storm. "These discoveries illustrate the significant advantage of studying cosmic ray muons as probes of the upper atmosphere,” Sarkar says.

Sunil Gupta, who devised the method to use muons to measure thunderclouds.

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The finding may also help researchers explain the mystery of high energy gamma-ray flashes sometimes observed during thunderstorms and first discovered 25 years back, Gupta says.

Tariq Aziz, a retired professor of high energy physics at TIFR, agrees. "Possibly the discovery will go far beyond explaining gamma ray flashes in understanding cosmic phenomenon at grander scale."

Gupta’s team is now setting up gamma-ray detectors around G3MT, hoping to catch the terrestrial gamma-ray flashes in coincidence with a gigavolt-level thunderstorm.

The highest energy observed to date in a terrestrial gamma-ray flash (TGF) is 100 MeV, says Narayana Bhat, an astrophysicist at the University of Alabama in Huntsville, USA “Such high electric potentials required to accelerate electrons to GeV energies has not been measured so far nor was it thought possible to generate such high fields. This paper demonstrates the presence of such gigavolt electric potentials in the terrestrial atmosphere for the first time,” he says.

Bhat says the clever ground based experimental technique as opposed to using balloon-borne instruments used to demonstrate the existence of such potential is unique and powerful.


References

1. Hariharan, B. et al. Measurement of the electrical properties of a thundercloud through muon Imaging by the GRAPES-3 experiment. Phys. Rev. Lett. 122, 105101 (2019) doi: 10.1103/PhysRevLett.122.105101