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Billion-volt thunderstorm studied using muons

26 Mar 2019
Lightning
Particle deflector: the paths of muons are affected by the huge voltages found in thunderclouds. (Courtesy: iStock/prudkov)

A thundercloud with a record-breaking voltage of 1.3 GV has been observed by physicists in India and Japan.  Sunil Gupta at the Tata Institute of Fundamental Research in Mumbai and colleagues calculated the voltage from changes in the intensity of atmospheric muons detected by the GRAPES-3 muon telescope. The existence of such high voltages could explain the origin of the mysterious, high-energy gamma-ray flashes, which are occasionally seen in cloud tops during thunderstorms.

Thunderstorm clouds are normally studied by flying weather balloons and aeroplanes straight through their centres.  Indeed, a balloon was used several decades ago to measure the previous record high voltage of 130 MV – which was observed inside a thunderstorm over New Mexico.  Such a voltage is high enough to create atmospheric particle accelerators that can generate X-rays and low-energy gamma rays. However, it is not high enough to create high-energy (about 100 MeV) gamma rays that are sometimes detected during thunderstorms.

In the 1920s, Scottish physicist and meteorologist Charles Wilson predicted that thunderstorms could induce far larger potentials; on scales of billions of volts. Voltages this large could only form across storm clouds that are several kilometres high and his prediction had been untested because balloons and aeroplanes are not able to measure voltages on such length scales.

Deflecting fields

Gupta and colleagues have got around this problem by using the GRAPES-3 muon telescope at Ooty in southern India to measure voltages across entire clouds. The telescope detects muons created when cosmic rays smash into the atmosphere. Muons are charged particles and are therefore deflected by the electric fields associated with huge voltages in thunderclouds. This means that fewer muons should be detected by GRAPES-3 when thunderclouds are nearby – which the team verified by studying 184 thunderstorms over three years.

To understand their results, the team modelled a thundercloud as a colossal parallel-plate capacitor– with the plates representing positively- and negatively-charged cloud layers that are separated by several kilometres. Using this model to interpret muon observations gathered during a storm in December 2014, they concluded that a voltage of 1.3 GV had developed between cloud layers – confirming Wilson’s prediction.

The voltage appears to be large enough to create flashes of gamma rays with energies as high as 100 MeV – which the team could not detect with their set-up. The researchers now plan to install  gamma-ray detectors close to GRAPES-3, allowing them to pick up gamma rays in coincidence with gigavolt-scale thunderstorms.

The study is described in Physical Review Letters.

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