Billion-volt thunderstorm studied using muons
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.
