A thunderstorm in India produced an electric potential of 1.3 billion volts 鈥� 10 times聽the highest voltage previously recorded. The finding could help explain how high-energy gamma rays are produced during storms.
Inside a thundercloud, positively charged water droplets are carried upward while negatively charged hail pellets are drawn down. This difference in charge between the top and the bottom of the cloud聽creates a vertical electric field that intensifies as the cloud gets larger. In thunderclouds that are several kilometres thick, the electric potential can reach billions of volts.
That is what happened in December 2014, when the record-breaking voltage was detected in a storm over Ooty in India. It was caused by clouds聽11 kilometres above sea level covering an area聽about 22 kilometres across, though there was no lightning detected.
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鈥淭hunderstorms are really big places, and it鈥檚 hard to get enough instruments inside them to measure the whole thing,鈥� says Joseph Dwyer at the University of New Hampshire, who wasn鈥檛 involved in the work. 鈥淭he act of making the measurement with a balloon or airplane can collapse the electric field and artificially initiate lightning.鈥�
Instead of going up into the complex mess of clouds, Sunil Gupta at the Tata Institute of Fundamental Research in India and his colleagues measured the electric potential with messengers in the form of muons 鈥� subatomic particles generated in the atmosphere by powerful cosmic rays from outside the solar system. They pass through the thundercloud in a few microseconds and experience the electric potential produced inside.
The team聽calculated the storm鈥檚 power using G3MT, a muon telescope in southern India. As short-lived muons pass through the electric field, they gain energy and more of them make it to the sensors on the ground than usual. Measuring this flux in muons allowed the team to simulate the voltage produced by the thunderstorm overhead.
The reason the calculations were so delayed was because Gupta and his colleagues don鈥檛 normally study thunderstorms. It was only when they were combing through data from the telescope that they decided to model the storm.
The high potentials inside storm clouds could explain the puzzle of how thunderstorms initiate聽terrestrial gamma ray flashes (TGFs). 鈥淭hese gamma rays can blast their way through the atmosphere and temporarily blind a satellite, but they come from garden variety thunderstorms,鈥� says Dwyer. This new measurement suggests that the potentials are larger than we had thought, which means there is more energy to impart to electrons within the thundercloud.
Gupta says electrons with an energy of 1.3 billion volts can easily radiate gamma rays of more than 100 million volts, and explain the production of the highest energy terrestrial gamma ray flashes we see.
Maribeth Stolzenburg at the University of Mississippi doesn鈥檛 agree.聽鈥淚t is surprising to me that this largest thundercloud potential on record occurs in the absence of major lightning. After 30 years of studying thunderstorms, I find this very hard to understand,鈥� she says. 鈥淚t is additionally perplexing because there is a claim made in the paper that this large potential estimate can somehow explain the production of TGFs. As far as I am aware, the present understanding of TGF production is closely tied to the initiation stage of particular intracloud lightning flashes.鈥�
Physical Review Letters
