Ooty’s muon detection facility measures potential of thundercloud

At 1.3 gigavolts, this cloud had ten times higher potential than the previous record in a cloud

For the first time in the world, researchers at the GRAPES-3 muon telescope facility in Ooty have measured the electrical potential, size and height of a thundercloud that passed overhead on December 1, 2014. At 1.3 gigavolts (GV), this cloud had 10 times higher potential than the previous record in a cloud. This is not because clouds with such high potentials are a rarity, but rather, because the methods of detection have not been successful so far.

Cloud structure

Clouds have negative charges along their lower side and positive charges on top and can be several kilometres thick. If balloons are used to measure the potential difference between the top and bottom, they will take hours to traverse the distance. Unfortunately, thunderstorms last only for about 15-20 minutes, and this method fails.

The Ooty group did not really set out to measure the cloud’s potential. Sunil Gupta from TIFR, Mumbai and corresponding author of the paper published in Physical Review Letters, says that he was first intrigued by the way the muon intensity dipped briefly in a manner correlated with the thunderstorm. Though it was known that thunderstorms had an effect on muon intensity, it had not been probed in detail earlier. Dr Gupta urged the researchers in his team to study this carefully.

Threshold of detection

Muons and other particles are produced when cosmic rays bombard air particles surrounding the earth. The muons produced can have positive or negative charge. When a positively charged muon falls through a cloud, it loses energy. If its energy falls below 1 giga electron volt (GeV), which is the threshold of detection of the GRAPES-3 muon telescope, it goes undetected. On the contrary, a negatively charged muon gains energy when falling through the cloud and gets detected. Since there are more positive than negative muons produced in nature, the two effects don’t cancel out, and a net change in intensity is detected.

From April 2011 to December 2014, the group studied the variation of muon intensity during 184 thunderstorms. In seven events they came across thunderclouds that corresponded to a large change in muon intensity, of above 0.4%. They also simultaneously monitored the profiles of the clouds using four ground-based electric field monitors. Only the cloud that crossed on December 1, 2014, had a profile that was simple enough to simulate.

Using a computer simulation and the observed muon intensity variations, the group worked out the relationship with the electric potential of the cloud. They calculated that the potential of the cloud they were studying was approximately 1.3 GV. “To best of our knowledge no one has ever measured potential, size and height of a thundercloud simultaneously. That is the reason for all the excitement,” says Dr Gupta.

Clue to puzzle

Dr Gupta and his colleagues surmise that this method can be used to solve a 25-year-old puzzle of terrestrial gamma ray bursts — huge flashes of light that accompany lightnings, but which have not been explained in theory until now.

Learning about the properties of thunderclouds can be useful in navigation of aircraft and preventing short circuits. This serendipitous discovery might provide the means to making headway in this direction.