Cosmic-ray particles help probe powerful electric fields inside thunderstorms

An instrument designed to study cosmic rays, known as GRAPES-3, has been found to be a valuable tool for investigating thunderstorms. Initial observations with GRAPES-3 revealed that thunderclouds can develop electrical potential differences far greater than previously measured. However, a puzzling observation was made: the instrument detected significantly more thunderstorm events in the eastern part of its field of view compared to the western part.

A new study published in the Journal of Cosmology and Astroparticle Physics has now explained this asymmetry. The asymmetry is not due to a greater frequency of thunderstorms in the east, but rather due to the way the geomagnetic field affects the cosmic rays traveling through Earth's magnetosphere. This results in GRAPES-3 being more sensitive to thunderstorms from certain directions than others.

Cosmic rays are high-energy particles originating from outer space, primarily protons. They are deflected by Earth's magnetic field, making their trajectories unpredictable. When cosmic rays enter the atmosphere, they collide with atomic nuclei, creating secondary particles called muons. Muons are highly penetrating particles and are the primary focus of GRAPES-3.

The GRAPES-3 instrument, located in Ooty, India, consists of a 560-square-meter muon telescope that records around 4 billion muons daily. The large number of muons detected allows GRAPES-3 to probe thunderstorms, despite not being designed for this purpose. When muons pass through regions with intense electric fields, such as those found inside thunderclouds, their behavior is affected differently depending on whether they are positive or negative.

This causes a small but measurable change in the total muon flux, which can be used to infer information about the electrical potential within the thundercloud. Previous measurements of thunderstorm potentials have been significantly lower than the 1.3 gigavolts detected by GRAPES-3. The directional asymmetry in the observed thunderstorm events is explained by the geomagnetic cutoff effect, where positively charged cosmic rays are more easily filtered out in the west than in the east.

This directional difference in cosmic ray entry results in a larger imbalance between positive and negative muons in the east, making GRAPES-3 more sensitive to the electrical effects of thunderstorms in that direction.