Geomagnetic field explains east-west bias in muon telescope thunderstorm detections

An instrument built to study cosmic rays has proved to be a useful tool for investigating thunderstorms. Earlier observations with the GRAPES-3 muon telescope showed that thunderclouds can develop electrical potential differences far greater than any measured directly. They also turned up a puzzle: the instrument detected many more thunderstorm events in the eastern part of its field of view than in the western part. A new study by the same team, published in the Journal of Cosmology and Astroparticle Physics (JCAP), now explains why.

The asymmetry does not mean that thunderstorms are more common to the east. It arises from the way the geomagnetic field affects cosmic rays travelling through Earth's magnetosphere, which makes GRAPES-3 more sensitive to thunderstorms from some directions than others. The result strengthens the case for using these cosmic-ray particles as a natural probe of thunderstorm electricity.

Cosmic rays are a continuous flux of extremely high-energy particles arriving at Earth from outer space. Most primary cosmic rays are protons. Because they are electrically charged, their trajectories are deflected by Earth's magnetic field. When they enter the atmosphere they collide with atomic nuclei in the air, generating showers of secondary particles.

Among these are muons, short-lived charged particles produced at high energies and travelling at close to the speed of light. "The cosmic rays we detect at ground level consist predominantly of muons," said Sunil Gupta of the Tata Institute of Fundamental Research (TIFR) in Mumbai, India, one of the authors of the study. "They're highly penetrating particles."

GRAPES-3, located in Ooty, India, includes a 560-square-metre muon telescope that records around four billion muons every day. Positive and negative muons do not arrive at the detector in equal numbers. Since primary cosmic rays are mostly positively charged, positive muons are also more abundant in the atmosphere. Physicists describe this imbalance using the muon charge ratio - the ratio of positive to negative muons - which is normally greater than one.

That imbalance lets GRAPES-3 do something it was not designed to do: probe thunderstorms.

Inside a thundercloud, electric charges become separated, producing intense electric fields and large potential differences. When muons cross these regions, the electric field affects positive and negative particles in opposite ways, generally slowing positive muons while accelerating negative ones. If equal numbers of positive and negative muons were present, these effects would largely cancel, leaving almost no detectable change in the total muon flux.

"If nature provided equal numbers of positive and negative muons throughout the field of view, we wouldn't be able to observe the thunderstorm phenomenon with the current setup," said Hari Haran Balakrishnan of TIFR, first author of the new study.

Because positive muons are more abundant, thunderstorms produce a small but measurable change in the total muon flux, allowing researchers to infer the electrical potential inside the thundercloud.

"Muons are actually an ideal gift for doing these kinds of studies," said Gupta. "They are like an electric current flowing through the atmosphere."

In an earlier study, the GRAPES-3 team had already obtained a notable result. "We could measure a potential of 1.3 gigavolts in a thunderstorm," Gupta said.

The idea that thunderstorms might generate potentials on this scale is not new. As early as the 1920s, physicist Charles Thomson Rees Wilson, the 1927 Nobel laureate, suggested that thunderclouds could reach such voltages. Direct measurements, however, had previously reached only around 130 million volts, more than an order of magnitude lower.

Those observations revealed another unexpected result. Between April 2011 and December 2020, GRAPES-3 detected 487 thunderstorm events. Of these, 81.5% appeared in the eastern part of its field of view and only 13.7% in the west, a difference of almost six to one.

Local weather was not the cause. Using an independent array of instruments that monitors the atmospheric electric field, the researchers confirmed that actual thunderstorms showed no comparable preference for the east.

The answer lies in the east-west effect, known for nearly a century. Because charged particles are deflected by Earth's magnetic field, positively charged cosmic rays can reach the atmosphere more easily from some directions than others. Lower-energy positive particles arriving from the east are filtered out more strongly than those arriving from the west.

This directional threshold, known as the geomagnetic cutoff, varies significantly across GRAPES-3's field of view.

To test whether this could explain the observed asymmetry, the researchers used computer simulations to model the production and propagation of muons through the atmosphere. The simulations showed that the balance between positive and negative muons changes with direction: the muon charge ratio is about 1.37 in the east and 1.14 in the west.

"It's not that the voltage is different," Gupta said. Thunderstorms to the east are not necessarily more electrically powerful; the larger imbalance between positive and negative muons in that direction makes GRAPES-3 more sensitive to their electrical effects.

The simulations support this explanation. When the direction-dependent muon charge ratio is included, a strong east-west asymmetry emerges, similar to that seen in the observations. When the same ratio is imposed in both directions, the asymmetry disappears.

Understanding the effect is therefore essential for using GRAPES-3 as a reliable probe of thunderstorm electricity.

Direct measurements of electrical potential inside thunderclouds are difficult. They generally require instruments carried by aircraft or weather balloons into or near active storms, which makes it hard to sample a large, rapidly evolving phenomenon. Muons offer a different approach: they pass continuously through the atmosphere and can be monitored from the ground.

GRAPES-3 was built to study particles arriving from space and to investigate cosmic-ray physics. Those same particles are now helping researchers understand thunderstorms, a familiar phenomenon that is still not fully understood.

Research Report: The muon charge asymmetry and the directional distribution of thunderstorm events observed by the GRAPES-3 muon telescope

CONTACT: Federica Sgorbissa, Sissa Medialab, [email protected]