Researchers in India have explained the puzzling bias in data from the GRAPES-3 telescope, which showed thunderstorms east of the instrument appearing about six times more frequently than those detected to its west.
The magnetic field explains the detection bias
A study recently published in the Journal of Cosmology and Astroparticle Physics found that thunderstorms are not more numerous in the east. Rather, Earth’s magnetic field indirectly affects the cosmic particles used to detect electricity inside clouds. The findings explain the disparity recorded by the telescope over several years, after its data conflicted with weather measurements taken at ground level.
The findings also support using muons to measure the huge electrical potential differences generated inside thunderstorms, without having to send measuring instruments directly into the clouds’ interiors, where conventional observations are difficult.
Hari Haran Balakrishnan, a researcher at India’s Tata Institute of Fundamental Research and the study’s lead author, said the key to the phenomenon was that positive and negative muons did not reach the telescope in equal numbers. He added: “If nature provided equal numbers of positive and negative muons in all parts of the field of view, we would not be able to detect thunderstorms using the current system.”
The GRAPES-3 telescope’s specifications and detection capabilities
Muons are created when cosmic rays arriving from space collide with the atmosphere. They then travel at speeds close to that of light and reach instruments on Earth’s surface. When these particles pass through the strong electric field inside a thundercloud, positive and negative muons are affected in opposite directions.
However, a larger number of positive muons means the opposing effects do not completely cancel each other out, causing a limited change in the total number of muons reaching the telescope. By measuring this change, researchers can infer information about the electrical potential difference inside the storm.
Balakrishnan explained that muons’ ability to penetrate the atmosphere makes them suitable for this task, likening them to “an electric current flowing through the atmosphere”. The GRAPES-3 team had previously used this mechanism to estimate the potential difference in one storm at about 1.3 billion volts — far above the highest level that could be reached through direct measurements, which was about 130 million volts, according to the study.
Storm data reveal an east–west gap
But an analysis of the storm record revealed a disparity that had to be explained before the method could be used more widely. Between April 2011 and December 2020, the telescope detected 487 thunderstorms.
Some 81.5% of the detected events came from the eastern part of its field of view, while events recorded in the western part accounted for no more than 13.7%. To determine whether the difference reflected a genuine pattern in weather conditions, the team compared the telescope’s data with independent measurements of the electric field in the atmosphere.
The independent measurements did not show a similar increase in the number of storms in the east. This indicated that the cause of the bias was linked to the detection mechanism itself and led the researchers to study the effect of Earth’s magnetic field on cosmic rays and the muons they produce.
Deflected rays alter the telescope’s sensitivity to storms
Balakrishnan said cosmic rays carry electric charges, so their paths are deflected as they pass through Earth’s surrounding magnetic field. The energy these particles need to reach the atmosphere also varies according to the direction from which they arrive. This affects the characteristics of the muons that ultimately reach the telescope and its sensitivity to storms in each part of the field of view.
Using computer simulations of muon production and movement through the atmosphere, the researchers found that the ratio of positive to negative muons was about 1.37 in the east, compared with about 1.14 in the west. This disparity makes the detector more sensitive to changes caused by eastern storms, but does not mean those storms carry a higher electrical potential than storms west of the telescope.
According to the team’s explanation, the greater difference between the numbers of positive and negative muons makes the effect of the electric field inside the storm easier to detect in the east. When the different muon ratios were entered into the simulation, an east–west bias similar to that recorded in the actual data emerged.
When the same ratio was applied in both directions, the disparity disappeared, supporting a link to Earth’s magnetic field rather than to the distribution of storms. The study shows how an instrument built to detect cosmic rays arriving from space has become a tool for studying storms on Earth using the same particles.
Balakrishnan believes that explaining the difference between east and west gives researchers a more precise understanding of the method’s limitations and helps them use the billions of muons passing through the atmosphere each day to detect electricity inside thunderclouds and measure their huge potential differences.
