🔭 astrophysics

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

Using a decade of observations from the GRAPES-3 muon telescope and CORSIKA simulations, this study reveals that a directional asymmetry in thunderstorm-induced muon intensity variations is driven by the geomagnetic field's systematic modulation of the muon charge ratio, which enhances sensitivity to positively charged thunderstorm tops from the east.

Original authors: B. Hariharan, S. K. Gupta, Y. Hayashi, P. Jagadeesan, A. Jain, S. Kawakami, H. Kojima, P. K. Mohanty, Y. Muraki, P. K. Nayak, A. Oshima, M. Rameez, K. Ramesh, L. V. Reddy, S. Shibata

Published 2026-03-30

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CC BY 4.0

Original authors: B. Hariharan, S. K. Gupta, Y. Hayashi, P. Jagadeesan, A. Jain, S. Kawakami, H. Kojima, P. K. Mohanty, Y. Muraki, P. K. Nayak, A. Oshima, M. Rameez, K. Ramesh, L. V. Reddy, S. Shibata

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: A Cosmic Rain Gauge with a "Bias"

Imagine you are standing in a field with a giant, high-tech rain gauge (the GRAPES-3 Muon Telescope). But instead of measuring rain, this telescope is counting muons—tiny, ghost-like particles that rain down on Earth from outer space, created when cosmic rays hit our atmosphere.

The scientists noticed something very strange. When a thunderstorm happens, the number of muons hitting the ground changes. But here's the kicker: The telescope sees thunderstorms coming from the East six times more often than from the West.

You might think, "Well, maybe it just rains more in the East?" But that's not the case. Thunderstorms happen all around the sky. So, why does the telescope only "see" the ones coming from the East?

The answer isn't the weather; it's a cosmic traffic jam caused by the Earth's magnetic field.

The Cast of Characters

The Muons (The Messengers): Think of muons as messengers carrying a secret code. They come in two flavors: Positive (+) and Negative (-). In the universe, there are usually more positive messengers than negative ones (about 1.25 positives for every 1 negative).
The Earth's Magnetic Field (The Bouncer): The Earth acts like a giant magnet. It doesn't treat all messengers the same. It acts like a bouncer at a club who lets some people in easily but stops others.
- From the West: The magnetic field bends the path of the cosmic rays toward the Earth. It's like a slide that funnels more particles in.
- From the East: The magnetic field pushes the particles away. It's like a wall that blocks many of them.
The Thunderstorm (The Electric Storm): Inside a thunderstorm, there are massive electric fields (like a giant battery). When muons fly through this storm, the electric field speeds up the negative ones and slows down the positive ones.

The Mystery: Why the East?

The scientists realized that the "East vs. West" difference in muon counts was the key.

The Analogy of the Filter:
Imagine you have two buckets of marbles (muons).

The West Bucket: Because of the Earth's magnetic field, this bucket has a mix of marbles that is almost balanced (maybe 1.14 positive for every 1 negative).
The East Bucket: Because the magnetic field filtered out the negatives more aggressively, this bucket is heavily skewed toward positive marbles (1.37 positive for every 1 negative).

Now, imagine a thunderstorm is a giant magnet that repels positive marbles and attracts negative ones.

If you drop the West Bucket (balanced mix) into the storm, the storm repels some positives and attracts some negatives. The total number of marbles changes a little bit.
If you drop the East Bucket (mostly positives) into the storm, the storm repels a lot of those positives. The total number of marbles drops drastically.

The Result:
Because the East side has so many more positive muons, the thunderstorm's electric field causes a huge, noticeable drop in the muon count. The telescope easily spots this big drop and says, "Hey, a thunderstorm is coming from the East!"

On the West side, the drop is much smaller and harder to see. It's like trying to hear a whisper in a noisy room versus a shout. The telescope misses the "whispers" from the West, making it look like there are no storms there, even though there are.

The Detective Work (Computer Simulations)

To prove this wasn't just a glitch in their machine, the scientists built a virtual universe using a super-computer program called CORSIKA.

The Simulation: They simulated billions of muons flying through the atmosphere, passing through the Earth's magnetic field, and hitting their virtual telescope.
The Test: They ran the simulation twice:
- Test A: Let the magnetic field do its thing (changing the ratio of positive to negative muons).
- Test B: Force the ratio of positive to negative muons to be the same everywhere (no magnetic bias).
The Verdict:
- In Test A, the simulation showed 6 times more storms from the East, exactly matching the real data.
- In Test B, the East/West difference disappeared completely.

This proved that the "East Bias" is real physics, not a broken telescope. It's the Earth's magnetic field acting as a filter that makes the East side of the sky "louder" to the telescope.

Why Does This Matter?

This discovery is a big deal for two reasons:

It's a New Tool: Now that we understand this "East Bias," we can use muon telescopes to measure the electric power of thunderstorms (which can be billions of volts!) much more accurately. We just have to remember to look harder at the West to catch the quiet storms.
It Connects Two Worlds: It links Cosmic Rays (particles from deep space) with Weather (storms on Earth). It shows that the Earth's magnetic field, which protects us from space radiation, also changes how we see our own weather.

The Bottom Line

The GRAPES-3 telescope isn't broken; it's just wearing "magnetic glasses." These glasses make thunderstorms coming from the East look huge and obvious, while storms from the West look tiny and hard to spot. By understanding this trick of the light (or rather, the trick of the magnetic field), scientists can now use these muon detectors to map out the massive electric power of thunderstorms with incredible precision.

1. Problem Statement

The GRAPES-3 muon telescope (G3MT) in Ooty, India, has recorded a decade of thunderstorm data (April 2011–December 2020). A significant anomaly was observed in the directional distribution of these events: thunderstorm events were detected approximately six times more frequently from the East than from the West.

While atmospheric electric fields are known to modulate secondary cosmic ray muon intensity ( $\Delta I_\mu$ ), the physical mechanism behind this extreme directional asymmetry was not fully understood. The authors sought to determine if this asymmetry was an instrumental artifact, a meteorological coincidence, or a consequence of fundamental particle physics and geomagnetic interactions. Specifically, they investigated the role of the muon charge ratio ( $R_\mu = N_{\mu^+}/N_{\mu^-}$ ) and its variation across the telescope's field of view (FOV).

2. Methodology

The study employed a combination of long-term observational data analysis and extensive Monte Carlo simulations.

Observational Data:
- Dataset: 487 thunderstorm events recorded by the G3MT between April 2011 and December 2020.
- Instrumentation: The G3MT consists of 3712 proportional counters (PRCs) arranged in 16 modules, covering 560 m². It is paired with electric field mills (EFMs) to correlate muon intensity variations with atmospheric electric potentials.
- Selection Criteria: Events were selected based on a muon intensity variation threshold of $|\Delta I_\mu| > 0.3\%$ .
- Directional Binning: Data was analyzed in a 9-direction configuration (NW, N, NE, W, V, E, SW, S, SE) and aggregated into West (WG), Vertical (VG), and East (EG) groups.
Monte Carlo Simulations (CORSIKA):
- Framework: The COsmic Ray SImulations for KAscade (CORSIKA) package was used to simulate cosmic ray propagation through the atmosphere.
- Generators: Seven combinations of high-energy (SIBYLL, EPOS-LHC, QGSJETIII, DPMJETIII) and low-energy (FLUKA, UrQMD) hadronic interaction generators were tested to assess model dependence.
- Parameters: Simulations covered primary protons in the 10 GeV–10 TeV range. A uniform electric field (representing thunderstorm potentials from -3 GV to +3 GV) was applied between 8–10 km altitude.
- Objective: To calculate the muon charge ratio ( $R_\mu$ ) as a function of azimuth and zenith angles and to simulate the resulting $\Delta I_\mu$ under various thunderstorm potentials.

3. Key Contributions

Quantitative Link: The paper establishes the first quantitative link between the geomagnetic-field-induced muon charge asymmetry and the directional distribution of detected thunderstorms.
Mechanism Elucidation: It demonstrates that the East-West asymmetry is not due to meteorological factors but is driven by the geomagnetic East-West effect.
Hadronic Model Sensitivity: The study systematically evaluates the impact of different hadronic interaction models on the derived thunderstorm potentials, providing an uncertainty envelope for future studies.

4. Results

A. Observational Asymmetry

Event Distribution: Of the 487 events, 82% originated from the East (NE, E, SE), while only 14% came from the West (NW, W, SW). The ratio of East-to-West events is 5.9 ± 0.8.
Charge Ratio Variation: The simulations revealed that the muon charge ratio ( $R_\mu$ $R_{μ}$ ) varies systematically across the FOV due to geomagnetic cutoff rigidities (ranging from ~15 GV in the West to ~24 GV in the East).
- West (WG): $R_\mu \approx 1.14$
- East (EG): $R_\mu \approx 1.37$
- Vertical (VG): $R_\mu \approx 1.25$

B. Physical Mechanism

Selection Effect: Most thunderstorms possess a positive top potential, which decelerates positive muons ( $\mu^+$ ) and accelerates negative muons ( $\mu^-$ ).
Net Intensity Change: Since $R_\mu > 1$ (more $\mu^+$ than $\mu^-$ ), the deceleration of the majority species leads to a net deficit in muon intensity ( $\Delta I_\mu < 0$ ).
Sensitivity: A higher $R_\mu$ (as found in the East) results in a larger magnitude of intensity deficit for the same thunderstorm potential. Consequently, the detection threshold ( $|\Delta I_\mu| > 0.3\%$ ) is more easily exceeded for events coming from the East, leading to a higher detection rate.

C. Simulation Validation

Reproducing Asymmetry: When simulations used the direction-dependent $R_\mu$ values derived from CORSIKA, the simulated East-to-West event ratio was 5.09 ± 0.02, closely matching the observed 5.9.
Control Test: When simulations were run with a constant $R_\mu$ (uniform across all directions), the East-West asymmetry disappeared (ratio $\approx 1.0$ ), confirming that the charge imbalance is the dominant driver.

D. Hadronic Model Dependence

Generator Spread: Different combinations of hadronic generators produced a spread in $R_\mu$ of $\lesssim 7\%$ at zero potential.
Potential Uncertainty: This variation translates to a $\lesssim 14\%$ variation in the derived thunderstorm potentials.
Conservative Estimate: The SIBYLL-FLUKA combination yielded the highest $R_\mu$ (1.251 for the full FOV), providing the most conservative (lowest) estimate of thunderstorm potentials.

5. Significance

Validation of G3MT: The results reinforce the validity of the GRAPES-3 experiment as a unique tool for probing gigavolt-scale electric structures in thunderstorms, confirming that the observed asymmetry is a physical phenomenon rather than an instrumental bias.
Refined Potential Estimation: By accounting for the geomagnetic modulation of $R_\mu$ , the study provides a more accurate framework for converting muon intensity variations into atmospheric electric potentials.
Fundamental Physics: The work highlights the interplay between high-energy hadronic physics, geomagnetic field effects, and atmospheric electrodynamics, demonstrating how ground-based muon telescopes can serve as probes for both cosmic ray composition and severe weather phenomena.

In conclusion, the paper resolves the mystery of the East-West thunderstorm asymmetry observed by GRAPES-3, attributing it to the geomagnetic filtering of primary cosmic rays which creates a directional gradient in the muon charge ratio, thereby enhancing the sensitivity of the telescope to thunderstorms from the East.