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

Abstract

Abstract The intense electric fields inside thunderstorms can significantly modify the intensity of secondary cosmic ray muons at the ground level, producing measurable variations in their intensity (|ΔI μ | >0.3%). By utilizing the decade-long observations of thunderstorms (April 2011–December 2020) by the GRAPES-3 muon telescope (G3MT), a directional asymmetry in |ΔI μ | is observed, with nearly six times more events being detected from the east than the west directions. Using detailed CORSIKA Monte Carlo simulations, we present that this asymmetry is caused by the variations of the muon charge ratio R μ (N μ + /N μ - ). The anisotropic R μ in turn, is caused by the systematic changes in geomagnetic effects, and subsequent selective filtering of predominantly positively charged primary cosmic rays. As a consequence, the R μ increases systematically from west to east across the G3MT field of view, enhancing the sensitivity of east directions to charged thunderstorms. Independent meteorological validation using ground-based electric field mill data confirms that the thunderstorm trajectories exhibit no eastward preference, ruling out localized storm clustering as a source of the asymmetry. Monte Carlo simulations with constant R μ show that the directional asymmetry disappears, indicating the muon charge imbalance to be the dominant driver of the observed asymmetry. Furthermore, we evaluated the systematic robustness of the framework against variations in hadronic interaction generators, thundercloud configuration, and |ΔI μ |. The dependence of R μ on the hadronic interaction is studied extensively by comparing seven combinations of high-, and low-energy hadronic interaction generators, which show a ≲7% spread in R μ , and ≲14% variation in the derived thunderstorm potentials. These results provide the first quantitative link between the muon charge asymmetry caused by the geomagnetic field, and the directional distribution of thunderstorms, reinforcing the role of muon observations as a probe of gigavolt potentials in atmospheric electrical structures.