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

TL;DR

This study utilizes a decade of muon observations from the GRAPES-3 telescope to reveal a directional asymmetry in muon intensity related to thunderstorm electric fields, driven by muon charge ratio variations influenced by geomagnetic effects.

Contribution

It demonstrates that muon charge ratio variations caused by geomagnetic cutoff rigidities explain the observed directional asymmetry in thunderstorm-related muon intensity changes.

Findings

Nearly six times more thunderstorm events detected from the east than the west.

Muon charge ratio R$_mbda$ increases from west to east, affecting sensitivity to thunderstorm potentials.

Simulations show the asymmetry disappears with constant R$_mbda$, confirming charge imbalance as the main cause.

Abstract

The electric fields inside thunderstorms can significantly modify the intensity of secondary cosmic ray muons at the ground level, producing measurable variations in their intensity ($\Delta$I$_{\mu}$). By utilizing the decade-long observations of thunderstorms (April 2011-December 2020) by the GRAPES-3 muon telescope (G3MT), a directional asymmetry in $\Delta$I$_{\mu}$ is observed, with nearly six times more events being detected from the east than the west directions. Using detailed CORSIKA Monte Carlo simulations, it is shown that this asymmetry is caused by the variations of the muon charge ratio R$_\mu$ (N$_{\mu^+}$/N$_{\mu^-}$). The anisotropic R$_\mu$ in turn, is caused by the systematic changes in geomagnetic cutoff rigidities, and subsequent selective filtering of predominantly positively charged primary cosmic rays. As a consequence, the R$_\mu$ increases systematically from west to east across the G3MT field of view, enhancing the sensitivity of east directions to positively charged thunderstorm top. Monte Carlo simulations with constant R$_\mu$ show that the directional asymmetry disappears, demonstrating the muon charge imbalance to be the dominant driver of the observed asymmetry. The dependence of R$_\mu$ on the hadronic interaction is also studied by comparing seven combinations high-, and low-energy hadronic interaction generators, which show a $\lesssim$7% spread in R$_\mu$, and $\lesssim$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.