Monitoring the upper atmospheric temperature and interplanetary magnetic field with the GRAPES-3 muon telescope

Abstract

We study the influence of variations in the upper atmospheric temperature and interplanetary magnetic field on the cosmic ray induced atmospheric muon flux measured by the GRAPES-3 experiment over 22 years (2001--2022) of data; spanning three solar cycles: the declining phase of Solar Cycle 23, the full Cycle 24, and the rising and maximum phases of Cycle 25. Located in Ooty- India, the GRAPES-3 large area (560\,$m^2$) muon telescope detects $\sim$4 billion muons daily above 1\,GeV, with an angular resolution of $\sim$4$^\circ$, enabling a statistical precision $<$0.01\% on the hourly muon rate. After accounting for the effect of atmospheric pressure variations, we compare this data with the upper atmospheric temperature inferred from NASA's MERRA-2 dataset as well as magnetic field data from the ACE and WIND spacecraft at Lagrange point L1. A simultaneous iterative fitting method employing Fast Fourier Transforms and a narrow band-pass filter reveals the temperature and magnetic field coefficients to be $\alpha_T=-\,0.2241\,\pm\,0.04\, (stat.)\,\pm\,0.0220\, (syst.)\,\%\,K^{-1}$ and $\gamma_{M}=-\,0.574\,\pm\,0.027\, (stat.)\,\pm\,0.011\, (syst.)\,\%\,\text{nT}^{-1}$, respectively, for an assumed hadronic attenuation length $\lambda$=120 g cm$^{-2}$, underscoring the potential of the GRAPES-3 muon telescope to serve as a real time monitor of the upper atmospheric temperature or interplanetary magnetic field.