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

TL;DR

The study uses 22 years of GRAPES-3 muon telescope data to jointly constrain upper-atmospheric temperature and interplanetary magnetic-field effects on ground-level muon flux. By applying FFT-based isolation of the annual temperature signal and band-pass filtering, it derives a temperature coefficient $oldsymbol{ ext{α}_T}$ that quantifies muon-rate sensitivity to $T_{eff}$, and subsequently determines a magnetic-field coefficient $oldsymbol{ ext{γ}}_{ ext{M}}$ that links muon variations to the IMF magnitude. An iterative deconvolution refines both coefficients, yielding $oldsymbol{ ext{α}_T}=-0.2241\pm0.0003\,(stat)\pm0.0220\,(syst)\%\,\text{K}^{-1}$ and $oldsymbol{ ext{γ}}_{ ext{M}}=-0.574\pm0.027\,(stat)\pm0.011\,(syst)\%\,\text{nT}^{-1}$ (for $oldsymbol{λ=120\,g\,cm^{-2}}$). The results demonstrate the feasibility of using atmospheric-muon fluxes as live monitors of the upper atmosphere and of the IMF, with potential for temperature imaging and improved climate forecasting when combined with real-time solar-wind data.

Abstract

Galactic Cosmic Rays (GCRs) have to travel through the heliosphere before they interact with the Earth's atmosphere. During this, they are deflected by the Sun's magnetic field, causing variations in this field to imprint on the flux, spectrum and angular distribution of GCRs detected at or near Earth. Studies of these variations over the past several decades have revealed the impact of both transient phenomena such as solar flares, coronal holes, sunspot activity and coronal mass ejections (CMEs) as well as their effects such as Forbush Decreases (FDs), precursors and Ground-Level Enhancements (GLEs). Periodic variations, such as due to the solar diurnal modulation, the 27-day solar rotation, the 11-year solar cycle, and the 22-year solar magnetic cycle have also been characterized. These Sun-induced phenomena are most prominent in GCR intensity variations up to $\sim$30 GeV/nuc, beyond which the influence of solar modulation decreases rapidly as the gyro-radii of GCRs exceed the characteristic size of the heliosphere ($\sim$100 AU).

Figures (20)

• Figure 1: A schematic of a 4-layer tracking muon telescope with 58 PRCs per layer.
• Figure 2: The 22-year average vertical temperature profile obtained from NASA's MERRA-2 dataset at the location ($\theta$ = 11.5$^\circ$, $\phi$ = 76.825$^\circ$) near the GRAPES-3 site, shown by the red line, as a function of pressure level. Temperatures are presented at 22 different pressure levels ranging from 775 hPa to 10 hPa above sea level.
• Figure 3: Temporal variations of (a) the muon rate, (b) the effective temperature (T$_\text{eff}$; assuming $\lambda$=120 g cm$^{-2}$), and (c) the interplanetary magnetic field $(\text{B}_{\text{scalar}}=\sqrt{\text{B}_{x}^{2}+\text{B}_{y}^{2}+\text{B}_{z}^{2}})$ observed by the ACE and WIND spacecraft 1998SSRv...86..613S1995SSRv...71..207L2005JGRA..110.2104K at Lagrange point L1, during 22 years (2001--2022). The left panels show the 3-hourly averaged data, while the right panels present the same datasets after applying a 60-day running average to suppress short term variations.
• Figure 4: Variation of (a) muon flux (in $\%$), and (b) $\Delta{\text{T}_{\text{eff}}}$ (in K; assuming $\lambda$=120 g cm$^{-2}$), obtained using a 60-day low-pass filter (implemented via a running average) during 22 years (2001--2022).
• Figure 5: Fast Fourier transform (FFT) spectra of (a) muon flux variation (in $\%$) and (b) $\Delta{\text{T}_{\text{eff}}}$ (in K; assuming $\lambda$=120 g cm$^{-2}$) over the 22 years (2001--2022). Red and blue solid lines represent the original spectra for panels (a) and (b), respectively, while black solid lines indicate the corresponding filtered spectra.