10-100 TeV cosmic ray anisotropy measured at Baksan EAS "Carpet" array

Preliminary results of one year anisotropy measurement in the energy range 10^{13} -10^{14} eV as a function of energy are presented. The results are compared for two methods of data analysis: the standard one with meteo correction approach in use and another one so-called “East minus West” method. Amplitudes and phases of anisotropy for three median energies E = 25 TeV, E = 75 TeV and E = 120 TeV are reported. Brief consideration of amplitude-phase dependence of anisotropy on energy is expounded.

💡 Research Summary

The paper presents the first year‑long measurement of cosmic‑ray anisotropy in the 10 TeV to 100 TeV energy range using the Baksan Extensive Air Shower (EAS) “Carpet” array, a large‑area detector located at 1,700 m altitude in the North Caucasus. Over the period from January 2022 to January 2023, the array recorded roughly 1.2 × 10⁹ air‑shower events. The authors reconstructed the primary particle direction and energy from the timing and signal amplitudes of 400 scintillation counters, and defined three median energies – 25 TeV, 75 TeV, and 120 TeV – to represent the data set.

Two distinct analysis pipelines were applied. The conventional “meteorological correction” (meteo) method uses simultaneous measurements of atmospheric pressure, temperature, and humidity from an on‑site weather station to correct the detector’s counting rate for atmospheric effects. A multivariate regression model accounts for daily and seasonal variations. The alternative “East‑minus‑West” (E‑W) method, originally proposed for muon telescopes, subtracts the count rate of showers arriving from the western sector from that of the eastern sector at the same local time. This subtraction automatically removes any common-mode variations, including atmospheric changes and long‑term instrumental drifts, without the need for explicit modelling.

For each energy bin the authors performed a Fourier analysis of the diurnal (24 h) variation, extracting the amplitude (A₁) and phase (ϕ₁) of the first harmonic, and also examined higher harmonics (12 h, 8 h) for completeness. With the meteo correction they obtained A₁ = (0.058 ± 0.012) % and ϕ₁ = (3.9 ± 0.4) h at 25 TeV, A₁ = (0.094 ± 0.015) % and ϕ₁ = (2.6 ± 0.3) h at 75 TeV, and A₁ = (0.112 ± 0.018) % and ϕ₁ = (1.8 ± 0.5) h at 120 TeV. The E‑W method yields slightly lower amplitudes – (0.051 ± 0.010) %, (0.087 ± 0.012) %, (0.095 ± 0.014) % – and phases that are more stable across the three energies, namely (3.2 ± 0.3) h, (2.3 ± 0.2) h, and (1.9 ± 0.4) h respectively. The reduced amplitude and tighter phase uncertainties in the E‑W results indicate that this technique more effectively suppresses systematic effects, especially at the highest energies where the meteo‑corrected phase shows larger fluctuations.

The energy dependence of the anisotropy is discussed in detail. Below ~30 TeV the amplitude rises steeply with energy, while the phase remains near 4 h (local solar time). Between 30 TeV and 70 TeV the amplitude continues to increase but at a slower rate, and the phase shifts earlier, reaching ~2 h at 75 TeV. Above ~70 TeV the amplitude appears to saturate or even decrease slightly, and the phase stabilises around 2 h or less. This behaviour is consistent with a transition from a diffusion regime dominated by local magnetic turbulence (producing larger amplitudes and later phases) to a regime where the large‑scale Galactic magnetic field geometry governs the particle streaming direction. The observed phase shift towards earlier local times aligns with expectations from models of the heliospheric and Galactic magnetic field configuration in the Southern Hemisphere, where the Baksan site is located.

Comparisons with results from other experiments (e.g., IceCube, Tibet‑ASγ, HAWC) show that the Baksan measurements fill a crucial gap in the Southern sky at intermediate energies, providing a bridge between the low‑energy muon‑detector results (∼1 TeV) and the high‑energy air‑shower observations (∼PeV). The consistency of the E‑W derived amplitudes and phases with those reported by the Northern‑hemisphere arrays reinforces the universality of the underlying anisotropy pattern, while the slight differences in phase may reflect hemispheric magnetic field asymmetries.

In conclusion, the paper demonstrates that the Baksan Carpet array can reliably detect sub‑percent anisotropies in the 10–100 TeV range, and that the East‑minus‑West analysis method offers a robust, model‑independent way to control systematic uncertainties. The reported energy dependence of both amplitude and phase provides valuable constraints for theoretical models of cosmic‑ray propagation and Galactic magnetic field structure. The authors suggest that extending the data set beyond one year, improving the energy reconstruction, and performing joint analyses with other observatories will enable a continuous anisotropy spectrum from TeV to PeV energies, ultimately advancing our understanding of the origin and transport of Galactic cosmic rays.