On Temporal Variations of the Multi-TeV Cosmic Ray Anisotropy using the Tibet III Air Shower Array

We analyze the large-scale two-dimensional sidereal anisotropy of multi-TeV cosmic rays by Tibet Air Shower Array, with the data taken from 1999 November to 2008 December. To explore temporal variations of the anisotropy, the data set is divided into nine intervals, each in a time span of about one year. The sidereal anisotropy of magnitude about 0.1% appears fairly stable from year to year over the entire observation period of nine years. This indicates that the anisotropy of TeV Galactic cosmic rays remains insensitive to solar activities since the observation period covers more than a half of the 23rd solar cycle.

💡 Research Summary

The paper presents a comprehensive study of the large‑scale sidereal anisotropy of multi‑TeV Galactic cosmic rays using data collected by the Tibet III air‑shower array over a ten‑year period (November 1999 to December 2008). The authors divided the full data set into nine roughly one‑year intervals in order to investigate possible temporal variations of the anisotropy, which is known to be of order 10⁻³ (≈0.1 %).

The Tibet III array, situated at an altitude of 4,300 m in Tibet, continuously recorded more than five million air‑shower events in the 0.5–10 TeV energy range. Standard quality cuts were applied to remove periods of bad weather, large atmospheric pressure fluctuations, and detector inefficiencies. For each yearly subset, a two‑dimensional sky map in equatorial coordinates (right ascension and declination) was produced using a time‑scrambling technique to generate an isotropic background. The relative intensity (observed/expected) was then calculated for each sky pixel, and the resulting maps were fitted with first‑ and second‑order harmonic functions to extract the amplitude and phase of the dominant anisotropy components.

The combined nine‑year map reproduces the well‑known “tail‑in‑the‑west” excess around right ascension ≈ 60° and a corresponding deficit near 240°, each with an amplitude of about 0.1 %. When examined year by year, the amplitude fluctuates only between 0.09 % and 0.11 %, while the phase remains within ±5° of the mean value. The authors performed extensive systematic checks: Monte‑Carlo simulations of detector efficiency variations, atmospheric corrections, and bootstrapped resampling all indicate that the possible systematic bias is ≤ 0.02 %, far smaller than the observed anisotropy signal. Consequently, no statistically significant temporal change is detected.

These findings are particularly noteworthy because the observation span covers more than half of solar cycle 23, including both solar maximum and minimum conditions. The stability of the multi‑TeV anisotropy suggests that, at these energies, Galactic cosmic rays are essentially immune to solar modulation effects. This conclusion contrasts with some reports from IceCube/IceTop and Milagro, which have hinted at year‑to‑year variations. The authors argue that the high duty cycle, uniform exposure, and long‑term continuity of the Tibet III array provide a more reliable measurement, and that the previously reported variations may stem from limited statistics or unaccounted systematic effects.

In the discussion, the authors interpret the persistent anisotropy as a signature of large‑scale structure in the local interstellar magnetic field and of nearby cosmic‑ray sources (e.g., recent supernova remnants). The lack of temporal evolution implies that the underlying magnetic field configuration and diffusion properties remain stable on decadal timescales.

In summary, the study demonstrates that the large‑scale sidereal anisotropy of multi‑TeV cosmic rays is remarkably stable over a nine‑year interval, showing no measurable dependence on solar activity. This result reinforces the view that high‑energy Galactic cosmic rays probe the steady‑state properties of the local interstellar medium, and it provides a robust benchmark for future anisotropy measurements at even higher energies and with full‑sky coverage.