Cosmic-ray Anisotropy

Introduction

The IceCube and the HAWC observatories have established themselves as leaders in studying Galactic cosmic-ray anisotropy in the TeV–PeV energy range. IceCube captures anisotropy amplitudes with high precision by mapping cosmic-ray arrival directions relative to an isotropic reference. The IceTop surface array detects showers above 500 TeV, while the deep in-ice array records muons down to 10 TeV, both closely aligning with primary cosmic-ray directions. HAWC gamma-ray array detects showers above 1-10 TeV. IceCube and HAWC’s continuous sky observation enhances measurement stability, enabling energy-dependent anisotropy studies and spherical harmonic expansion analysis.

Recent findings highlight the dipole component’s amplitude and phase as indicators of cosmic-ray diffusion in interstellar plasma. The angular power spectrum at different energies reflects pitch angle scattering processes. IceCube has submitted results from 12 years (2011–2023) of cosmic-ray muon data, refining event selection for improved stability. High-resolution sky maps will explore temporal anisotropy variations and cross-check muon and shower data consistency.

IceCube also analyzes the Compton-Getting effect for calibration and cosmic-ray spectral index measurement. Given individual experiments’ limited sky coverage, full-sky measurements via collaborations with HAWC, GRAPES-3, TALE, and KASCADE aim to provide a comprehensive view of anisotropy. These efforts will improve understanding of cosmic-ray diffusion and heliospheric influence on observed distributions.

Publications

This is the list of cosmic-ray anisotropy results published by the team:

Observation of Cosmic-Ray Anisotropy in the Southern Hemisphere with 12 yr of Data Collected by the IceCube Neutrino Observatory

In our latest IceCube publication, we report a definitive, high-statistics view of TeV–PeV cosmic-ray anisotropy in the Southern Hemisphere using 7.92×10¹¹ cosmic-ray–induced muon events collected by the fully built detector between May 13, 2011 and May 12, 2023. With improved simulations and uniform, year-by-year processing, systematic uncertainties are reduced to levels below statistical fluctuations. The data confirm the long-noted evolution of the anisotropy’s angular structure between ~10 TeV and 1 PeV—most prominently across 100–300 TeV—and, for the first time in IceCube, present energy-dependent angular power spectra that show comparatively diminished large-scale power at the highest energies while retaining significant medium/small-scale structure down to ~6°.

Methodologically, the analysis builds reference maps via 24-hour time-scrambling and quantifies relative intensity per sky pixel, with top-hat smoothing (typically 20°; 5° for certain low-energy residual maps) used only for visualization; the median angular resolution is ~3° and reaches ~1° above 100 TeV. Systematics from the interference between sidereal anisotropy and the solar Compton–Getting effect are controlled by analyzing full calendar years and by checking the antisidereal frame; the resulting systematic spreads are now comparable to statistical errors—a marked improvement over earlier IceCube work.

Physically, the horizontal (equatorial) dipole component behaves as in other experiments: its amplitude decreases from ~10 TeV to just above 100 TeV—reaching a minimum of ≈2×10⁻⁴ while its phase flips from ~RA 50° to ~RA 260°—then rises again with energy. At ~5.3 PeV, the dipole is only marginally distinguishable from isotropy, yielding a 99%-CL upper limit of 3.34×10⁻³. The measured dipole phase and amplitude align with Northern-hemisphere results once field-of-view biases in one-dimensional fits are corrected, reinforcing a coherent, largely horizontal large-scale pattern across both hemispheres.

IceCube’s event selection and reconstruction underpin these results: the detector triggers at 2.0–2.3 kHz on muon bundles that track the parent cosmic-ray directions to within a few degrees, and a compact “DST” data stream preserves the essential kinematics for anisotropy analyses over the full 12-year exposure. Residual maps—after subtracting dipole and quadrupole—reveal persistent smaller-scale structure at both low (<18 TeV) and high (>320 TeV) energies. Together, these advances deliver the most precise Southern-sky anisotropy maps to date and a clean baseline for future joint, full-sky studies that probe how the anisotropy’s scale mix evolves with energy.

Time Variation in the TeV Cosmic Ray Anisotropy with IceCube and Energy Dependence of the Solar Dipole

Report on the analyses of the time variations in the cosmic-ray anisotropy and preliminary results on the measurement of the orbital Compton-Getting Effect observed by IceCube, presented at the ICRC 2025 Conference in Geneva. NSF award #2209483.

Investigating Energy-Dependent Anisotropy in Cosmic Rays with IceTop Surface Array

Report on the analysis of the cosmic-ray anisotropy measured by the IceTop surface array, presented at the ICRC 2025 Conference in Geneva. NSF award #2209483.

All-Sky Cosmic-Ray Anisotropy Update at Multiple Energies

Results on the combined analysis of cosmic-ray anisotropy with the HAWC observatory and with HAWC and IceCube observatories and the demonstration of rigidity scaling, presented at the ICRC 2025 Conference in Geneva. NSF award #2310092.