Galactic Cosmic Ray Anisotropy: Calculations of the Angular Power Spectrum

We present simulations of the cosmic-ray (CR) anisotropy down to TeV energies, employing turbulence parameters consistent with those inferred from interstellar observations. We compute the angular power spectra $C_{\ell}$ of the CR anisotropy obtained from the simulations. Our results demonstrate that the power spectrum exhibits a clear dependence on cosmic-ray energy and is particularly sensitive to the observer's location at small $\ell$ multipoles. Angular power spectrum are found to flatten at large $\ell$, and can be modelled by a broken power-law, exhibiting a break at $\ell \approx 4$. Our computed power spectrum at $\sim 10\,$TeV fits well HAWC and IceCube measurements. We further compute all spherical harmonic coefficients and derive the angular power spectrum component projected along the local magnetic field direction. Our analysis reveals that gyrotropic effects are more significant at lowerer CR energies and smaller $\ell$ multipole moments.