Anisotropy in Cosmic-Ray Arrival Directions in the Southern Hemisphere Based on Six Years of Data From the Icecube Detector

Abstract

The IceCube Neutrino Observatory accumulated a total of 318 billion cosmic-ray-induced muon events between 2009 May and 2015 May. This data set was used for a detailed analysis of the sidereal anisotropy in the arrival directions of cosmic rays in the TeV to PeV energy range. The observed global sidereal anisotropy features large regions of relative excess and deficit, with amplitudes of the order of 10 −3 up to about 100 TeV. A decomposition of the arrival direction distribution into spherical harmonics shows that most of the power is contained in the lowmultipole (ℓ4) moments. However, higher multipole components are found to be statistically significant down to an angular scale of less than 10°, approaching the angular resolution of the detector. Above 100 TeV, a change in the morphology of the arrival direction distribution is observed, and the anisotropy is characterized by a wide relative deficit whose amplitude increases with primary energy up to at least 5 PeV, the highest energies currently accessible to IceCube. No time dependence of the large-and small-scale structures is observed in the period of six years covered by this analysis. The high-statistics data set reveals more details of the properties of the anisotropy and is potentially able to shed light on the various physical processes that are responsible for the complex angular structure and energy evolution.

Cosmic-Ray Small-Scale Anisotropies and Local Turbulent Magnetic Fields

“…The spectrum quantitatively indicates which multipole moments ℓ in the spherical harmonic expansion contribute to the observed map. The IceCube observatory provided a power spectrum of their Southern Hemisphere observation in the 10 TeV scale (Abbasi et al 2011a;Aartsen et al 2016) and the HAWC gamma-ray observatory provided one for the northern hemisphere in the TeV scale (Abeysekara et al 2014). The angular power spectrum is determined using the anafast tool in the HealPix framework.…”

TeV Cosmic-Ray Anisotropy from the Magnetic Field at the Heliospheric Boundary

“…Each infinitesimal rigidity interval produces a characteristic fingerprint pattern similar to those in the figures, deeply dependent on the properties of the heliospheric magnetic field. Observations reveal the overlapping of those characteristic distributions, and in fact, there is experimental evidence that coexisting anisotropy features originate from different energy ranges (see, e.g., Bartoli et al 2013;Aartsen et al 2016). Even though in this study we provide a window on the possible effects of CR composition, a detailed prediction of the observation requires fine-tuning of several effects, which is not within the scope of this work.…”

“…From an observational point of view, a statistically significant anisotropy has been observed by a variety of experiments, sensitive to different energy ranges (from tens of GeV to a few PeV), located on or below the Earth's surface in the Northern Hemisphere (Nagashima et al 1998;Hall et al 1999;Amenomori et al 2005Amenomori et al , 2006Amenomori et al , 2011Guillian et al 2007;Abdo et al 2009;Aglietta et al 2009;Zhang et al 2009;Munakata et al 2010;Cui et al 2011;de Jong et al 2011;Bartoli et al 2015) and in the Southern Hemisphere (Abbasi et al 2010(Abbasi et al , 2011(Abbasi et al , 2012Aartsen et al 2013Aartsen et al , 2016.…”

“…As expected, the shape of the angular power spectrum depends on particle rigidity. As in LX16, the figures shows the angular power spectrum from the IceCube observatory (Santander et al 2013;Aartsen et al 2016), the power spectrum from Ahlers (2014), and that corresponding to an isotropic arrival direction distribution of the same number of particles. In the figures, the power spectra are normalized to each other at the dipole component (i.e., ℓ=1).…”

“…The actual distribution is a combination of several angular scales (Amenomori et al 2007;Abdo et al 2008;Abbasi et al 2011;Bartoli et al 2013;Abeysekara et al 2014;Aartsen et al 2016) that can be studied by decomposing it into individual spherical harmonic contributions. This makes it possible to determine the angular power spectrum of the observed arrival distribution.…”