IceCube and HAWC unite efforts to dissect the cosmic-ray anisotropy

It was only a few years ago that IceCube provided the first view of the arrival direction distribution of cosmic rays in the Southern Hemisphere. Observations in the Northern Hemisphere, including those from the HAWC gamma-ray observatory earlier this year, had already shown that the number of cosmic rays hitting the atmosphere varied depending on their direction and energy. The anisotropy patterns found in the Southern Hemisphere supported models that pointed to the local interstellar magnetic field as the origin of the dominant effects of this observation.

In an attempt to better understand the anisotropy, the IceCube Neutrino Observatory and HAWC have united their efforts to study cosmic-ray arrival directions in both hemispheres at the same primary energy. The goal of this combined observation was to get a nearly full-sky coverage to study the propagation of cosmic rays with median energy of 10 TeV through our local interstellar medium as well as the interactions between interstellar and heliospheric magnetic fields. Results have just been accepted for publication in The Astrophysical Journal and include measurements on how the anisotropy modulations are distributed over different angular scales.

The all-sky distribution in relative intensity of 10 TeV cosmic rays (CR) obtained with the HAWC and IceCube observation. Blue means deficit with respect to the mean CR flux and red excess. On the left, the white arrow indicates the direction of motion of the solar system through the local interstellar medium; the black lines indicate the local interstellar magnetic field lines outside of the heliosphere. On the right, the view of the opposite side of the sky.

Cosmic rays swirling through space constantly bombard Earth from every direction. Out of every 1,000 cosmic rays there is at most one cosmic ray with a preferred (nonrandom) arrival direction. We refer to this as anisotropy, and this tiny 0.1% effect is what scientists would like to decipher.

The variations are small but significant and show two different amplitude scales, a large-scale anisotropy with variations of one per mille and a small-scale anisotropy with variations of one per ten thousand.

The cosmic-ray anisotropy is associated with the distribution of the cosmic ray sources

and with the properties of the magnetic fields through which the cosmic rays propagate. However, the limited field of view of any ground-based experiment prevents us from capturing the anisotropy features that are wider than the observable sky.

The angular variations of this anisotropy support the contribution of two different mechanisms: the mean propagation along the turbulent interstellar magnetic field, which is expected to isotropically diffuse cosmic rays, and the deflection in nearby magnetic fields—the local interstellar magnetic field (LIMF) and the heliosphere—whose relative contribution depends on energy.

Ground-based experiments typically require averaging the number of cosmic rays along each declination band, to estimate its response to a perfectly isotropic flux. This has the effect of washing out the vertical (north-south) component of the anisotropy. On the other hand, the heliospheric deflections induced on the cosmic-ray particle distribution by the long interstellar propagation are partially aligned along the LIMF and not significantly affected by the north-south blindness.

In this study, IceCube and HAWC joined efforts to get a full-sky coverage that captures for the first time a full, unbiased picture of the cosmic-ray anisotropy. The work used five years of IceCube data, from May 2011 to May 2016, and two years of HAWC data, from May 2015 and May 2017.

The fit of the IceCube-HAWC observed anisotropy at 10 TeV shows the expected alignment with the LIMF. Researchers then used this deviation to derive the north-south component of the dipole anisotropy.

Previous studies of the anisotropy have shown that the dominant dipole variation starts to decrease around 10 TeV and then to abruptly increase again at energies around 100 TeV. This had been explained as a possible effect of the heliosphere, which has a much larger impact for lower energy cosmic rays.

Deviations of the anisotropy from the LIMF could be due to the motion of the observer and/or to the effects of the heliosphere on the LIMF. However, only a full-sky study of the cosmic-ray anisotropy at different energies will make it possible to distinguish between these or other possible effects, thus enabling a deeper understanding of the properties of the LIMF and the heliosphere.

+ info “All-Sky Measurement of the Anisotropy of Cosmic Rays at 10 TeV and Mapping of the Local Interstellar Magnetic Field,” The HAWC and IceCube Collaborations: A.U. Abeysekara et al. The Astrophysical Journal 871, (2019) 96,