Solution to the cosmic ray anisotropy problem

TL;DR

This paper investigates the discrepancy between predicted and observed cosmic ray anisotropy, showing that magnetic field configurations can suppress the expected dipole amplitude and misalignments complicate source identification.

Contribution

It demonstrates that specific magnetic field orientations can reconcile theoretical predictions with observations and highlights the impact of magnetic field misalignments on source searches.

Findings

Dipole amplitude can be suppressed when magnetic field and gradient are nearly perpendicular.

The dipole direction often does not align with the CR density gradient.

Magnetic field configurations influence the interpretation of cosmic ray anisotropy data.

Abstract

In the standard diffusive picture for transport of cosmic rays (CRs), a gradient in the CR density induces a typically small, dipolar anisotropy in their arrival directions. This has been widely advertised as a tool for finding nearby sources. However, the predicted dipole amplitude at TeV and PeV energies exceeds the measured one by almost two orders of magnitude. Here, we critically examine the validity of this prediction which is based on averaging over an ensemble of turbulent magnetic fields. We focus (1) on the deviations of the dipole in a particular random realisation from the ensemble average and (2) the possibility of a misalignment between the regular magnetic field and the CR gradient. We find that if the field direction and the gradient direction are close to $\sim 90^\circ$, the dipole amplitude is considerably suppressed and can be reconciled with observations, which sheds light on a long-standing problem. Furthermore, we show that the dipole direction in general does not coincide with the gradient direction, thus hampering the search for nearby sources.