Solution to the cosmic ray anisotropy problem

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.

5 pages, 5 figures; extended discussion; published in PRL

References in corpus (9)

• Observation of an anomalous positron abundance in the cosmic radiation
• The Large Scale Cosmic-Ray Anisotropy as Observed with Milagro
• Evolution of the cosmic ray anisotropy above 10^{14} eV
• Observation of Cosmic Ray Anisotropy with the IceTop Air Shower Array
• A common solution to the cosmic ray anisotropy and gradient problems
• A Cosmic Ray Positron Anisotropy due to Two Middle-Aged, Nearby Pulsars?
• Numerical propagation of high energy cosmic rays in the Galaxy I: technical issues
• Implications of the Cosmic Ray Electron Spectrum and Anisotropy measured with Fermi-LAT
• CRT: A numerical tool for propagating ultra-high energy cosmic rays through Galactic magnetic field models

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• Escape model for Galactic cosmic rays and an early extragalactic transition
• Interpretation of the cosmic ray positron and antiproton fluxes
• Large-Scale Cosmic-Ray Anisotropy as a Probe of Interstellar Turbulence
• An observable electron-positron anisotropy cannot be generated by dark matter
• Comic ray flux anisotropies caused by astrospheres