Solution to the cosmic ray anisotropy problem.

Area of Science:

• Astrophysics
• Cosmic Ray Physics

Background:

• The standard diffusive model for cosmic ray (CR) transport predicts a dipolar anisotropy in their arrival directions.
• This anisotropy is considered a tool for identifying nearby CR sources.
• Observed dipole amplitudes at TeV and PeV energies are significantly smaller than predicted by models.

Purpose of the Study:

• To critically examine the validity of the standard CR transport prediction.
• To investigate the reasons for the discrepancy between predicted and observed dipole amplitudes.
• To reconcile theoretical predictions with observational data.

Main Methods:

• Analyzing deviations of the dipole in specific turbulent magnetic field realizations from ensemble averages.
• Investigating the impact of misalignment between the regular magnetic field and the CR density gradient.

Main Results:

• A significant suppression of the dipole amplitude occurs when the magnetic field and CR gradient directions are nearly perpendicular (∼90°).
• This suppression reconciles the predicted dipole amplitude with observational data.
• The dipole direction generally does not align with the CR gradient direction.

Conclusions:

• The discrepancy between predicted and observed CR dipole anisotropies can be explained by considering specific magnetic field configurations and gradient misalignments.
• The findings resolve a long-standing problem in cosmic ray physics.
• The misalignment between dipole and gradient directions complicates the use of anisotropy as a direct probe for nearby cosmic ray sources.