Diffusion-Drift Transition as the Origin of the Cosmic Ray Knee

We performed test-particle simulations of cosmic rays (CRs) propagating in synthetic turbulent magnetic fields using the Monte Carlo code CRPropa 3. We evaluated the role of gradient-curvature drifts in the propagation and escape of Galactic CRs, as well as its connection to antisymmetric diffusion. We computed the escape timescales of CRs and found that for low turbulence levels and large halo sizes, a 'knee'-like spectral feature is predicted, attributed to the domination of drifts over perpendicular diffusion for CR escape. For a regular Galactic magnetic field with azimuthal symmetry, the predicted grammage at PeV energies is $\sim 1$ g cm$^{-2}$, larger than its extrapolated value considering the grammage at TeV energies estimated from direct CR observations. A possible alternative to reduce the CR grammage is the addition of a small field component in direction perpendicular to the Galactic Plane, a topology of the regular field we aim to study in the future.