Cosmic ray propagation in sub-Alfvénic magnetohydrodynamic turbulence

Context. The propagation of cosmic rays or energetic charged particles in magnetized turbulence is a complex problem which involves non-linear wave-particle interactions and chaotic magnetic field lines transport. This problem has been addressed until recently using either analytical calculations or simulations using prescribed turbulence models. With the advent of super computers it is now possible to investigate energetic charged particle propagation using direct computation of electromagnetic fields. This is in particular the case for high-energy particles propagation in magnetohydrodynamic turbulence.
Aims: This work has the main objective to provide a detailed investigation of cosmic ray propagation in magnetohydrodynamic turbulent fields generated by forcing the fluid velocity field at large scales. It provides a derivation of the particle mean free path dependences in terms of the turbulence level described by the Alfvénic Mach number and in terms of the particle rigidity.
Methods: We use an upgrade version of the magnetohydrodynamic code RAMSES which includes a forcing module and a kinetic module and solve the Lorentz equation for each particle. The simulations are performed using a 3 dimension periodical box in the test-particle and magnetostatic limits. The forcing module is implemented using an Ornstein-Uhlenbeck process. An ensemble average over a large number of particle trajectories is applied to reconstruct the particle mean free paths.
Results: We derive the cosmic ray mean free paths in terms of the Alfvénic Mach numbers and particle reduced rigidities in different turbulence forcing geometries. The reduced particle rigidity is ρ = r_L/L where r_L is the particle Larmor radius and L is the simulation box length related to the turbulence coherence or injection scale L_inj by L ~ 5 L_inj. We have investigated with a special attention compressible and solenoidal forcing geometries.
Conclusions: We find that compressible forcing solutions are compatible with the quasi-linear theory or more advanced non-linear theories which predict a rigidity dependence as ρ^1/2 or ρ^1/3. Solenoidal forcing solutions at least at low or moderate Alfvénic numbers are not compatible with the above theoretical expectations and require more refined arguments to be interpreted. It appears especially for Alfvénic Mach numbers close to one that the wandering of field lines controls perpendicular mean free path solutions whatever the forcing geometry.