  The transport of charged particles in various astrophysical environments
permeated by magnetic fields is described in terms of a diffusion process,
which relies on diffusion-tensor parameters generally inferred from Monte-Carlo
simulations. Based on a red-noise approximation to model the two-point
correlation function of the magnetic field experienced by charged particles
between two successive times, the diffusion-tensor coefficients were previously
derived in the case of pure turbulence. In this contribution to ECRS2022, the
derivation is extended to the case of a mean field on top of the turbulence.
The results are applicable to a variety of astrophysical environments in
regimes where the Larmor radius of the particles is resonant with the power
spectrum of the turbulence wavelength (gyro-resonant regime), or where the
Larmor radius is greater than the largest turbulence wavelength (high-rigidity
regime).
