Pitch-Angle Scattering of Charged Particles in Random Magnetic Fields

We discuss the physics of pitch-angle scattering of charged particles in random magnetic fields. We perform numerical simulations of the interaction of charged particles with synthesized turbulent magnetic fields which are time stationary and vary only in the direction along the mean field (the slab approximation). We confirm that the well-known quasi-linear theory remains valid for cases in which the energy contained in the random component of the magnetic field is much smaller than that in the mean field. However, the power in the region near the resonant wavenember can be very small even if the total integrated power is on the order of the mean field (squared). This is the case when the correlation length of the random magnetic field is much larger than the particle gyroradius. We show that for this case there is a disagreement between numerical simulations and the quasi-linear theory. This is not related to the scattering of the particles near ninety degrees pitch angle. We find, instead, that the effects of magnetic mirroring is more important. We also show that the pitch angle measured with respect to the local magnetic field exhibits diffusive behavior on a time scale which is much less than the time it takes to traverse a correlation length (Jokipii, Astrophys. J., 194, 465, 1974.). The relevant time scale is on the order of several gyroperiods. This is important with regards to numerical studies since the integration time can be significantly less than previously thought.