Pitch-angle diffusion through localized interactions with sharp magnetic field bends in MHD turbulence

When a particle crosses a region of space where the curvature radius of the magnetic field line shrinks below the gyroradius $r_{\rm g}$, it experiences a non-adiabatic (magnetic moment violating) change in pitch-angle. The present paper carries that observation into MHD turbulence to examine the influence of intermittent, sharp bends of the magnetic field lines on particle transport. On the basis of dedicated measurements in a simulation of incompressible turbulence, it is argued that regions of sufficiently large curvature may exist in sufficient numbers on all scales to promote pitch-angle diffusion. The parallel mean free path predicted by the powerlaw statistics of the curvature strength scales as $\sim r_{\rm g}^{0.3}\,\ell_{\rm c}^{0.7}$ ($\ell_{\rm c}$ coherence scale of the turbulence), of direct interest to cosmic-ray phenomenology. Particle tracking in that numerical simulation confirms that the magnetic moment diffuses through localized, violent interactions, in agreement with the above picture. Correspondingly, the overall transport process is non-Brownian up to length scales $\gtrsim\ell_{\rm c}$.