Pitch-angle diffusion of ions via nonresonant interaction with Alfvénic turbulence

The present discussion revisits the problem of nonresonant heating of ions by Alfvenic turbulence. It is shown that in the limit of weak Alfvenic turbulence it is appropriate to describe the nonresonant heating of protons as perpendicular pseudoheating. However, in a more general situation it is demonstrated that the more appropriate view of the nonresonant heating process is the pitch-angle scattering in the wave frame. The purpose of this paper is to generalize the earlier theory to the case in which the energy density of the turbulent Alfven waves is not necessarily very low. For weakly turbulent situation the present analysis confirms the earlier finding by Wu and Yoon [Phys. Rev. Lett. 99, 075001 (2007)], according to whom the nonresonant Alfven wave heating is described as leading to perpendicular pseudoheating of the protons. However, for more general situation the present paper demonstrates that pitch-angle scattering plays the principal role in the Alfven wave pseudoheating process, and thereby shows that the perpendicular heating discussed by Wu and Yoon is kinetic in nature, not attributable to fluid motion.