Stochastic Fermi acceleration in turbulent fields with non-vanishing wave helicities

In the previous paper [Astropart. Phys. 10 (1999) 121] we showed that the opposite helicity circularly polarized Alfvén waves of finite amplitudes provide conditions to forward–backward asymmetry of particle scattering. In this letter we present an analytic solution of kinematic equation proving the enhancement of stochastic acceleration efficiency due to regular (asymmetry) term. The process is controlled by the ratio of the regular and the ordinary diffusion term.