Chaotic cosmic ray acceleration in nonlinear Alfven waves

Abstract

We demonstrate that motion of a particle in a single coherent linearly polarized (LP) Alfven wave with relative amplitude $\delta =B_w/B_0\gtrsim 0.25$ becomes chaotic. As a result, a sufficiently strong, near resonant Alfven waves ($r_L{k}_{\parallel }\sim 1$, $r_L$ is relativistic cyclotron radius, $k_{\parallel }$ is wavevector) can lead to transient trapping and phase mixing, imitating classical scattering. Evolution of pitch angle via Alfven chaos is non-diffusive - a particle on trapped trajectory can reverse its velocity in one scattering event. This both avoids the problem of the 90${}^{\circ }$ barrier, and increases the acceleration rate for those special particle.