Non-resonant heating of ions by Alfven waves with a spectrum

Non-resonant heating of ions by a monochromatic low-frequency Alfven wave, which propagates along the background magnetic field, has recently been investigated in a low beta plasma [Lu and Li, Phys. Plasmas 14, 042303(2007)]. Here, the monochromatic Alfven wave is generalized to a spectrum of Alfvén wave with random phase. It is found that ions can be non-resonantly heated with the same physical mechanism. First, ions are picked up in the transverse direction, and then phase difference (randomization) between ions due to their different parallel thermal motions leads to heating of ions. The heating is dominant in the direction perpendicular to the background magnetic field. The temperatures of ions at the asymptotic stage do not depend on individual waves in the spectrum, but are determined by the total wave amplitude. The effect of the initial ion bulk flow in the parallel direction on the heating is also considered.