Stochastic acceleration and heating of ions by a non-sinusoidal magnetosonic wave

The stochastic non-resonant energization of ions in non-sinusoidal coherent plane-polarized magnetosonic waves is investigated. It is found that, as in the case of sinusoidal magnetosonic waves, the process does not follow quasi-linear theory, i.e. it is a diffusion in velocity space, but with a diffusion coefficient D ν^-α, α ≈ 1, instead of D ν^-3. Moreover, it is found that the presence of strong harmonics lowers the stochasticity threshold of the wave amplitude by as much as one order of magnitude (depending on the form of the power spectrum), but at the same time it suppresses the diffusion rate of the ions in velocity space. Our results indicate that waves with a saw-tooth wave form are the most efficient for ion energization.